Case Study/

Integrating hydrogeochemical, hydrogeological, and environmental tracer data to understand groundwater flow for a karstified aquifer system by Igor Pavlovskiy1 and Benny Selle1,2,3

Abstract For karstified aquifer systems, numerical models of groundwater flow are difficult to setup and parameterize. However, a system understanding useful for groundwater management may be obtained without applying overly complicated models. In this study, we demonstrate for a karstified carbonate aquifer in south-western Germany that a combination of methods with moderate data requirements can be used to infer flowpaths and transit times of groundwater to production wells.

Introduction Delineation of capture zones for drinking-water production wells and a quantification of associated transit times of water are essential for a proper management of groundwater. However, a reliable quantification of groundwater flow patterns remains difficult, particularly for karstified carbonate aquifers. For such systems, hydrogeochemical, hydrogeological, and environmental tracer data were often collected for different purposes and time periods; and it is then necessary to integrate these heterogeneous data sets into a coherent conceptual model of groundwater flow (Drew and Goldscheider 2007). There are two distinct approaches for this data integration. The first approach is to setup a numerical model of groundwater flow and solute transport; and to calibrate this model against different observations such as groundwater heads and solute concentrations (Ghasemizadeh et al. 2012 and references therein). This approach 1

Water & Earth System Science (WESS) Competence Cluster, ¨ ¨ c/o University of Tubingen, H¨olderlinstaße 12, Tubingen, Germany. 2 Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Straße 24-25, Potsdam-Golm, Germany. 3 Corresponding author: Karl-Liebknecht-Straße 24-25, D-14476 Potsdam-Golm, Germany; +49 331 977 2388; fax: +49 331 977 2092; [email protected] Received February 2014, accepted July 2014. © 2014, National Ground Water Association. doi: 10.1111/gwat.12262

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is difficult for karstified systems as both groundwater flow and solute transport are influenced by fractures and karst conduits (Sauter et al. 2006), and diffusion processes between conductive units and the low-permeability matrix play a role (Cook et al. 2005). Matrix, fractures, and conduits need to be represented explicitly in the groundwater model to fit the observed state variables in a meaningful way; but adequate information to effectively parameterize the various flow compartments is often missing. A useful and relatively simple alternative to develop an understanding of groundwater flow in carbonate aquifers is the integrated analysis of the available data sources using a combination of methods with relatively low data requirements. This type of approach was adopted by several studies, for example, Zanini et al. (2000), Gooddy et al. (2006), or Land and Huff (2010). However, it remains a challenge to integrate the qualitatively different data sources into one coherent interpretation. No widely accepted methodology for such data integration exists. The objective of this study was to obtain a conceptual understanding of groundwater flow using qualitatively different information sources for the Ammer River catchment in south-western Germany, which is used for drinking-water production from a karstified carbonate aquifer. For this purpose, we systematically analyzed and interpreted hydrogeochemical and hydrogeological information, as well as, environmental tracer data. Groundwater

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Figure 1. Study area, that is, the area contributing groundwater flow to the Ammer River upstream of Pf¨affingen gauging station (dotted outline). Dashed outline (grey) indicates position of the inset in the lower left corner, which displays hydraulic heads for the Upper Muschelkalk aquifer from Villinger (1982). Dashed outline (black) indicates position of the map displayed in Figure 5. Solid line indicates the approximate location of the inset in the lower right corner, which shows a geological cross section. Area with thick Keuper overburden is shaded.

Materials and Methods Study Area and Available Data The study area to demonstrate our integrated data analysis is situated in the central part of the state of Baden-W¨urttemberg in south-western Germany. It comprises the upper Ammer River catchment (Figure 1). There are six drinking-water production wells within the study area: two well pairs near Poltringen and Entringen, and two solo wells close to Breitenholz and Altingen; and for these wells, flowpaths and transit times of groundwater were analyzed. All wells tap the Upper Muschelkalk which forms an 80 to 90-m thick aquifer in the area. The catchment contributing groundwater discharge from the Upper Muschelkalk aquifer into the Ammer River 2

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(Figure 1, dotted outline) has been investigated in detail by Harreß (1973), Villinger (1982), and Pl¨umacher (1999). However, the source areas of recharge for the Upper Muschelkalk aquifer were insufficiently known, in particular for the groundwater production wells. The hydrogeological setting of the drinking-water production wells is summarized in Figures 1 and 2. In the study area, the primary discharge of the aquifer system is to the Ammer River and its tributaries, which mainly happens via major springs from both the Upper Muschelkalk and the Gipskeuper aquifers (Selle et al. 2013a). Hydrogeological units in the area generally dip toward South-East where the Upper Muschelkalk becomes confined and is increasingly overlain by Keuper layers. NGWA.org

Figure 2. Geological profiles of the study area. Columns (top right) represent strathigraphies at drinking-water production wells (Ammertal-Sch¨onbuchgruppe, personal communication). Well screens are indicated by dotted lines.

These layers are—from top to bottom—the Stubensandstein (30 m), the Bunte Mergel (25 m), the Schilfsandstein (20 m), the Gipskeuper (up to 100 m), and the Lettenkeuper (20 m). They mainly consist of terrestrial sediments, mostly alternating marl and sandstone strata. The Gipskeuper is characterized by gypsum layers (Figure 2). Keuper overburden is hundreds of metres thick in the area of the Sch¨onbuch plateau (Figure 1). The stratigraphic dip affects groundwater flow in the area. However, several faults and folds complicate groundwater flow, for example, the Reusten anticline (“Reustener Sattel” just south of Altingen), a geological structure, which is around 3 km wide and is perpendicular to the general dip direction in the area. Another important feature of the study area is the karstification of the aquifers, which is typical for both the Upper Muschelkalk and the Gipskeuper formations (Harreß 1973). The extent of the karst development is reported to depend on the overburden thickness both in the Upper Muschelkalk (Simon 1986) and Gipskeuper (Krause 1988) strata, with thick overburden inhibiting karstification. There is a history of research into groundwater flow patterns in the Ammer catchment (Harreß 1973, Villinger 1982) and the adjacent territories (Kriele 1976, Ufrecht 1994, Neeb and Wittkopp 1998). Despite a relative abundance of data, the complexity of the hydrogeological NGWA.org

setting and, particularly, the karstification of the aquifers prevented an unambiguous characterization of groundwater flow to the groundwater-production wells in the Ammer valley. The available hydrogeological information such as piezometric maps, geological cross sections, and pumping test data were inadequate to develop a reliable conceptual model of groundwater flow. Information on the location and extent of the specific karst features in the Ammer catchment were unavailable. Note that two recently published papers by Selle et al. (2013a, 2013b) used some of the data that were also utilized in our study, but they had different objectives and outcomes. The study by Selle et al. (2013a) focussed on the sensitivity of a numerical groundwater model to changes in groundwater discharge and recharge patterns. The study by Selle et al. (2013b) identified processes governing water quality for both ground and surface water using principal component analysis. There were several data sources available for the study area, which may be subdivided into hydrogeochemical, environmental tracer, and hydrogeological information. The former two data sources provided information on the chemical and isotopic composition of the groundwater including environmental tracer concentrations. The latter data source consisted of hydraulic I. Pavlovskiy and B. Selle Groundwater

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Table 1 Summary of Different Data Sets Used for This Study Data

Location

Time Period

Source

Major ion chemistry, pH, and temperature

36 Lettenkeuper, 16 Gipskeuper, and 14 Upper Muschelkalk springs in the study area Poltringen, Entringen, Altingen, and Breitenholz wells Poltringen wells

April 1971, October 1971, April 1972

Harreß (1973)

December 2004

Personal communication ASG

December 2004, June and September 2006, February, May, and July 2007, September 2011 December 2004 1970s

Personal communications ASG (data 2004 to 2007) and Dr. Karsten Osenbr¨uck (data 2011) Personal communication ASG Villinger (1982)

March to October 2012 (discharge), 1999 to 2009 (pumping)1931 to 1960 (recharge)

Li (2013, discharge), personal communication ASG (pumping), Villinger (1982, recharge)

Major ion chemistry, pH, and temperature SF6 and tritium concentrations

SF6 and tritium concentrations Groundwater levels

Groundwater discharge, pumping, and recharge

Entringen, Altingen, and Breitenholz wells Upper Muschelkalk wells and springs in the study area (Figure 1) Relevant parts of the Upper Muschelkalk (see section “groundwater balance”)

ASG, Ammertal-Sch¨onbuchgruppe.

heads and data of groundwater pumping, discharge, and recharge for relevant parts of the Upper Muschelkalk aquifer. The available data sets used for our integrated analysis are summarized in Table 1. Interpretation of Hydrogeochemical Data For the interpretation of hydrogeochemical data, we constructed ternary plots and computed saturation indices using PHREEQC (Parkhurst and Appelo 1999). More specifically, PHREEQC was applied to calculate saturation indices for calcite, dolomite and gypsum, and the partial pressure of carbon dioxide from the available well and spring data sets on major ions, pH, and temperature (Table 1). Interpretation of Environmental Tracer Data The analysis of environmental tracers consisted of two steps. First, time series of environmental tracers—that were only available for the Poltringen wells—were analyzed. A transit time distribution for the Poltringen wells was obtained by fitting a lumped parameter model to the observed time series of environmental tracers. For more detail on lumped parameter models, we refer to the textbook by Leibundgut et al. (2009, 161 ff.). For our application, a dispersion model was fitted as it can accommodate most hydrogeological settings and it can also account for several mixing processes (Małoszewski and Zuber 1982). The dispersion model was previously applied for karstified aquifers (Maloszewski et al. 2002; Long and Putnam 2006). The parameters of the dispersion model were estimated by manually fitting model parameters to the observed concentrations of both tritium and SF6 in the Poltringen wells using a MatLab script. In this context, 4

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we would like to mention that tritium is a hydrogen isotope, whose elevated concentrations in precipitation were associated with thermonuclear bomb tests starting in 1951 and SF6 is an anthropogenic gas with steadily increasing atmospheric concentrations over the last 60 years. For SF6 input time series, measured concentrations in air (Rigby et al. 2010) were converted to aquatic concentrations using a partitioning coefficient of 0.39 × 10−3 mol/L/atm, which is applicable for non-saline waters at 10 ◦ C (Bullister et al. 2002). The assumption of a water temperature of 10 ◦ C was reasonable as it conceptually represented the temperature of recharging water for which no differences between the various wells were anticipated. Input functions for tritium and SF6 were seasonal (summer and winter) and annual time series, respectively. A seasonal input function for tritium was obtained by using a ratio of the fractions of precipitation entering the groundwater system of summer to winter of 0.74. Second, spatially distributed concentrations of environmental tracers for the different wells at a single time point were analyzed. This data set was interpreted using a binary mixing model of water of different ages, which additionally used chloride concentrations. In this context, chlorides were used as a supplementary tracer of relatively young water, as their elevated concentrations are strongly associated with modern agriculture (Broers 2004). Interpretation of Hydrogeological Data The interpretation of hydrogeological data was conducted using a water balance for the relevant part of the Upper Muschelkalk aquifer which contained the groundwater production wells. Furthermore, Darcy’s law was used to infer transmissivities for different parts of the Upper Muschelkalk aquifer. More specifically, the flow of NGWA.org

used for large-scale modeling purposes (Pl¨umacher 1999) or to compare different parts of an aquifer.

Results and Discussion

Figure 3. Anionic composition of the spring water measured in the 1970s (Harreß 1973) and well water measured in December 2004 (personal communication AmmertalSch¨onbuchgruppe). Clusters in the plot are denoted as groups I to III which are explained in the text.

groundwater through two different cross sections into the balanced area (derived from a water balance) together with the hydraulic gradient were used to estimate the aquifer transmissivity using Darcy’s law: Q = w × T × i ⇔ T = Q/ (w × i)

(1)

where Q is the discharge through the cross section, w, the width of the cross section, i , the gradient of hydraulic head, and T = m × K, the transmissivity with K being the hydraulic conductivity, and m being the thickness of the aquifer. The hydraulic gradient was estimated from data on the hydraulic heads in the Upper Muschelkalk aquifer (Villinger 1982, Figure 1). While, strictly speaking, Darcy’s law is not applicable to karstified systems, the transmissivity of the equivalent porous aquifer may be

Interpretation of Hydrogeochemical Data The analysis of the anionic composition of spring water (Figure 3) resulted in three distinct clusters: (group I) springs located in the area covered by Gipskeuper that were of the sulfate-bicarbonate type; (group II) springs located in the area covered by either Upper Muschelkalk or Upper Muschelkalk with a thin Lettenkeuper overburden that were of the bicarbonate-sulfate type and (group III) springs in the area covered by either Lettenkeuper or Upper Muschelkalk with bicarbonate comprising the majority of the dissolved anions. Note that groundwater from the production wells belonged to group II. From the clustering of major anion chemistry, it may be inferred that the Gipskeuper and Upper Muschelkalk aquifers are only weakly connected. If groundwater from the production wells was a mix of groups I (Gipskeuper) and III (Lettenkeuper and Upper Muschelkalk) waters then production wells contained a proportion of Gipskeuper water that could be about 10%. This proportion was estimated from a mixing analysis using average sulfate concentrations of groups I and III springs and the production wells. Therefore, it may be inferred that the infiltration of groundwater from the Gipskeuper-covered area into the underlying Upper Muschelkalk was relatively small. This finding supported the conceptual model that the upper marl strata of the Lettenkeuper (Figure 2) act as an aquitard, thus prompting groundwater discharge from the Gipskeuper aquifer. Note that the fraction of the sulfates in the Breitenholz well was lower than in the other wells, despite the Gipskeuper overburden. Unless the Lettenkeuper indeed prevented the downward flow from the Gipskeuper, the groundwater in this well featured much higher sulfate concentrations. For production wells, application of PHREEQC provided consistent values of calcite, dolomite, and gypsum saturation indices, and carbon dioxide partial pressure (Table 2).

Table 2 Computed Saturation Indices (SI) and Partial Pressures of Carbon Dioxide (pco2 in atm) for Drinking-Water Production Wells Calcite Production Well Altingen 3 Entringen 1 Entringen 2 Poltringen 1 Poltringen 2 Breitenholz

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SI 0.056 0.031 −0.047 0.050 0.007 0.003

Dolomite

Gypsum

CO2

10SI (%)

SI

10SI (%)

SI

10SI (%)

log10 pco2

Volumetric Fraction (%)

113.6 107.5 89.7 112.1 101.6 100.7

−0.17 −0.25 −0.43 −0.27 −0.36 −0.24

68.2 56.0 36.7 54.2 43.3 57.9

−1.46 −1.29 −1.19 −1.18 −1.06 −1.47

3.5 5.2 6.5 6.7 8.7 3.4

−1.54 −1.51 −1.43 −1.42 −1.43 −1.48

2.9 3.1 3.7 3.8 3.8 3.3

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The low saturation indices for gypsum confirmed that recharge to the wells did not significantly flowed though gypsum-bearing strata. Samples from the wells were in equilibrium with calcite but undersaturated with dolomite, which is a typical feature of dedolomotization processes. Computed volumetric contents of carbon dioxide of 3 to 9% implied that water from the Ammer River did not contribute significantly to groundwater pumped at the drinking water production wells, as surface water is in equilibrium with the atmosphere with a volumetric carbon dioxide content of circa 0.3%. For the Breitenholz well, nitrates were undetected whereas all other wells had significant nitrate concentrations. Together with the low chloride concentrations of the Breitenholz well, this may indicate that groundwater recharged when fertilizer inputs within the catchment zone were of minor importance such as at the time before the 1950s. The temperature of groundwater in the Breitenholz well—routinely measured by the water supply company—was 15.2 to 15.3 ◦ C, whereas temperatures in other wells were between 11.3 and 12.3 ◦ C (Ammertal-Sch¨onbuchgruppe, personal communication). (a)

These differences may be due to the geothermal gradient, which was reported to be circa 4 ◦ C/100 m to the north of the study area and circa 6 ◦ C/100 m to the east of the study area (Wolff and Hellenthal 2005). These gradients imply that the flowpaths of groundwater to the Breitenholz well were several tens of metres below the ground surface. Interpretation of Environmental Tracer Data For the Poltringen wells, environmental tracer data had several notable characteristics (Figure 4a and 4b). There was a marked variability of the SF6 concentration; it varied both between the two drinking-water production wells and between different time points. Magnitude of these fluctuations was equivalent to a difference in piston flow ages of up to 5 years; and fluctuations were independent from the rising temporal trend in the SF6 data. Tritium concentrations displayed similar fluctuations. The observed magnitude of concentration change was 4.6 TU (tritium units) and 3.4 TU in the Poltringen 1 and Poltringen 2 wells, respectively. At a single time point, the highest difference between the wells was 2.9 TU. (b)

(c)

(e)

(d)

(f)

Figure 4. Environmental tracer data used in this study. Tritium (a) and SF6 (b) concentrations in the Poltringen wells with measurement errors as vertical bars if this information was available (Ammertal-Sch¨onbuchgruppe and Dr. Karsten Osenbruck, personal communications). Time series of input concentrations are also displayed for both tritium (c) and SF6 (d). ¨ Relationship between chloride and tritium (e), chloride and SF6 (f) concentrations measured in December 2004 for drinking water production wells (Ammertal-Sch¨onbuchgruppe, personal communication).

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There are several possible reasons for the abovementioned variability in both tritium and SF6 concentrations. The likely reason for the observed variability of both SF6 and tritium concentrations were significant temporal variations in the relative weight of different groundwater flowpaths in the Upper Muschelkalk aquifer. Such effects were previously reported by Toran and White (2005). Fitting of the dispersion model to the observed tracer concentrations resulted in a mean transit time of 15 years for tritium and 3 years for SF6. For tritium, the fitted dispersion parameters was approximately 0.5. This parameter value suggested exponential-like flow in the catchments of the Poltringen wells associated with a long tailing of the weighting function and a predominant contribution of young water. This type of behavior with extremely short and also very long transit times pointed to an unconfined catchment (Małoszewski and Zuber 1982) of the Poltringen production well such as the one west of the Ammer River (Villinger 1982). A pronounced tailing of tracers is often associated with triple porosity systems (conduits-fractures-matrix) which are typical for many karstified aquifers (Sauter et al. 2006). To summarize, the Poltringen wells featured a mix of waters of different ages with a significant fraction of relatively young water. Therefore, the groundwater in the Poltringen wells will be referred to as “young” groundwater from now on. Note that the fit to the SF6 data resulted in shorter mean transit times than the fit to the tritium data; and there may be several possible reasons why the parameter estimates differed. However, we believe that the differences were most likely due to different equilibration times of SF6 and tritium along the same flowpaths. Tritium input concentrations are determined at the moment of a precipitation event, whereas SF6 equilibrates with the air in the vadose zone. In other words, the tritium signal in the groundwater relates to a moment of precipitation; in contrast, equilibration with SF6 may occur in the unsaturated zone well below the ground surface. From the geological map and the hydraulic heads in the Upper Muschelkalk aquifer, it may be inferred that the thickness of the unsaturated zone is circa 50 m in the potential recharge source area west of the Ammer River (Villinger 1982). Therefore, longer transit times for tritium than for SF6 could be expected. Concentrations of tritium, SF6, and chloride measured in December 2004 varied significantly between the different production wells (Figure 4e and 4f). The Breitenholz well had the lowest SF6 and chloride concentrations of all wells; and the tritium content in this well was also well below the concentrations found in contemporary precipitation. These results indicate that the groundwater in this well mostly originated from the time before the tritium peak in the 1960s and the intensive use of nitrogen fertilizer. However, we were unable to accurately quantify the transit time for the Breitenholz well because a time series was unavailable. The presence of tritium in the absence of SF6 may be interpreted as a sign that the groundwater composition in this well included a low fraction of water from the time of or shortly after the tritium peak. NGWA.org

However, adjusted for the decay up to 2004 (the sampling date), a peak concentration of more than 3000 TU corresponds to a concentration of more than 300 TU. Even a few per cent of the water from the time of or shortly after the tritium peak mixed with tritium-free, pre-1950s water would be observable in tritium concentrations, whereas concentrations of chloride and SF6 remained undisturbed. We therefore believe that the Breitenholz well predominately received pre-1950s water; and the groundwater in the Breitenholz wells will be referred to as “old” groundwater from now on. Previous studies (Villinger 1982) suggested that the area north of the Sch¨onbuch plateau belongs to the Ammer catchment and, thus, may act as a source zone for the Breitenholz well. This flowpath of groundwater below the Sch¨onbuch plateau agreed with our previously mentioned interpretation of the elevated groundwater temperature in the Breitenholz well. The Entringen wells occupied an intermediate position between the “Breitenholz” and “Poltringen” extreme cases. Comparison between concentrations of different tracers suggested that binary mixing took place (Figure 4e and 4f). The “young” and “old” components of the mixing corresponded to the Poltringen- and Breitenholz-type of water, respectively. The drinking-water production wells near Entringen featured approximately equal proportions of the “old” and the “young” components, which also agreed with the location of the Entringen wells between Breitenholz and Poltringen. Similar mixing processes of groundwater from distant recharge areas are reported in the literature (Land and Huff 2010). For the Altingen well, we found a discrepancy between the different environmental tracers. While chloride and tritium data corresponded to the ones of the “young” groundwater, the SF6 content suggested a significant fraction of the “old” component. On the basis of the chloride data one may speculate that the groundwater from the Altingen well probably did not have a significant pre-1950s component. However, both SF6 and tritium concentrations differed from the Poltringen wells and consequently, the Altingen well likely had a distinct transit time distribution. A precise quantification of the transit time distribution would require a time series of environmental tracers which was unavailable. Interpretation of Hydrogeological Data According to a groundwater balance, the inflow of water has to match its outflow in the steady state. For simplicity, a balance was calculated only for the part of the Upper Muschelkalk aquifer which included the drinkingwater production wells (Figure 5). It was assumed that the balanced area gained groundwater across its northern and western borders. This assumption was justified by the groundwater levels in the Upper Muschelkalk aquifer (Villinger 1982). Groundwater recharge within the Gipskeuper-free zone of the balanced area also contributed to inflows. We conceptualized that water could leave the balanced area via pumping in the drinking-water production wells, groundwater discharge into the Ammer River, and as outflow across the eastern border. I. Pavlovskiy and B. Selle Groundwater

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Figure 5. Conceptual model of groundwater flow to the groundwater production wells for the Upper Muschelkalk aquifer. The area used for a groundwater balance of the Upper Muschelkalk are all three “groundwater zones” together. Locations of baseflow measurements (Li 2013), which were used for the groundwater balance of the Upper Muschelkalk aquifer, are also displayed.

Two components of the groundwater balance, that is, groundwater discharge into the Ammer River and pumping, were quantified directly. Discharge into the Ammer River was measured by Li (2013) using OTT acoustic digital current meter as the difference in Ammer baseflow over a relevant river segment (the Altingen-Reusten segment, Figure 5). Note that downstream of Reusten there was insignificant discharge from the Upper Muschelkalk in the balanced area, which could be inferred from streamflow measurements and an endmember mixing analysis (Li 2013) at Pf¨affingen gauging station (Figure 5). It is important to note that the Ammer River was gaining water in the examined segments at all times. For pumping at the drinking-water production wells, long-term average rates were available (Table 1). Groundwater recharge and subsurface outflow rates were estimated, thus being the main sources of uncertainty in our water balance. Environmental tracer data suggested the existence of the two subzones within the balanced area with distinct recharge source zones. These subzones corresponded to the “old” Breitenholz- and the “young” Poltringen-type of groundwater. Therefore, the analysis was subdivided into a balance of “old” and “young” water (Table 3). This subdivision allowed estimating the transmissivity of the Upper Muschelkalk aquifer at two different cross sections, that is, at the northern and the western borders of the balanced area. From the baseflow measurements (Li 2013), discharge of “young” groundwater from the Upper Muschelkalk aquifer into the Ammer River ranged from 0.05 to 0.15 m3 /s. Pumping of the “young” water corresponded to the sum of the pumping rates at the Poltringen 8

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Table 3 Summary of a Groundwater Balance for the Area Shown in Figure 5 Component

Input

Output

“young” water

•Inflow across

•Pumping of the

the western border •Recharge within the balanced area

“old” water

•Inflow across

northern border

“young” water •Groundwater

discharge into the Ammer River •Outflow of the “young” water •Pumping of the “old” water •Outflow of the “old” water

and Altingen wells (33 L/s each) plus half of the pumping rate at the Entringen wells (11 L/s). It yielded a pumping rate of “young” groundwater of 0.077 m3 /s. The total Gipskeuper-free area available for the recharge west of the Ammer River was roughly 30 km2 . Using a recharge rate of 0.007 m3 /s/km2 (Villinger 1982), this yielded the upper limit of the inflow of “young” groundwater of 0.21 m3 /s. Groundwater recharge over the balanced area was calculated as the product of the Gipskeuper-free area in the balanced zone (6 km2 ) and the abovementioned recharge rate; the result was 0.042 m3 /s. This implied that the subsurface outflow of the “young” water could be in the order of tens of liters per second. NGWA.org

A similar calculation of a groundwater balance for the “old” groundwater was hampered by the fact that no information on the area of the recharge zone was available. However, considering that “old” water was sampled only in the Breitenholz well and it was significantly diluted in the Entringen well, subsurface outflow of the “old” water did probably not exceed the one of the “young” water. For transmissivity calculations at the western border of the balanced area, we used 0.2 m3 /s as an estimate of the inflow of “young” groundwater (see above). The possible error of this estimate was well within an order of magnitude and, consequently, had little impact on the interpretation of the aquifer transmissivity values. The width of the Altingen-Reusten segment, which was the cross section used for our transmissivity calculation, is 2.5 km. The resultant value of transmissivity using a gradient of 7‰ was circa 1 × 10−2 m2 /s. This value reinforced the hypothesis of the developed karst in the areas west of the Ammer River. For the estimation of transmissivity at the northern border of the balanced area, a lower limit of the transmissivity was estimated by assuming the minimum possible value of the discharge, which was equal to the groundwater pumping of 0.021 m3 /s (sum of the pumping rates for the Breitenholz well of 10 L/s plus half of the pumping rate for the Entringen wells of 11 L/s), and the maximum possible width of the cross section (distance of 3 km between Breitenholz and Altingen wells). The hydraulic gradient was 12‰. This approach yielded the lower limit of the transmissivity which was 6 × 10−4 m2 /s. The real value may be several times higher. Assuming that the width and discharge were respectively overand underestimated by the factor of 2 each, it yielded a transmissivity of 2.2 × 10−3 m2 /s which displayed a surprisingly good match with the value of 5 × 10−3 m2 /s used by Pl¨umacher (1999) for a groundwater flow model in the Upper Muschelkalk aquifer. This transmissivity value suggested that the karst may be less developed in the areas with a thick overburden below the Sch¨onbuch plateau compared to the unconfined areas west of the Ammer River.

Concluding Remarks and Synthesis There were three findings from our analysis that turned out to be particularly relevant for both the flowpaths and travel times of groundwater to the wells. First, we noted that water from all production wells had substantially lower sulfate concentrations compared to water from the Gipskeuper springs. This indicates that Gipskeuper overburden played a minor role in the recharge source area of the production wells. Thus the area not covered by Gipskeuper is the potential recharge zone for the drinking-water production wells. Second, we closely examined additional data for two selected production well sites. The Breitenholz site had elevated groundwater temperatures and relatively low tracer concentrations for both tritium and SF6. This suggested a long, >50 years and deep groundwater flowpath with NGWA.org

insignificant recharge on the way from its recharge area to the well. This can only be the area north of the Sch¨onbuch plateau (Figure 1). In contrast, the Poltringen well site had a continuous transit time distribution with relatively short and also long transit times, and a median transit time of 6 to 11 years. This indicated a different recharge source area than for Breitenholz site, which is likely the remaining Gipskeuper-free area west of the Ammer River plus the Reusten anticline. Finally, we found that water in the other production well sites, other than the two sites discussed previously, can be represented as a mixture of two types of water. These types of water were the old water from the Breitenholz type and the young water from the Poltringen type. Piecing all three findings together (Figure 5), there were two main sources of groundwater to the wells: relatively clean groundwater, mostly recharged >50 years ago from north of the Sch¨onbuch, and a relatively young groundwater component coming from west of the Ammer River and the Reusten anticline. The young groundwater component is contaminated by agrochemicals and mixes with the old groundwater at each well site with different proportions. It should, however, be noted that our conceptual model of groundwater flow is probably most representative for low to normal flows as sampling was primarily done under those conditions. Relevant information for the development of the conceptual understanding of groundwater flow to the production wells was predominantly contributed by the analysis of hydrogeochemical and environmental tracer data. Our examination of hydrogeological data involved several assumptions and was hence relatively uncertain. We therefore believe that measured concentrations of solutes such as major ions or environmental tracers could be more or at least equally useful for an understanding of groundwater flow in karstified aquifers than hydrogeological information more traditionally used in groundwater modeling.

Acknowledgments We thank the Ammertal-Sch¨onbuchgruppe (local water supplier) for providing data. Furthermore, we would like to thank Dr. Karsten Osenbr¨uck (WESS Competence Cluster) for technical discussions, the tracer input functions and environmental tracer measurements for the Poltringen wells. We are also grateful to Tiansheng Li (University of T¨ubingen) who measured the Ammer River baseflows. We would like to thank Dr. Marc Schwientek (WESS Competence Cluster) and Bernhard Keim (engineering company kup) for technical discussions as well as Dr. Andreas Musolff (Helmholtz Centre for Environmental Research UFZ) for his constructive criticism on an earlier draft of this paper. This work was supported by a grant from the Ministry of Science, Research and Arts of Baden-W¨urttemberg (AZ Zu 33-721.3-2) and the Helmholtz Center for Environmental Research, Leipzig (UFZ). Finally, we would like to thank the executive editor Kristine Uhlman and the three anonymous reviewers for the useful comments that significantly improved the presentation of our study. I. Pavlovskiy and B. Selle Groundwater

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References Broers, H.P. 2004. The spatial distribution of groundwater age for different geohydrological situations in the Netherlands: Implications for groundwater quality monitoring at the regional scale. Journal of Hydrology 299, no. 1-2: 84–106. Bullister, J.L., D.P. Wisegarver, and F.A. Menzia. 2002. The solubility of the sulfur hexafluoride in water and seawater. Deep-Sea Research 49, no. 1: 175–187. Cook, P.G., A.J. Love, N.I. Robinson, and C.T. Simmons. 2005. Groundwater ages in fractured rock aquifers. Journal of Hydrology 308, no. 1-4: 284–301. Drew, D., and N. Goldscheider. 2007. Combined use of methods. In Methods in Karst Hydrogeology, ed. N. Goldscheider and D. Drew, 223–228. London, UK: Taylor & Francis. Ghasemizadeh, R., F. Hellweger, C. Butscher, I. Padilla, D. Vesper, M. Field, and A. Alshawabkeh. 2012. Review: Groundwater flow and transport modeling of karst aquifers, with particular reference to the North Coast Limestone aquifer system of Puerto Rico. Hydrogeology Journal 20, no. 8: 1441–1461. Gooddy, D.C., W.G. Darling, C. Abesser, and D.J. Lapworth. 2006. Using chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) to characterise groundwater movement and residence time in a lowland chalk catchment. Journal of Hydrology 330, no. 1-2: 44–52. Harreß, H.M. 1973. Hydrogeologische Untersuchungen im Oberen G¨au. PhD thesis, University of T¨ubingen. Krause, H. 1988. Zur Ausbildung und Tiefenlage von Gips- und Anhydritspiegel im Gipskeuper BadenW¨urttembergs. Jahreshefte des Geologischen Landesamtes Baden-W¨urttemberg 30, no. 1: 285–299. Kriele, W. 1976. Hydrogeologische Untersuchungen im Muschelkalkkarst des westlichen unteren G¨aus zwischen Pforzheim, Vaihingen/Enz und Sindelfingen. PhD thesis. University of T¨ubingen. Land, L., and G.F Huff. 2010. Multi-tracer investigation of groundwater residence time in a karstic aquifer: Bitter Lakes National Wildlife Refuge, New Mexico, USA. Hydrogeology Journal 18, no. 2: 455–472. Leibundgut, C., P. Maloszewski, and C. K¨ulls. 2009. Tracers in Hydrology. Chichester, UK: Wiley-Blackwell. Li, T. 2013. Quantifying streamflow contributions from different sources into the Ammer River using End-Member Mixing Analysis. Master thesis. University of T¨ubingen. Long, A.J., and L.D. Putnam. 2006. Translating CFC-based piston ages into probability density functions of groundwater age in karst. Journal of Hydrology 330, no. 3–4: 735–747. Małoszewski, P., and A. Zuber. 1982. Determining the turnover time of groundwater systems with the aid of environmental tracers. 1. Models and their applicability. Journal of Hydrology 57, 3-4: 207–231. Maloszewski, P., W. Stichler, A. Zuber, and D. Rank. 2002. Identifying the flow systems in a karstic-fissuredporous aquifer, the Schneealpe, Austria, by modelling of

10

I. Pavlovskiy and B. Selle Groundwater

environmental 18O and 3H isotopes. Journal of Hydrology 256, no. 1–2: 48–59 Neeb, I., and B. Wittkopp. 1998. Untersuchungen des Grundwassers im Oberen Muschelkalk im Raum Sindelfingen. Schriftenreihe des Amtes f¨ur Umweltschutz Stuttgart 1998, no. 1: 29–60. Parkhurst, D.L., and C.A.J. Appelo. 1999. User’s guide to PHREEQC (Version 2)—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. Technical report. Reston, Virginia: U.S. Geological Survey. Pl¨umacher, J. 1999. Kalibrierung eines regionalen Grundwasserstr¨omungsmodells mit Hilfe von Umwetltisotopeninformation. PhD thesis. ETH Z¨urich. Rigby, M., J. M¨uhle, B.R. Miller, R.G. Prinn, P.B. Krummel, L.P. Steele, P.J. Fraser, P.K. Salameh, C.M. Harth, R.F. Weiss, B.R. Greally, S. O’Doherty, P.G. Simmonds, M.K. Vollmer, S. Reimann, J. Kim, K.R. Kim, H.J. Wang, E.J. Dlugokencky, G.S. Dutton, B.D. Hall, and J.W. Elkins. 2010. History of atmospheric SF6 from 1973 to 2008. Atmospheric Chemistry and Physics 10, no. 11: 10305–10320. Sauter, M., A. Kov´acs, T. Geyer, and G. Teutsch. 2006. Modellierung der Hydrodynamik von Karstgrundwasserleitern ¨ Eine Ubersicht. Grundwasser 11, no. 3: 143–156. Selle, B., K. Rink, and O. Kolditz. 2013a. Recharge and discharge controls on groundwater travel times and flow paths to production wells for the Ammer catchment in SW Germany. Environmental Earth Sciences 69, no. 2: 443–452. Selle, B., M. Schwientek, and G. Lischeid. 2013b. Understanding processes governing water quality in the Ammer catchment in SW Germany. Journal of Hydrology 486, no. 1: 31–38. Simon, T. 1986. Schwebende Schichtgrundwasser-Stockwerke im Oberen Muschelkalk und ihre Bedeutung f¨ur die Verkastung. Jahreshefte des Geologischen Landesamtes Baden-W¨urttemberg 28, no. 1: 245–265. Toran, L., and W.B. White. 2005. Variation in nitrate and calcium as indicators of recharge pathways in Nolte Spring, PA. Environmental Geology 48, no. 7: 854–860. Ufrecht, W. 1994. Das Mineralwasser von Stuttgart-Bad Cannstatt und Berg – eine Einf¨uhrung in die Geologie, Geohydraulik und Hydrochemie des Systems. Schriftenreihe des Amtes f¨ur Umweltschutz Stuttgart 1994, no. 2: 13–48. Villinger, E. 1982. Grundwasserbilanzen im Karstaquifer des Oberen Muschelkalks im Oberen G¨au (BadenW¨urttemberg). Geologische Jahrb¨ucher - Reihe C 32, no. 1: 43–61. Wolff, G., and N. Hellenthal. 2005. Nutzung der Geothermie in Stuttgart. Schriftenreihe des Amtes f¨ur Umweltschutz Stuttgart 2005, no. 1: 1–88. Zanini, L., K.S. Novakowski, P. Lapcevic, G.S. Bickerton, J. Voralek, and C. Talbot. 2000. Groundwater flow in a fractured carbonate aquifer inferred from combined hydrogeological and geochemical measurements. Ground Water 38, no. 3: 350–360.

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