Water Research 93 (2016) 289e295
Contents lists available at ScienceDirect
Water Research journal homepage: www.elsevier.com/locate/watres
Multi-dimensional water quality assessment of an urban drinking water source elucidated by high resolution underwater towed vehicle mapping €ger f Alan Lock a, b, *, Graeme Spiers b, c, Blair Hostetler d, James Ray e, Dirk Wallschla a
Environmental and Life Sciences Graduate Program, Trent University, Peterborough, Ontario, Canada MIRARCO, Laurentian University, Sudbury, Ontario, Canada School of the Environment, Departments of Chemistry, Earth Sciences and Biology, Laurentian University, Sudbury, Ontario, Canada d Department of Earth and Planetary Sciences, Macquarie University, North Ryde, Australia e Aquapath Canada Limited, Temagami, Ontario, Canada f Environmental and Resource Sciences Program, Department of Chemistry and Water Quality Centre, Trent University, Peterborough, Ontario, Canada b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 September 2015 Received in revised form 27 January 2016 Accepted 30 January 2016 Available online 13 February 2016
Spatial surveys of Ramsey Lake, Sudbury, Ontario water quality were conducted using an innovative underwater towed vehicle (UTV) equipped with a multi-parameter probe providing real-time water quality data. The UTV revealed underwater vent sites through high resolution monitoring of different spatial chemical characteristics using common sensors (turbidity, chloride, dissolved oxygen, and oxidation/reduction sensors) that would not be feasible with traditional water sampling methods. Multiparameter probe vent site identification is supported by elevated alkalinity and silica concentrations at these sites. The identified groundwater vent sites appear to be controlled by bedrock fractures that transport water from different sources with different contaminants of concern. Elevated contaminants, such as, arsenic and nickel and/or nutrient concentrations are evident at the vent sites, illustrating the potential of these sources to degrade water quality. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Monitoring UTV Vent detection Water quality Source water protection
1. Introduction Freshwater bodies, now recognized as a resource, require protection to ensure clean, sustainable drinking water for future generations. The first step in protecting this resource is to monitor water quality and the interpretation of the water quality monitoring results dictates if action is needed to protect or improve the water quality. Traditional monitoring techniques may include sampling tributaries, sampling at discrete depths in the water column, composite sampling (physically mixing of individual sample units) and water column volume weighted composite samples. These often labour intensive sampling methods can limit the number of sampling locations that are feasible in a given study. There is an implicit assumption that each sample location characterizes a region of a water body whose extent is limited by half the distance to the next sample location, often on the scale of
* Corresponding author. Environmental and Life Sciences Graduate Program, Trent University, Peterborough, Ontario, Canada. E-mail address: [email protected] (A. Lock). http://dx.doi.org/10.1016/j.watres.2016.01.059 0043-1354/© 2016 Elsevier Ltd. All rights reserved.
kilometres for large water bodies. This approach may produce results that are not representative of the true health of the system and not suited for accurate determination of potential contaminant sources. Steissberg et al. (2005) illustrate the spatial and vertical heterogeneity or patchiness for clarity, heat and general water quality associated with in-lake circulation of Lake Tahoe using synoptic satellite images. A major conclusion from this study is that in-situ, set point monitoring is unlikely to accurately represent the true water quality of a density stratified lake or reservoir. Howell (2006) shows that much near shore depth-related and locationdependent variability in water quality may not be detected using conventional methods. High resolution water quality monitoring using an underwater towed vehicle (UTV) with common water quality sensors provides a technique to better assess water quality and identify potential contaminant sources, such as groundwater vents, through changes in chemical characteristics that is less labour intensive than traditional methods, is more cost effective and increases data accuracy. Areas identified by the UTV as particular can be sampled discretely to further elucidate water quality concerns, such as, source waters enriched in nutrients, As and Ni.
290
A. Lock et al. / Water Research 93 (2016) 289e295
UTV technology has greatly improved over the last decade and has proved to be an operational asset for a variety of measurement tasks related to water column and seabed mapping (Hostetler and Ray, 2005; Statham et al., 2005; Yoerger et al., 2002; Yu et al., 2002; Mindell and Bingham, 2001; Whitcomb and Yoerger, 1993). These vehicles facilitate the identification of anomalies, like groundwater vent sites, in the water column and/or sediment surface by measuring chemical gradients and/or acoustic responses at high spatial resolutions while traversing an area. Although applications of this growing technology have focused on marine environments, initially used in the late 1970s by oil companies exploring the continental shelves for upwelling hydrocarbon traces, the physical and chemical signatures of surface runoff and upwelling groundwater in freshwater bodies can be identified by UTVs as anomalies. Reusch et al. (2015) identified a giant vent 10 m deep and 160 m diameter in Lake Neuchatel, Switzerland, using acoustic mapping. However, acoustic mapping for the purpose of identifying vent sites is limited because this technique measures physical characteristics like the formation of a basin, thus vent sites must be composed of material sufficiently fine (silt and clay) to be transported, the vent system must provide high energy fluvial transport and the formation must be of sufficient size to be identified by the meters to 10s of meters resolution for acoustic mapping. Monitoring with chemical sensors provides a tool with greater sensitivity for groundwater detection that permits identification of small and moderate sized groundwater vents. Groundwater vent sites are often discussed in the geological community when referring to hydrothermal structures or processes related to geysers or seabed vents. However, groundwater vent sites can occur in the bottom of freshwater bodies providing that the water table is higher than the lake bottom and that a permeable transport network, such as gravel or fractured bedrock, cuts through the substrate of the lake bottom (Church, 1996). Historically, groundwater has been considered a clean source of freshwater, but this false sense of security is rapidly changing as more groundwater is identified as contaminated with organic and inorganic pollutants. Baird (1999) provides a summary of the fate and transport of common groundwater contaminants. Groundwater residence times of 10's to 10's of thousands of years have the potential for long-term release of contaminants (Baird, 1999). A simple and accurate means of identifying groundwater that is upwelling in freshwater bodies is crucial to monitoring surface water resources. To conduct this research using traditional methods of water sample collection and discrete point sensor monitoring is not feasible due to time and financial limitations needed to obtain the thousands of data points used here to identify water quality anomalies. The purpose of this study is to identify anomalous groundwater quality inflows to Ramsey Lake using an underwater towed vehicle (UTV) hosting an array of common water quality monitoring sensors, assess the potential of the inflow as a source of As, Ni and nutrient contamination and indicate the potential land use contaminant source. Chemical differences between groundwater and surface water have been previously observed (Knowlers et al., 2010, Bussmanni et al., 1999). Areas of upwelling groundwater are generally characterized by higher levels of dissolved silica, alkalinity, turbidity, as well as lower dissolved oxygen (DO) and oxidation-reduction potential (ORP) than lake water, making these areas distinctive. Hostetler and Ray (2005) use a similar parameter array to identify groundwater vent sites that were sampled for mineral exploration. As the residence time for groundwater is high, rockewater interactions occur and, thus, the composition of groundwater is modified by the minerals in contact with it (Faure, 1991), for example carbonate rocks may increase groundwater alkalinity and the partial pressure of CO2 and silicates increase the Si
concentration. Diatoms efficiently remove dissolved silica from lake water through metabolic processes (Stumm and Morgan, 1996), thereby making dissolved silica-rich vent sites a more obvious anomaly. Alkalinity is often higher in groundwater because of microbial respiration causing an increase in the partial pressure of CO2 (Faure, 1991). Microbially mediated mineralization of organic matter in groundwater results in depletion of DO and lower ORP compared to lake waters (Stumm and Morgan, 1996). Because groundwater develops a “finger print” of the host substrate it travels through, potential contaminant sources can be elucidated. Turbidity, DO, and ORP sensors on the UTV provide good monitoring parameters for identifying areas of upwelling groundwater. However, the sensor array also included pH, temperature, Cl and conductivity that provide supporting or clarifying data. Water samples obtained from the identified upwelling groundwater sites have been analyzed for silica and alkalinity to confirm results obtained from the UTV. Arsenic, Fe, Ni, and NO3 analysis of these water samples is used to identify potential groundwater contaminants. 1.1. Physiography Ramsey Lake, covers nearly 800 ha and is located in the urban centre of the City of Greater Sudbury (CGS), Ontario at latitude 46 290 N and longitude 80 570 W (Fig. 1). The lake is approximately 8.5 km long and 4 km wide with the fetch orientated east to west. The west end of the lake is characterized by deeper water, with a maximum depth of approximately 22 m, and is surrounded by densely built residential properties. The eastern part of the lake is shallower with a typical depth of 8 m and residential properties are limited to the north shore, with the southern shoreline being a conservation area. Historically inflow has been suspected to be dominantly through groundwater sources, with the majority of springs in the eastern part of the lake. The north northeast trending fractures in the Nipissing Gabbro at the eastern end of the lake may be significant groundwater conduits. Northeast trending lenses of glaciofluvial material, extending from the northeastern shore, may also transport groundwater. The prominent Creighton Fault, trending east across the southern part of the lake potentially controls sites of groundwater eruption on the lake bottom in this area. Northwest trending fractures at the west end of the lake appear to control 2 or 3 sites of groundwater eruption. Ephemeral streams dominantly drain the conservation area and the urban infrastructure, however some small permanently flowing streams exist. 1.2. Background Ramsey Lake is the centre of a very important watershed in the City of Greater Sudbury (CGS) because of its role as a drinking water reservoir for over 50,000 residents. Since the late 19th century this watershed has been subject to acidic and metal-laden smelter emissions, leakage from septic systems, and runoff from a local sawmill, fertilizer and pesticide-rich golf course, residential lawns, and regional transportation networks. The result is a unique “living laboratory”, the survivor of over a century of extreme environmental insult, with an accumulation of 50e60 cm of metal and organic-rich sediment causing seasonally sub-oxic hypolimnic water at depth that must be managed and monitored to enable Ramsey Lake to be a continuing source of potable water for the city. In the past, dissolved metals became enriched in the hypolimnic water as oxygen was increasingly depleted with the progression of summer. In 2000, the intake pipe for drinking water was found to be drawing metal-enriched water into the treatment plant. After chlorination, oxidation of metals produced a gray precipitate that was observable at residential faucets. This water was deemed not
A. Lock et al. / Water Research 93 (2016) 289e295
291
Fig. 1. Map of Ramsey Lake high lighting shoreline use, geological structures (dashed lines represent faults) and groundwater vent sites.
potable. The simple, short-term solution was to raise the intake pipe to a depth above the seasonally sub-oxic level. This event was compounded by the identification of greater reservoir protection needs in light of the E. coli outbreak in nearby Walkerton, Ontario (Hrudey et al., 2002) resulting in the death of 7 people. The CGS had to increase their understanding of the chemical and physical dynamics of the reservoir to elucidate sustainability. To ensure the sustainability of this reservoir, a stronger understanding of the diverse input sources from the watershed was required. Although surface contaminant sources have been indicated by persistently elevated nutrient and metal concentrations in specific inflow streams, groundwater sources have the greatest potential to control water quality in this reservoir due largely to greater inflow volumes. Despite not having a hydrologic mass balance model, historically it had been suspected that groundwater inputs greatly exceeded surface water inputs. This was supported by consistently stable water levels during the summer and winter months when surface inputs are derived from small, dominantly ephemeral streams and large outflow volumes of drinking water, ca 400,000 m3/month (Greater Sudbury, 2009), are drawn. Using a mass balance approach, the maintenance of constant volume with known significant discharge, sufficient to supplying 50,000 residence with potable water, must be balanced by a significant inflow (Chapra, 1997). With no observed surface inflow transporting significant quantities of water during most summer and winter months, groundwater inflows are likely the dominant inflow sources. 2. Methods A custom-built underwater towed vehicle (UTV) tethered to a boat was used to map water quality recording a suite of parameters every 30 s while slowly traversing the water body. The UTV is constructed of an open body stainless steel rectangular prism frame, long axis running front to back, with a vertical fin attached on the upper rear portion of the frame to provide stability during towing. Four equal length chains, each attached to the top corner of the UTV and join at a central point above the UTV where the tether from the vessel is fastened. The UTV provides a structure to house and protect equipment mounted within the framework, rapid circulation of water around sensors and sufficient weight to sink while
in tow. Equipment mounted in the UTV includes a data sonde, underwater camera and high intensity light. A differential global positioning system tracks the location of the vessel, a marine depth sounder identifies the water depth at the vessel location and a depth sensor incorporated into the data sonde that is attached to the UTV monitors the depth that water quality parameters are recorded. The lag of the UTV behind the boat is calculated from the monitored depth of the UTV and the length of tethered rope released. The calculated lag distance is converted into a time delay using the traversing speed and corrected for by off setting the GPS and sonde results by the calculated time lag. An underwater camera and high intensity light attached to the front of the UTV allow an operator to visually monitor the UTV distance from the lake sediments. The UTV tether is attached to a winch mounted on the vessel that the operator controls to maintain the desired 1 m depth above the lake sediments. The sonde parameters are temperature, dissolved oxygen (DO), pH, conductivity, oxidation-reduction potential (ORP), turbidity, and chloride. All sensors are standard factory supplied sensors and daily calibrations were conducted following methods outlined in the user manual (Hydrolab Corporation, 1997). During the July 2002 mapping event the UTV was suspended 1 m above the sediment to identify upwelling groundwater. While traversing, the sonde records a suite of data from the sensor array every 30 s. A total of 2451 locations were sampled for the 7 parameters resulting in 17,157 readings recorded during the 3 day mapping period. The lake-bottom map tracking is illustrated in Fig. 2. The individual dots making up the map tracking represent the locations that water quality data were recorded. The water logger data were complimented by 24 water samples taken on the fourth day from strategic locations identified by the underwater towed sensor array as having anomalous water quality and areas having widespread water quality. Discrete samples were taken at select depths and locations by pumping water to the surface using a peristaltic pump. The sampling tube was thoroughly flushed with water from the desired location before taking the sample. The samples were filtered in the field using a syringe and 0.45 mm filters and submitted to the Ontario Geological Laboratory for analysis of anions using ion chromatograph (Pamer, 2011) and major and trace metals by ICP-OES (Pamer, 2007) and ICP-MS (Burnham, 2002). Interpolated data were plotted using Surfer, a graphing software package.
292
A. Lock et al. / Water Research 93 (2016) 289e295
Fig. 4. Turbidity measurements 1 m above the sediment for July 2002. Elevated values (dark) at sites 1 to 6 indicate areas of upwelling groundwater. Fig. 2. Underwater towed vehicle map tracking for July 2002.
Ramsey Lake is dimictic, vertically mixing in the spring and fall. Summer thermo-stratification divides the lake into two laminar flow circulation cells; the epilimnion and hypolimnion. The epilimnion, susceptible to wind, has a higher turbulence than the hypolimnion that is shielded by the thermocline. Areas with higher turbulence have greater potential to dilute the upwelling groundwater “finger print” whereas areas with low turbulence and mixing permit detection of upwelling groundwater indicators. In July 2002, the top of the thermocline was near 6 m (Fig. 3), permitting potential upwelling groundwater detection throughout most of the lake. The background chemistry of hypolimnic waters in many urban lakes and groundwater vents sites have similar chemical characteristics observed by the UTV sensor array. However, turbidity is a vent site indicator that does not reveal similar results to deep basin water quality. Turbidity is an indirect measure of water clarity and is often related to flow rate. Higher flows may increase the amount of suspended material, decreasing water clarity. Chloride concentration typically increases with depth due to greater density. DO and ORP commonly decrease with depth and have a minimum at the greatest depth. Ramsey Lake is an excellent location to validate
this groundwater vent detection technique because many of the source locations are not the deepest parts of the lake, hence there should be a discernable chemical contrast between groundwater vents and ambient lake water. Therefore, the relative differences of chloride, DO and ORP measured at vent sites should be compared to mean lake values from the same depths as the vent sites. Figs. 4 and 5, respectively, show elevated turbidity and chloride and Figs. 6 and 7, respectively, show low DO and ORP on the east end between the two small islands (site 1), south of site 1 along the Creighton fault (site 2), southern most bay (site 3), and sites 4e6 at the west end of the lake. Vent site locations are clearly depicted on Fig. 1. These results are supported by water samples taken about 1 m above the lake bottom at the vent sites. Alkalinity and dissolved silica are excellent parameters to distinguish between lake and groundwater because there are no diatoms in groundwater and, therefore, silica concentrations only reflect equilibrium with quartz or clay. The partial pressure of carbon dioxide is higher in groundwater than in lake water. Table 1 shows greater alkalinity and silica with respect to mean lake values (n ¼ 24) at all the vent sites. Water samples were not taken directly above site 5 because during mapping sites 5 and 6 were suspected to be a single site. However, after data processing 2 distinct sites were identified. The alkalinity concentration directly above vent sites 1 and 2 are, respectively, 8.9 (±4.8) and 8.2 (±4.6) mg/L greater than the mean lake concentration plus 2 times the standard deviation (38.6 mg/L), illustrating with 95% confidence these samples are significantly different than the sampled population (McGrew and Monroe, 1993). Although alkalinity at vent sites 3, 4 and 6 do not have
Fig. 3. Temperature distribution of monitored points for July 2002 revealing approximate lake thermal structure.
Fig. 5. Chloride concentrations 1 m above the sediment for July 2002. Elevated concentrations (dark) at sites 1 to 6 indicate areas of upwelling groundwater.
3. Results/discussion
A. Lock et al. / Water Research 93 (2016) 289e295
Fig. 6. Percent saturation of dissolved oxygen 1 m above the sediment for July 2002. Low values (dark) at sites 1 to 6 indicate upwelling groundwater.
Fig. 7. Oxidation-reduction potential 1 m above the sediment for July 2002. Low values (dark) at sites 1 to 6 indicate upwelling groundwater.
293
located 20 km southwest of vent sites 1 and 2 (Ministry of Northern Development and Mines (1983)), intercepting the strike lines of off set faults at these vent sites, thus providing a likely source for the elevated As concentration. Metal concentrations at this site (Fig. 8, 8 m depth) are well above median lake concentrations (Table 1) showing this is a source of contamination to the lake and must be investigated in greater detail to ascertain what degree this may impact the lake. The concentrations in Fig. 8 for Ni, As, Fe, alkalinity, Si, and nitrate are normalized so the maximum concentration of each metal is represented by a value of 1 and lower concentrations are proportional to this normalization. This serves 2 purposes; the first, all parameters can be plotted on the same graph and second, slopes of the concentration gradients can be directly compared between each of the metals. Observed decrease in alkalinity and Si concentrations with increasing distance above the vent site support these parameters are indicators of groundwater vent sites that has previous been discussed above. Nitrate concentration is similar 1 m and 3 m above vent site 1 revealing this is not a significant source at this location. Nickel, As and Fe transported from this vent site may be of environmental concern, thus discussed in greater detail below. The rapid decrease in concentration of Ni, As and Fe away from the vent is due to mixing with lake water resulting in dilution and/ or a removal mechanism from the water column like adsorption and sedimentation, reaching ca. mean lake concentrations (Table 1) below 1.5 m depth. Ferrous iron rapidly oxidizes and forms (oxy) (hydr)oxides in natural lake waters (Peters and Blum, 2003). This particulate readily binds arsenate and settles to the lake bottom therefore significantly decreasing arsenic bioavailability, however arsenite does not complex with iron (oxy) (hydr)oxides in natural waters as readily as arsenate (Peters and Blum, 2003) and arsenite oxidation in water with dissolved oxygen as the oxidizing agent requires 1e2 months (Cherry et al., 1979). The near identical slopes
Table 1 Mean lake concentrations compared to 1 m above the sediment surface at vent sites 1, 2, 3, 4, and 6 for July 2002 (sd ¼ standard deviation). The analytical standard deviation is presented beside the individual site concentrations and the whole lake sample population standard deviation is presented beside the mean concentrations. Analytical Parameter
Mean (n ¼ 24)
Site 1
Site 2
Site 3
Site 4
Site 5
Site 6
Alkalinity (mg/L as CaCO3) Silica (mg/L) Arsenic (mg/L) Iron (mg/L) Nickel (mg/L) Nitrate (mg/L)
35.2 (1.7sd) 450 (436sd) 2 (3.1sd) 28 (462sd) 71 (22.1sd) 100 (727sd)
47.5 (2.4sd) 2500 (125sd) 12 (0.4sd) 1603 (80sd) 118 (6sd) 20 (1sd)
46.8 (2.3sd) 2400 (120sd) 13 (0.5sd) 1742 (87sd) 120 (6sd) 140 (7sd)
37.8 (1.9sd) 1700 (85sd) 3 (0.1sd) 149 (8sd) 129 (7sd) 60 (3sd)
41.3 (2.1sd) 2400 (120sd) 4 (0.2sd) 229 (12sd) 106 (5sd) 490 (25sd)
N/A N/A N/A N/A N/A N/A
41.3 (2.1sd) 2000 (100sd) 3 (0.1sd) 167 (8sd) 94 (5sd) 3630 (182sd)
95% confidence of being significantly different than the sampled population they have 68% confidence of it being significantly different. Silica concentration directly above all sampled vent sites reveals with 95% confidence that these samples are significantly different than the sampled population. Corroboration of these findings confirms the UTV method for identification of upwelling groundwater is valid for some lakes. Groundwater emergence sites 1 and 2 have typically strongest signals shown in Figs. 4e7 and highest alkalinity and silica concentrations (Table 1) indicating this site has significant potential for contaminant loading from groundwater sources. The gradient of a profile at site 1 for select parameters (Fig. 8) strongly indicates Ni, As, Fe, alkalinity, and Si are transported with the groundwater to the lake water, whereas nitrate is not significantly transported at this vent site. The high abundance of local, metal-rich bedrock suggests these metal sources are likely natural. For example, an abandoned gold mine with ca 250,000 tons of As bearing tailing is
Fig. 8. Select chemical profiles from the water column at vent site 1, July 2002. The maximum concentration is normalized to a value of 1 and other concentrations are proportional to this normalization for each metal.
294
A. Lock et al. / Water Research 93 (2016) 289e295
of As and Fe depletion (Fig. 8) suggests co-transport, thus arsenate is the dominant species at depths shallower than the 6 m. Compared to Fe and As, Ni is removed less efficiently from the water column illustrated by the lower slope of Ni from 1.5 to 6 m (Fig. 8). Lock et al. (2003) observed nickel primarily remaining dissolved in the nearby Kelly Lake, but suggest that a fraction of nickel does adsorb to manganese oxides and settles to the sediment. With dilution acting equally on Fe, As and Ni, the lower removal rate for Ni must be due to less adsorption and sedimentation. Moderate to low intensities of vent detection parameters at site 3 (Figs. 4e7 and alkalinity and silica concentrations from Table 1) suggest this site does not have as great a potential to have a deleterious impact on lake water quality as vent site 1. Of the contaminants examined in this study, only nickel concentration is significantly higher at this site (Table 1). The water chemistry from this vent is different from vent 1. The transport network for vent site 1 is an off set fault and for vent site 3 it is a dyke (Fig. 1). The difference in chemistry may be due to different rockewater interactions or the water origins may be different. Water sampled 1 m above vent sites 4 and 6 have 4.9 and 36 times more nitrate, respectively, than the mean lake concentration (Table 1) indicating this may be a significant source of contaminated groundwater. These vent sites are located directly above northwest trending faults. Highly plausible sources of the nitrate is the golf course located south of the vent sites 5 and 6 and Bethel Lake, a eutrophic water body located southeast of vent site 4 (Fig. 1). Nitrate is often considered the secondary limiting nutrient for lake eutrophication (Schnoor, 1996), and, therefore, should be monitored closely. This data may be beneficial here to guide land use management. Elevated Ni concentrations, with mean lake concentrations (Table 1) nearly 3 times the provincial water quality objectives (25 ug/L) (Ontario Ministry of the Environment, 1994), reveals this resource must be monitored and managed to ensure a safe and sustainable drinking water source. The sensitivity of this system, under its current use, is likely high because there are multiple and natural sources of Ni that would be difficult to isolate from the lake and seasonally sub-oxic hypolimnic water limiting the lake's potential to efficiently remove Ni from the water column and store it in the sediments. Identified sources of large nutrient pools seeping into the lake that feed the eutrophication process will sustain a seasonally sub-oxic hypolimnic environment that can increase Ni mobility. The action of the City of Greater Sudbury to increase the height of the drinking water in-take pipe to a depth above the hypolimnion provided a drinking water source with increased water quality and should increase the sustainability of this reservoir. Results from the UTV mapping are also being used to assist with land use planning in the drinking water reservoir watershed. 4. Conclusions Areas of upwelling groundwater were detected in Ramsey Lake using an UTV hosting dissolved oxygen, oxidation-reduction potential, turbidity, and chloride sensors that would most likely be missed using only traditional sampling methods. Elevated silica and alkalinity near the groundwater vent sites indicated by the UTV corroborated the validity of this method for upwelling groundwater site identification. Groundwater vent site identification using an UTV is facilitated in environments with limited turbulent flow like that found below the thermocline. Although these tools have primarily been used in marine environments and for resource exploration, freshwater and environmental applications may prove to be beneficial especially for drinking water source protection. Potential surface runoff and
groundwater contaminant sources have been identified in Ramsey Lake and this information will be used to guide future research. Additional mapping to elucidate potential seasonal differences of groundwater flow during spring and fall would compliment the July data presented here. Current results show individual vent sites have different water chemistry that is partly due to different water source origins and partly due to different rockewater interactions. Ramsey Lake water quality is heterogeneous both horizontally and vertically; therefore traditional discrete sampling methods have the potential to not determine water quality in Ramsey Lake accurately due to the less obvious inflow sources, such as, groundwater vents. It is strongly recommended that UTVs are used during early stages of drinking water source protection research to elucidate hidden inflow sources and assist in project development, but seasonal variation may be significant. The high resolution data obtain with UTV mapping facilitates in-lake simulations by providing a high resolution data set for model calibration and validation. Acknowledgments This research was partly funded by the City of Greater Sudbury, Ontario, Canada and NSERC Industrial Postgraduate Scholarship (IPS). References Baird, C., 1999. Environmental Chemistry. W.H. Freeman and Company, New York, NY, p. 555. Burnham, O., 2002. 34. Improvements in the Accuracy and Sensitivity of Water Analyses by the Application of Inductivity Coupled Plasma Dynamic Reaction Cell™ Mass Spectrometry. Ontario Geological Survey, Open File Report, vol. 6100, 34e41e34e39. Bussmanni, I., Dando, P., Niven, S., Suess, E., 1999. Groundwater seepage in the marine environment: role for mass flux and bacterial activity. Mar. Ecol. Prog. Ser. 178, 169e177. Chapra, S., 1997. Surface Water-Quality Modeling. The McGraw-Hill Companies, Inc., New York, NY, p. 382. Cherry, J., Shaikh, D., Tallman, D., Nicholson, R., 1979. Arsenic species as an indicator of redox conditions in groundwater. J. Hydrol. 43, 373e392. Church, T., 1996. An underground route for the water cycle. Nature 380, 579e580. Faure, G., 1991. Principles and Applications of Inorganic Geochemistry. MacmillanPublishing Company, New York, NY, p. 626. Greater Sudbury, 2009. Water Works Summary Report. Large Municipal-Residential Systems, 2012. http://www.greatersudbury.ca/limkservid/919E70E7-98. Hostetler, P., Ray, J., 2005. Mineral exploration by systematic analysis of groundwaters upwelling in lakes and rivers. In: Ontario Mineral Exploration Technologies Program. P02-03-036a. Howel, E., 2006. Fine-scale analysis of water quality on the SE shores of Lake Huron: relevance and approach. In: Proceedings 41st Central Canadian Sympossium on Water Quality Research, 20. Hrudey, S., Huck, P., Payment, P., Gillham, R., Hrudey, E., 2002. Walkerton: lessons learned in comparison with waterbourne outbreaks in the developed world. J. Environ. Eng. Sci. 1, 397e407. Hydrolab Corporation, 1997. DataSonde 4 and MiniSonde Water Quality Multiprobes User Manual, Quality Multiprobes User Manual, 12921 Burnet Road, Austin, Texas, USA, 278. Knowlers, L., Katz, B., Toth, D., 2010. Using multiple chemical indicators to characterize and determine the age of groundwater from select vents of the silver springs group, central Florida USA. Hydrogeol. J. 18 (8), 1825e1838. Lock, A., Pearson, D., Spiers, G., 2003. Early diagenesis of sediment from Kelly Lake, Sudbury, Ontario e a lake contaminated by sewage effluent and high levels of copper and nickel from mining and smelting. In: Proceedings Mining and the Environment III, Sudbury 2003,7-D. McGrew, C., Monroe, C., 1993. In: Meyers, M. (Ed.), An Introduction to Statistical Problem Solving in Geography. Wm. C. Brown Communications, Inc,, Dubuque, IA, p. 305. Mindell, D., Bingham, B., 2001. New archaeological uses of autonomous underwater vehicles. In: Proceedings Institute of Electrical and Electronic Engineers, Modular Training Systems, pp. 555e558. Ministry of Northern Development and Mines, 1983. MDI File MDI41I06SE00002. http://www.geologyontario.mndmf.gov.on.ca/gosportal/gos? command¼mndmsearchdetails:mdi&uuid¼MDI41I06SE00002. Ontario Ministry of the Environment, 1994. Water Management, Ontario Water Quality Objectives. Queen’s Printer for Ontario. ISBN 0-7778-8473-9rev. http:// www.ene.gov.on.ca/envision/gp/3303e.htm.
A. Lock et al. / Water Research 93 (2016) 289e295 Pamer, L., 2011. 34. Analysis of Anions in Waters by Ion Chromatography at the Geoscience Laboratories. Ontario Geological Survey, Open File Report, vol. 6270, 34e1e34e5. Pamer, L., 2007. 32. Major and Trace Element Analysis of Water Samples by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-OES) at the Geoscience Laboratories: Preliminary Data. Ontario Geological Survey, Open File Report 6213, 32e1e32e7. Peters, S., Blum, J., 2003. The source and transport of arsenic in a bedrock aquifer, New Hampshire, USA. Appl. Geochem. 18, 1773e1787. Reusch, A., Loher, M., Bouffard, D., Moernaut, J., Hellmich, F., Anselmetti, F., Bernasconi, S., Hilbe, M., Kopf, A., Lilley, M., Meinecke, G., Strasser, M., 2015. Giant lacustrine pockmarks with subaqueous groundwater discharge and subsurface sediment mobilization. Geophys. Res. Lett. 42, 3465e3473. Schnoor, J., 1996. In: Schnoor, J., Zehnder, A. (Eds.), Environmental Modeling: Fate and Transport of Pollutants in Water,air, and Soil. John Wiley and Sons, Inc, New York, NY, p. 682. Statham, P., Connelly, D., German, C., Brand, T., Overnell, J., Bulukin, E., Millard, N., Mcphai, S., Pebody, M., Perrett, J., Squire, M., Stevenson, P., Webb, A., 2005.
295
Spatially complex distribution of dissolved manganese in a fjord as revealed by high-resolution in situ sensing using the autonomous underwater vehicle autosub. Environ. Sci. Technol. 39, 9440e9445. Steissberg, T., Hook, S., Schladow, G., 2005. Characterizing partial upwellings and surface circulation at Lake Tahoe, California eNevada, USA with thermal infrared images. Remote Sens. Environ. 99, 2e15. Stumm, W., Morgan, J., 1996. Aquatic Chemistry e Chemical Equilibria and Rates in Natural Waters, 1040 Natural Waters. John Wiley and Sons Inc,, New York, NY, p. 1040. Whitcomb, L., Yoerger, D., 1993. A new distributed real-time control system for the jason underwater robot. In: Proceedings Institute of Electrical and Electronic Engineers, Intelligent Robots and Systems, pp. 368e374. Yoerger, D., Collier, R., Bradley, A., 2002. Hydrothermal vent plume discovery and survey with an autonomous underwater vehicle. In: Proceedings American Geophysical Union, T11C-1261. Yu, X., Dickey, T., Bellingham, J., Manov, D., Streitlien, K., 2002. The application of autonomous underwater vehicles for interdisciplinary measurements in Massachusetts and cape cod bays. Cont. Shelf Res. 22, 2225e2245.
Contents lists available at ScienceDirect
Water Research journal homepage: www.elsevier.com/locate/watres
Multi-dimensional water quality assessment of an urban drinking water source elucidated by high resolution underwater towed vehicle mapping €ger f Alan Lock a, b, *, Graeme Spiers b, c, Blair Hostetler d, James Ray e, Dirk Wallschla a
Environmental and Life Sciences Graduate Program, Trent University, Peterborough, Ontario, Canada MIRARCO, Laurentian University, Sudbury, Ontario, Canada School of the Environment, Departments of Chemistry, Earth Sciences and Biology, Laurentian University, Sudbury, Ontario, Canada d Department of Earth and Planetary Sciences, Macquarie University, North Ryde, Australia e Aquapath Canada Limited, Temagami, Ontario, Canada f Environmental and Resource Sciences Program, Department of Chemistry and Water Quality Centre, Trent University, Peterborough, Ontario, Canada b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 September 2015 Received in revised form 27 January 2016 Accepted 30 January 2016 Available online 13 February 2016
Spatial surveys of Ramsey Lake, Sudbury, Ontario water quality were conducted using an innovative underwater towed vehicle (UTV) equipped with a multi-parameter probe providing real-time water quality data. The UTV revealed underwater vent sites through high resolution monitoring of different spatial chemical characteristics using common sensors (turbidity, chloride, dissolved oxygen, and oxidation/reduction sensors) that would not be feasible with traditional water sampling methods. Multiparameter probe vent site identification is supported by elevated alkalinity and silica concentrations at these sites. The identified groundwater vent sites appear to be controlled by bedrock fractures that transport water from different sources with different contaminants of concern. Elevated contaminants, such as, arsenic and nickel and/or nutrient concentrations are evident at the vent sites, illustrating the potential of these sources to degrade water quality. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Monitoring UTV Vent detection Water quality Source water protection
1. Introduction Freshwater bodies, now recognized as a resource, require protection to ensure clean, sustainable drinking water for future generations. The first step in protecting this resource is to monitor water quality and the interpretation of the water quality monitoring results dictates if action is needed to protect or improve the water quality. Traditional monitoring techniques may include sampling tributaries, sampling at discrete depths in the water column, composite sampling (physically mixing of individual sample units) and water column volume weighted composite samples. These often labour intensive sampling methods can limit the number of sampling locations that are feasible in a given study. There is an implicit assumption that each sample location characterizes a region of a water body whose extent is limited by half the distance to the next sample location, often on the scale of
* Corresponding author. Environmental and Life Sciences Graduate Program, Trent University, Peterborough, Ontario, Canada. E-mail address: [email protected] (A. Lock). http://dx.doi.org/10.1016/j.watres.2016.01.059 0043-1354/© 2016 Elsevier Ltd. All rights reserved.
kilometres for large water bodies. This approach may produce results that are not representative of the true health of the system and not suited for accurate determination of potential contaminant sources. Steissberg et al. (2005) illustrate the spatial and vertical heterogeneity or patchiness for clarity, heat and general water quality associated with in-lake circulation of Lake Tahoe using synoptic satellite images. A major conclusion from this study is that in-situ, set point monitoring is unlikely to accurately represent the true water quality of a density stratified lake or reservoir. Howell (2006) shows that much near shore depth-related and locationdependent variability in water quality may not be detected using conventional methods. High resolution water quality monitoring using an underwater towed vehicle (UTV) with common water quality sensors provides a technique to better assess water quality and identify potential contaminant sources, such as groundwater vents, through changes in chemical characteristics that is less labour intensive than traditional methods, is more cost effective and increases data accuracy. Areas identified by the UTV as particular can be sampled discretely to further elucidate water quality concerns, such as, source waters enriched in nutrients, As and Ni.
290
A. Lock et al. / Water Research 93 (2016) 289e295
UTV technology has greatly improved over the last decade and has proved to be an operational asset for a variety of measurement tasks related to water column and seabed mapping (Hostetler and Ray, 2005; Statham et al., 2005; Yoerger et al., 2002; Yu et al., 2002; Mindell and Bingham, 2001; Whitcomb and Yoerger, 1993). These vehicles facilitate the identification of anomalies, like groundwater vent sites, in the water column and/or sediment surface by measuring chemical gradients and/or acoustic responses at high spatial resolutions while traversing an area. Although applications of this growing technology have focused on marine environments, initially used in the late 1970s by oil companies exploring the continental shelves for upwelling hydrocarbon traces, the physical and chemical signatures of surface runoff and upwelling groundwater in freshwater bodies can be identified by UTVs as anomalies. Reusch et al. (2015) identified a giant vent 10 m deep and 160 m diameter in Lake Neuchatel, Switzerland, using acoustic mapping. However, acoustic mapping for the purpose of identifying vent sites is limited because this technique measures physical characteristics like the formation of a basin, thus vent sites must be composed of material sufficiently fine (silt and clay) to be transported, the vent system must provide high energy fluvial transport and the formation must be of sufficient size to be identified by the meters to 10s of meters resolution for acoustic mapping. Monitoring with chemical sensors provides a tool with greater sensitivity for groundwater detection that permits identification of small and moderate sized groundwater vents. Groundwater vent sites are often discussed in the geological community when referring to hydrothermal structures or processes related to geysers or seabed vents. However, groundwater vent sites can occur in the bottom of freshwater bodies providing that the water table is higher than the lake bottom and that a permeable transport network, such as gravel or fractured bedrock, cuts through the substrate of the lake bottom (Church, 1996). Historically, groundwater has been considered a clean source of freshwater, but this false sense of security is rapidly changing as more groundwater is identified as contaminated with organic and inorganic pollutants. Baird (1999) provides a summary of the fate and transport of common groundwater contaminants. Groundwater residence times of 10's to 10's of thousands of years have the potential for long-term release of contaminants (Baird, 1999). A simple and accurate means of identifying groundwater that is upwelling in freshwater bodies is crucial to monitoring surface water resources. To conduct this research using traditional methods of water sample collection and discrete point sensor monitoring is not feasible due to time and financial limitations needed to obtain the thousands of data points used here to identify water quality anomalies. The purpose of this study is to identify anomalous groundwater quality inflows to Ramsey Lake using an underwater towed vehicle (UTV) hosting an array of common water quality monitoring sensors, assess the potential of the inflow as a source of As, Ni and nutrient contamination and indicate the potential land use contaminant source. Chemical differences between groundwater and surface water have been previously observed (Knowlers et al., 2010, Bussmanni et al., 1999). Areas of upwelling groundwater are generally characterized by higher levels of dissolved silica, alkalinity, turbidity, as well as lower dissolved oxygen (DO) and oxidation-reduction potential (ORP) than lake water, making these areas distinctive. Hostetler and Ray (2005) use a similar parameter array to identify groundwater vent sites that were sampled for mineral exploration. As the residence time for groundwater is high, rockewater interactions occur and, thus, the composition of groundwater is modified by the minerals in contact with it (Faure, 1991), for example carbonate rocks may increase groundwater alkalinity and the partial pressure of CO2 and silicates increase the Si
concentration. Diatoms efficiently remove dissolved silica from lake water through metabolic processes (Stumm and Morgan, 1996), thereby making dissolved silica-rich vent sites a more obvious anomaly. Alkalinity is often higher in groundwater because of microbial respiration causing an increase in the partial pressure of CO2 (Faure, 1991). Microbially mediated mineralization of organic matter in groundwater results in depletion of DO and lower ORP compared to lake waters (Stumm and Morgan, 1996). Because groundwater develops a “finger print” of the host substrate it travels through, potential contaminant sources can be elucidated. Turbidity, DO, and ORP sensors on the UTV provide good monitoring parameters for identifying areas of upwelling groundwater. However, the sensor array also included pH, temperature, Cl and conductivity that provide supporting or clarifying data. Water samples obtained from the identified upwelling groundwater sites have been analyzed for silica and alkalinity to confirm results obtained from the UTV. Arsenic, Fe, Ni, and NO3 analysis of these water samples is used to identify potential groundwater contaminants. 1.1. Physiography Ramsey Lake, covers nearly 800 ha and is located in the urban centre of the City of Greater Sudbury (CGS), Ontario at latitude 46 290 N and longitude 80 570 W (Fig. 1). The lake is approximately 8.5 km long and 4 km wide with the fetch orientated east to west. The west end of the lake is characterized by deeper water, with a maximum depth of approximately 22 m, and is surrounded by densely built residential properties. The eastern part of the lake is shallower with a typical depth of 8 m and residential properties are limited to the north shore, with the southern shoreline being a conservation area. Historically inflow has been suspected to be dominantly through groundwater sources, with the majority of springs in the eastern part of the lake. The north northeast trending fractures in the Nipissing Gabbro at the eastern end of the lake may be significant groundwater conduits. Northeast trending lenses of glaciofluvial material, extending from the northeastern shore, may also transport groundwater. The prominent Creighton Fault, trending east across the southern part of the lake potentially controls sites of groundwater eruption on the lake bottom in this area. Northwest trending fractures at the west end of the lake appear to control 2 or 3 sites of groundwater eruption. Ephemeral streams dominantly drain the conservation area and the urban infrastructure, however some small permanently flowing streams exist. 1.2. Background Ramsey Lake is the centre of a very important watershed in the City of Greater Sudbury (CGS) because of its role as a drinking water reservoir for over 50,000 residents. Since the late 19th century this watershed has been subject to acidic and metal-laden smelter emissions, leakage from septic systems, and runoff from a local sawmill, fertilizer and pesticide-rich golf course, residential lawns, and regional transportation networks. The result is a unique “living laboratory”, the survivor of over a century of extreme environmental insult, with an accumulation of 50e60 cm of metal and organic-rich sediment causing seasonally sub-oxic hypolimnic water at depth that must be managed and monitored to enable Ramsey Lake to be a continuing source of potable water for the city. In the past, dissolved metals became enriched in the hypolimnic water as oxygen was increasingly depleted with the progression of summer. In 2000, the intake pipe for drinking water was found to be drawing metal-enriched water into the treatment plant. After chlorination, oxidation of metals produced a gray precipitate that was observable at residential faucets. This water was deemed not
A. Lock et al. / Water Research 93 (2016) 289e295
291
Fig. 1. Map of Ramsey Lake high lighting shoreline use, geological structures (dashed lines represent faults) and groundwater vent sites.
potable. The simple, short-term solution was to raise the intake pipe to a depth above the seasonally sub-oxic level. This event was compounded by the identification of greater reservoir protection needs in light of the E. coli outbreak in nearby Walkerton, Ontario (Hrudey et al., 2002) resulting in the death of 7 people. The CGS had to increase their understanding of the chemical and physical dynamics of the reservoir to elucidate sustainability. To ensure the sustainability of this reservoir, a stronger understanding of the diverse input sources from the watershed was required. Although surface contaminant sources have been indicated by persistently elevated nutrient and metal concentrations in specific inflow streams, groundwater sources have the greatest potential to control water quality in this reservoir due largely to greater inflow volumes. Despite not having a hydrologic mass balance model, historically it had been suspected that groundwater inputs greatly exceeded surface water inputs. This was supported by consistently stable water levels during the summer and winter months when surface inputs are derived from small, dominantly ephemeral streams and large outflow volumes of drinking water, ca 400,000 m3/month (Greater Sudbury, 2009), are drawn. Using a mass balance approach, the maintenance of constant volume with known significant discharge, sufficient to supplying 50,000 residence with potable water, must be balanced by a significant inflow (Chapra, 1997). With no observed surface inflow transporting significant quantities of water during most summer and winter months, groundwater inflows are likely the dominant inflow sources. 2. Methods A custom-built underwater towed vehicle (UTV) tethered to a boat was used to map water quality recording a suite of parameters every 30 s while slowly traversing the water body. The UTV is constructed of an open body stainless steel rectangular prism frame, long axis running front to back, with a vertical fin attached on the upper rear portion of the frame to provide stability during towing. Four equal length chains, each attached to the top corner of the UTV and join at a central point above the UTV where the tether from the vessel is fastened. The UTV provides a structure to house and protect equipment mounted within the framework, rapid circulation of water around sensors and sufficient weight to sink while
in tow. Equipment mounted in the UTV includes a data sonde, underwater camera and high intensity light. A differential global positioning system tracks the location of the vessel, a marine depth sounder identifies the water depth at the vessel location and a depth sensor incorporated into the data sonde that is attached to the UTV monitors the depth that water quality parameters are recorded. The lag of the UTV behind the boat is calculated from the monitored depth of the UTV and the length of tethered rope released. The calculated lag distance is converted into a time delay using the traversing speed and corrected for by off setting the GPS and sonde results by the calculated time lag. An underwater camera and high intensity light attached to the front of the UTV allow an operator to visually monitor the UTV distance from the lake sediments. The UTV tether is attached to a winch mounted on the vessel that the operator controls to maintain the desired 1 m depth above the lake sediments. The sonde parameters are temperature, dissolved oxygen (DO), pH, conductivity, oxidation-reduction potential (ORP), turbidity, and chloride. All sensors are standard factory supplied sensors and daily calibrations were conducted following methods outlined in the user manual (Hydrolab Corporation, 1997). During the July 2002 mapping event the UTV was suspended 1 m above the sediment to identify upwelling groundwater. While traversing, the sonde records a suite of data from the sensor array every 30 s. A total of 2451 locations were sampled for the 7 parameters resulting in 17,157 readings recorded during the 3 day mapping period. The lake-bottom map tracking is illustrated in Fig. 2. The individual dots making up the map tracking represent the locations that water quality data were recorded. The water logger data were complimented by 24 water samples taken on the fourth day from strategic locations identified by the underwater towed sensor array as having anomalous water quality and areas having widespread water quality. Discrete samples were taken at select depths and locations by pumping water to the surface using a peristaltic pump. The sampling tube was thoroughly flushed with water from the desired location before taking the sample. The samples were filtered in the field using a syringe and 0.45 mm filters and submitted to the Ontario Geological Laboratory for analysis of anions using ion chromatograph (Pamer, 2011) and major and trace metals by ICP-OES (Pamer, 2007) and ICP-MS (Burnham, 2002). Interpolated data were plotted using Surfer, a graphing software package.
292
A. Lock et al. / Water Research 93 (2016) 289e295
Fig. 4. Turbidity measurements 1 m above the sediment for July 2002. Elevated values (dark) at sites 1 to 6 indicate areas of upwelling groundwater. Fig. 2. Underwater towed vehicle map tracking for July 2002.
Ramsey Lake is dimictic, vertically mixing in the spring and fall. Summer thermo-stratification divides the lake into two laminar flow circulation cells; the epilimnion and hypolimnion. The epilimnion, susceptible to wind, has a higher turbulence than the hypolimnion that is shielded by the thermocline. Areas with higher turbulence have greater potential to dilute the upwelling groundwater “finger print” whereas areas with low turbulence and mixing permit detection of upwelling groundwater indicators. In July 2002, the top of the thermocline was near 6 m (Fig. 3), permitting potential upwelling groundwater detection throughout most of the lake. The background chemistry of hypolimnic waters in many urban lakes and groundwater vents sites have similar chemical characteristics observed by the UTV sensor array. However, turbidity is a vent site indicator that does not reveal similar results to deep basin water quality. Turbidity is an indirect measure of water clarity and is often related to flow rate. Higher flows may increase the amount of suspended material, decreasing water clarity. Chloride concentration typically increases with depth due to greater density. DO and ORP commonly decrease with depth and have a minimum at the greatest depth. Ramsey Lake is an excellent location to validate
this groundwater vent detection technique because many of the source locations are not the deepest parts of the lake, hence there should be a discernable chemical contrast between groundwater vents and ambient lake water. Therefore, the relative differences of chloride, DO and ORP measured at vent sites should be compared to mean lake values from the same depths as the vent sites. Figs. 4 and 5, respectively, show elevated turbidity and chloride and Figs. 6 and 7, respectively, show low DO and ORP on the east end between the two small islands (site 1), south of site 1 along the Creighton fault (site 2), southern most bay (site 3), and sites 4e6 at the west end of the lake. Vent site locations are clearly depicted on Fig. 1. These results are supported by water samples taken about 1 m above the lake bottom at the vent sites. Alkalinity and dissolved silica are excellent parameters to distinguish between lake and groundwater because there are no diatoms in groundwater and, therefore, silica concentrations only reflect equilibrium with quartz or clay. The partial pressure of carbon dioxide is higher in groundwater than in lake water. Table 1 shows greater alkalinity and silica with respect to mean lake values (n ¼ 24) at all the vent sites. Water samples were not taken directly above site 5 because during mapping sites 5 and 6 were suspected to be a single site. However, after data processing 2 distinct sites were identified. The alkalinity concentration directly above vent sites 1 and 2 are, respectively, 8.9 (±4.8) and 8.2 (±4.6) mg/L greater than the mean lake concentration plus 2 times the standard deviation (38.6 mg/L), illustrating with 95% confidence these samples are significantly different than the sampled population (McGrew and Monroe, 1993). Although alkalinity at vent sites 3, 4 and 6 do not have
Fig. 3. Temperature distribution of monitored points for July 2002 revealing approximate lake thermal structure.
Fig. 5. Chloride concentrations 1 m above the sediment for July 2002. Elevated concentrations (dark) at sites 1 to 6 indicate areas of upwelling groundwater.
3. Results/discussion
A. Lock et al. / Water Research 93 (2016) 289e295
Fig. 6. Percent saturation of dissolved oxygen 1 m above the sediment for July 2002. Low values (dark) at sites 1 to 6 indicate upwelling groundwater.
Fig. 7. Oxidation-reduction potential 1 m above the sediment for July 2002. Low values (dark) at sites 1 to 6 indicate upwelling groundwater.
293
located 20 km southwest of vent sites 1 and 2 (Ministry of Northern Development and Mines (1983)), intercepting the strike lines of off set faults at these vent sites, thus providing a likely source for the elevated As concentration. Metal concentrations at this site (Fig. 8, 8 m depth) are well above median lake concentrations (Table 1) showing this is a source of contamination to the lake and must be investigated in greater detail to ascertain what degree this may impact the lake. The concentrations in Fig. 8 for Ni, As, Fe, alkalinity, Si, and nitrate are normalized so the maximum concentration of each metal is represented by a value of 1 and lower concentrations are proportional to this normalization. This serves 2 purposes; the first, all parameters can be plotted on the same graph and second, slopes of the concentration gradients can be directly compared between each of the metals. Observed decrease in alkalinity and Si concentrations with increasing distance above the vent site support these parameters are indicators of groundwater vent sites that has previous been discussed above. Nitrate concentration is similar 1 m and 3 m above vent site 1 revealing this is not a significant source at this location. Nickel, As and Fe transported from this vent site may be of environmental concern, thus discussed in greater detail below. The rapid decrease in concentration of Ni, As and Fe away from the vent is due to mixing with lake water resulting in dilution and/ or a removal mechanism from the water column like adsorption and sedimentation, reaching ca. mean lake concentrations (Table 1) below 1.5 m depth. Ferrous iron rapidly oxidizes and forms (oxy) (hydr)oxides in natural lake waters (Peters and Blum, 2003). This particulate readily binds arsenate and settles to the lake bottom therefore significantly decreasing arsenic bioavailability, however arsenite does not complex with iron (oxy) (hydr)oxides in natural waters as readily as arsenate (Peters and Blum, 2003) and arsenite oxidation in water with dissolved oxygen as the oxidizing agent requires 1e2 months (Cherry et al., 1979). The near identical slopes
Table 1 Mean lake concentrations compared to 1 m above the sediment surface at vent sites 1, 2, 3, 4, and 6 for July 2002 (sd ¼ standard deviation). The analytical standard deviation is presented beside the individual site concentrations and the whole lake sample population standard deviation is presented beside the mean concentrations. Analytical Parameter
Mean (n ¼ 24)
Site 1
Site 2
Site 3
Site 4
Site 5
Site 6
Alkalinity (mg/L as CaCO3) Silica (mg/L) Arsenic (mg/L) Iron (mg/L) Nickel (mg/L) Nitrate (mg/L)
35.2 (1.7sd) 450 (436sd) 2 (3.1sd) 28 (462sd) 71 (22.1sd) 100 (727sd)
47.5 (2.4sd) 2500 (125sd) 12 (0.4sd) 1603 (80sd) 118 (6sd) 20 (1sd)
46.8 (2.3sd) 2400 (120sd) 13 (0.5sd) 1742 (87sd) 120 (6sd) 140 (7sd)
37.8 (1.9sd) 1700 (85sd) 3 (0.1sd) 149 (8sd) 129 (7sd) 60 (3sd)
41.3 (2.1sd) 2400 (120sd) 4 (0.2sd) 229 (12sd) 106 (5sd) 490 (25sd)
N/A N/A N/A N/A N/A N/A
41.3 (2.1sd) 2000 (100sd) 3 (0.1sd) 167 (8sd) 94 (5sd) 3630 (182sd)
95% confidence of being significantly different than the sampled population they have 68% confidence of it being significantly different. Silica concentration directly above all sampled vent sites reveals with 95% confidence that these samples are significantly different than the sampled population. Corroboration of these findings confirms the UTV method for identification of upwelling groundwater is valid for some lakes. Groundwater emergence sites 1 and 2 have typically strongest signals shown in Figs. 4e7 and highest alkalinity and silica concentrations (Table 1) indicating this site has significant potential for contaminant loading from groundwater sources. The gradient of a profile at site 1 for select parameters (Fig. 8) strongly indicates Ni, As, Fe, alkalinity, and Si are transported with the groundwater to the lake water, whereas nitrate is not significantly transported at this vent site. The high abundance of local, metal-rich bedrock suggests these metal sources are likely natural. For example, an abandoned gold mine with ca 250,000 tons of As bearing tailing is
Fig. 8. Select chemical profiles from the water column at vent site 1, July 2002. The maximum concentration is normalized to a value of 1 and other concentrations are proportional to this normalization for each metal.
294
A. Lock et al. / Water Research 93 (2016) 289e295
of As and Fe depletion (Fig. 8) suggests co-transport, thus arsenate is the dominant species at depths shallower than the 6 m. Compared to Fe and As, Ni is removed less efficiently from the water column illustrated by the lower slope of Ni from 1.5 to 6 m (Fig. 8). Lock et al. (2003) observed nickel primarily remaining dissolved in the nearby Kelly Lake, but suggest that a fraction of nickel does adsorb to manganese oxides and settles to the sediment. With dilution acting equally on Fe, As and Ni, the lower removal rate for Ni must be due to less adsorption and sedimentation. Moderate to low intensities of vent detection parameters at site 3 (Figs. 4e7 and alkalinity and silica concentrations from Table 1) suggest this site does not have as great a potential to have a deleterious impact on lake water quality as vent site 1. Of the contaminants examined in this study, only nickel concentration is significantly higher at this site (Table 1). The water chemistry from this vent is different from vent 1. The transport network for vent site 1 is an off set fault and for vent site 3 it is a dyke (Fig. 1). The difference in chemistry may be due to different rockewater interactions or the water origins may be different. Water sampled 1 m above vent sites 4 and 6 have 4.9 and 36 times more nitrate, respectively, than the mean lake concentration (Table 1) indicating this may be a significant source of contaminated groundwater. These vent sites are located directly above northwest trending faults. Highly plausible sources of the nitrate is the golf course located south of the vent sites 5 and 6 and Bethel Lake, a eutrophic water body located southeast of vent site 4 (Fig. 1). Nitrate is often considered the secondary limiting nutrient for lake eutrophication (Schnoor, 1996), and, therefore, should be monitored closely. This data may be beneficial here to guide land use management. Elevated Ni concentrations, with mean lake concentrations (Table 1) nearly 3 times the provincial water quality objectives (25 ug/L) (Ontario Ministry of the Environment, 1994), reveals this resource must be monitored and managed to ensure a safe and sustainable drinking water source. The sensitivity of this system, under its current use, is likely high because there are multiple and natural sources of Ni that would be difficult to isolate from the lake and seasonally sub-oxic hypolimnic water limiting the lake's potential to efficiently remove Ni from the water column and store it in the sediments. Identified sources of large nutrient pools seeping into the lake that feed the eutrophication process will sustain a seasonally sub-oxic hypolimnic environment that can increase Ni mobility. The action of the City of Greater Sudbury to increase the height of the drinking water in-take pipe to a depth above the hypolimnion provided a drinking water source with increased water quality and should increase the sustainability of this reservoir. Results from the UTV mapping are also being used to assist with land use planning in the drinking water reservoir watershed. 4. Conclusions Areas of upwelling groundwater were detected in Ramsey Lake using an UTV hosting dissolved oxygen, oxidation-reduction potential, turbidity, and chloride sensors that would most likely be missed using only traditional sampling methods. Elevated silica and alkalinity near the groundwater vent sites indicated by the UTV corroborated the validity of this method for upwelling groundwater site identification. Groundwater vent site identification using an UTV is facilitated in environments with limited turbulent flow like that found below the thermocline. Although these tools have primarily been used in marine environments and for resource exploration, freshwater and environmental applications may prove to be beneficial especially for drinking water source protection. Potential surface runoff and
groundwater contaminant sources have been identified in Ramsey Lake and this information will be used to guide future research. Additional mapping to elucidate potential seasonal differences of groundwater flow during spring and fall would compliment the July data presented here. Current results show individual vent sites have different water chemistry that is partly due to different water source origins and partly due to different rockewater interactions. Ramsey Lake water quality is heterogeneous both horizontally and vertically; therefore traditional discrete sampling methods have the potential to not determine water quality in Ramsey Lake accurately due to the less obvious inflow sources, such as, groundwater vents. It is strongly recommended that UTVs are used during early stages of drinking water source protection research to elucidate hidden inflow sources and assist in project development, but seasonal variation may be significant. The high resolution data obtain with UTV mapping facilitates in-lake simulations by providing a high resolution data set for model calibration and validation. Acknowledgments This research was partly funded by the City of Greater Sudbury, Ontario, Canada and NSERC Industrial Postgraduate Scholarship (IPS). References Baird, C., 1999. Environmental Chemistry. W.H. Freeman and Company, New York, NY, p. 555. Burnham, O., 2002. 34. Improvements in the Accuracy and Sensitivity of Water Analyses by the Application of Inductivity Coupled Plasma Dynamic Reaction Cell™ Mass Spectrometry. Ontario Geological Survey, Open File Report, vol. 6100, 34e41e34e39. Bussmanni, I., Dando, P., Niven, S., Suess, E., 1999. Groundwater seepage in the marine environment: role for mass flux and bacterial activity. Mar. Ecol. Prog. Ser. 178, 169e177. Chapra, S., 1997. Surface Water-Quality Modeling. The McGraw-Hill Companies, Inc., New York, NY, p. 382. Cherry, J., Shaikh, D., Tallman, D., Nicholson, R., 1979. Arsenic species as an indicator of redox conditions in groundwater. J. Hydrol. 43, 373e392. Church, T., 1996. An underground route for the water cycle. Nature 380, 579e580. Faure, G., 1991. Principles and Applications of Inorganic Geochemistry. MacmillanPublishing Company, New York, NY, p. 626. Greater Sudbury, 2009. Water Works Summary Report. Large Municipal-Residential Systems, 2012. http://www.greatersudbury.ca/limkservid/919E70E7-98. Hostetler, P., Ray, J., 2005. Mineral exploration by systematic analysis of groundwaters upwelling in lakes and rivers. In: Ontario Mineral Exploration Technologies Program. P02-03-036a. Howel, E., 2006. Fine-scale analysis of water quality on the SE shores of Lake Huron: relevance and approach. In: Proceedings 41st Central Canadian Sympossium on Water Quality Research, 20. Hrudey, S., Huck, P., Payment, P., Gillham, R., Hrudey, E., 2002. Walkerton: lessons learned in comparison with waterbourne outbreaks in the developed world. J. Environ. Eng. Sci. 1, 397e407. Hydrolab Corporation, 1997. DataSonde 4 and MiniSonde Water Quality Multiprobes User Manual, Quality Multiprobes User Manual, 12921 Burnet Road, Austin, Texas, USA, 278. Knowlers, L., Katz, B., Toth, D., 2010. Using multiple chemical indicators to characterize and determine the age of groundwater from select vents of the silver springs group, central Florida USA. Hydrogeol. J. 18 (8), 1825e1838. Lock, A., Pearson, D., Spiers, G., 2003. Early diagenesis of sediment from Kelly Lake, Sudbury, Ontario e a lake contaminated by sewage effluent and high levels of copper and nickel from mining and smelting. In: Proceedings Mining and the Environment III, Sudbury 2003,7-D. McGrew, C., Monroe, C., 1993. In: Meyers, M. (Ed.), An Introduction to Statistical Problem Solving in Geography. Wm. C. Brown Communications, Inc,, Dubuque, IA, p. 305. Mindell, D., Bingham, B., 2001. New archaeological uses of autonomous underwater vehicles. In: Proceedings Institute of Electrical and Electronic Engineers, Modular Training Systems, pp. 555e558. Ministry of Northern Development and Mines, 1983. MDI File MDI41I06SE00002. http://www.geologyontario.mndmf.gov.on.ca/gosportal/gos? command¼mndmsearchdetails:mdi&uuid¼MDI41I06SE00002. Ontario Ministry of the Environment, 1994. Water Management, Ontario Water Quality Objectives. Queen’s Printer for Ontario. ISBN 0-7778-8473-9rev. http:// www.ene.gov.on.ca/envision/gp/3303e.htm.
A. Lock et al. / Water Research 93 (2016) 289e295 Pamer, L., 2011. 34. Analysis of Anions in Waters by Ion Chromatography at the Geoscience Laboratories. Ontario Geological Survey, Open File Report, vol. 6270, 34e1e34e5. Pamer, L., 2007. 32. Major and Trace Element Analysis of Water Samples by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-OES) at the Geoscience Laboratories: Preliminary Data. Ontario Geological Survey, Open File Report 6213, 32e1e32e7. Peters, S., Blum, J., 2003. The source and transport of arsenic in a bedrock aquifer, New Hampshire, USA. Appl. Geochem. 18, 1773e1787. Reusch, A., Loher, M., Bouffard, D., Moernaut, J., Hellmich, F., Anselmetti, F., Bernasconi, S., Hilbe, M., Kopf, A., Lilley, M., Meinecke, G., Strasser, M., 2015. Giant lacustrine pockmarks with subaqueous groundwater discharge and subsurface sediment mobilization. Geophys. Res. Lett. 42, 3465e3473. Schnoor, J., 1996. In: Schnoor, J., Zehnder, A. (Eds.), Environmental Modeling: Fate and Transport of Pollutants in Water,air, and Soil. John Wiley and Sons, Inc, New York, NY, p. 682. Statham, P., Connelly, D., German, C., Brand, T., Overnell, J., Bulukin, E., Millard, N., Mcphai, S., Pebody, M., Perrett, J., Squire, M., Stevenson, P., Webb, A., 2005.
295
Spatially complex distribution of dissolved manganese in a fjord as revealed by high-resolution in situ sensing using the autonomous underwater vehicle autosub. Environ. Sci. Technol. 39, 9440e9445. Steissberg, T., Hook, S., Schladow, G., 2005. Characterizing partial upwellings and surface circulation at Lake Tahoe, California eNevada, USA with thermal infrared images. Remote Sens. Environ. 99, 2e15. Stumm, W., Morgan, J., 1996. Aquatic Chemistry e Chemical Equilibria and Rates in Natural Waters, 1040 Natural Waters. John Wiley and Sons Inc,, New York, NY, p. 1040. Whitcomb, L., Yoerger, D., 1993. A new distributed real-time control system for the jason underwater robot. In: Proceedings Institute of Electrical and Electronic Engineers, Intelligent Robots and Systems, pp. 368e374. Yoerger, D., Collier, R., Bradley, A., 2002. Hydrothermal vent plume discovery and survey with an autonomous underwater vehicle. In: Proceedings American Geophysical Union, T11C-1261. Yu, X., Dickey, T., Bellingham, J., Manov, D., Streitlien, K., 2002. The application of autonomous underwater vehicles for interdisciplinary measurements in Massachusetts and cape cod bays. Cont. Shelf Res. 22, 2225e2245.
