A model of cells as practical approach to simulate spring flow in the Itxina karstic aquifer, Basque Country, Spain

The aim of this study is to apply a parsimonious hydrologic model to the Itxina karstic aquifer that can predict changes in discharge resulting from variable inputs (recharge). The Itxina Aquifer was divided into four cells corresponding to different recharge areas. Each cell was treated as a tank to characterize the conditions within the cell. In the model, when the reservoir boundaries coincide with the position of the siphons, the signal simulated is sensitive to input pulses of the recharge. This supports the hypothesis that the siphons are the controlling mechanism in the flow system of the aquifer. The good agreement between predicted and measured discharges demonstrates the ability of the model to simulate the flow in the Itxina Aquifer. These results demonstrated that the hydraulic conductivity increases downstream within the aquifer. The hydraulic conductivities obtained by calibration varied between 4.2 x 10(-3) m/s upstream of the aquifer, 6.0 x 10(-2) m/s in the central region, and 9.5 x 10(-1) m/s in the lower region of the aquifer. These values seem reasonable because the underground features in the principal caves show that the density of caves increases downstream in the Itxina Aquifer. The simple representation of the system produced results comparable to traditional ground water models with fewer data requirements and calibration parameters.     

Hydrogeologic Characteristics of Shallow Glacial Drift Aquifers in Dissected Till Plains (North-Central Missouri)

The dissected till plains physiographic province contains extensive areas of pre-Illinoian drift in Kansas, Nebraska, Iowa, and Missouri. The drift is typically a clay-rich, oxidized, and highly weathered till. In many areas of Missouri, this shallow drift represents the only readily available aquifer suitable for domestic and small agricultural users. There are, however, only few published data on the region's hydrogeology. Water budget analyses indicate that approximately 30% of mean annual rainfall is discharged by stream flow and 70% by evapotranspiration. The shallow drift flow systems are best considered unconfined, steady-state systems. Each watershed corresponds roughly to a local ground-water system. There is, typically, little hydraulic connection between the shallow drift systems and underlying strata. The drift hydraulic conductivity varies from approximately 10¹¹ m/sec (laboratory tests) to 10⁻⁶ to 10⁻⁷ m/sec (field tests) when permeable sand lenses or joints are intersected. Ground-water discharge is concentrated in the immediate vicinity of stream channels, and recharge is concentrated on nearly horizontal hillslope summits. The ground water is generally potable (averaging 455 mg/1 total dissolved solids), but may be high in iron or polluted locally by organic wastes.     

Aquifer vulnerability to pesticide migration through till aquitards

This study investigates the influence of key factors-mainly recharge rate and degradation half-life--on downward migration of the widely used pesticide mecoprop (MCPP) through a typical clayey till aquitard. The study uses the numerical model FRAC3Dvs, which is a three-dimensional discrete fracture/matrix diffusion (DFMD) numerical transport model. The model was calibrated with laboratory and field data from a site near Havdrup, Denmark, but the overall findings are expected to be relevant to many other sites in similar settings. Fracture flow and MCPP transport parameters for the model were obtained through calibration using well-characterized laboratory experiments with large (0.5 m diameter by 0.5 m high) undisturbed columns of the fractured till and a field experiment. A second level of upscaling and sensitivity analysis was then carried out using data on hydraulic head, fracture spacing, and water budget from the field site. The simulations of downward migration of MCPP show that MCPP concentration and mass flux into the underlying aquifer, and hence the aquifer vulnerability to this pesticide compound, is mainly dependent on the degradation rate of the pesticide, the overall aquitard water budget, and the ground water recharge rate into the aquifer. The influence of flow rate, matrix diffusion, and degradation rate are intertwined. This results in one to four orders of magnitude higher MCPP flux into the aquifer from aquifer recharge rates of 20 and 120 mm/yr, respectively, for no degradation and MCPP half-life of 0.5 yr. From a sensitivity analysis it was found that the range of MCPP flux into the aquifer varied less than one order of magnitude due to (1) changing fracture spacing from 1 to 10 m, or (2) preferential flow along inclined thin sand layers, which represent common conditions for the current and other settings of clayey till in Denmark and other glaciated areas in Europe and North America. The results indicate that for aquifers overlain by fractured clayey tills, the vulnerability to contamination with pesticides (pesticide flux into the aquifer) and other widespread agricultural contaminants is going to vary strongly in the watershed as a function of the distribution of aquitard water budget (flow rate) and aquitard redox environment (controlling contaminant degradation rates), even if the thickness of the till is relatively constant. DFMD modeling of cause-effect relationships within such systems has great potential to support decisions in planning, regulation, and contaminant remediation.     

Geochemical quantification of semiarid mountain recharge

Analysis of a typical semiarid mountain system recharge (MSR) setting demonstrates that geochemical tracers help resolve the location, rate, and seasonality of recharge as well as ground water flowpaths and residence times. MSR is defined as the recharge at the mountain front that dominates many semiarid basins plus the often-overlooked recharge through the mountain block that may be a significant ground water resource; thus, geochemical measurements that integrate signals from all flowpaths are advantageous. Ground water fluxes determined from carbon-14 ((14)C) age gradients imply MSR rates between 2 x 10(6) and 9 x 10(6) m(3)/year in the Upper San Pedro Basin, Arizona, USA. This estimated range is within an order of magnitude of, but lower than, prior independent estimates. Stable isotopic signatures indicate that MSR has a 65% +/- 25% contribution from winter precipitation and a 35% +/- 25% contribution from summer precipitation. Chloride and stable isotope results confirm that transpiration is the dominant component of evapotranspiration (ET) in the basin with typical loss of more than 90% of precipitation-less runoff to ET. Such geochemical constraints can be used to further refine hydrogeologic models in similar high-elevation relief basins and can provide practical first estimates of MSR rates for basins lacking extensive prior hydrogeologic measurements.     

Estimate of recharge of a rising water table in semiarid Niger from 3H and 14C modeling

A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.     

Michigan basin regional ground water flow discharge to three Great Lakes

Ground water discharge to the Great Lakes around the Lower Peninsula of Michigan is primarily from recharge in riparian basins and proximal upland areas that are especially important to the northern half of the Lake Michigan shoreline. A steady-state finite-difference model was developed to simulate ground water flow in four regional aquifers in Michigan's Lower Peninsula: the Glaciofluvial, Saginaw, Parma-Bayport, and Marshall aquifers interlayered with the Till/"red beds," Saginaw, and Michigan confining units, respectively. The model domain was laterally bound by a continuous specified-head boundary, formed from lakes Michigan, Huron, St. Clair, and Erie, with the St. Clair and Detroit River connecting channels. The model was developed to quantify regional ground water flow in the aquifer systems using independently determined recharge estimates. According to the flow model, local stream stages and discharges account for 95% of the overall model water budget; only 50% enters the lakes directly from the ground water system. Direct ground water discharge to the Great Lakes' shorelines was calculated at 36 m3/sec, accounting for 5% of the overall model water budget. Lowland areas contribute far less ground water discharge to the Great Lakes than upland areas. The model indicates that Saginaw Bay receives only approximately 1.13 m3/sec ground water; the southern half of the Lake Michigan shoreline receives only approximately 2.83 m3/sec. In contrast, the northern half of the Lake Michigan shoreline receives more than 17 m3/sec from upland areas.     

Geochemical imaging of flow near an artificial recharge facility, Orange County, California

Critical for the management of artificial recharge operations is detailed knowledge of ground water dynamics near spreading areas. Geochemical tracer techniques including stable isotopes of water, tritium/helium-3 (T/3He) dating, and deliberate gas tracer experiments are ideally suited for these investigations. These tracers were used to evaluate flow near an artificial recharge site in northern Orange County, California, where approximately 2.5 x 10(8) m3 (200,000 acre-feet) of water are recharged annually. T/3He ages show that most of the relatively shallow ground water within 3 km of the recharge facilities have apparent ages < 2 years; further downgradient apparent ages increase, reaching > 20 years at approximately 6 km. Gas tracer experiments using sulfur hexafluoride and xenon isotopes were conducted from the Santa Ana River and two spreading basins. These tracers were followed in the ground water for more than two years, allowing subsurface flow patterns and flow times to be quantified. Results demonstrate that mean horizontal ground water velocities range from < 1 to > 4 km/year. The leading edges of the tracer patch moved at velocities about twice as fast as the center of mass. Leading edge velocities are important when considering the potential transport of microbes and other "time sensitive" contaminants and cannot be determined easily with other methods. T/3He apparent ages and tracer travel times agreed within the analytical uncertainty at 16 of 19 narrow screened monitoring wells. By combining these techniques, ground water flow was imaged with time scales on the order of weeks to decades.     

Ground water recharge and flow characterization using multiple isotopes

Stable isotopes of delta(18)O, delta(2)H, and (13)C, radiogenic isotopes of (14)C and (3)H, and ground water chemical compositions were used to distinguish ground water, recharge areas, and possible recharge processes in an arid zone, fault-bounded alluvial aquifer. Recharge mainly occurs through exposed stream channel beds as opposed to subsurface inflow along mountain fronts. This recharge distribution pattern may also occur in other fault-bounded aquifers, with important implications for conceptualization of ground water flow systems, development of ground water models, and ground water resource management. Ground water along the mountain front near the basin margins contains low delta(18)O, (14)C (percent modern carbon [pmC]), and (3)H (tritium units [TU]), suggesting older recharge. In addition, water levels lie at greater depths, and basin-bounding faults that locally act as a flow barrier may further reduce subsurface inflow into the aquifer along the mountain front. Chemical differences in ground water composition, attributed to varying aquifer mineralogy and recharge processes, further discriminate the basin-margin and the basin-center water. Direct recharge through the indurated sandstones and mudstones in the basin center is minimal. Modern recharge in the aquifer is mainly through the broad, exposed stream channel beds containing coarse sand and gravel where ground water contains higher delta(18)O, (14)C (pmC), and (3)H (TU). Spatial differences in delta(18)O, (14)C (pmC), and (3)H (TU) and occurrences of extensive mudstones in the basin center suggest sluggish ground water movement, including local compartmentalization of the flow system.     

A first estimate of ground water ages for the deep aquifer of the Kathmandu Basin, Nepal, using the radioisotope chlorine-36

The Kathmandu Basin in Nepal contains up to 550 m of Pliocene-Quaternary fluvio-lacustrine sediments which have formed a dual aquifer system. The unconfined sand and gravel aquifer is separated by a clay aquitard, up to 200 m thick, from the deeper, confined aquifer, comprised of Pliocene sand and gravel beds, intercalated with clay, peat, and lignite. The confined aquifer currently provides an important water supply to the central urban area but there are increasing concerns about its sustainability due to overexploitation. A limited number of determinations of the radioisotope 36Cl have been made on bore waters in the basin, allowing us to postulate on the age of ground water in the deeper, confined aquifer. Ground water evolution scenarios based on radioisotope decay, gradual dissolution of formational salts as the ground waters move downgradient, and flow velocity estimations produce comparable ground water ages for the deep waters, ranging from 200,000 to 400,000 years. From these ages, we deduce a mean ground water flow velocity of only 45 mm/year from recharge in the northeast to the main extraction region 15 km to the southwest. We thus estimate current recharge at about 5 to 15 mm/year, contributing 40,000 to 1.2 million m3/year to the ground water system. Current ground water extraction is estimated to be 20 times this amount. The low specific discharge confirms that the resource is being mined, and, based on current projections, reserves will be used up within 100 years.     

Multiple-aquifer characterization from single borehole extensometer records

Measurement and analysis of aquifer-system compaction have been used to characterize aquifer and confining unit properties when other techniques such as flow modeling have been ineffective at adequately quantifying storage properties or matching historical water levels in environments experiencing land subsidence. In the southeastern coastal plain of Virginia, high-sensitivity borehole pipe extensometers were used to measure 24.2 mm of total compaction at Franklin from 1979 through 1995 (1.5 mm/year) and 50.2 mm of total compaction at Suffolk from 1982 through 1995 (3.7 mm/year). Analysis of the extensometer data reveals that the small rates of aquifer-system compaction appear to be correlated with withdrawals of water from confined aquifers. One-dimensional vertical compaction modeling indicates measured compaction is the result of nonrecoverable hydrodynamic consolidation of the fine-grained confining units and interbeds, as well as recoverable compaction and expansion of coarse-grained aquifer units. The calibrated modeling results indicate that nonrecoverable specific storage values decrease with depth and range from 1.5 x 10(-5)/m for aquifer units to 1.5 x 10(-4)/m for confining units and interbeds. The aquifer and Potomac system recoverable specific storage values were all estimated to be 4.5 x 10(-6)/m, while the confining units and interbeds had values of 6.0 x 10(-6)/m. The calibrated vertical hydraulic conductivity values of the confining units and interbeds ranged from 6.6 x 10(-4) m/year to 2.0 x 10(-3) m/year. These parameter values will be useful in future management and modeling of ground water in the Virginia Coastal Plain.     

Analytical and numerical models to explain steady rates of spring flow

Flow from some springs in former glacial lakebeds of the Upper Midwest is extremely steady throughout the year and does not increase significantly after precipitation events or seasonal recharge. Analytical and simplified numerical models of spring systems were used to determine whether preferential ground water flow through high-permeability features in shallow sandstone aquifers could produce typical values of spring discharge and the unusually steady rates of spring flow. The analytical model is based on a one-dimensional solution for periodic ground water flow. Solutions to this model suggest that it is unlikely that a periodic forcing due to seasonal variations in areal recharge would propagate to springs in a setting where high-permeability features exist. The analytical model shows that the effective length of the aquifer, or the length of flowpaths to a spring, and the total transmissivity of the aquifer have the greatest potential to impact the nature of spring flow in this setting. The numerical models show that high-permeability features can influence the magnitude of spring flow and the results demonstrate that the lengths of ground water flowpaths increase when high-permeability features are explicitly modeled, thus decreasing the likelihood for temporal variations in spring flow.     

Agricultural pollutant penetration and steady state in thick aquifers

The leakage of pollutants from agricultural lands to aquifers has increased greatly, driven by increasing fertilizer and pesticide use. Because this increase is recent, ground water pollutant concentrations, loads, and exports may also be increasing as pollutants penetrate more deeply into aquifers. We established in an aquifer profile a ground water recharge and pollutant leakage chronology in an agricultural landscape where 30 m of till blankets a 57-m thick sandstone aquifer. Pollutant concentrations increased from older ground water (1963) at the aquifer base to younger ground water (1985) at its top, a signal of increasing pollutant leakage. Nitrate-N increased from 0.9 to 13.2 mg/L, implying that leakage increased from 1.9 to 16.5 kg/ha/year. Nitrate load and export could increase from 130% to 230% before reaching a steady state in 20 to 40 years. Chloride increases were similar. Pesticide residues alachlor ethane sulfonic acid (ESA), metolachlor ESA, and atrazine residues partially penetrated the aquifer profile. Their concentration-age-date patterns exhibited an initial increase and then a leveling corresponding to the timing of product adoption and leveling of demand. Unlike NO(3), projecting pesticide residue steady states is complicated by the phasing in and out of pesticide products over time; for example, neither alachlor nor atrazine is currently used in the area, and newer products, which have not had time to transit to the aquifer, have been adopted. The circumstances that resulted in the lack of a pollutant steady state are not rare; thus, the lack of steady states in agricultural region aquifers may not be uncommon.     

Ground water discharge and nitrate flux to the Gulf of Mexico

Ground water samples (37 to 186 m depth) from Baldwin County, Alabama, are used to define the hydrogeology of Gulf coastal aquifers and calculate the subsurface discharge of nutrients to the Gulf of Mexico. The ground water flow and nitrate flux have been determined by linking ground water concentrations to 3H/3He and 4He age dates. The middle aquifer (A2) is an active flow system characterized by postnuclear tritium levels, moderate vertical velocities, and high nitrate concentrations. Ground water discharge could be an unaccounted source for nutrients in the coastal oceans. The aquifers annually discharge 1.1 +/- 0.01 x 10(8) moles of nitrate to the Gulf of Mexico, or 50% and 0.8% of the annual contributions from the Mobile-Alabama River System and the Mississippi River System, respectively. In southern Baldwin County, south of Loxley, increasing reliance on ground water in the deeper A3 aquifer requires accurate estimates of safe ground water withdrawal. This aquifer, partially confined by Pliocene clay above and Pensacola Clay below, is tritium dead and contains elevated 4He concentrations with no nitrate and estimated ground water ages from 100 to 7000 years. The isotopic composition and concentration of natural gas diffusing from the Pensacola Clay into the A3 aquifer aids in defining the deep ground water discharge. The highest 4He and CH4 concentrations are found only in the deepest sample (Gulf State Park), indicating that ground water flow into the Gulf of Mexico suppresses the natural gas plume. Using the shape of the CH4-He plume and the accumulation of 4He rate (2.2 +/- 0.8 microcc/kg/1000 years), we estimate the natural submarine discharge and the replenishment rate for the A3 aquifer.     

Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA

Surface-water and ground-water flow are coupled in the central Everglades, although the remoteness of this system has hindered many previous attempts to quantify interactions between surface water and ground water. We modeled flow through a 43,000 ha basin in the central Everglades called Water Conservation Area 2A. The purpose of the model was to quantify recharge and discharge in the basin's vast interior areas. The presence and distribution of tritium in ground water was the principal constraint on the modeling, based on measurements in 25 research wells ranging in depth from 2 to 37 m. In addition to average characteristics of surface-water flow, the model parameters included depth of the layer of ‘interactive’ ground water that is actively exchanged with surface water, average residence time of interactive ground water, and the associated recharge and discharge fluxes across the wetland ground surface. Results indicated that only a relatively thin (8 m) layer of the 60 m deep surfical aquifer actively exchanges surface water and ground water on a decadal timescale. The calculated storage depth of interactive ground water was 3.1 m after adjustment for the porosity of peat and sandy limestone. Modeling of the tritium data yielded an average residence time of 90 years in interactive ground water, with associated recharge and discharge fluxes equal to 0.01 cm d⁻¹. ³H/³He isotopic ratio measurements (which correct for effects of vertical mixing in the aquifer with deeper, tritium-dead water) were available from several wells, and these indicated an average residence time of 25 years, suggesting that residence time was overestimated using tritium measurements alone. Indeed, both residence time and storage depth would be expected to be overestimated due to vertical mixing. The estimate of recharge and discharge (0.01 cm d⁻¹) that resulted from tritium modeling therefore is still considered reliable, because the ratio of residence time and storage depth (used to calculated recharge and discharge) is much less sensitive to vertical mixing compared with residence time alone. We conclude that a small but potentially significant component of flow through the Everglades is recharged to the aquifer and stored there for years to decades before discharged back to surface water. Long-term storage of water and solutes in the ground-water system beneath the wetlands has implications for restoration of Everglades water quality.     

Ground water flow parameterization of an Appalachian coal mine complex

We examined a large (240 km2) northern Appalachian bituminous coal basin (Irwin Syncline, Westmoreland County, Pennsylvania) comprising 27 mine complexes with nine major (> 2.5 x 10(3) L/min) discharges. The synclinal basin was divided into seven subbasins based on equilibrium hydraulic relationships established during the past 25 years. Recharge rates, mine pool velocity, and residence times respond to hydraulic changes in the overburden induced by mine subsidence. The estimated maximum depth for subsidence fractures is 60 m (30 times mined thickness) with recharge rates decreasing significantly in subbasins with thicker overburden (> 75 m). Calculated subbasin recharge rates range from 2 to 6 x 10(-4) L/min/m2 and are significantly lower than the previously used rate for the basin. Residence time of ground water in the Irwin subbasins calculated using average linear velocity ranged from one to five years and were more consistent with field observations than estimates obtained using discharge and basin volume area. A positive correlation (r2 = 0.80) exists between net alkalinity of the mine water-impacted discharges and residence time in the mine pools. Our results for the Irwin coal basin suggest that use of a subbasin approach incorporating overburden depth, mining methodology, and the extent of postmining inundation will lead to improved determination of ground water flow parameters in mined watersheds in northern Appalachia and elsewhere.     

Estimating groundwater residence time and recharge patterns in a saline coastal aquifer

A detailed study using environmental tracers such as chloride (Cl^?) and tritium (³H), deuterium (²H) and oxygen (¹⁸O) isotopes was performed in an alluvial coastal aquifer in two contrasting environments (urban and agricultural). These environmental tracers combined with a high‐resolution multi‐level sampling approach were used to estimate groundwater residence time and recharge patterns and to validate the hydrogeochemical conceptual model already proposed in previous studies. δ¹⁸O and δ²H combined with Cl^? data proved that the hypersaline groundwater present in the deepest part of the aquifer was sourced from the underlying hypersaline aquitard via an upward flux. Both chemical and isotopic data were employed to calibrate a density‐dependent numerical model based on SEAWAT 4.0, where ³H and Cl^? were helped quantifying solutes transport within the modelled aquifer. Model results highlighted the differences on estimated recharge in the two contrasting environments, with the urban one exhibiting concentrated recharge because of preferential infiltration associated to the storm water drains network, while scarce local recharge characterized the agriculture setting. In the urban field site, is still possible to recognize at 9 m b.g.l. the input of the atmospheric anthropogenic ³H generated by testing of thermonuclear weapons, while in the agricultural field site, the ³H peak has been washed out at 6 m b.g.l. because the groundwater circulation is restricted only to the upper fresh part of the aquifer, drained by the reclamation system. The presented approach that combined high‐resolution field monitoring, environmental tracers and numerical modelling, resulted effective in validating the conceptual model of the aquifer salinization. Copyright © 2016 John Wiley & Sons, Ltd.     

Contamination of the Sandstone Aquifer of Prince Edward Island, Canada by Aldicarb and Nitrogen Residues

Prince Edward Island is wholly dependent upon ground water from a highly permeable fractured sandstone aquifer for all industrial, domestic, agricultural, and potable uses. The contamination of this aquifer by agricultural residues, principally aldicarb and nitrate, has caused concern among Islanders. Ground water quality was monitored between 1985 and 1988 beneath two potato fields to which aldicarb (Temik) was applied at planting once or twice between 1983 and 1986. In May of 1988,12 percent of 48 monitoring well samples exceeded the drinking water guideline of 9μg/L for total aldicarb. Furthermore 32 percent of all samples exceeded the nitrate guideline of 10 mg/L. Aldicarb persistence appears related to its application at planting when soil temperatures are low and recharge is high and to the inhibiting pH effect that ammonium (from fertilizers and soil organic nitrogen) oxidation has on its degradation. Therefore, based on the research of others, it is recommended that aldicarb be applied at plant emergence when degradation is more rapid and recharge is lower.     

The vertical hydraulic conductivity of an aquitard at two spatial scales

Aquitards protect underlying aquifers from contaminants and limit recharge to those aquifers. Understanding the mechanisms and quantity of ground water flow across aquitards to underlying aquifers is essential for ground water planning and assessment. We present results of laboratory testing for shale hydraulic conductivities, a methodology for determining the vertical hydraulic conductivity (K(v)) of aquitards at regional scales and demonstrate the importance of discrete flow pathways across aquitards. A regional shale aquitard in southeastern Wisconsin, the Maquoketa Formation, was studied to define the role that an aquitard plays in a regional ground water flow system. Calibration of a regional ground water flow model for southeastern Wisconsin using both predevelopment steady-state and transient targets suggested that the regional K(v) of the Maquoketa Formation is 1.8 x 10(-11) m/s. The core-scale measurements of the K(v) of the Maquoketa Formation range from 1.8 x 10(-14) to 4.1 x 10(-12) m/s. Flow through some additional pathways in the shale, potential fractures or open boreholes, can explain the apparent increase of the regional-scale K(v). Based on well logs, erosional windows or high-conductivity zones seem unlikely pathways. Fractures cutting through the entire thickness of the shale spaced 5 km apart with an aperture of 50 microns could provide enough flow across the aquitard to match that provided by an equivalent bulk K(v) of 1.8 x 10(-11) m/s. In a similar fashion, only 50 wells of 0.1 m radius open to aquifers above and below the shale and evenly spaced 10 km apart across southeastern Wisconsin can match the model K(v).     

Trench infiltration for managed aquifer recharge to permeable bedrock

Managed aquifer recharge to permeable bedrock is increasingly being utilized to enhance resources and maintain sustainable groundwater development practices. One such target is the Navajo Sandstone, an extensive regional aquifer located throughout the Colorado Plateau of the western United States. Spreading‐basin and bank‐filtration projects along the sandstone outcrop's western edge in southwestern Utah have recently been implemented to meet growth‐related water demands. This paper reports on a new cost‐effective surface‐infiltration technique utilizing trenches for enhancing managed aquifer recharge to permeable bedrock. A 48‐day infiltration trench experiment on outcropping Navajo Sandstone was conducted to evaluate this alternative surface‐spreading artificial recharge method. Final infiltration rates through the bottom of the trench were about 0·5 m/day. These infiltration rates were an order of magnitude higher than rates from a previous surface‐spreading experiment at the same site. The higher rates were likely caused by a combination of factors including the removal of lower permeability soil and surficial caliche deposits, access to open vertical sandstone fractures, a reduction in physical clogging associated with silt and biofilm layers, minimizing viscosity effects by maintaining isothermal conditions, minimizing chemical clogging caused by carbonate mineral precipitation associated with algal photosynthesis, and diminished gas clogging associated with trapped air and biogenic gases. This pilot study illustrates the viability of trench infiltration for enhancing surface spreading of managed aquifer recharge to permeable bedrock. Published in 2010 by John Wiley & Sons, Ltd.     

Predictability of surface mine spoil hydrologic properties in the Appalachian Plateau

Prevention of acid mine drainage at surface coal mines in the Appalachian region relies to an extent on minimizing ground water contact with acid-forming materials, and maximizing ground water contact with alkalinity-yielding materials. Acid-forming materials are often selectively handled to minimize or prevent contact with ground water. Controlling ground water contact with acidic or alkaline materials depends on forecasting the level and range of fluctuation of the postmining water table within the mine backfill. Physical measurements and aquifer testing of more than 120 wells from 18 reclaimed mines in Kentucky, Ohio, Pennsylvania, and West Virginia have led to improved forecasting of the postmining ground water system. Factors that influence the ground water regime include spoil lithology and particle size, age of reclamation, spoil thickness, distance from the final highwall, and pit floor dip angle and direction. Spoil hydraulic conductivity (K) exhibits a 95% confidence interval range of six orders of magnitude about a mean K of 1.7 x 10(-5) m/sec. Spoil aquifer saturated thickness is related to the overall thickness of the spoil, the lithology of the spoil, dip of the pit floor, and distance to the highwall. Saturated spoil thickness has a 95% confidence interval of 2.2 to 3.6 m about the mean of 2.9 m. The predicted saturated zone averages 19% of the total spoil thickness.