Showing papers on "Groundwater flow published in 1990"

Estimating groundwater exchange with lakes: 1. The stable isotope mass balance method

Abstract: Groundwater inflow and outflow contributions to the hydrologic budget of lakes can be determined using a stable isotope (18O/16O) mass balance method. The stable isotope method provides a way of integrating the spatial and temporal complexities of the flow field around a lake, thereby offering an appealing alternative to the traditional time and labor intensive methods using seepage meters and an extensive piezometer network. In this paper the method is applied to a lake in northern Wisconsin, demonstrating that it can be successfully applied to lakes in the upper midwest where thousands of similar lakes exist. Inflow and outflow rates calculated for the Wisconsin lake using the isotope mass balance method are 29 and 54 cm/yr, respectively, which compare well to estimates, derived independently using a three-dimensional groundwater flow and solute transport model, of 20 and 50 cm/yr. Such a favorable comparison lends confidence to the use of the stable isotope method to estimate groundwater exchange with lakes. In addition, utilization of stable isotopes in studies of groundwater-lake systems lends insight into mixing processes occurring in the unsaturated zone and in the aquifer surrounding the lake and verifies assumed flow paths based on head measurements in piezometers.

An experimental investigation of variable density flow and mixing in homogeneous and heterogeneous media

Abstract: This study is an experimental investigation of variable density groundwater flow in homogeneous, layered and lenticular porous media. At the scale of the experiments the flow of dissolved mass in water depends upon both forced and free convection. In addition, density differences as low as 0.0008 g/cm{sup 3} (1,000 mg/L NaCl) between a plume of dense water and ambient groundwater in a homogeneous medium produces gravitational instabilities at realistic groundwater velocities. These instabilities are manifest by lobe-shaped protuberances that formed first along the bottom edge of the plume and later within the plume. As the density difference increases to 0.0015 g/cm{sup 3} (2,000 mg/L NaCl), 0.037 g/cm{sup 3} (5,000 mg/L NaCl), or higher, this unstable mixing due to convective dispersion significantly alters the spreading process. In a layered medium, reductions in hydraulic conductivity of the order of half an order of magnitude or less can influence the flow of the dense plume. Dense water may accumulate along bedding interfaces, which when dipping can result in plume migration velocities larger than ambient groundwater velocities. In a lenticular medium the combination of convective dispersion and nonuniform flow due to heterogeneities result in relatively large dispersion. Scale considerations, further, indicate that convective dispersionmore » may provide an important component of mixing at the field scale.« less

Hydrogeochemical Response of a Forested Watershed to Storms: Effects of Preferential Flow Along Shallow and Deep Pathways

Abstract: Hydrology and solute chemistry were monitored during four storms in a forested watershed in karst terrain in eastern Tennessee at spatial scales ranging from a 0.5-ha upper hillslope subcatchment to a 38.4-ha watershed drained by a first-order stream. Storm flow was generated from large portions of the watershed as a result of subsurface preferential flow initiated by zones of perched saturation in the soil profile. For most storms, runoff per unit area increased with increasing catchment size, indicating that preferential flow was primarily vertical along relatively deep pathways that emerged in the lower portions of the watershed. However, when antecedent soil moisture was high and rainfall was large, runoff was similar across catchment sizes, indicating that preferential flow was largely lateral along shallow paths (<2.5 m deep). Concentrations of some solutes differed for shallow and deep flow paths because dominant sources and sinks are stratified vertically in the watershed. Runoff generated by preferential flow along shallow flow paths resulted in higher exports of Cl, Al, SO4, and dissolved organic carbon and lower exports of alkalinity, Ca, Mg, and soluble reactive phosphorus than an equivalent amount of runoff generated by preferential flow via deeper flow paths.

Simulation of biodegradable organic contaminants in groundwater. 1. Numerical formulation in principal directions.

Abstract: Groundwater contamination by organic chemicals is of concern because of the widespread use of these compounds and because even low concentrations may be very harmful. Dissolved organic contaminants are affected by advection, dispersion, sorption, and biological transformations in groundwater systems; however, biological degradation by indigeneous bacterial populations is the only mechanism whereby contaminant mass can be naturally removed from an aquifer. The purpose of this study is to develop a physically and biochemically based numerical solution for the transport of biodegradable organic solutes with emphasis on an efficient numerical approach. A dual-Monod relationship, combined with the advection-dispersion equation, is used to represent the biological and physical processes affecting the organic solute, electron acceptor, and microbial population. The three resulting differential equations are nonlinearly coupled through the Monod decay terms. By employing an iterative principal direction finite-element technique, efficiency is achieved by decoupling each of the two-dimensional transport equations into a series of one-dimensional equations. This decoupling should easily allow for extension of the model to three dimensions. An iterative solution is adopted because a purely sequential technique was observed to greatly underestimate the dissolved mass of an organic plume. Comparison of numerical results with the results of a laboratory column experiment shows that the model equations adequately describe the behavior of toluene, dissolved oxygen, and the bacterial population, without considering solute diffusion through stagnant fluid layers or biofilms. In a two-dimensional shallow aquifer setting an organic plume experiences mass loss, spreading controlled by the availability of dissolved oxygen, and skewing in the direction of groundwater flow. These features would be lost if the interactions between the organic contaminant, electron acceptor, and microbial population were ignored in the mathematical formulation of the problem.

Nitrogen inputs to a marine embayment: the importance of groundwater

Abstract: We examined the importance of nitrogen inputs from groundwater and runoff in a small coastal marine cove on Cape Cod, MA, USA. We evaluated groundwater inputs by three different methods: a water budget, assuming discharge equals recharge; direct measurements of discharge using bell jars; and a budget of water and salt at the mouth of the Cove over several tidal cycles. The lowest estimates were obtained by using a water budget and the highest estimates were obtained using a budget of water and salt at the Cove mouth. Overall there was more than a five fold difference in the freshwater inputs calculated by using these methods. Nitrogen in groundwater appears to be largely derived from on site septic systems. Average nitrate concentrations were highest in the region where building density was greatest. Nitrate in groundwater appeared to behave conservatively in sandy sediments where groundwater flow rates were high (> 11/m2/h), indicating that denitrification was not substantially reducing external nitrogen loading to the Cove. Nitrogen inputs from groundwater were approximately 300 mmol-N/m3/y of Cove water. Road runoff contributed an additional 60 mmol/m3/y. Total nitrogen inputs from groundwater and road runoff to this cove were similar in magnitude to river dominated estuaries in urbanized areas in the United States.

Ground water flow paths in relation to nitrogen chemistry in the near-stream zone

Abstract: Interactions between ground water flow paths and water chemistry were studied in the riparian zone of a small headwater catchment near Toronto, Ontario. Significant variations in oxygen — 18 and chloride indicated the presence of distinct sources of water in the ground water flow system entering the near-stream zone. Shallow ground water at the upland perimeter of the riparian zone had nitrate-N, chloride and dissolved oxygen concentrations which ranged between 100–180 µg L−1, 1.2–1.8 mg L−1 and 4.6–9.1 mg L−1 respectively. Concentrations of nitrate — N in deep ground water flowing upward beneath the riparian wetland were < 10 µg L−1, whereas chloride and dissolved oxygen ranged between 0.6–0.9 mg L−1 and 0.4–2.2 mg L−1 respectively. Ammonium — N concentrations (20–60 µg L−1) were similar in shallow and deep ground water. Ground water was transported through the wetland to the stream by three hydrologic pathways. 1) Shallow ground water emerged as springs near the base of the hillslope producing surface rivulets which crossed the riparian zone to the stream. 2) Deep ground water flowed upward through organic soils and entered the rivulets within the wetland. 3) Deep ground water reached the stream as bed and bank seepage. Springs were higher in nitrate and chloride than rivulets entering the stream, whereas bank seeps had lower concentrations of nitrate and chloride and considerably higher ammonium concentrations than the rivulets. These contrasts in nitrate and chloride concentrations were related to initial differences in the ion chemistry of shallow and deep ground water rather than to element transformations within the riparian wetland. Differences in ammonium concentration between seeps and rivulets were caused by immobilization of ammonium in the substrates of aerobic rivulets, whereas little ammonium depletion probably occurred in deep ground water flowing upward through reduced subsurface organic soils around the stream perimeter.

Simulation of biodegradable organic contaminants in groundwater: 2. Plume behavior in uniform and random flow fields

Abstract: The behavior of dissolved organic contaminants in groundwater is often difficult to interpret in field settings because of the complex interaction of physical, chemical, and biological processes. The organic solute transport model presented by MacQuarrie et al. (this issue) is used to examine plume migration in a shallow aquifer where dissolved oxygen is the sole electron acceptor. In uniform groundwater flow a plume originating from a high-concentration source will experience more spreading and slower normalized mass loss than a plume from a lower initial concentration source because dissolved oxygen is more quickly depleted. Low background dissolved oxygen concentrations also cause organic mass loss to decrease, while the initial size of the microbial population has little effect. Large groundwater velocities produce increases in the rate of organic solute mass loss because of increased mechanical mixing of the organic plume with oxygenated groundwater. Because sorbed organic mass is unavailable for biodegradation, increasing the retardation factor of an organic solute causes slower mass loss. For easily biodegraded organics the mass loss is only weakly dependent on the kinetic biodegradation parameters because the amount of mixing of the organic and dissolved oxygen controls the biodegradation process. This implies that for these types of compounds the kinetic parameters do not need to be known with precision. For all simulations the rate of mass loss for the organic plumes decreased with time because the concentrations of organic and dissolved oxygen decrease with time. In complex-structured flow fields caused by heterogeneity, the bulk velocity, position, and rate of spreading of an organic plume is strongly dependent on local-scale transport parameters such as the groundwater velocity in individual beds. Because an organic plume emanating from a small to moderate sized source does not evolve in a manner similar to a conservative plume in heterogeneous materials due to a decrease in the size of the plume, it is suggested that average transport parameters such as macrodispersivity may not be applicable to such organic plume transport. If this is the case, then reliable predictions concerning the ultimate fate of organics in groundwater exemplifies the need for obtaining knowledge about the bedding features and degree of heterogeneity of the host geologic materials.

Estimating groundwater exchange with lakes: 2. Calibration of a three‐dimensional, solute transport model to a stable isotope plume

Abstract: A three-dimensional groundwater flow and solute transport model was calibrated to a plume of water described by measurements of δ18O and used to calculate groundwater inflow and outflow rates at a lake in northern Wisconsin. The flow model was calibrated to observed hydraulic gradients and estimated recharge rates. Calibration of the solute transport submodel to the configuration of a stable isotope (18O) plume in the contiguous aquifer on the downgradient side of the lake provides additional data to constrain the model. A good match between observed and simulated temporal variations in plume configuration indicates that the model closely simulated the dynamics of the real system. The model provides information on natural variations of rates of groundwater inflow, lake water outflow, and recharge to the water table. Inflow and outflow estimates compare favorably with estimates derived by the isotope mass balance method (Krabbenhoft et al., this issue). Model simulations agree with field observations that show groundwater inflow rates are more sensitive to seasonal variations in recharge than outflow.

An integrated approach to quantify groundwater transport of phosphorus to Narrow Lake, Alberta

Abstract: An integrated approach was used to quantify groundwater phosphorus flux to Narrow Lake, a smallglacial-terrain lake in central Alberta. Data from a drilling program, major ion concentrations, environmental isotopes, and computer simulations indicated that the lake gains water through the nearshore region from a small, shallow groundwater flow system; at deep offshore regions, water moves from the lake to the groundwater flow system. Seepage flux was quantified by water budget, Darcy’s equation with data from wells near the lake, Darcy’s equation with data from minipiezometers in the lake, and seepage meters. Whole-lake seepage flux determined from minipiezometer data (30 mm yr-I) was only lO-25% of the other estimates (mean, 221 mm yr-I; range, 133-332 mm yr- l from seepage meter and water budget data, respectively). Groundwater contributed - 30% of the annual water load to the lake. The P concentration, [PI, in pore water from lake sediments (mean, 175 mg m-‘) was 8 times higher than groundwater from wells near the lake (mean, 2 1 mg m-3). Thus, if well water was used to estimate the [P] of the seepage water, the rate of groundwater P loading to the lake would be underestimated. The rate of groundwater P loading to the lake computed from average seepage flux and average pore-water [P] was 39 mg m-2 yr-I, and groundwater may be the largest single source of P to epilimnetic water in the lake.

Groundwater contribution to the nutrient budget of Tomales Bay, California

Abstract: Tomales Bay, a graben structure along the San Andreas Fault, was selected for modeling ecosystem nutrient dynamics because of its linear, one-dimensional morphology and relatively pristine state Groundwater is a potentially important term in the nutrient budget The geologic complexities created by the San Anreas Fault, however, complicate the hydrogeology and require the area to be subdivided into three regions: granite to the west, Franciscan Formation to the east, and alluvial fill in the trough Nutrient concentrations in the groundwater were determined through extensive well sampling; groundwater discharge was estimated using both Darcy's Law calculations and a soil moisture budget Results indicate that groundwater discharge is of the same order of magnitude as summer streamflow into the Bay, while being significantly less than other freshwater inputs in winter Dissolved nutrient (phosphate, nitrate + nitrite, ammonium, silica and DIC) concentrations in groundwater were consistently higher (by as much as an order of magnitude) than in surface water discharges During the summer months, groundwater flow contributes about as much nutrient load to the bay as does streamflow During the winter, the groundwater contribution to nutrient loading is about 20% of the streamflow contribution Our findings indicate that groundwater is a significant component of terrestrial nutrient and freshwater loading to Tomales Bay, particularly so during the summer months However, neither groundwater nor streamflow nutrient fluxes are large in comparison to the mixing flux at the bay mouth or the flux of N2 gas across the air-water interface

Hydrology of a headwater basin wetland: Groundwater discharge and wetland maintenance

Abstract: The link between groundwater and surface hydrology in a small headwater drainage basin in the zone of glacial deposition of southern Ontario south of the Precambrian Shield was examined for two years. The basin is situated in a discharge zone of a regional aquifer and contains a small treed spring-fed swamp. The swamp exists because of the groundwater and has little effect on the maintenance of streamflow. Groundwater input to the swamp is an order of magnitude larger than precipitation. Groundwater of local and regional origin passes through the swamp by two routes: surface streamlets, where groundwater that emerges at specific seepage points in the swamp is conveyed over the ground surface with little interaction with the swamp itself, and by diffuse seepage in the swamp and through the bed of the stream. While the diffuse seepage input is the smaller component of groundwater it maintains the swamp's saturation. Groundwater input to the swamp from the specific seepage points and diffuse flow varies little over a year; therefore the saturation of the swamp and baseflow from the basin display little seasonal variation compared to other wetland types. The existence of the valley bottom in the headwater basin alters the seasonal and storm hydrology and is important to biogeochemical transformation of emerging groundwater.

Heat flow in the Oregon Cascade Range and its correlation with regional gravity, Curie point depths, and geology

Abstract: New heat flow data for the Oregon Cascade Range are presented and discussed. Heat flow measurements from several deep wells (up to 2500 m deep), as well as extensive new data from industry exploration efforts in the Breitenbush and the Santiam Pass-Belknap/Foley areas are described. The regional heat flow pattern is similar to that discussed previously. The heat flow is about 100 mW m−2 in the High Cascade Range and at the eastern edge of the Western Cascade Range, It is about 40–50 mW m−2 to the west in the outer arc block of the subduction zone. In the high heat flow zone the heat flow is low at shallow depths in young volcanic rocks due to the high permeability of the rocks and the resultant rapid groundwater flow. Below a depth of 200–400 m much of the area appears to be dominated by conductive heat transfer at least to 2–2.5 km depth. There are perturbations to the regional heat flow in the vicinity of the hot springs where values are up to twice the background. The gravity field in the Cascade Range has characteristics that can be closely related to the heat flow pattern. The relationship may be causal, and to examine the relationship in more detail, earlier two-dimensional modeling is extended to three dimensions. Consideration of the effects of a midcrustal density anomaly, such as might be associated with a region with at least areas of partial melt, has two major consequences. The first of these is that a high-frequency gravity gradient near the Western Cascade Range/High Cascade Range boundary is explained. Second, the negative gravity anomaly associated with the north half of the High Cascade Range can be removed, and as a result, the prominent northeast/southwest striking regional Bouguer gravity anomaly associated with the north edge of the Blue Mountains becomes continuous across the Cascade Range with a similar feature along the north side of the Klamath Mountains. Apparently, this zone is a major crustal feature upon which the negative gravity anomaly coincident with the high heat flow is superimposed. The correlation, or lack thereof, of the heat flow, depth to Curie point, gravity field, crustal electrical resistivity, crustal seismic velocity, and geology in the High/Western Cascade Ranges is summarized. Many of the data show aspects that can be interpreted in relation to possible high temperatures in the midcrust of the Cascade Range. The High Cascade Range midcrust has unusually high temperatures and contains a zone of magma staging at 10±2 km depth that can also be identified in subdued form in the Cascade Range in Washington and British Columbia.

Interbed storage changes and compaction in models of regional groundwater flow

Abstract: Water released from permanent compaction of compressible fine-grained interbeds within confined aquifers may be a significant source of pumped water. Permanent or inelastic compaction of the interbeds occurs when head declines from pumping cause the effective stress or grain-to-grain load to exceed the elastic limits of the interbeds. As a result, the grains in the interbeds rearrange and excess water is expelled. The amount of inelastic compaction generally is proportional to the increase in effective stress and the decrease in head. A common approach for the simulation of elastic and inelastic compaction in groundwater flow models is to assume that head changes in the aquifer result in instantaneous storage changes in the compressible interbeds. A term is added to the groundwater flow equation to account for the storage changes. Changes in specific storage may be computed explicitly from the results at the previous time step in the finite difference formulation of the groundwater flow equation. A better approach is to implicitly apportion storage changes between elastic and inelastic components within a time step. If storage changes cannot be considered to occur instantaneously with change in head in the aquifer, another approach is taken. Equations for horizontal flow in the aquifer are coupled with equations for vertical flow in the compressible interbeds to simulate flow in half a representative doubly draining interbed. Storage changes and compaction are calculated for the representative half thickness and are extrapolated to the entire thickness of all interbeds within each cell. This approach was applied to an existing model of groundwater flow in the Central Valley of California. The simulation demonstrates that solving coupled systems of equations is feasible for a regional flow model.

Thermal effects of compaction‐driven groundwater flow from overthrust belts

Abstract: The thermal consequences of compaction-driven groundwater flow resulting from overthrusting are studied with a two-dimensional numerical model. The model represents a foreland basin 5 km deep and 400 km wide and is used to estimate quantitatively the magnitude, direction, and thermal consequences of fluid expulsion. Model simulations in which the permeability structure is homogeneous lead to maximum Darcy velocities of the order of 1 cm/yr; temperature in the foreland increases by less than 5°C. A sensitivity analysis reveals that temperature in the foreland may be increased an additional 2°C by increasing porosity, speed of thrusting, or heat flow. However, if the area underneath the thrust sheet is not effectively sealed, fluid escapes upward and thermal perturbations in the foreland are negligible. Models with basal and midlevel aquifers produce maximum Darcy velocities of the order of 4 cm/yr, but temperature in the foreland again increases by less than 5°C. Models in which hot fluids at depth are channeled directly upward through a high-permeability pathway can produce temperature perturbations as high as 50°C, over limited areas. Modeling results suggest that the expulsion of pore fluids from orogenic zones through the process of sediment compaction is likely to produce significant thermal perturbations in adjacent forelands only over areas that are spatially restricted, or spatially and temporally restricted.

Characterization and simulation of rainfall-runoff relations for headwater basins in western King and Snohomish Counties, Washington

Abstract: The characteristics of rainfall-runoff relations were hypothesized for the study area as a whole by using existing information. In undisturbed areas, shallow-subsurface flow from hillslopes mantled with glacial till, groundwater flow from glacial outwash deposits, and saturation overland flow from depressions, stream bottoms, and till-capped hilltops are the important runoff mechanisms. In disturbed, primarily urban areas, Horton overland flow, which is runoff generated from rain falling at a greater rate than the infiltration rate of the soil, is a significant mechanism, along with overland flow from impervious surfaces. These hypothesized characteristics were incorporated into the Hydrologic Simulation Program-FORTRAN (HSPF) simulation model, and the model was calibrated concurrently at 21 stream-gage sites in the study area with hydrologic data from the 1985-86 water years. The calibration resulted in 12 sets of generalized HSPF parameters, one set for each land-segment type with a unique hydrologic response. The generalized parameters can be used with HSPF to simulate runoff from most headwater basins within the study area. The average standard errors of estimate for calibrated streamflow simulation at all 21 sites were 7.9 percent for annual runoff, 11.2 percent for winter runoff, 13.1 percent for spring runoff, 40.1 percent for summer runoff, 21.7 percent for storm peak discharge, 21.4 percent for storm runoff volume, and 42.3 percent for all daily mean discharges. High flows were simulated more accurately than were low flows. The simulation errors were not large enough to reject the hypothesized rainfall-runoff relations.

Heat flow in the Great Plains of the United States

Abstract: Anomalous high heat flow previously reported for the Great Plains is inconsistent with the tectonic setting and requires reexamination. Forty-six new heat flow measurements, 12 revised heat flow values, and several hundred geothermal gradient measurements indicate extensive geothermal anomalies with heat flows ranging from 80 to 140 mW m−2 in the northern and central Great Plains. Heat flow in the Great Plains outside the geothermally anomalous regions ranges from 40 − 60 mW m−2. The heat flow anomalies result from the thermal effects of regional groundwater flow where it moves upward either within a dipping aquifer or by cross-formational flow through fractures. The gravitational driving force for the groundwater flow derives from the eastward sloping surface of the Great Plains, and the locations of the geothermal amonalies are determined by the structures of the aquifers and the crystalline basement rocks. The most widespread and largest-amplitude geothermal anomaly occurs in southern South Dakota and northern Nebraska. Another large anomaly occurs on the eastern flank of the Denver Basin, and small anomalies occur on structures such as the Billings and Nesson anticlines in the Williston Basin. Previous reports of high heat flow in the Great Plains generally are supported by the results of this study. However, the source of anomalous heat is shown to be nontectonic, and theoretical arguments for normal continental heat flow in the Great Plains are supported. Another difference from the results of previous heat flow studies is that the thermal conductivities of shales in the Mesozoic strata in the Great Plains are about 40% lower than the conductivities that commonly have been used for shales. This observation and recent studies which have suggested lower thermal conductivities for shales in the Great Plains are the reasons for revision of some previous heat flow calculations. A significant result of revising some of the previous heat flow values is that the high heat flow zone that previously has been shown to extend through North Dakota into Canada is a region of normal continental heat flow.

Groundwater seepage along a Barrier Island

Abstract: Resistivity and water level measurements were made on a barrier island on the south shore of Long Island, New York to examine the distribution of fresh groundwater and the potential for recirculation of saline groundwater. The depth to the base of the freshwater lens was overpredicted by calculations of the static-equilibrium depth to a sharp interface apparently because of the sensitivity of the calculation to the low water-table elevations which are in turn sensitive to variations in sea level because of the existence of a transition zone between fresh and saline groundwater. Mixing and recirculation of saline groundwater at the base of the lens produced a transition zone up to 9.65 m thick. Measurements also support model forecasts of a mean bay level several centimeters above sea level, augmented by atmospheric forcing and wave setup. A time lag of about 8 hours between the response of the ocean level to longshore winds and the corresponding response of the bay level can result in a difference in elevation between the bay and the ocean that is up to four times that produced by other agents such as Stokes transport and density differences. In the presence of differential hydraulic head, bay and ocean water may be exchanged via groundwater flow between the base of the freshwater lens under the barrier beach and a deeper clay layer.

Stream functions and equivalent freshwater heads for modeling regional flow of variable‐density groundwater: 2. Application and implications for modeling strategy

Abstract: Stream functions and equivalent freshwater heads are used to simulate steady state flow of variable-density groundwater in a regional, cross-sectional flow model through the Palo Duro Basin, Texas, where fluid densities vary between 1.0 and 1.15 g/cm3. Centroid-consistent velocities computed from the stream function solution allow a more precise interpretation of local flow patterns in cross-sectional models than those from the head solution. Effects of significant fluid density variation on the regional groundwater flow pattern are studied by comparing simulation results that incorporate spatially varying, time-invariant densities with those that assume uniform density. Modeling shows that the regional groundwater flow pattern in the Palo Duro Basin is not significantly affected by variations in fluid densities, indicating that the topographically driven flow component dominates buoyancy forces associated with dense brines. An exception is near the eastern boundary where high fluid densities cause stronger downward flow. However, simulated equivalent freshwater heads in the variable-density model differ significantly from simulated heads in the freshwater density model, which is important for model calibration. In addition to the well-known problems of hydraulic parameter and boundary condition uncertainties, modeling strategies for regional flow systems require consideration of uncertainties associated with fluid density and evaluation of equivalent freshwater head data.

Groundwater and Wetland Contributions to Stream Acidification: An Isotopic Analysis

Abstract: Stream water pH may be influenced by (1) the flow paths and (2) the residence time of water that contributes to streamflow, when these hydrologic factors interact with the biogeochemical processes that neutralize H+ ions in the catchment. This paper presents measures of the volumes of groundwater contributing to streamflow, the groundwater residence times, and the sources of stream water acidity found during spring runoff in three basins on the Canadian Shield. Isotopic hydrograph separations were used to estimate the relative contributions of groundwater to spring runoff. The contributions of old (premelt) groundwater to spring runoff were greater (60%) in a well-buffered, third-order basin than in a more acidic first-order basin (49%). Using a simple mixing model, a larger groundwater reservoir (420 mm unit depth) and longer residence time (162 days) were estimated in the third-order basin. The lowest stream pH (4.8) was observed in a second-order basin with a wetland that collects drainage from about 79% of the basin. In this basin the principal source of H+ ions was the conifer-sphagnum wetland. We conclude that the hypotheses that the pH of these streams was proportional to (1) a fraction of streamflow contributed by groundwater or (2) the residence time of water in a basin are rejected. More attention must be focused upon the source of acidity generated in wetlands, since these are ubiquitous in small basins.