Vertical Hydraulic Gradient Research Articles

Impact of Vertical Hydraulic Gradient on Rill Erodibility and Critical Shear Stress

Field observations and subsequent support from controlled lab experiments have shown the linkage between vertical hydraulic gradients and initiation of rills or incised channels. Nevertheless, current erosion prediction models do not include the hydraulic gradient effect in the erodibility parameters. The objectives of this study were to: (i) quantify the impact of a subsurface hydraulic gradient on rill erodibility and critical shear stress; and (ii) determine the intrinsic critical shear stress of the soil, which is the critical shear stress of the soil without the influence of the vertical hydraulic gradient. Two series of concentrated flow experiments were conducted on a silt loam soil subjected to hydraulic gradients varying from −2 to 2 m m−1 The first series measured soil loss at a given shear stress, while the second was a direct measurement of the critical shear stress by calculating the shear stress at the point of incipient motion of particles. The intrinsic critical shear stress was determined using a fluidized bed approach. We found that the average erodibility under an upward or positive hydraulic gradient was 5.64 times larger than that under a downward hydraulic gradient. The critical shear stress decreased from 1.0 to 0.2 Pa as the hydraulic gradient was increased from −2 to 2 m m−1 Using the fluidized bed approach, we obtained an inherent critical shear stress value of 1.61 Pa. We concluded that the presence of positive pore pressure near the soil surface was a major factor in reducing soil cohesion. We also concluded that with some improvements, the fluidized bed concept can be applied to determine the critical shear stress independent of the hydraulic gradient effect.

Variations in saturated and unsaturated water movement through an upland floodplain wetland, mid-Wales, UK

Summary Variations in inferred water movement are examined during a two-year study of an upland floodplain wetland at Afon Llwyd, in mid-Wales, UK in 2001 and 2002. Soil–water pressures ( ψ ) were monitored by six tensiometer nests, each comprising two sensors at depths of 30 and 60 cm below the surface. Detailed sedimentary sections were produced and changes in saturated and unsaturated hydraulic conductivity (and their relationship to ψ ) were estimated. Seasonal trends in ψ were identified and vertical hydraulic gradients within different tensiometer nests were derived and interpreted in the context of the local channel and floodplain sedimentology, channel bedform morphology and hydro-climatic conditions. Although there is considerable variation in ψ , the results display characteristic trends at several levels: (1) in relationship to precipitation; (2) reflecting changes in river stage (with downstream stage providing base-level control through a riffle-pool couplet); and (3) marked spatial variations in the direction and rate of water movement. These controls influence water redistribution through the wetland, which reflects the relative position of distinct sedimentary units within the floodplain. The results demonstrate that water movement is episodic and characterised by relatively rapid horizontal water fluxes during and immediately following individual rain events, followed by residual seepage to the river and water movement under a local soil moisture gradient. The implications of these results include elucidating the role of upland floodplains in buffering sub-surface drainage, and attenuating patterns of river flow under base flow conditions. This has the potential to advance the basis for future upland floodplain wetland and channel restoration schemes.

Incorporating a Watershed-Based Summary Field Exercise into an Introductory Hydrogeology Course

We have developed and implemented a summary field exercise for an introductory hydrogeology course without a laboratory section. This exercise builds on lectures and problem sets that use pre-existing field data. During one day in April, students measure hydraulic heads, stream and spring flow, and stream-bed seepage within the rural watershed of a third-order perennial stream in western Kentucky. Students calculate net specific discharge at various scales, map groundwater flow in the watershed, and calculate vertical hydraulic gradients at the mouth of the watershed, where the stream enters a reservoir (Kentucky Lake). Distinctive features of the exercise include hydraulic head measurements in large-diameter domestic wells and in piezometers installed in the reservoir embayment. Kentucky Lake is raised ~ 2 m shortly before the field trip, thus providing an analog of bank storage. Former students who responded to a questionnaire indicated that the exercise was worthwhile. The exercise was based at a biological field station but could be completed at any field site where long-term hydrologic monitoring is in place or could be initiated.

Observation of bioturbation and hyporheic flux in streambeds

In the Elkhorn River, burrows, tubes, and sediment mounds created by invertebrate bioturbation were observed in the exposed streambed and commonly concentrated on the fine-sediment patches, which consist of silt, clay, and organic matter. These invertebrate activities could loosen the thin layer of clogging sediments and result in an increase of pore size in the sediments, leading to greater vertical hydraulic conductivity of the streambed (Kv). The measurements of the vertical hydraulic gradient across the submerged streambed show that vertical flux in the hyporheic zone can alter directions (upward versus downward) for two locations only a few meters apart. In situ permeameter tests show that streambed Kv in the upper sediment layer is much higher than that in the lower sediment layer, and the calculated Kv in the submerged streambed is consistently greater than that in the clogged sediments around the shorelines of the sand bars. Moreover, a phenomenon of gas bubble release at the water-sediment interface from the subsurface sediments was observed in the groundwater seepage zone where flow velocity is extremely small. The bursting of gas bubbles can potentially break the thin clogging layer of sediments and enhance the vertical hydraulic conductivity of the streambed.

Interplay of geomorphic and hydrogeologic features at reach- and channel unit-scales on riverbed hydrology and hydrochemistry: a conceptual model in the Lower Coal Measures, South Yorkshire, UK

A conceptual model of groundwater and surface-water interactions in areas of minor aquifers has been developed. It assesses the interplay of reach-scale subsurface flow paths (RSSF), controlled by the lateral extent of the alluvial valley, and channel unit-scale hyporheic flow paths (CUSHF), controlled by riffle and run/pool sequences, and their impacts on the spatial variability of riverbed flow and solute exchange. A network of riverbed mini-piezometers and multi-level samplers in different reach- and channel-unit scale settings of the River Don (South Yorkshire, UK) is monitored to: (1) estimate vertical hydraulic gradients (VHGs) and specific discharge; (2) discriminate subsurface flow paths from conservative natural tracers; and (3) deduce biogeochemical processes. In a constrained context (downstream end of the alluvial valley), RSSF discharge favours a homogeneous riverbed hydrochemistry with limited biogeochemical processes and shallow CUSHF. In an unconstrained (open alluvial valley) or asymmetric (bedrock outcropping on one bank) context, low VHGs favour deep CUSHF and a vertical stratification of RSSF. Reducing conditions intensify with depth, and superimpose with mixing in riffles. This good approximation of flow and solute behaviour in minor aquifers provides a practical framework to understand nutrient and contaminant fate and develop cost-effective monitoring programmes across the groundwater/surface-water interface.

Habitat use by spinous loach (Cobitis shikokuensis) in southwestern Japan: importance of subsurface interstices

AbstractWe examined habitat use by spinous loach (Cobitis shikokuensis), an endangered benthic fish in Japan, in relation to distance to the stream bank, water depth, current velocity, substrate types and bed‐subsurface conditions (fine‐sediment volume, vertical hydraulic gradient). In the study reach (Shigenobu River in Shikoku Island), spinous loach exhibited a patchy distribution within the channel, being limited to sloping bed of channel margins. Although high selectivities were detected for three variables (close to the stream bank, pebble‐dominant substrates, and low fine‐sediment volume) from univariate perspective, decision‐tree analysis indicated that their distribution pattern was best explained by the two variables representing subsurface conditions. Locations occupied by spinous loach were characterized by extremely low fine‐sediment volume (≤1.5%), or by moderate fine‐sediment volume (1.5–9.8%) with positive vertical hydraulic gradient (indicative of upwelling). Our results suggest that subsurface interstices are essential habitat for spinous loach and that prevention of excessive inputs and deposition of fine sediments, which cause interstitial sedimentation, is crucial for conservation of this endangered species.

A Wet/Wet Differential Pressure Sensor for Measuring Vertical Hydraulic Gradient

Vertical hydraulic gradient is commonly measured in rivers, lakes, and streams for studies of groundwater-surface water interaction. While a number of methods with subtle differences have been applied, these methods can generally be separated into two categories; measuring surface water elevation and pressure in the subsurface separately or making direct measurements of the head difference with a manometer. Making separate head measurements allows for the use of electronic pressure sensors, providing large datasets that are particularly useful when the vertical hydraulic gradient fluctuates over time. On the other hand, using a manometer-based method provides an easier and more rapid measurement with a simpler computation to calculate the vertical hydraulic gradient. In this study, we evaluated a wet/wet differential pressure sensor for use in measuring vertical hydraulic gradient. This approach combines the advantage of high-temporal frequency measurements obtained with instrumented piezometers with the simplicity and reduced potential for human-induced error obtained with a manometer board method. Our results showed that the wet/wet differential pressure sensor provided results comparable to more traditional methods, making it an acceptable method for future use.

Transient storage processes in a steep headwater stream

AbstractThis study examined hyporheic exchange flow and transient storage processes within a steep headwater stream, using both hydrometric and tracer approaches. Vertical hydraulic gradients varied systematically with location within step‐pool units, with upwelling located in a relatively narrow zone immediately below the step. Downward hydraulic gradients were greatest immediately above channel steps, and in that zone, gradients were also correlated with step height. Hydraulic conductivity tended to be lower in downwelling zones than in upwelling and neutral locations. Transient storage areas determined from reach‐scale tracer tests tended to increase with discharge, whereas transient exchange coefficients were not correlated with discharge. Transient storage exchange fluxes estimated from the reach‐scale tracer tests were an order of magnitude greater than hyporheic exchange fluxes estimated by scaling up local estimates based on Darcy's Law and piezometer measurements. This difference could be explained by the presence of lateral hyporheic flow paths and/or the influences of transient storage in pools. Tracer injections into infiltrometers using Rhodamine WT and sodium chloride allowed visual determination of hyporheic discharge zones and calculation of residence times for hyporheic flow through the step (pool inflow) and transport through the downstream pool (pool outflow). These tests indicated that residence time in pools can be comparable to hyporheic residence times, although the lack of replication in time and space limits the drawing of strong inferences. Copyright © 2009 John Wiley & Sons, Ltd.

Spatio‐temporal variations of hyporheic flow in a riffle‐step‐pool sequence

AbstractSubsurface flow in streambeds can vary at different scales in time and space. Recognizing this variability is critical for understanding biogeochemical and ecological processes associated with the hyporheic zone. The aim of this study was to examine the variability of hydraulic conductivity (K), vertical hydraulic gradients (VHGs), and subsurface fluxes, over a riffle–step–pool sequence and at a high spatio‐temporal resolution. A 20 m reach was equipped with a network of piezometers in order to determine the distribution of VHGs and K. During a summer month, temporal variations of VHGs were regularly surveyed and, for a subset of piezometers, the water level was automatically recorded at 15 min intervals by logging pressure transducers. Additionally, point‐dilution tests were carried out on the same subset of piezometers. Whereas the distribution of vertical fluxes can be derived from K and VHG values, point‐dilution tests allow for the estimation of horizontal fluxes where no VHG is detectable. Results indicate that, spatially, VHGs switched from upwelling to downwelling across lateral as well as longitudinal sections of the channel. Vertical fluxes appeared spatially more homogeneous than VHGs, suggesting that the latter can be a poor indicator of the intensity of flow. Finally, during flow events, some VHGs showed little or no fluctuations; this was interpreted as the result of a pressure wave propagating from upstream through highly diffusive alluvial sediments. Copyright © 2009 John Wiley & Sons, Ltd.

Influence of the hyporheic zone on the phosphorus dynamics of a large gravel‐bed river, Garonne River, France

AbstractPhosphorus (P) concentrations in sediments and in surface and interstitial water from three gravel bars in a large river (Garonne River, southern France) were measured daily, downstream of a wastewater treatment plant for a city of 740 000 inhabitants (Toulouse). Measurements were made of vertical hydraulic gradient (VHG), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP) and total phosphorus (TP) in water and of three extractable forms of phosphorus (water extractable, NaOH extractable and H2SO4 extractable) in hyporheic sediments from the gravel bars. Dissolved phosphorus was the major contributor to TP (74–79%) in both interstitial and surface waters on all sampling dates, and in most cases surface water P concentrations were significantly higher than interstitial concentrations. Hyporheic sediment TP concentrations ranged between 269 and 465 µg g−1 and were highest in fine sediment fractions. Acid‐extractable P, a non‐bioavailable form, represented at least 95% of sediment TP. A positive relationship was observed between VHG and TP in two of the gravel bars, with wells that were strongly downwelling having lower TP concentrations. These results suggest that in downwelling zones, hyporheic sediments can trap surface‐derived dissolved P, and that much of this P becomes stored in refractory particulate forms. Bioavailable P is mainly present in dissolved form and only occupies a small fraction of total P, with particulate P comprising the majority of total P. Copyright © 2009 John Wiley & Sons, Ltd.

Heterogeneous characteristics of streambed saturated hydraulic conductivity of the Touchet River, south eastern Washington, USA

AbstractTraditionally a streambed is treated as a layer of uniform thickness and low saturated hydraulic conductivity (K) in surface‐ and ground‐water studies. Recent findings have shown a high level of spatial heterogeneity within a streambed and such heterogeneity directly affects surface‐ and ground‐water exchange and can have ecological implications for biogeochemical transformations, nutrient cycling, organic matter decomposition, and reproduction of gravel spawning fish. In this study a detailed field investigation of K was conducted in two selected sites in Touchet River, a typical salmon spawning stream in arid south eastern Washington, USA. In‐stream slug tests were conducted to determine K following the Bouwer and Rice method. For the upper and lower sites, each 50 m long and 9 m wide and roughly 20 m apart, a sampling grid of 5 m longitudinally and 3 m transversely was used. The slug tests were performed for each horizontal coordinate at 0·3–0·45, 0·6–0·75, 0·9–1·05 and 1·2–1·35 m depth intervals unless a shallower impenetrable obstruction was encountered. Additionally, water levels were measured to obtain vertical hydraulic gradient (VHG) between each two adjacent depth intervals. Results indicated that K ranged over three orders of magnitude at both the upper and lower sites and differed between the two sites. At the upper site, K did not differ significantly among different depth intervals based on nonparametric statistical tests for mean, median, and empirical cumulative distribution, but the spatial pattern of K varied among different depth intervals. At the lower site, K for the 0·3–0·45 m depth interval differed statistically from those at other depth intervals, and no similar spatial pattern was found among different depth intervals. Zones of upward and downward water flow based on VHG also varied among different depth intervals, reflecting the complexities of the water flow regime. Detailed characterization of the streambed as attempted in this study should be helpful in providing information on spatial variations of streambed hydraulic properties as well as surface‐ and ground‐water interaction. Copyright © 2009 John Wiley & Sons, Ltd.

Parameters affecting the occurrence of quicksand and the drying up of large diameter wells that gain water from the bottom: a case study from Iran

Large diameter fully cased wells that gain water from the bottom are often dug in sandy and collapsible aquifers. They have cylindrical vertical walls lined with brick or concrete. The well bottom is partially filled with aquifer material through which the flow is vertically upward. When the vertical hydraulic gradient reaches a critical value, quicksand occurs and the well structure can be destroyed. Another difficulty encountered is drawdown in the wellbore and the drying up of the well. To overcome these problems, the flow around and beneath these wells is numerically simulated. The simulation results are used to investigate the effect of well and aquifer parameters on quicksand and drawdown. For practical purposes, the dimensionless drawdown-time and dimensionless vertical gradient-time curves are developed. It was found that the ratio of filling material thickness to well radius affects the shape of these type curves. The type curves may be used to predict the time after pumping commences when quicksand occurs and the well dries up. They are also useful to design the safe pumping rate and duration as well as the optimum well radius. These are demonstrated by analyzing the pumping test data from a case study in the arid Chah Kutah region, southern Iran.

Surface–subsurface water exchange rates along alluvial river reaches control the thermal patterns in an Alpine river network

Summary1. Water temperature is a key characteristic of stream ecosystems that is gaining scientific and managerial relevance as maximum temperatures in aquatic ecosystems increase worldwide.2. To assess the effect of surface–subsurface water exchange on stream water temperature patterns, four alluvial reaches in the Tagliamento River basin (NE Italy), constrained by geomorphic knickpoints at the upper and lower end, and two to four hyporheic flowpaths within each reach, were continuously studied during summer 2007 and winter 2007–08. Water temperature was continuously monitored at the upstream and downstream knickpoints of the floodplains, as well as at discrete upwelling areas within each reach. Discharge and vertical hydraulic gradient were measured along the alluvial reaches, and the residence time and chemistry of upwelling water were assessed four times during the study.3. Discharge variation along the study reaches revealed that massive hyporheic exchange occurred in all sites, ranging from 21% in reach 2–52% in reach 1. End member mixing analysis showed little influence of ground water, as almost all upwelling water was freshly infiltrated hyporheic water. Importantly, hyporheic exchange flows shaped surface temperature at the upwelling locations in all study reaches, providing potential thermal refugia for aquatic biota. At sites with highest hyporheic flow rates, net temperature change was also reflected at the floodplain scale.4. The magnitude of the thermal change along a hyporheic flowpath was not related to the flowpath length but to the estimated 222Rn water age. Reduction in the diel thermal amplitude by hyporheic flows rather than net temperature change, reduced temperature extremes. Therefore, restoration activities to create thermal refugia should consider the role of hyporheic flows and enhance the exchange between surface and hyporheic waters.

Biased Monitoring of Fresh Water‐Salt Water Mixing Zone in Coastal Aquifers

In coastal aquifers, significant vertical hydraulic gradients are formed where fresh water and underlying salt water discharge together upward to the seafloor. Monitoring boreholes may act as "short circuits" along these vertical gradients, connecting between the higher and the lower hydraulic head zones. When a sea tide is introduced, the fluctuations of both the water table and the depth of the mixing zone are also biased due to this effect. This problem is intensified in places of long-screen monitoring boreholes, which are common in many places in the world. For example, all approximately 500 boreholes of the fresh water-salt water mixing zone in the coastal aquifer of Israel are installed with 10 to 50 m long screens. We present field measurements of these fluctuations, along with a three-dimensional numerical model. We find that the in-well fluctuation magnitude of the mixing zone is an order of magnitude larger than that in the porous media of the actual aquifer. The primary parameters that affect the magnitude of this bias are the anisotropy of the aquifer conductivity and the borehole hydraulic parameters. With no sea tide, borehole interference is higher for the anisotropic case because the vertical hydraulic gradients are high. When tides are introduced, the amplitude of the mixing zone fluctuation is higher for the isotropic case because the overall effective hydraulic conductivity is greater than the conductivity in the anisotropic case. In the aquifer, the fresh water-salt water mixing zone fluctuations are dampened, and tens of meters inland from the shoreline, the fluctuations are on the order of few centimeters.

Treating causes not symptoms: restoration of surface - groundwater interactions in rivers

Many river restoration projects seek to address issues associated with impaired hydrological and ecological connectivity in longitudinal (e.g. effects of dams, weirs) or lateral (e.g. alienated floodplain) dimensions. Efforts to restore the vertical dimension of impaired stream–groundwater exchange are rare, hampered by limited understanding of the factors controlling this linkage in natural alluvial rivers. We propose a simplified two-axis model of the ‘primary drivers’ (sediment structure and vertical hydraulic gradient) of stream–groundwater exchange that acknowledges their interaction and provides a practical template to help researchers and river managers pose hypothesis-driven solutions to restoration of damaged or lost vertical connectivity. Many human activities impact on one or both of these drivers, and we review some of the tools available for treating the causes (rather than symptoms) in impacted stream reaches. For example, creating riffle-pool sequences along stream reaches will enhance vertical hydraulic gradient, whereas flushing flows can remove clogging layers and sustain sediment permeability. Our model is a first step to specifying mechanisms for recovery of lost vertical connectivity. Assessing results of river restoration using this approach at reach to catchment scales will provide scientific insights into the interplay of hydrology, fluvial geomorphology and river ecosystem function at appropriately broad scales.

Development of an Electronic Seepage Chamber for Extended Use in a River

Seepage chambers have been used to characterize the flux of water across the water-sediment interface in a variety of settings. In this work, an electronic seepage chamber was developed specifically for long-term use in a large river where hydraulic gradient reversals occur frequently with river-stage variations. A bidirectional electronic flowmeter coupled with a seepage chamber was used to measure temporal changes in the magnitude and direction of water flux across the water-sediment interface over an 8-week period. The specific discharge measured from the seepage chamber compared favorably with measurements of vertical hydraulic gradient and previous specific discharge calculations. This, as well as other supporting data, demonstrates the effectiveness of the electronic seepage chamber to accurately quantify water flux in two directions over a multimonth period in this setting. The ability to conduct multimonth measurements of water flux at a subhourly frequency in a river system is a critical capability for a seepage chamber in a system where hydraulic gradients change on a daily and seasonal basis.

Ambient vertical flow in long-screen wells: a case study in the Fontainebleau Sands Aquifer (France)

A tritium (3H) profile was constructed in a long-screened well (LSW) of the Fontainebleau Sands Aquifer (France), and the data were combined with temperature logs to gain insight into the potential effects of the ambient vertical flow (AVF) of water through the well on the natural aquifer stratification. AVF is commonly taken into account in wells located in fracture aquifers or intercepting two different aquifers with distinct hydraulic heads. However, due to the vertical hydraulic gradient of the flow lines intercepted by wells, AVF of groundwater is a common process within any type of aquifer. The detection of 3H in the deeper parts of the studied well (approximate depth 50 m), where 3H-free groundwater is expected, indicates that shallow young water is being transported downwards through the well itself. The temperature logs show a nearly zero gradient with depth, far below the mean geothermal gradient in sedimentary basins. The results show that the age distribution of groundwater samples might be biased in relation to the age distribution in the surroundings of the well. The use of environmental tracers to investigate aquifer properties, particularly in LSWs, is then limited by the effects of the AVF of water that naturally occurs through the well.

Groundwater’s significance to changing hydrology, water chemistry, and biological communities of a floodplain ecosystem, Everglades, South Florida, USA

The Everglades (Florida, USA) is one of the world’s larger subtropical peatlands with biological communities adapted to waters low in total dissolved solids and nutrients. Detecting how the pre-drainage hydrological system has been altered is crucial to preserving its functional attributes. However, reliable tools for hindcasting historic conditions in the Everglades are limited. A recent synthesis demonstrates that the proportion of surface-water inflows has increased relative to precipitation, accounting for 33% of total inputs compared with 18% historically. The largest new source of water is canal drainage from areas of former wetlands converted to agriculture. Interactions between groundwater and surface water have also increased, due to increasing vertical hydraulic gradients resulting from topographic and water-level alterations on the otherwise extremely flat landscape. Environmental solute tracer data were used to determine groundwater’s changing role, from a freshwater storage reservoir that sustained the Everglades ecosystem during dry periods to a reservoir of increasingly degraded water quality. Although some of this degradation is attributable to increased discharge of deep saline groundwater, other mineral sources such as fertilizer additives and peat oxidation have made a greater contribution to water-quality changes that are altering mineral-sensitive biological communities.

Consequences of a biogeomorphic regime shift for the hyporheic zone of a Sonoran Desert stream

Summary1. Feedbacks between vegetation and geomorphic processes can generate alternative stable states and other nonlinear behaviours in ecological systems, but the consequences of these biogeomorphic interactions for other ecosystem processes are poorly understood. In this study, we describe the changes in the hydrological, geomorphic and biogeochemical characteristics of the hyporheic zone of a Sonoran desert stream (Sycamore Creek, Arizona, U.S.A.) in response to a transition from an unvegetated gravel‐bed state to densely vegetated wetlands (ciénegas).2. A survey of the entire length of Sycamore Creek indicated that ciénegas occupied c. 18% of the stream, and were disproportionately represented in constrained canyons rather than wide, unconstrained valleys.3. Vegetated patches were characterized by low concentrations of dissolved oxygen (DO) and nitrate and high concentrations of carbon dioxide and methane in the hyporheic zone. In contrast to unvegetated areas, hyporheic DO in ciénegas exhibited no relationship with vertical hydraulic gradients.4. Increases in hyporheic DO following removal of vegetation by floods supports the hypothesis that these reduced conditions were the result of biogeochemical and geomorphic changes associated with vegetation establishment. In locations where vegetation persisted, hyporheic DO exhibited no response to flooding; in sections where vegetation was removed hyporheic DO closely tracked post‐flood increases in surface stream DO.5. Shallow sediments in vegetated patches were finer and more organic‐rich than in unvegetated patches, due to increased deposition during floods. Conservative tracer additions indicated that hydrological exchange between the surface stream and hyporheic zone was much lower in ciénegas than in gravel‐bed reaches.6. Vegetation establishment in desert streams not only alters the physical and chemical characteristics of the hyporheic zone, but also the nature of interactions between surface and hyporheic subsystems.

Mixing-induced groundwater denitrification beneath a manured field in southern Alberta, Canada

Abstract Contamination of shallow groundwater by NO 3 - from manure may occur under fields where manure is spread as fertilizer and for disposal. Attenuation of NO 3 - in groundwater occurs through denitrification under certain conditions, or NO 3 - -contaminated younger groundwater may mix with older groundwater, lowering the NO 3 - concentration. In this study, δ15N and δ18O values of NO 3 - , and δ18O and δ2H values in groundwater under a manured field were evaluated to determine if groundwater NO 3 - concentrations were influenced through mixing of shallower, manure-impacted groundwater with older groundwater, or if denitrification was reducing NO 3 - concentrations. The younger groundwater showed clear evidence of manure impact with elevated Cl− (∼85 mg L−1) and NO 3 - concentrations (∼50 mg NO3–N L−1), and δ15N and δ18O values of NO 3 - consistent with a manure source. Vertical hydraulic gradients and δ18O and δ2H values in groundwater suggest older, more reduced groundwater is upwelling locally and mixing with the shallow groundwater. Decreasing NO3:Cl ratios, decreasing dissolved O2 concentrations, and increasing δ15N and δ18O values of NO 3 - suggest that denitrification occurs locally in the aquifer. The extent of denitrification is proportional to the fraction of deeper groundwater in the aquifer. Denitrification apparently does not proceed in the younger, manure-impacted groundwater in the absence of mixing.