Methods Note/
Field Observation of Diurnal Dissolved Oxygen Fluctuations in Shallow Groundwater by Keith E. Schilling1 and Peter Jacobson2
Abstract Dissolved oxygen (DO) concentrations influence many biogeochemical processes in groundwater systems but studies of temporal variability in DO are lacking. In this study, we used an optical DO probe to measure rapid changes in concentration due to plant-groundwater interaction at an alluvial aquifer field site in Iowa. Diurnal DO concentrations were observed during mid- to late-summer when soil conditions were dry, fluctuating approximately 0.2 to 0.3 mg/L on a daily basis. DO fluctuations in groundwater were out-of-phase with diurnal water table fluctuations, increasing during the day and decreasing at night. DO consumption at night is likely due to increased soil autotrophic and heterotrophic respiration linked with patterns of carbon supply derived from daytime photosynthetic activity, and consistent with available literature on diurnal soil respiration patterns. Although more work is needed to quantify specific processes, our results indicate the potential usefulness of the new optical DO technology to reveal insights regarding many ecohydrological processes.
Introduction Many biogeochemical processes occurring in groundwater environments are influenced by the presence of dissolved oxygen (DO) concentrations, including microbial respiration (Sierra and Renault 1995; Chapelle 2000), oxidation-reduction processes (Champ et al. 1979), mineral reactions (Langmuir 1997), denitrification (Burgin and Groffman 2012), contaminant biodegradation (Baedecker and Back 1979), among many other examples. While variations in groundwater DO concentrations have been evaluated in terms of spatial patterns (e.g., Rose and Long 1988; Chen and Liu 2003; Wassenaar and Hendry 2007) or as an indicator of other processes (Chapelle et al. 2012), studies of temporal changes in groundwater DO are lacking. Continuous measurements of DO, long used to characterize aquatic metabolism and primary production in streams (Odum 1956; Venkiteswaran 1
Corresponding author: Iowa Geological Survey, 109 Trowbridge Hall, Iowa City, IA 52242-1319; 319-335-1575; fax: 319-335-2754; [email protected] 2 Department of Biology, Grinnell College, Grinnell, IA 50112; 641-269-3027; fax: 641-269-4285; [email protected] Received October 2013, accepted March 2014. © 2014, National Ground Water Association. doi: 10.1111/gwat.12218
NGWA.org
et al. 2008), are being increasingly used in groundwater systems where rapid changes in DO are expected, such as beneath infiltration basins (Datry et al. 2004; Foulquier et al. 2010) or in hyporheic zones (Soulsby et al. 2009). New monitoring techniques using an optical DO technology are allowing for greater temporal resolution of measurements with improved accuracy and less maintenance during long-term deployments. In shallow groundwater settings typical of alluvial floodplains, water tables are known to fluctuate diurnally in response to plant water demand (Engel et al. 2005; Loehide et al. 2005; Butler et al. 2007; Schilling and Jacobson 2009). Diurnal water table fluctuations are produced when plants transpire during the daylight hours and use shallow groundwater as a water source. During the night when plant stomata are closed, transpiration decreases and the water table rebounds owing to net inflow from the surrounding aquifer. These rapid water table changes, albeit small (often on the order of less than 1 cm in magnitude), reveal hydraulic interaction between the root zone and the shallow groundwater system. To our knowledge, the relation of diurnal water table fluctuations to groundwater DO concentrations has not been previously reported. This methods paper reports on the application of an optical DO probe to capture rapid changes in groundwater
Vol. 53, No. 3–Groundwater–May-June 2015 (pages 493–497)
493
DO concentration due to plant-groundwater interaction. We hypothesized that groundwater DO levels would fluctuate in response to diurnal variations in plant and microbial physiological processes. Our results provide a glimpse of the potential usefulness of the new technology to reveal insights regarding biogeochemical processes that operate at the interface between ecology and hydrology (Bond 2003).
Methods Our study was conducted at The Nature Conservancy (TNC) Swamp White Oak (SWO) preserve located on the floodplain of the Cedar River in Muscatine County, Iowa (lat 41◦ 24 36 , long 91◦ 17 06 ). The floodplain savanna is characterized by alluvial, often sandy soils, high water tables, and an overstory vegetation dominated by Quercus bicolor (Swamp white oak), Quercus macrocarpa (Bur oak), and Quercus velutina (Black oak) and sand prairie species in the understory. Additional site details can be found in Schilling and Jacobson (2009, 2011). A monitoring well was installed to a depth of approximately 2 m in the central region of the SWO preserve beneath the canopy of a large Swamp White Oak approximately 2 m from the tree trunk. The soil boring was made using a 152-mm hand auger and a 1.5-m long factoryslotted polyvinyl chloride (PVC) well screen and 1.5-m long PVC riser was installed in the borehole. The borehole was allowed to collapse around the well screen for a natural sand pack. An YSI Model 600 XLMV2 multiprobe sonde equipped with an optical DO probe was deployed in the monitoring well to record 15-min geochemical variations in shallow groundwater. The YSI sonde also measured and logged readings of hydraulic head. The sonde was submerged approximately 0.2 to 0.4 m below the water table surface to ensure readings could be maintained during late summer when hydrologic conditions were dry. The unit was calibrated before deployment and checked for calibration during well sampling activities. Accuracy of the measurements was ±0.02 mg/L for DO and ±0.01 m for head. The sonde was deployed across the May to October growing seasons in 2009 and 2010.
Results Throughout the growing season, we observed diurnal water table fluctuations in the shallow alluvial aquifer, which were expected given previous monitoring (Schilling and Jacobson 2009). Unexpected were observations of diurnal DO concentrations in shallow groundwater (Figure 1). Diurnal DO patterns were not observed on a continuous basis during the monitoring seasons but were only evident in the mid- to late-summer when soil conditions were dry and were not observed during non-leaf-out periods. When diurnal patterns were observed, DO concentrations typically fluctuated between approximately 0.2 and 0.3 mg/L on a daily cycle. DO fluctuations in groundwater were out-of-phase with diurnal water table fluctuations (Figure 2). Water depths 494
Figure 1. Water table and dissolved oxygen fluctuations in a shallow (2-m) monitoring well installed into a sandy alluvial aquifer on the Cedar River floodplain, Iowa.
(A)
(B)
Figure 2. (A) Depiction of water and nutrient flow between the overlying vegetation and shallow groundwater. (B) Water table depths and DO concentrations measured across a 2-d period in August 2010.
declined from about 10 AM to 6 PM and increased during the nighttime hours, whereas DO concentrations increased during the day and decreased at night.
Discussion Long-term deployment of an optical DO probe provided us an opportunity to measure the variations in DO levels that were not evident during periodic spot measurements (Figure 3). Further, when we examined the
K.E. Schilling and P. Jacobson Groundwater 53, no. 3: 493–497
NGWA.org
Figure 3. Comparison of measurements of DO in the monitoring well collected during periodic well sampling with DO measurements collected using the optical DO probe.
continuous DO record at subdaily timescales, previously unknown diurnal fluctuations were observed. Hence, the optical DO technology is revealing new insights on both the long-term temporal variations in DO concentrations and short-term variations in DO resulting from interaction of tree physiological and soil biogeochemical processes with shallow groundwater. Studies have shown diurnal cycles of root respiration in laboratory studies (Williams et al. 2010; Dong et al. 2011) and in the soil zone at field sites (Cardon et al. 2002; Tang et al. 2005). Our study represents an extension of this work to include the effects of these diurnal cycles on shallow groundwater DO at the field scale. We can hypothesize on the mechanisms involved with the clear understanding that additional investigation is needed (Figure 2). Changes in groundwater temperature are not suspected as the cause of the diurnal DO fluctuations. Deeper groundwater DO concentrations at our field site (mean 2.2 mg/L; n = 20) are lower than concentrations at the water table (5.8 mg/L, n = 20) ruling out the possibility that deeper, colder water with higher DO levels is the source of increasing DO at the water table (i.e., hydraulic lift; Ishikawa and Bledsoe 2000). Minor diurnal changes in groundwater temperature (
Field Observation of Diurnal Dissolved Oxygen Fluctuations in Shallow Groundwater by Keith E. Schilling1 and Peter Jacobson2
Abstract Dissolved oxygen (DO) concentrations influence many biogeochemical processes in groundwater systems but studies of temporal variability in DO are lacking. In this study, we used an optical DO probe to measure rapid changes in concentration due to plant-groundwater interaction at an alluvial aquifer field site in Iowa. Diurnal DO concentrations were observed during mid- to late-summer when soil conditions were dry, fluctuating approximately 0.2 to 0.3 mg/L on a daily basis. DO fluctuations in groundwater were out-of-phase with diurnal water table fluctuations, increasing during the day and decreasing at night. DO consumption at night is likely due to increased soil autotrophic and heterotrophic respiration linked with patterns of carbon supply derived from daytime photosynthetic activity, and consistent with available literature on diurnal soil respiration patterns. Although more work is needed to quantify specific processes, our results indicate the potential usefulness of the new optical DO technology to reveal insights regarding many ecohydrological processes.
Introduction Many biogeochemical processes occurring in groundwater environments are influenced by the presence of dissolved oxygen (DO) concentrations, including microbial respiration (Sierra and Renault 1995; Chapelle 2000), oxidation-reduction processes (Champ et al. 1979), mineral reactions (Langmuir 1997), denitrification (Burgin and Groffman 2012), contaminant biodegradation (Baedecker and Back 1979), among many other examples. While variations in groundwater DO concentrations have been evaluated in terms of spatial patterns (e.g., Rose and Long 1988; Chen and Liu 2003; Wassenaar and Hendry 2007) or as an indicator of other processes (Chapelle et al. 2012), studies of temporal changes in groundwater DO are lacking. Continuous measurements of DO, long used to characterize aquatic metabolism and primary production in streams (Odum 1956; Venkiteswaran 1
Corresponding author: Iowa Geological Survey, 109 Trowbridge Hall, Iowa City, IA 52242-1319; 319-335-1575; fax: 319-335-2754; [email protected] 2 Department of Biology, Grinnell College, Grinnell, IA 50112; 641-269-3027; fax: 641-269-4285; [email protected] Received October 2013, accepted March 2014. © 2014, National Ground Water Association. doi: 10.1111/gwat.12218
NGWA.org
et al. 2008), are being increasingly used in groundwater systems where rapid changes in DO are expected, such as beneath infiltration basins (Datry et al. 2004; Foulquier et al. 2010) or in hyporheic zones (Soulsby et al. 2009). New monitoring techniques using an optical DO technology are allowing for greater temporal resolution of measurements with improved accuracy and less maintenance during long-term deployments. In shallow groundwater settings typical of alluvial floodplains, water tables are known to fluctuate diurnally in response to plant water demand (Engel et al. 2005; Loehide et al. 2005; Butler et al. 2007; Schilling and Jacobson 2009). Diurnal water table fluctuations are produced when plants transpire during the daylight hours and use shallow groundwater as a water source. During the night when plant stomata are closed, transpiration decreases and the water table rebounds owing to net inflow from the surrounding aquifer. These rapid water table changes, albeit small (often on the order of less than 1 cm in magnitude), reveal hydraulic interaction between the root zone and the shallow groundwater system. To our knowledge, the relation of diurnal water table fluctuations to groundwater DO concentrations has not been previously reported. This methods paper reports on the application of an optical DO probe to capture rapid changes in groundwater
Vol. 53, No. 3–Groundwater–May-June 2015 (pages 493–497)
493
DO concentration due to plant-groundwater interaction. We hypothesized that groundwater DO levels would fluctuate in response to diurnal variations in plant and microbial physiological processes. Our results provide a glimpse of the potential usefulness of the new technology to reveal insights regarding biogeochemical processes that operate at the interface between ecology and hydrology (Bond 2003).
Methods Our study was conducted at The Nature Conservancy (TNC) Swamp White Oak (SWO) preserve located on the floodplain of the Cedar River in Muscatine County, Iowa (lat 41◦ 24 36 , long 91◦ 17 06 ). The floodplain savanna is characterized by alluvial, often sandy soils, high water tables, and an overstory vegetation dominated by Quercus bicolor (Swamp white oak), Quercus macrocarpa (Bur oak), and Quercus velutina (Black oak) and sand prairie species in the understory. Additional site details can be found in Schilling and Jacobson (2009, 2011). A monitoring well was installed to a depth of approximately 2 m in the central region of the SWO preserve beneath the canopy of a large Swamp White Oak approximately 2 m from the tree trunk. The soil boring was made using a 152-mm hand auger and a 1.5-m long factoryslotted polyvinyl chloride (PVC) well screen and 1.5-m long PVC riser was installed in the borehole. The borehole was allowed to collapse around the well screen for a natural sand pack. An YSI Model 600 XLMV2 multiprobe sonde equipped with an optical DO probe was deployed in the monitoring well to record 15-min geochemical variations in shallow groundwater. The YSI sonde also measured and logged readings of hydraulic head. The sonde was submerged approximately 0.2 to 0.4 m below the water table surface to ensure readings could be maintained during late summer when hydrologic conditions were dry. The unit was calibrated before deployment and checked for calibration during well sampling activities. Accuracy of the measurements was ±0.02 mg/L for DO and ±0.01 m for head. The sonde was deployed across the May to October growing seasons in 2009 and 2010.
Results Throughout the growing season, we observed diurnal water table fluctuations in the shallow alluvial aquifer, which were expected given previous monitoring (Schilling and Jacobson 2009). Unexpected were observations of diurnal DO concentrations in shallow groundwater (Figure 1). Diurnal DO patterns were not observed on a continuous basis during the monitoring seasons but were only evident in the mid- to late-summer when soil conditions were dry and were not observed during non-leaf-out periods. When diurnal patterns were observed, DO concentrations typically fluctuated between approximately 0.2 and 0.3 mg/L on a daily cycle. DO fluctuations in groundwater were out-of-phase with diurnal water table fluctuations (Figure 2). Water depths 494
Figure 1. Water table and dissolved oxygen fluctuations in a shallow (2-m) monitoring well installed into a sandy alluvial aquifer on the Cedar River floodplain, Iowa.
(A)
(B)
Figure 2. (A) Depiction of water and nutrient flow between the overlying vegetation and shallow groundwater. (B) Water table depths and DO concentrations measured across a 2-d period in August 2010.
declined from about 10 AM to 6 PM and increased during the nighttime hours, whereas DO concentrations increased during the day and decreased at night.
Discussion Long-term deployment of an optical DO probe provided us an opportunity to measure the variations in DO levels that were not evident during periodic spot measurements (Figure 3). Further, when we examined the
K.E. Schilling and P. Jacobson Groundwater 53, no. 3: 493–497
NGWA.org
Figure 3. Comparison of measurements of DO in the monitoring well collected during periodic well sampling with DO measurements collected using the optical DO probe.
continuous DO record at subdaily timescales, previously unknown diurnal fluctuations were observed. Hence, the optical DO technology is revealing new insights on both the long-term temporal variations in DO concentrations and short-term variations in DO resulting from interaction of tree physiological and soil biogeochemical processes with shallow groundwater. Studies have shown diurnal cycles of root respiration in laboratory studies (Williams et al. 2010; Dong et al. 2011) and in the soil zone at field sites (Cardon et al. 2002; Tang et al. 2005). Our study represents an extension of this work to include the effects of these diurnal cycles on shallow groundwater DO at the field scale. We can hypothesize on the mechanisms involved with the clear understanding that additional investigation is needed (Figure 2). Changes in groundwater temperature are not suspected as the cause of the diurnal DO fluctuations. Deeper groundwater DO concentrations at our field site (mean 2.2 mg/L; n = 20) are lower than concentrations at the water table (5.8 mg/L, n = 20) ruling out the possibility that deeper, colder water with higher DO levels is the source of increasing DO at the water table (i.e., hydraulic lift; Ishikawa and Bledsoe 2000). Minor diurnal changes in groundwater temperature (
