Geoelectrical Imaging of Hyporheic Exchange and Mixing of River

Dec 31, 2010 - electrical resistivity (ER) imaging in a 60-m wide and 0.7-m deep alluvial river whose stage periodically varied by 0.7 m due to dam op...
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Geoelectrical Imaging of Hyporheic Exchange and Mixing of River Water and Groundwater in a Large Regulated River M. Bayani Cardenas* and Michael S. Markowski 1 University Station C9000, Geological Sciences, The University of Texas at Austin, Austin, Texas 78712, United States

bS Supporting Information ABSTRACT: Hyporheic mixing and surface water-groundwater interactions are critical processes in aquatic environments. Yet, there is a lack of methods for assessing the spatial extent and distribution of these mixing zones. This study applied time-lapse electrical resistivity (ER) imaging in a 60-m wide and 0.7-m deep alluvial river whose stage periodically varied by 0.7 m due to dam operations to assess dynamic hyporheic mixing and surface water-groundwater interactions. Sixteen channel-spanning repeat ER tomograms (2D sections) over one flood cycle captured the dynamic ER distribution. We mapped a laterally discontinuous hyporheic zone, which had mainly river water circulating through it, several meters into the bed. Underneath the hyporheic zone was a transitional mixing zone intermittently flushed by mixing river water and deep groundwater. Minimally mixed groundwater dominated the deepest areas. ER imaging allows for unraveling hyporheic and deep mixing zone dynamics in large regulated rivers.

’ INTRODUCTION The hyporheic zone, the subsurface volume where river water and groundwater actively mix in alluvial material, also defined by flow paths originating from and ending at the river, is a common feature of flowing aquatic systems. The hyporheic zone provides myriad biogeochemical and ecological services integral to the river corridor in both natural and human-impacted systems.1-6 The hyporheic zone is where waters with different composition (river and groundwater) converge which presents a unique environment, going from oxic to anoxic, allowing for critical reactions such as denitrification (e.g., refs 1, 7, 8, and 10) and other redox processes such as of metals to occur.9 The hyporheic zone is a favored site for metal uptake, metal precipitation as oxides, metal sorption, and transport and deposition as ions or with colloids.5,11-13 Therefore, quantifying rates of coupled fluid and biogeochemical mixing and reactions within the hyporheic zone is critical.4,11,14 The dynamic nature of the hyporheic zone and its relative inaccessibility in large rivers makes physical mapping of the hyporheic zone, let alone spatial characterization of biogeochemical processes, a formidable task. A few techniques allow for mapping the hyporheic zone. The most direct approach is to map flow paths by mapping pressure or head gradients or mapping chemical boundaries between mixing waters. Because it is tedious, this can be done in mostly smaller streams or at a limited number of points in larger rivers.6,14,15 An alternative is to identify flow paths with tracers. Injected solute tracing is very useful but is time-consuming and typically only a few observation points are possible.16 Heat tracing is becoming a favored technique since dynamic heat signals are omnipresent (e.g., diel and seasonal variations) and logging temperature probes have become affordable.17,18 Such studies rarely go beyond a few decimeters since thermal signals may be too obscure at larger scales. Electrical resistivity imaging (ERI) or tomography is a promising method for mapping hyporheic zone extent and dynamics r 2010 American Chemical Society

when there is a suitable electrical contrast between the river water and deeper groundwater, especially when they mix in the interstices of the hyporheic zone. This technique has been applied in coastal areas where fresh groundwater and seawater mix, leading to a pronounced contrast in bulk ER of the saturated sediment.19 Acworth et al.20 mapped the mixing zone between saltier water coming from a tidal creek and fresher groundwater via ER. Fewer studies have attempted to study the interaction of groundwater and surface water in a noncoastal environment using ER imaging. For example, Nyquist et al.21 conducted two repeat ER surveys about four months apart in the same longitudinal transect along a stream. They found that areas exhibiting more pronounced relative change in ER have higher permeability and are preferential groundwater discharge points into the stream. Singha et al.22 theoretically showed the potential of ERI to map the movement of an in-stream injected saline tracer through the hyporheic zone and for quantifying mass transfer rates. Ward et al.23 followed-up the theoretical work with a field study where they were able to successfully image a saline tracer moving through the hyporheic zone and which was released into a ∼1.75 m wide and 0.15 m deep stream. In regulated or dammed major rivers, which account for 60% of major rivers worldwide,24,25 hyporheic zones can be very extensive, have high exchange rates, and are dynamic.25,26 Few studies have presented observations from hyporheic zones of regulated systems due to challenging field conditions; measurements need to be conducted across a broad range of river discharge and depth. Arntzen et al.27 presented head measurements within the riverbed of the regulated Columbia River where it has been postulated that the hyporheic zone in the Columbia Received: October 12, 2010 Accepted: December 15, 2010 Revised: December 10, 2010 Published: December 31, 2010 1407

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Figure 1. Map of and location of the study site (indicated by `Hornsby Bend site’) with locations of electrical resistivity survey transect and the groundwater well where electrical resistivity was measured.

River could be contributing to attenuation of Cr(VI).14 Sawyer et al.26 used water table fluctuations in the bank of a regulated river to quantify lateral hyporheic exchange rates. Boutt and Fleming28 conducted numerical simulations of near-stream groundwater flow and transport processes in a regulated river. Their simulations were patterned after some field observations. These previous studies show that river stage fluctuations efficiently pump river water in and out of the surrounding sediment with implications for heat and chemical transport. Hyporheic zone processes in regulated rivers are understudied partly due to inadequacy of observational techniques developed for much smaller rivers. The main objective of this study is to test the application of ERI for mapping large-scale and dynamic hyporheic mixing and surface water-groundwater interactions in a difficult setting. The novel aspects of this study are as follows: 1) it is conducted at a large spatial scale but at relatively hightemporal resolution, 2) it is conducted in a regulated river, representing the majority of large rivers, and 3) no introduced ionic tracers are used, unlike previous similar studies.

’ STUDY SITE The study site is located along a section of the Colorado River near Austin, Texas, USA approximately 10 km east from downtown Austin (Figure 1). The Colorado River is ∼60 m wide and ∼0.7 m deep at the study site. Upstream of the site is Tom Miller Dam, which releases water daily for irrigation and to generate hydroelectric power. Further downstream, near downtown Austin, Longhorn Dam holds water back but is mainly a flow-through dam operated partly in tandem with the Tom Miller Dam. The release of water from Tom Miller Dam caused stage fluctuations of about 0.7 m during the study period at the site (Figure 2) which is located 17 km downstream from Longhorn Dam. At the study site, a shallow alluvial aquifer underlies the Colorado River, and this aquifer consists of Tertiary and Quaternary gravels, sands, and silts. The alluvium is incised into the regionally extensive Cretaceous Taylor Clay unit that is exposed in some parts of the river. Under natural conditions, the Colorado River at the study site is gaining and regional groundwater flow through the shallow alluvial aquifer is toward the river (Figure 1).

Figure 2. River stage, river resistivity, and time-lapse tomograms for the transverse transect imaged over one flood cycle. The transect is indicated by the red line in Figure 1. The boxed number labels in the river stage curve correspond to the start of the tomographic surveys for the numbered tomograms. Black lines indicate the sediment-water interface or land surface. A more complete version (images at 16 times) is presented in the Supporting Information.

’ DATA COLLECTION, INVERSION, AND ANALYSIS Electrical resistivity imaging is accomplished by injecting electrical currents into the subsurface using two electrodes, while two additional electrodes measure electrical potential. Each measurement using four electrodes, following Ohm’s Law, interrogates an effective point in the subsurface for its dielectric properties. Changing the locations of these electrode pairs allows for mapping a 2D (or 3D) set of points in the subsurface. The collection of these points are then used as targets for an inverse model which solves for the electrical resistivity field. Over the course of two days (November 7-8, 2009), electrical resistivity data were collected along a fixed transverse transect that starts on the southeast bank of the Colorado River and ends 1408

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near the northwest bank (Figure 1). All ER surveys were conducted over a 24-h time period so that the time-series covered a complete dam storage-release cycle. A dipole-dipole configuration for the electrodes was used where a pair of adjacent electrodes injects the current, while a separate pair some distance away from the current electrodes measures potential. This configuration allows for a good balance of resolution, depth of penetration, and reasonably fast survey times. All ER data are collected using an Advanced Geosciences, Inc. SuperSting R8 unit with submersible cables. The cables had 56 graphite electrodes spaced every 1.5 m; the SuperSting R8 automatically switches and monitors current and potential electrodes following the prescribed survey design. The electrodes were laid directly on the streambed with some electrodes fastened to rods for stability. The electrode locations and elevations were surveyed to within a few mm. Each ER survey took about 1-1.5 h to complete, resulting in 16 surveys over a 24-h period. River water ER, depth, and temperature were monitored with an In-situ Aqua Troll 200 probe. The ER of groundwater inside a sufficiently purged well hydraulically up-gradient from the river was measured. In order to constrain temperature effects on resistivity, riverbed temperatures were monitored along a vertical profile with HOBO TMC thermistors placed inside a steel pipe from 45 to 105 cm, spaced every 15 cm, below the sedimentwater interface. Individual data sets from the surveys were inverted with EarthImager2D software. The raw data were truncated prior to inversion. There was a discrete zone with very high resistivity on the west bank. We removed the electrodes whose measurements included this zone, leaving only 40 electrodes spanning from bank to bank. The resulting raw data set still covered a majority of the channel. Following Day-Lewis et al.,19 the inversion used water column depth and resistivity as constraints since this minimizes artifacts resulting from the inversion.

’ RESULTS AND DISCUSSION The resistivity of groundwater in the well hundreds of meters from the river was 8.25 Ωm. Throughout the survey period, the resistivity of river water varied by 1.86 Ωm, ranging from 14.30 to 16.16 Ωm (Figure 2). Therefore, the pore fluid in the alluvium adjacent to the river may vary from 8.25 Ωm to 16.16 Ωm, with a maximum range of 7.91 Ωm. In this extreme range scenario, at some point in time, the pores become completely occupied either by river water or the groundwater end member. Since the imaging is conducted repeatedly at the exact same location, any changes in bulk resistivity are mostly if not solely due to changes in pore water resistivity (effectively chemistry) or bulk temperature. We first quantified the contributions of changing water temperature to bulk resistivity. River water temperature fluctuated between 20 and 22.4 °C during the study period. The effects of changing temperature on the resistivity of water can be calculated following Smolen and Lightsey29 Fw2 ¼

Fw1 ðT1 þ 21:5Þ ðT2 þ 21:5Þ

ð1Þ

where Fw and T are resistivity and temperature of a given water sample. To get a conservative range of resistivity change (Fw2Fw1) with temperature, the conductive groundwater end member and the most resistive river end member water samples were used

for Fw1 in calculations with eq 1. The temperature observed at 45 cm depth in the bed ranged from 20.46 to 21.03 °C; the mean temperature gets slightly warmer with depth but was well within the observed temperature range for river water. These values were used for T1 and T2. At 0.45 m into the bed, expected resistivity changes due to dynamic temperature are 0.09 Ωm and 0.22 Ωm, using the groundwater and river water end members, respectively. The effect becomes smaller at 0.60 m depth based on temperature data from that depth. Below 0.75 m, very small temperature changes were observed, and any changes in bulk resistivity can be attributed to changes in pore fluid resistivity. In the shallowest areas, the maximum change in resistivity due to temperature dynamics was ∼0.22 Ωm. The range of expected change in bulk resistivity due to changes in pore fluid resistivity is calculated following a simple form of Archie’s Law F ð2Þ F ¼ b ¼ φ-m Fw where F is the formation factor, Fb is bulk resistivity, Fw is water resistivity, φ is porosity, and m is the cementation factor. The alluvial material at the site has φ = 0.385.30 Assuming m = 1.3, typical for unconsolidated sands,31 leads to F = 3.46. Based on this, the Fb of groundwater-saturated sediment should be ∼28.5 Ωm, and sediment saturated with river water should have Fb ∼ 52.7 Ωm. Note that these are qualitative thresholds since the geology and porosity may vary. In fact, even for sands, F may reach as high as 8.31 The time-series ER tomograms captured the dynamics of hyporheic and mixing zones (Figure 2). In this study, we refer to zones within the riverbed dominated by river water as comprising the `hyporheic zone’, whereas those that clearly show variation between river water and groundwater are referred to as `mixing zones’. The first tomogram is from 1:00 p.m., just as the stage begins to rise. The fourth tomogram is from 5:25 p.m. when the stage is at its peak. The remaining four tomograms show the resistivity profiles as the river stage slowly falls again. A more complete sequence of tomograms (16 total) is available as Supporting Information. The tomograms show two distinct zones. The top zone is about 2-2.5 m thick with Fb ranging from ∼25-100 Ωm, consistent with what is expected for river water-saturated sediment. This zone consists of mixed gravels, sand, with some mixed clay. Below this relatively resistive zone is a more conductive zone with Fb generally below ∼25-30 Ωm which is consistent with groundwater-saturated sediment. This zone likely consists of finer grains and clay and may include some of the Taylor Clay unit. Comparing the time-lapse tomograms shows noticeable Fb changes, especially within the top 2-2.5 m of the riverbed (Figure 2). A shallow resistive zone contracts vertically and horizontally with the dynamic river stage. The extent of this increasingly resistive zone increases through tomograms 1-4 and contracts afterward. Its spatial growth coincides with increasing stage, when river water is infiltrating into the bed. Bulk resistivity in this zone is lowest during surveys 1 and 8, when the river is at a low stage once more and under gaining conditions. This suggests that more conductive groundwater is rising toward the river, flushing out the more resistive river water. Assuming a maximum change in Fw of 7.91 Ωm between the groundwater and river end members and F = 3.46, Archie’s Law predicts that the maximum possible Fb change in the alluvium is 1409

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Figure 3. a) Range in resistivity change calculated from the sixteen tomograms in Figure 2 and b) sensitivity plot for dipole-dipole ER surveys. The red contour line within the bed indicates the threshold for a less variable zone flushed mainly by river water at shallow depths (the hyporheic zone) and a static groundwater-dominated zone in deeper areas. The area bounded by the red contour line showing large changes in resistivity is a transitional zone where river and groundwater mix. This excludes areas in the bank where the resistivity variations are due to a fluctuating water table and wetting/drying zone.

∼27.4 Ωm. Since the river water Fw varies by 1.86 Ωm, the Fb of alluvium which is sufficiently flushed by pure river water, i.e. a hyporheic zone, should exhibit a maximum change of ∼6.4 Ωm, also following Archie’s Law. This threshold Fb value is indicated by the red contour lines in Figure 3a; below this threshold, the change can be solely attributed to changes in Fw of infiltrating river water. The distribution of observed changes in Fb over the 16 surveys highlights areas of dynamic fluid mixing (Figure 3a). There is a shallow zone immediately beneath the river with small Fb change observed. This zone is underlain by a broad area found at intermediate depth, which is vertically extensive in some locations, with large changes in Fb. The deepest zones are characterized by little change in Fb. (Note that the banks also show large changes in Fb partly due to a fluctuating water table. While interesting, this study is focused on processes in the bed which stays saturated.) The shallow resistive areas above this line constitute the hyporheic zone, except of course areas in the bank. The area at intermediate depth subjected to the largest changes in Fb is interpreted to be a zone where river water and groundwater mix. This is some sort of transition zone which may include parts of the hyporheic zone since some river water also flows through it. In some spots, this zone extends to the river. This suggests that these are preferential flow paths with higher permeability. Beneath the transitional mixing zone is another mostly static zone. This is interpreted as mostly hosting groundwater with little to no mixing. The ER images shows that the hyporheic zone is several meters deep and extends across the channel. It is much larger than it would be if the river were under its natural groundwater baseflow-fed conditions; baseflow may even prevent the formation of any hyporheic zones. Sensitivity of the dipole-dipole survey is calculated by taking the diagonal of the product of the sensitivity matrix and its transpose. Its distribution (Figure 3b) shows decreasing sensitivity with distance/depth from the electrodes and is more or less uniform horizontally. Higher sensitivity values lead to higher confidence in inversion results. The sensitivity values are similar to those in Ward et al.23 The ER changes observed in the shallower zones where we infer a very active hyporheic zone is therefore more reliable than those at depth. Detecting any resistivity changes in areas with reduced sensitivity becomes more difficult. While this might be contributing to the observed minimal-change zones at

depth in the tomograms, the inverted tomograms still indicate zones of pronounced changes in resistivity suggesting that these areas are in fact subjected to measurable variation. It is important to note that tomography with hydrogeophysical observations naturally lead to smoothed or diffuse images and inaccurately represent abrupt boundaries.32 While we apply ER tomography to mixing zones where boundaries may not be very prominent, the thresholds or boundaries outlined and suggested are to be taken as qualitative. For this reason, we refrain from specifically quantifying cross-sectional areas of the hyporheic zone, as accurate extraction of spatial properties from tomograms is known to have limitations.32

’ IMPLICATIONS FOR USE OF ELECTRICAL RESISTIVITY IMAGING IN DAM-IMPACTED AQUATIC ENVIRONMENTS Since the majority of the world’s rivers are dammed and regulated, with many more dams being constructed and planned for construction, hyporheic and transitional zone dynamics in these regulated rivers need to be investigated further. This is critical for understanding the consequences and benefits of river regulation on hydraulic and biogeochemical functioning of the hyporheic zone. Questions such as; Does river regulation increase or decrease nutrient or metal uptake? ; need to be addressed. This requires placing equal emphasis on both physical and chemical processes. Here, we have shown that electrical imaging allows for unraveling both physical and chemical mixing processes in a ubiquitous setting that has been challenging to study. We only presented an example application, but the technique can be used to assess even larger scale and threedimensional patterns allowing for mapping for hyporheic zone volumes across nested scales. The only limitation in this case would be cable length and the duration of the study. ’ ASSOCIATED CONTENT

bS

Supporting Information. Supplement 1 shows the complete 16 tomograms. It is an expanded version of Figure 2. This material is available free of charge via the Internet at http://pubs. acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Phone: (512)471-6897. Fax: (512)471-9425. E-mail: cardenas@ jsg.utexas.edu.

’ ACKNOWLEDGMENT This research was partly supported by the Geology Foundation at the University of Texas. We thank three anonymous reviewers and Associate Editor David Dzombak for their constructive comments. ’ REFERENCES (1) Brunke, M.; Gonser, T. The ecological significance of exchange processes between rivers and groundwater. Freshwater Biol. 1997, 37, 1–33. (2) Boulton, A. J.; Findlay, S.; Marmonier, P.; Stanley, E. H.; Valett, H. M. The functional significance of the hyporheic zone in streams and rivers. Annu. Rev. Ecol. Syst. 1998, 29, 59–81. (3) Hester, E. T.; Gooseff, M. N. Moving Beyond the Banks: Hyporheic Restoration Is Fundamental to Restoring Ecological Services and Functions of Streams. Environ. Sci. Technol. 2010, 44 (5), 1521– 1525. (4) Benner, S. G.; Smart, E. W.; Moore, J. N. Metal behavior during surface groundwater interaction, Silver-Bow Creek, Montana. Environ. Sci. Technol. 1995, 29 (7), 1789–1795. (5) Oram, L. L.; Strawn, D. G.; Morra, M. J.; Moller, G. Selenium biogeochemical cycling and fluxes in the hyporheic zone of a miningimpacted stream. Environ. Sci. Technol. 2010, 44 (11), 4176–4183. (6) Ryan, R. J.; Boufadel, M. C. Lateral and longitudinal variation of hyporheic exchange in a piedmont stream pool. Environ. Sci. Technol. 2007, 41 (12), 4221–4226. (7) Battin, T. J.; Kaplan, L. A.; Findlay, S.; Hopkinson, C. S.; Marti, E.; Packman, A. I.; Newbold, J. D.; Sabater, F. Biophysical controls on organic carbon fluxes in fluvial networks. Nat. Geosci. 2008, 1 (2), 95–100. (8) Mulholland, P. J.; Helton, A. M.; Poole, G. C.; Hall, R. O.; Hamilton, S. K.; Peterson, B. J.; Tank, J. L.; Ashkenas, L. R.; Cooper, L. W.; Dahm, C. N.; Dodds, W. K.; Findlay, S. E. G.; Gregory, S. V.; Grimm, N. B.; Johnson, S. L.; McDowell, W. H.; Meyer, J. L.; Valett, H. M.; Webster, J. R.; Arango, C. P.; Beaulieu, J. J.; Bernot, M. J.; Burgin, A. J.; Crenshaw, C. L.; Johnson, L. T.; Niederlehner, B. R.; O’Brien, J. M.; Potter, J. D.; Sheibley, R. W.; Sobota, D. J.; Thomas, S. M. Stream denitrification across biomes and its response to anthropogenic nitrate loading. Nature 2008, 452 (7184), 202–U46. (9) Harvey, J. W.; Fuller, C. C. Effect of enhanced manganese oxidation in the hyporheic zone on basin-scale geochemical mass balance. Water Resour. Res. 1998, 34 (4), 623–636. (10) Battin, T. J.; Kaplan, L. A.; Newbold, J. D.; Hansen, C. M. E. Contributions of microbial biofilms to ecosystem processes in stream mesocosms. Nature 2003, 426 (6965), 439–442. (11) Fuller, C. C.; Harvey, J. W. Reactive uptake of trace metals in the hyporheic zone of a mining-contaminated stream, Pinal Creek, Arizona. Environ. Sci. Technol. 2000, 34 (7), 1150–1155. (12) Gandy, C. J.; Smith, J. W. N.; Jarvis, A. P. Attenuation of mining-derived pollutants in the hyporheic zone: A review. Sci. Total Environ. 2007, 373 (2-3), 435–446. (13) Ren, J. H.; Packman, A. I. Coupled stream-subsurface exchange of colloidal hematite and dissolved zinc, copper, and phosphate. Environ. Sci. Technol. 2005, 39 (17), 6387–6394. (14) Moser, D. P.; Fredrickson, J. K.; Geist, D. R.; Arntzen, E. V.; Peacock, A. D.; Li, S. M. W.; Spadoni, T.; McKinley, J. P. Biogeochemical processes and microbial characteristics across groundwater-surface water boundaries of the Hanford Reach of the Columbia River. Environ. Sci. Technol. 2003, 37 (22), 5127–5134.

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