Drift Aquifers Research Articles

Use of nitrogen isotopes and other geochemical tools to evaluate the source of ammonium in a confined glacial drift aquifer, Ottawa County, Michigan, USA

This study utilized several isotopic tracers and geochemical parameters to characterize the source and hydrogeochemical setting of elevated ammonium (NH4+) concentrations (>2 mg/L) in a confined glacial drift aquifer in Ottawa County, Michigan. Results from this investigation indicate that NH4+ is attributed to degrading in situ organic matter and not from surficial anthropogenic sources. Tritium, δ18O, and δ2H values suggest that primary recharge in the confined aquifer occurred during the middle to late Holocene and before the mid-1900's. Geochemical constituents, including redox parameters and a suite of common ions, demonstrate a compositional difference between the confined and unconfined aquifers in the study area and therefore further support the isolation of the confined aquifer from a significant amount of recent recharge (post 1950's). Insignificant δ15N- NH4+ fractionation (+0.81‰ to +1.38‰) implies that NH4+ in the confined aquifer originated from degrading in situ organic matter. A complete Rotosonic glacial sediment core, collected to characterize the physical setting of the NH4+-rich aquifer, confirmed the presence of in situ organic matter in the confined aquifer. A limited set of δ15N-NO3 and δ18O-NO3 values suggest that the application of manure to row crops may be contributing nitrate (up to 10 mg/L) to nearby shallow residential wells. As implemented in this study, the use of multiple geochemical and isotopic parameters, including δ15N-NH4, can be applied to evaluate the potential source of NH4+ in an aquifer.

Constraining groundwater flow in the glacial drift and saginaw aquifers in the Michigan Basin through helium concentrations and isotopic ratios

Abstract3He and 4He concentrations in excess of those in water in solubility equilibrium with the atmosphere by up to two and three orders of magnitude are observed in the shallow Glacial Drift and Saginaw aquifers in the Michigan Basin. A simplified He transport model shows that in situ production is negligible and that most He excesses have a source external to the aquifer. Simulated results show that 3He and 4He fluxes entering the bottom of the Saginaw aquifer are 7.5 × 10−14 and 6.1 × 10−7 cm3STPcm−2 yr−1, both of which are lower than fluxes entering the underlying Marshall aquifer, 1.0 × 10−13 and 1.6 × 10−6 cm3STPcm−2 yr−1 for 3He and 4He, respectively. In contrast, He fluxes entering the Saginaw aquifer are higher than fluxes entering the overlying Glacial Drift aquifer of 5.2 × 10−14 and 1.5 × 10−7 cm3STPcm−2 yr−1 for 3He and 4He, respectively. The unusually high He fluxes and their decreasing values from the lower Marshall to the upper Glacial Drift aquifer strongly suggest the presence of an upward cross‐formational flow, with increasing He dilution toward the surface by recharge water. These fluxes are either comparable to or far greater than He fluxes in deeper aquifers around the world. Model simulations also suggest an exponential decrease in the horizontal groundwater velocity with recharge distance. Horizontal velocities vary from 13 to 2 myr−1 for the Saginaw aquifer and from 18 to 6 myr−1 for the Marshall aquifer. The highly permeable Glacial Drift aquifer displays a greater velocity range, from 250 to 5 myr−1. While Saginaw 4He ages estimated based on the simulated velocity field display an overall agreement with 14C ages, 14C and 4He ages in the Glacial Drift and Marshall aquifers deviate significantly, possibly due to simplifications introduced in the He transport model leading to calculation of first‐order approximation He ages and high uncertainties in Glacial Drift 14C ages.

Alternatives to Reduce Pumping Effects in Glacial Stratified Drift Aquifers During Periods of Low Stream Flow

Low stream flows in the Fenton River, part of a hydrogeological setting characterized by glacial stratified drift, forces the University of Connecticut to frequently reduce groundwater withdrawals during the months of June–October. The objective of this study was to investigate stream/aquifer interactions in such a hydrogeologic system in order to increase water withdrawals while minimizing adverse impacts to in-stream flow. A groundwater flow model was developed using MODFLOW to investigate the influence of well location and pumping timing on in-stream flow in the vicinity of the water supply wells. The numerical model comprised detailed geophysical data and decadal hydrologic data (2000–2009) to assess well placement, rest periods and cyclical pumping. The relocation of a water supply well up to 228 m from the river had a positive but minimal improvement to stream flows (<2.83 L/s). When the well field was shut off for more than 45 days, stream flows returned to the no pumping condition with only slight impact at 30 days, whereas a 30 day rest period gave 4 weeks of dampened pumping influence on stream flows. A management scenario of 1 week cyclical pumping between two water supply wells following a 45 day rest period can allow for current restriction thresholds to be reduced by 28.3 L/s with minimal impact to stream flows (7.36 L/s) and would allow additional water to be pumped for all years in which there was a demand for water.

Impacts of Pleistocene glaciation on large-scale groundwater flow and salinity in the Michigan Basin

Pleistocene melting of kilometer-thick continental ice sheets significantly impacted regional-scale groundwater flow in the low-lying stable interiors of the North American and Eurasian cratons. Glacial meltwaters penetrated hundreds of meters into the underlying sedimentary basins and fractured crystalline bedrock, disrupting relatively stagnant saline fluids and creating a strong disequilibrium pattern in fluid salinity. To constrain the impact of continental glaciation on variable density fluid flow, heat and solute transport in the Michigan Basin, we constructed a transient two-dimensional finite-element model of the northern half of the basin and imposed modern versus Pleistocene recharge conditions. The sag-type basin contains up to approximately 5 km of Paleozoic strata (carbonates, siliciclastics, and bedded evaporites) overlain by a thick veneer (up to 300 m) of glacial deposits. Formation water salinity increases exponentially from 350 g l−1 TDS at over 800 m depth. Model simulations show that modern groundwater flow is primarily restricted to shallow glacial drift aquifers with discharge to the Great Lakes. During the Pleistocene, however, high hydraulic heads from melting of the Laurentide Ice Sheet reversed regional flow patterns and focused recharge into Paleozoic carbonate and siliciclastic aquifers. Dilute waters (<20 g l−1 TDS) migrated approximately 110 km laterally into the Devonian carbonate aquifers, significantly depressing the freshwater-saline water mixing zones. These results are consistent with 14C ages and oxygen isotope values of confined groundwaters in Devonian carbonates along the basin margin, which reflect past recharge beneath the Laurentide Ice Sheet (14–50 ka). Constraining the paleohydrology of glaciated sedimentary basins, such as the Michigan Basin, is important for determining the source and residence times of groundwater resources, in addition to resolving geologic forces responsible for basinal-scale fluid and solute migration.

Redox evolution in glacial drift aquifers: role of diamicton units in reduction of Fe(III)

High iron concentrations create water quality problems for municipal use in glacial drift aquifer units. The chemical evolution of oxic groundwater in shallow aquifer units to anoxic groundwater in deeper aquifer units, in which soluble Fe(II) is stable, is attributed to coupled reduction of Fe(III) on aquifer solids with oxidation of organic carbon. The objective of this study was to characterize the distribution of organic carbon in aquifer and aquitard sediments to determine the availability of potential electron donors to drive these reactions. To do this, four complete rotasonic cores in a glacial aquifer/aquitard system were sampled at close intervals for analyses of grain-size distribution and organic carbon content. The results indicate significantly higher organic carbon concentrations in diamicton (till) units that function as aquitards, relative to coarse-grained aquifer units. In addition, readily reducible iron content in the diamicton units and lower aquifer unit materials is sufficient to produce far more dissolved iron than is present in the aquifer. Groundwater evolves to the level of iron reduction as a terminal electron-accepting process as it moves downward through aquitard units along flow paths from upland recharge areas to downgradient discharge areas. Deeper aquifer units are therefore unlikely to contain groundwater with low iron concentration.

Paleowaters in Silurian-Devonian carbonate aquifers: Geochemical evolution of groundwater in the Great Lakes region since the Late Pleistocene

Changes in the climatic conditions during the Late Quaternary and Holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the Great Lakes region. Increased hydraulic gradients from melting of kilometer-thick Pleistocene ice sheets reorganized regional-scale groundwater flow in Paleozoic aquifers in underlying intracratonic basins. Here, we present new elemental and isotopic analyses of 134 groundwaters from Silurian-Devonian carbonate and overlying glacial drift aquifers, along the margins of the Illinois and Michigan basins, to evaluate the paleohydrology, age distribution, and geochemical evolution of confined aquifer systems. This study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the Midcontinent region, and extends into deeper portions of the Illinois and Michigan basins, focused on the freshwater–saline water mixing zones. In addition, the hydrogeochemical data from Silurian-Devonian aquifers were integrated with deeper basinal fluids, and brines in Upper Devonian black shales and underlying Cambrian-Ordovician aquifers to reveal a regionally extensive recharge system of Pleistocene-age waters in glaciated sedimentary basins. Elemental and isotope geochemistry of confined groundwaters in Silurian-Devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals, dissolution of halite and anhydrite, cation exchange, microbial processes, and mixing with basinal brines. Carbon isotope values of dissolved inorganic carbon (DIC) range from −10 to −2‰, 87Sr/ 86Sr ratios range from 0.7080 to 0.7090, and δ 34 S – SO 4 values range from +10 to 30‰. A few waters have elevated δ 13C DIC values (>15‰) from microbial methanogenesis in adjacent organic-rich Upper Devonian shales. Radiocarbon ages and δ 18O and δD values of confined groundwaters indicate they originated as subglacial recharge beneath the Laurentide Ice Sheet (14–50 ka BP, −15 to −13‰ δ 18O). These paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake-bed clays and/or shales. The presence of isotopically depleted waters in Paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional-scale groundwater flow. Modern groundwater flow in the Great Lakes region is primarily restricted to shallow unconfined glacial drift aquifers. Recharge waters in Silurian-Devonian and unconfined drift aquifers have δ 18O values within the range of Holocene precipitation: −11 to −8‰ and −7 to −4.5‰ for northern Michigan and northern Indiana/Ohio, respectively. Carbon and Sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions ( δ 13C DIC = −14.7 to−11.1‰ and 87Sr/ 86Sr = 0.7080–0.7103). The distinct elemental and isotope geochemistry of Pleistocene- versus Holocene-age waters further confirms that surficial flow systems are out of contact with the deeper basinal-scale flow systems. These results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes, which are important for determining the sustainability of present-day water resources and stability of saline fluids in sedimentary basins.

Glacial Sediment Causing Regional‐Scale Elevated Arsenic in Drinking Water

In the upper Midwest, USA, elevated arsenic concentrations in public drinking water systems are associated with the lateral extent of northwest provenance late Wisconsin-aged drift. Twelve percent of public water systems located within the footprint of this drift (212 of 1764) exceed 10 microg/L arsenic, which is the U.S. EPA's drinking water standard. Outside of the footprint, only 2.4% of public water systems (52 of 2182) exceed 10 microg/L arsenic. Both glacial drift aquifers and shallow bedrock aquifers overlain by northwest provenance late Wisconsin-aged sediment are affected by arsenic contamination. Evidence suggests that the distinct physical characteristics of northwest provenance late Wisconsin-aged drift--its fine-grained matrix and entrained organic carbon that fosters biological activity--cause the geochemical conditions necessary to mobilize arsenic via reductive mechanisms such as reductive desorption and reductive dissolution of metal oxides. This study highlights an important and often unrecognized phenomenon: high-arsenic sediment is not necessary to cause arsenic-impacted ground water--when "impacted" is now defined as >10 microg/L. This analysis also demonstrates the scientific and economic value of using existing large but imperfect statewide data sets to observe and characterize regional-scale environmental problems.

Arsenic mobility in groundwater/surface water systems in carbonate-rich Pleistocene glacial drift aquifers (Michigan)

Within the Lower Peninsula of Michigan, groundwaters from the Marshall Formation (Mississippian) contain As derived from As-rich pyrites, often exceeding the World Heath Organization drinking water limit of 10 μg/L. Many Michigan watersheds, established on top of Pleistocene glacial drift derived from erosion of the underlying Marshall Formation, also have waters with elevated As. The Huron River watershed in southeastern Lower Michigan is a well characterized hydrogeochemical system of glacial drift deposits, proximate to the Marshall Fm. subcrop, which hosts carbonate-rich groundwaters, streams, and wetlands (fens), and well-developed soil profiles. Aqueous and solid phase geochemistry was determined for soils, soil waters, surface waters (streams and fens) and groundwaters from glacial drift aquifers to better understand the hydrogeologic and chemical controls on As mobility. Soil profiles established on the glacial drift exhibit enrichment in both Fe and As in the oxyhydroxide-rich zone of accumulation. The amounts of Fe and As present as oxyhydroxides are comparable to those reported from bulk Marshall Fm. core samples by previous workers. However, the As host in core samples is largely unaltered pyrite and arsenopyrite. This suggests that the transformation of Fe sulfides to Fe oxyhydroxides largely retains As and Fe at the oxidative weathering site. Groundwaters have the highest As values of all the waters sampled, and many were at or above the World Health limit. Most groundwaters are anaerobic, within the zones of Fe 3+ and As(V) reduction. Although reduction of Fe(III) oxyhydroxides is the probable source of As, there is no correlation between As and Fe concentrations. The As/Fe mole ratios in drift groundwaters are about an order of magnitude greater than those in soil profiles, suggesting that As is more mobile than Fe. This is consistent with the dominance of As(III) in these groundwaters and with the partitioning of Fe 2+ into carbonate cements. Soil waters have very low As and Fe contents, consistent with the stability of oxyhydroxides under oxidizing vadose conditions. When CO 2 charged groundwaters discharge in streams and fens, dissolved As is effectively removed by adsorption onto Fe-oxides or carbonate marls. Although Fe does not display conservative behavior with As in groundwaters, a strong positive correlation exists between As and Sr concentrations. As water–rock interactions proceed, the As/Fe and Sr/Ca ratios would be expected to increase because both As and Sr behave as incompatible elements. Comparisons with groundwater chemistries from other drift-hosted aquifers proximate to the Marshall sandstone are consistent with these relations. Thus, the Sr content of carbonate-rich groundwaters may provide useful constraints on the occurrence, origin and evolution of dissolved As in such systems.

Impact of Carbonate Precipitation on Riverine Inorganic Carbon Mass Transport from a Mid-continent, Forested Watershed

Physiochemical controls on the carbonate geochemistry of large river systems are important regulators of carbon exchange between terrestrial and marine reservoirs on human time scales. Although many studies have focused on large-scale river carbon fluxes, there are few investigations of mechanistic aspects of carbonate mass balance and transport at the catchment scale. We determined elemental and carbonate geochemistry and mass balances for net carbonate dissolution fluxes from the forested, mid-latitude Huron River watershed, established on carbonate-rich unconfined glacial drift aquifers. Shallow groundwaters are near equilibrium with respect to calcite at pCO2 values up to 25 times atmospheric values. Surface waters are largely groundwater fed and exhibit chemical evolution due to CO2 degassing, carbonate precipitation in lakes and wetlands, and anthropogenic introduction of road salts (NaCl and CaCl2). Because the source groundwater Mg2+/HCO3 − ratio is fairly constant, this parameter permits mass balances to be made between carbonate dissolution and back precipitation after groundwater discharge. Typically, precipitation does not occur until IAP/K calcite values exceed 10 times supersaturation. Stream chemistry changes little thereafter even though streams remain highly supersaturated for calcite. Our data taken together with historical United States Geological Survey (USGS) data show that alkalinity losses to carbonate precipitation are most significant during periods of lowest discharge. Thus, on an annual basis, the large carbon flux from carbonate dissolution in soil zones is only decreased by a relatively small amount by the back precipitation of calcium carbonate.

Arsenic in glacial drift aquifers and the implication for drinking water--lower Illinois River Basin.

The lower Illinois River Basin (LIRB) covers 47,000 km2 of central and western Illinois. In the LIRB, 90% of the ground water supplies are from the deep and shallow glacial drift aquifers. The deep glacial drift aquifer (DGDA) is below 152 m altitude, a sand and gravel deposit that fills the Mahomet Buried Bedrock Valley, and overlain by more than 30.5 m of clayey till. The LIRB is part of the USGS National Water Quality Assessment program, which has an objective to describe the status and trends of surface and ground water quality. In the DGDA, 55% of the wells used for public drinking-water supply and 43% of the wells used for domestic drinking water supply have arsenic concentrations above 10 micrograms/L (a new U.S. EPA drinking water standard). Arsenic concentrations greater than 25 micrograms/L in ground water are mostly in the form of arsenite (AsIII). The proportion of arsenate (AsV) to arsenite does not change along the flowpath of the DGDA. Because of the limited number of arsenic species analyses, no clear relations between species and other trace elements, major ions, or physical parameters could be established. Arsenic and barium concentrations increase from east to west in the DGDA and are positively correlated. Chloride and arsenic are positively correlated and provide evidence that arsenic may be derived locally from underlying bedrock. Solid phase geochemical analysis of the till, sand and gravel, and bedrock show the highest presence of arsenic in the underlying organic-rich carbonate bedrock. The black shale or coal within the organic-rich carbonate bedrock is a potential source of arsenic. Most high arsenic concentrations found in the DGDA are west and downgradient of the bedrock structural features. Geologic structures in the bedrock are potential pathways for recharge to the DGDA from surrounding bedrock.

Revisions to the Nomenclature for Kansas Aquifers

The nomenclature for Kansas aquifers needs revision. Many of the names currently in use were derived in an ad hoc manner without the benefit of a consistent set of hydrogeologically based guidelines. Because nomenclature forms the basis of scientific discussion of hydrologic issues, names should be clearly defined and consistently applied in order to prevent erroneous or unintended hydrologic interpretations. The nomenclature revisions presented in this paper are based on the guidelines developed by the U.S. Geological Survey's Water Resources Division (Laney and Davidson, 1986). According to these guidelines, aquifer names should be based on lithologic, lithostratigraphic, or geographic names. The following new aquifer names are recognized in Kansas: glacial drift aquifer, Niobrara aquifer, Morrison-Dockum aquifer, Day Creek aquifer, Cedar Hills-upper Salt Plain aquifer, Wellington aquifer, Flint Hills aquifer, Osage Cuestas aquifer system, and Ozark Plateau aquifer system.

Hydrogeochemical Interaction Between a Wetland and an Unconfined Glacial Drift Aquifer Southwestern Michigan

AbstractIn the glacial topography of southwestern Michigan, the water table does not always conform exactly to the land surface and flow‐through wetlands, those with both ground water recharge and discharge, are common. The W‐l wetland in Cass County, Michigan, a flow‐through system, shows a distinct contrast between upgradient and downgradient ground water quality. Ground water discharging into the wetland is oxic, has up to 40 mg/L NO3‐N derived from fertilizer and hog manure application to corn fields, and has major ion concentrations typical of the shallow unconfined aquifer in the area. In contrast, shallow ground water that originates as recharge from the wetland forms a plume extending downgradient that can be identified by isotopic enrichment in 18O and deuterium resulting from evaporation in the wetland prior to recharge and by distinct chemical characteristics similar to the wetland surface water (low conductivity and alkalinity, low concentrations of sulfate, nitrate and dissolved oxygen and high concentrations of ammonia, and DOC). As the wetland surface water infiltrates into the unconfined aquifer, conductivity and alkalinity increase due to carbonate mineral dissolution and iron concentrations increase as ferric iron in the aquifer solids serves as an electron acceptor in microbially mediated reactions. The other chemical characteristics, including the lack of nitrate, persist in the flow system for significant distances downgradient from the wetland.The chemical and isotopic composition of shallow ground water around wetlands can be used to spatially delineate areas of ground‐water discharge to the wetland and ground water recharge from the wetland, thereby providing a supplemental method to the use of sometimes inconclusive hydraulic head data in the determination of wetland recharge‐discharge function. In this study, the chemical and isotopic data confirm that the water table does not replicate surface topography around closed depressions in the landscape.

Transport of Agrichemicals to Ground and Surface Water in a Small Central Indiana Watershed

AbstractThe occurrence, distribution, concentrations, and pathways of agrichemicals in water were investigated in the Sugar Creek watershed, a poorly drained agricultural watershed typical of many watersheds in the midwestern USA. Water samples from Sugar Creek, two tile drains, and 11 wells along a groundwater flowpath to Sugar Creek were collected between May 1992 and August 1996 and analyzed for N and pesticide compounds. Nitrate was the principal N species and pesticides were common in alluvial water‐bearing units in the Sugar Creek floodplain. In the confined stratified drift aquifers, ammonia was the principal N species and pesticides were rare. Tile drains directly affected the water quality in Sugar Creek by transporting soil pore water and shallow groundwater containing high concentrations of nitrate (NO3) and pesticides to the creek. When tile drains were flowing (typically December through July), elevated NO3 concentrations (2–10 mg/L NO3N) in the creek correlated with high NO3 concentrations (2–23 mg/L NO3N) in tile drains discharging to the creek. Likewise, with concentrations of atrazine and atrazine metabolites, seasonal trends in the tile‐drain effluent were similar to seasonal trends in Sugar Creek. When tile drains went dry, NO3 concentrations in the creek were low, indicating most groundwater discharge to the creek consisted of old or denitrified water. Trace levels of pesticides in the creek at low flow probably were the result of seepage from alluvial water‐bearing units.

Iowa survey of waterborne 222Rn concentrations in private wells.

The major objective of the survey was to describe the distribution of waterborne 222Rn concentrations in Iowa's private well-water supplies. Well-water samples were obtained and analyzed for 222Rn from a random sample of 352 Iowa wells. The well-water 222Rn concentrations for the well sites were lognormally distributed and ranged from background concentrations to 87 Bq L-1, with a median value of 12 Bq L-1. The arithmetic mean 222Rn concentration for the sites was 16 Bq L-1 +/- 13 Bq L-1. The geometric mean 222Rn concentration was 12 Bq L-1 with a geometric standard deviation of 2.2. Over half of the samples (52%) exceeded 11 Bq L-1. Both well depth and indoor air 222Rn screening levels correlated with waterborne 222Rn concentrations; however, these correlations had very little predictive value. Glacial drift aquifers tended to have the highest 222Rn concentrations, although there was significant variance of 222Rn concentrations within all the aquifer classifications. In light of the estimate that 370 Bq L-1 of 222Rn in water may lead to 37 mBq L-1 in indoor air, the contribution of well-water derived indoor air 222Rn is minimal compared to ground sources in Iowa.

EVALUATION OF METHODS FOR DIAGNOSING CONTAMINATION IN RURALWELLS

Three diagnostic procedures were tested to determine their potential usefulness in identifying faulty rural wells: (1) monitoring wells were constructed at three depths near each of three rural wells having a history of nitratenitrogen and/or herbicide contamination, and all wells were sampled daily for four weeks and tested for nitrate-nitrogen, atrazine, alachlor, metolachlor, and chloride; (2) a chloride tracer solution was ponded around each of the water supply wells, and the shallowest monitoring well at each test site, for a period of 8 h during which the wells were continuously pumped and sampled for the tracer; and (3) nitrate-nitrogen and herbicide samples were collected from the water supply wells during the 8-h pumping period to observe contaminant variability during periods of continuous drawdown. Daily sampling revealed little temporal variability in the quality of water from the monitoring wells or the contaminated water supply wells. The monitoring wells, though limited in number, identified significant contaminant stratification within the shallow glacial drift aquifers supplying the water supply wells, and identified one water supply well that was producing water with much poorer quality than the shallow aquifer was capable of producing. The chloride tracer test was successful in distinguishing contaminant entry via preferential flow from that occurring through matrix flow in two of the case study wells, but proved ineffective on a third well where monitoring well data strongly suggested casing leakage. Nitrate-nitrogen and herbicide data showed little variability during the 8-h period of continuous well drawdown.

Fracture density estimates in glaciogenic deposits from P‐wave velocity reductions

Subsidence‐induced fracturing of glaciogenic deposits over coal mines in the southern Illinois basin alters hydraulic properties of drift aquifers and exposes these aquifers to surface contaminants. In this study, refraction tomography surveys were used in conjunction with a generalized form of a seismic fracture density model to estimate the vertical and lateral extent of fracturing in a 12-m thick overburden of loess, clay, glacial till, and outwash above a longwall coal mine at 90 m depth. This generalized model accurately predicted fracture trends and densities from azimuthal P‐wave velocity variations over unsaturated single‐ and dual‐parallel fractures exposed at the surface. These fractures extended at least 6 m and exhibited 10–15 cm apertures at the surface. The pre‐ and postsubsidence velocity ratios were converted into fracture densities that exhibited qualitative agreement with the observed surface and inferred subsurface fracture distribution. Velocity reductions as large as 25% were imaged over the static tension zone of the mine where fracturing may extend to depths of 10–15 m. Finally, the seismically derived fracture density estimates were plotted as a function of subsidence‐induced drawdown across the panel to estimate the average specific storage of the sand and gravel lower drift aquifer. This value was at least 20 times higher than the presubsidence (unfractured) specific storage for the same aquifer.

Interactions between ground water and wetlands, southern shore of Lake Michigan, USA

Wetlands between, and within, dune-beach complexes along the south shore of Lake Michigan are strongly affected by ground water. The hydrogeology of the glacial drift aquifer system in a 26 km 2 area was investigated to determine the effects of ground water on the hydrology and hydrochemistry of Cowles Bog and its adjacent wetlands. The investigation showed that ground water from intermediate- and regional-scale flow systems discharges to Cowles Bog from confined aquifers that underlie the wetland. These flow systems are recharged in moraines south of the dune-beach complexes. Water from the confined aquifers discharges into the surficial aquifer mainly by upward leakage through a buried till sheet that serves as the confining layer. However, the till sheet is breached below a raised peat mound in Cowles Bog, allowing direct upward discharge from the confinef aquifer into the surficial sand, marl, and peat. The shallow ground and wetland water in the area influenced by this leakage is a calcium magnesium bicarbonate type, with low tritium concentrations consistent with mixing of older ground water and more recent precipitation. Ground water and wetland water from surrounding areas are less mineralized and have higher tritium concentrations characteristic of precipitation in the late 1970s. The results of this study suggest that wetlands in complex hydrogeologic settings may be influenced by multiple ground-water flow systems that are affected by geomorphic features, stratigraphic discontinuities, and changes in sediment types. Discharge and recharge zones may both occur in the same wetland. Multidisciplinary studies incorporating hydrological, hydrochemical, geophysical, and sedimentological data are necessary to identify such complexities in wetland hydrology.

Groundwater quality variations in glacial drift and bedrock aquifers, Barry County, Michigan, U.S.A.

Groundwater samples from 288 domestic wells in Barry County, Michigan, were analyzed for 33 inorganic chemical parameters. Variations in chemical composition were investigated by considering the possible effects of human impact, aquifer type (bedrock vs glacial drift), chemical evolution along groundwater flow paths, and glacial landform type (moraine vs outwash). Approximately 25 percent of the glacial drift wells were classified as degraded by human impact and were excluded from further analysis of chemical variation. Two-sample tests comparing individual concentrations from drift and bedrock aquifers suggest that groundwater in the Marshall Sandstone aquifer is derived from local recharge through the glacial drift. This conclusion is supported by generalized groundwater flow patterns recognized for the two aquifers.

Potentiometric and Aquifer Property Changes Above Subsiding Longwall Mine Panels, Illinois Basin Coalfield

AbstractThis study examined the response of potentiometric levels and hydraulic properties to subsidence caused by a 725‐ft‐deep active longwall mine in southern Illinois. The overburden is mainly shale but includes a shallow sandstone aquifer, overlain by a shale aquitard, capped by thin drift. Pumping and packer tests indicate that subsidence fracturing increased the hydraulic conductivities of the sandstone aquifer by about an order of magnitude, and of certain horizons of the lower bedrock by several orders. The water table in the drift aquifer was unaffected by mining; however, heads in the bedrock units dropped sharply in response to subsidence, probably because tensional dilation of fractures increased storativities. The sandstone aquifer also displayed gradual potentiometric declines ahead of mining, and a rapid partial recovery afterwards. Depression of water levels and potentially higher well yields represent conflicting negative and positive aspects of the hydrologic impact of longwall mining.

PH and Redox Buffering Mechanisms in a Glacial Drift Aquifer Contaminated by Landfill Leachate

AbstractThe KL Landfill in Kalamazoo County, Michigan was closed in June 1979 because of ground‐water contamination. Study of water analyses from a monitoring‐well network has provided insight into a variety of pH and pe buffering reactions within the contaminant plume. The reactions occur when contaminated ground water containing dissolved organic acids from the landfill reacts with carbonate minerals and other solid phases in a glacial drift aquifer. Concentrations of Total Inorganic Carbon (TIC) in the plume, measured as the difference between Total Carbon (TC) and Total Organic Carbon (TOC), are lower than those obtained by alkalinity titrations. The difference results from the dissociation of organic acids in solution, producing organic anions that are included in the erroneously high titration alkalinity values.Organic acids cause dissolution of carbonate minerals in the drift and increased concentrations of calcium, magnesium, and bicarbonate in the plume. Geochemical modeling using WATEQF indicates that the plume is supersaturated with respect to calcite, dolomite, and siderite. The dissolution reactions buffer pH in the plume at near background levels.Redox potentials are below the level of sulfate reduction in the contaminant plume. Therefore, redox buffering mechanisms, reactions that inhibit changes in redox potential in the presence of a strong reducing agent, are much less effective than pH buffering in this system. Possible redox reactions occurring along the flow path include oxidation of organic compounds coupled with reduction of nitrate to ammonium and reduction of ferric iron in oxyhydroxide phases to ferrous iron.WATEQF simulations indicate that dissolved iron concentrations in the plume are well above saturation levels with respect to sulfide phases, moderately above saturation with respect to siderite, and below saturation with respect to iron hydroxide. It is likely that iron precipitates as sulfide phases until the dissolved sulfide is consumed. The concentration of dissolved iron is subsequently controlled by precipitation of siderite, which occurs when moderate levels of supersaturation are reached.These redox buffering reactions are insufficient to maintain high pe levels, and pe values drop to the level of sulfate reduction in the contaminant plume. Together, pH and redox buffering reactions control the concentrations of most major chemical species in the KL Landfill contaminant plume.