Recharge In Regions Research Articles

A large-scale simulation model to assess karstic groundwater recharge over Europe and the Mediterranean

Abstract. Karst develops through the dissolution of carbonate rock and is a major source of groundwater contributing up to half of the total drinking water supply in some European countries. Previous approaches to model future water availability in Europe are either too-small scale or do not incorporate karst processes, i.e. preferential flow paths. This study presents the first simulations of groundwater recharge in all karst regions in Europe with a parsimonious karst hydrology model. A novel parameter confinement strategy combines a priori information with recharge-related observations (actual evapotranspiration and soil moisture) at locations across Europe while explicitly identifying uncertainty in the model parameters. Europe's karst regions are divided into four typical karst landscapes (humid, mountain, Mediterranean and desert) by cluster analysis and recharge is simulated from 2002 to 2012 for each karst landscape. Mean annual recharge ranges from negligible in deserts to > 1 m a−1 in humid regions. The majority of recharge rates range from 20 to 50% of precipitation and are sensitive to subannual climate variability. Simulation results are consistent with independent observations of mean annual recharge and significantly better than other global hydrology models that do not consider karst processes (PCR-GLOBWB, WaterGAP). Global hydrology models systematically under-estimate karst recharge implying that they over-estimate actual evapotranspiration and surface runoff. Karst water budgets and thus information to support management decisions regarding drinking water supply and flood risk are significantly improved by our model.

A groundwater recharge perspective on locating tree plantations within low-rainfall catchments to limit water resource losses

Abstract. Despite the many studies that consider the impacts of plantation forestry on groundwater recharge, and others that explore the spatial heterogeneity of recharge in low-rainfall regions, there is little marriage of the two subjects in forestry management guidelines and legislation. Here we carry out an in-depth analysis of the impact of reforestation on groundwater recharge in a low-rainfall (< 700 mm annually), high-evapotranspiration paired catchment characterized by ephemeral streams. Water table fluctuation (WTF) estimates of modern recharge indicate that little groundwater recharge occurs along the topographic highs of the catchments (average 18 mm yr−1); instead the steeper slopes in these areas direct runoff downslope to the lowland areas, where most recharge occurs (average 78 mm yr−1). Recharge estimates using the chloride mass balance (CMB) method were corrected by replacing the rainfall input Cl− value with that for streamflow, because most recharge occurs from infiltration of runoff through the streambed and adjacent low gradient slopes. The calculated CMB recharge values (average 10 mm yr−1) are lower than the WTF recharge values (average 47 mm yr−1), because they are representative of groundwater that was mostly recharged prior to European land clearance (> BP 200 years). The tree plantation has caused a progressive drawdown in groundwater levels due to tree water use; the decline is less in the upland areas. The results of this study show that spatial variations in recharge are important considerations for locating tree plantations. To conserve water resources for downstream users in low-rainfall, high-evapotranspiration regions, tree planting should be avoided in the dominant zone of recharge, i.e. the topographically low areas and along the drainage lines, and should be concentrated on the upper slopes, although this may negatively impact the economic viability of the plantation.

A study of the characteristics of karst groundwater circulation based on multi-isotope approach in the Liulin spring area, North China

Due to the significance of karst groundwater for water supply in arid and semi-arid regions, the characteristics of the karst groundwater flow system in the Liulin spring area, North China, are analysed through isotopic tracing (δ2H, δ18O, δ13C and 3H) and dating approaches (14C). The results show that the primary recharge source of karst groundwater is precipitation. Evaporation during dropping and infiltration of rainfall results in a certain offset in the values of δ2H and δ18O in groundwater samples from the global meteoric water line (GMWL) and the local meteoric water line (LMWL). The altitudes of the recharge region calculated by δ18O range from 1280 to 2020 m above sea level, which is consistent with the altitudes of the recharge area. The Liulin spring groups could be regarded as the mixing of groundwater with long and short flow paths at a ratio of 4:1. In the upgradient of the Liulin spring, the groundwater represents modern groundwater features and its is mainly derived from dissolution of soil CO2, while in the downgradient of the Liulin spring, the 14C age of dissolved inorganic carbon (DIC) in groundwater shows an apparent increase and is mainly derived from the dissolution of carbonate rocks. The mean flow rate calculated by 14C ages of DIC between IS10 and IS12 is 1.23 m/year.

A field demonstration of groundwater vulnerability assessment using transport modeling and groundwater age modeling, Beijing Plain, China

Groundwater vulnerability is often regarded as the resistance of aquifers to pollutions, which is in fact the groundwater environment vulnerability. Similarly the renewable capacity of groundwater system can be regarded as the indicator for groundwater resources vulnerability. In this paper, the concept of groundwater vulnerability is extended to include both the groundwater environment vulnerability and groundwater resources vulnerability, and the extended overall groundwater vulnerability is assessed by examining the solute transport process from ground surface to the water table (for groundwater environment vulnerability), and evaluating the renewable capacity of groundwater system (for groundwater resources vulnerability). For this case study in Beijing Plain, a 1D-modeling, 2D-mapping model is constructed in HYDRUS to simulate the solute transport process through vadose zone. The transit times and solute concentrations at the water table are selected as the indicators for groundwater environment vulnerability. Meanwhile, a 3D groundwater age model in MODFLOW/MT3DMS is employed to model the age distribution in saturated zone, and the modeled ages are taken as the indicators for groundwater resources vulnerability. These two models are coupled together by exchanging their vertical infiltrations and water table elevations. Overall vulnerability is finally assessed by taking all the indicators into account. The result suggests the overall vulnerability can finely represent the synthetic impact of solute transport characteristics (groundwater environment vulnerability in vadose zone) and groundwater system renewability (groundwater resources vulnerability in saturated zone). The most vulnerable regions of Beijing Plain are mostly in agricultural areas, the medium vulnerable areas mostly appear at pumping sites, and the least vulnerable areas are mostly distributed in recharge regions.

GIS-based water balance modeling for estimating regional specific yield and distributed recharge in data-scarce hard-rock regions

In this study, a methodology is presented and demonstrated for combined estimation of regional specific yield and distributed recharge using double water-table fluctuation (DWTF) technique and geographical information system (GIS) in a hard-rock aquifer system of semi-arid regions. The study area was divided into 25 zones and groundwater budget components were computed for both wet and dry seasons using 11-year period (1996–2006) data. In each zone, the regional specific yield was estimated by applying the WTF technique for dry seasons and the rainfall recharge was estimated by applying the WTF technique for wet seasons. Zone-wise rainfall–recharge relationships were established using regression technique. Thereafter, the specific yield and recharge estimates were used with GIS to generate their maps. Surface-water bodies were found to significantly contribute to groundwater recharge. This finding underscores the need for adopting rainwater harvesting in the study area to enhance recharge. The regional specific yields were found to range from 0.038 to 0.002, whereas the mean rainfall recharge was found to vary from 0.5 to 10.9 cm. The box–whisker plots of z-scale transformed specific yield revealed the greatest spatial variation. The spatial and temporal variations of the rainfall recharge in the study area are statistically significant (p < 0.05 and CV > 30%). The developed rainfall–recharge relationships were found to be ‘highly significant’ (r2 ≥ 0.54, p < 0.05) in four zones, ‘moderately significant’ (0.54 > r2 ≥ 0.36, p < 0.01) in ten zones and ‘insignificant’ (r2 < 0.36) in the remaining zones.

Mapping Mountain Front Recharge Areas in Arid Watersheds Based on a Digital Elevation Model and Land Cover Types

A recent assessment that quantified potential impacts of solar energy development on water resources in the southwestern United States necessitated the development of a methodology to identify locations of mountain front recharge (MFR) in order to guide land development decisions. A spatially explicit, slope-based algorithm was created to delineate MFR zones in 17 arid, mountainous watersheds using elevation and land cover data. Slopes were calculated from elevation data and grouped into 100 classes using iterative self-organizing classification. Candidate MFR zones were identified based on slope classes that were consistent with MFR. Land cover types that were inconsistent with groundwater recharge were excluded from the candidate areas to determine the final MFR zones. No MFR reference maps exist for comparison with the study’s results, so the reliability of the resulting MFR zone maps was evaluated qualitatively using slope, surficial geology, soil, and land cover datasets. MFR zones ranged from 74 km2 to 1547 km2 and accounted for 40% of the total watershed area studied. Slopes and surficial geologic materials that were present in the MFR zones were consistent with conditions at the mountain front, while soils and land cover that were present would generally promote groundwater recharge. Visual inspection of the MFR zone maps also confirmed the presence of well-recognized alluvial fan features in several study watersheds. While qualitative evaluation suggested that the algorithm reliably delineated MFR zones in most watersheds overall, the algorithm was better suited for application in watersheds that had characteristic Basin and Range topography and relatively flat basin floors than areas without these characteristics. Because the algorithm performed well to reliably delineate the spatial distribution of MFR, it would allow researchers to quantify aspects of the hydrologic processes associated with MFR and help local land resource managers to consider protection of critical groundwater recharge regions in their development decisions.

Factors controlling terrigenic SF6 in young groundwater of the Odenwald region (Germany)

The mountainous Odenwald region in the federal state of Hesse/Germany is one of the main recharge areas for groundwater of the surrounding depressions, where substantial extraction for public water supply takes place. We investigated the groundwater to study residence times and mixing ratios of groundwater of different ages, define regions of groundwater recharge and understand the groundwater inflow from the Odenwald to the surrounding areas. This multi-tracer study included stable and radioactive gas and isotope tracers such as 2H, 18O, 3H, noble gases (He, Ne, Ar, Kr, Xe), 222Rn and SF6. Noble gases were used to calculate recharge temperatures of the groundwater and to correct all gas tracers for so called “excess air”. Comparing the results of the two independent dating methods – SF6 and 3H–3He – reveals that dating with SF6 is not possible in the crystalline region of the Odenwald, while the 3H–3He method gives robust groundwater ages. The results indicate that SF6 is influenced by a terrigenic source in the subsurface that varies with lithology. 222Rn data from part of the wells seem to be related to the terrigenic SF6, consistent with the idea of radiochemical SF6 production in rocks and release from the aquifer matrix.

Climatic controls on diffuse groundwater recharge across Australia

Abstract. Reviews of field studies of groundwater recharge have attempted to investigate how climate characteristics control recharge, but due to a lack of data have not been able to draw any strong conclusions beyond that rainfall is the major determinant. This study has used numerical modelling for a range of Köppen-Geiger climate types (tropical, arid and temperate) to investigate the effect of climate variables on recharge for different soil and vegetation types. For the majority of climate types, the correlation between the modelled recharge and total annual rainfall is weaker than the correlation between recharge and the annual rainfall parameters reflecting rainfall intensity. Under similar soil and vegetation conditions for the same annual rainfall, annual recharge in regions with winter-dominated rainfall is greater than in regions with summer-dominated rainfall. The importance of climate parameters other than rainfall in recharge estimation is highest in the tropical climate type. Mean annual values of solar radiation and vapour pressure deficit show a greater importance in recharge estimation than mean annual values of the daily mean temperature. Climate parameters have the lowest relative importance in recharge estimation in the arid climate type (with cold winters) and the temperate climate type. For 75% of all soil, vegetation and climate types investigated, recharge elasticity varies between 2 and 4 indicating a 20% to 40% change in recharge for a 10% change in annual rainfall. Understanding how climate controls recharge under the observed historical climate allows more informed choices of analogue sites if they are to be used for climate change impact assessments.

A method for estimating spatially variable seepage and hydraulic conductivity in channels with very mild slopes

AbstractInfiltration along ephemeral channels plays an important role in groundwater recharge in arid regions. A model is presented for estimating spatial variability of seepage due to streambed heterogeneity along channels based on measurements of streamflow‐front velocities in initially dry channels. The diffusion‐wave approximation to the Saint‐Venant equations, coupled with Philip's equation for infiltration, is connected to the groundwater model MODFLOW and is calibrated by adjusting the saturated hydraulic conductivity of the channel bed. The model is applied to portions of two large water delivery canals, which serve as proxies for natural ephemeral streams. Estimated seepage rates compare well with previously published values. Possible sources of error stem from uncertainty in Manning's roughness coefficients, soil hydraulic properties and channel geometry. Model performance would be most improved through more frequent longitudinal estimates of channel geometry and thalweg elevation, and with measurements of stream stage over time to constrain wave timing and shape. This model is a potentially valuable tool for estimating spatial variability in longitudinal seepage along intermittent and ephemeral channels over a wide range of bed slopes and the influence of seepage rates on groundwater levels. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Estimation of groundwater recharge in arid, data scarce regions; an approach as applied in the El Hawashyia basin and Ghazala sub-basin (Gulf of Suez, Egypt)

In this study, an approach for runoff and recharge estimations that can be applied in arid regions which suffer from lack of data is presented. Estimating groundwater recharge in arid regions is an extremely important but difficult task, the main reason is the scarcity of data in arid regions. This is true for the Eastern Egyptian Desert where groundwater is used for irrigation purposes in agricultural reclamation along the Red Sea coast line. As a result of the scarcity of hydrologic information, the relation between rainfall and runoff was calculated depending on the paleo-flood hydrology information. Two models were used to calculate the rainfall–runoff relationships for El Hawashyia basin and Ghazala sub-basin. Two computer programs known as Gerinne (meaning channel in German) and SMADA6 (Stormwater Management and Design Aid, version 6) were conjunctively used for this purpose. As a result of the model applied to El Hawashyia basin, a rainfall event of a total of 18.3 mm with duration 3 h at the station of Hurghada, which has an exceedance probability of 5–10 %, produces a discharge volume of 10.2 × 106 m3 at the delta, outlet of the basin, as 4.7 mm of the rainfall infiltrates (recharge). For the Ghazala sub-basin, the model yields a runoff volume of 3.16 × 106 m3 transferred from a total rainfall of 25 mm over a period of 3 h, as 3.2 mm of it was lost as infiltration.

Sand, salt and water in the Stampriet Basin, Namibia: Calculating unsaturated zone (Kalahari dunefield) recharge using the chloride mass balance approach

Estimating groundwater recharge rates in the Stampriet Basin is important for assessing the sustainability of the groundwater resource both within south-east Namibia, and across the borders of this transboundary resource into Botswana and South Africa. The 65 000 km2 basin contains a multi-layered aquifer system, of up to 9 superimposed aquifers, with a majorupper unconfined and 2 major artesian levels. The majority of water abstraction (66%) is from the unconfined Kalahari Aquifer. This research investigates the direct recharge pathway through the dune-sand cover of the southern Kalahari dunefield which overlies &#126;80% of the basin. This is undertaken using established chloride mass balance methods within 4unsaturated zone profiles (9 m to 11.5 m depth) from 2 regions of the basin. The results indicate that recent recharge rates are between 7 mm&#183;y-1 to 46 mm&#183;y-1, with profiles representing between 10 years and 30 years of rainfall infiltration. This is the first time that substantial direct (diffuse) recharge has been quantified for the Stampriet Basin, and this part of the southern Kalahari. These 4 profiles indicate potentially similar levels of spatial variability as seen in many other semi-arid, sand-rich unsaturated zones across the world. Further studies using the unsaturated zone should be employed in this region, to better inform our&#160; understanding of recharge mechanisms across the basin. The chloride mass balance technique provides information about the unsaturated zone, and important insights about recharge in various sand-covered regions of the basin for which very little is known, or is possible to determine using conventional water balance approaches. Direct recharge rates estimated in this way constitute a vital part of constraining the water balance of the basin, and particularly of the Kalahari Aquifer from which the majority of groundwater abstraction occurs.

Hydrogeochemical Modelling for Groundwater in Neyveli Aquifer, Tamil Nadu, India, Using PHREEQC: A Case Study

Sophisticated geochemical models have been used to describe and predict the chemical behaviour of complex natural waters and also to protect the groundwater resources from future contamination. One such model is used to study the hydrogeochemical complexity in a mine area. Extraction of groundwater from the coastal aquifer has been in progress for decades to mine lignite in Neyveli. This extraction has developed a cone of depression around the mine site. This cone of depression is well established by the geochemical nature of groundwater in the region. 42 groundwater samples were collected in a definite pattern and they were analysed for major cations, anions and trace elements. The saturation index (SI) of the groundwater for carbonate, sulphate and silica minerals was studied and it has been correlated with the recharge and the discharge regions. The SI of alumino silicates has been used to decipher the stage of weathering. The SIGibbsite − SIK-feldspar has been spatially distributed and the regions of discharge and recharge were identified. Then two flow paths A1 and A2 were identified and inverse modelling using PHREEQC were carried out to delineate the geochemical process that has taken place from recharge to discharge. The initial and final solutions in both the flow paths were correlated with the thermodynamic silicate stability diagrams of groundwater and it was found that the state of thermodynamic stability of the end solutions along the flow path were approaching similar states of equilibrium at the discharge.

14C dating of deep groundwater in the Bengal Aquifer System, Bangladesh: Implications for aquifer anisotropy, recharge sources and sustainability

Environmental isotopes and 14C dating were applied to estimate ages and recharge sources of deep groundwater (⩾150 m bgl) in south-east Bangladesh. With one exception, deep groundwater is shown to have been recharged more recently than 10 Ka (range 3–9 Ka, mean 7.6 Ka), under climatic conditions indicated by δ18O and δ2H as similar to the present day. Groundwater age distributions have been used to infer the scale of aquifer hydraulic anisotropy. 2D groundwater flow modelling is able to reproduce the observed vertical profiles of groundwater age, as determined at two locations, when the aquifer is assigned a permeability anisotropy (Kx/Kz) of at least 103. Under these conditions, deep groundwater originates as recharge in the hill regions at the eastern boundary of the basin. Recharge rates estimated from the groundwater ages are close to an estimate of the current rate of deep groundwater abstraction. Cautious development and careful monitoring are therefore necessary, as excessive deep groundwater pumping could draw dissolved arsenic from the shallow levels of the Bengal Aquifer System (BAS) and contaminate the deep groundwater resource.

Recharge signal identification based on groundwater level observations

This study applied a method of the rotated empirical orthogonal functions to directly decompose the space-time groundwater level variations and determine the potential recharge zones by investigating the correlation between the identified groundwater signals and the observed local rainfall records. The approach is used to analyze the spatiotemporal process of piezometric heads estimated by Bayesian maximum entropy method from monthly observations of 45 wells in 1999-2007 located in the Pingtung Plain of Taiwan. From the results, the primary potential recharge area is located at the proximal fan areas where the recharge process accounts for 88% of the spatiotemporal variations of piezometric heads in the study area. The decomposition of groundwater levels associated with rainfall can provide information on the recharge process since rainfall is an important contributor to groundwater recharge in semi-arid regions. Correlation analysis shows that the identified recharge closely associates with the temporal variation of the local precipitation with a delay of 1-2 months in the study area.

The “Morning Glory” Project: A Papua New Guinea–Queensland Australia Undersea Freshwater Pipeline

Australia's so-called “Dead Heart,” its hot arid interior landscape, can be brought into irrigated agricultural production with the importation of extracontinental freshwater supplies originating in Papua New Guinea. Despite Queensland's La Niña-caused historic and costly 2010 to 2011 river flooding induced, in great part by tropical cyclone Yasi, its westernmost torrid region is classed as arid, currently almost without any irrigation agriculture present. Freshwater importation from Papua New Guinea's Fly River via an undersea pipeline mostly skirting the Torres Strait could increase the value of Queensland's little-used dryland Outback and, perhaps, establish new overseas markets in Asia for exported agricultural products. An induced perennial Diamantina River could replenish groundwater recharge regions (Great Artesian Basin) and convert Lake Eyre to a permanent lake of slightly brackish water. Irrigated eucalyptus tree plantations might serve, in part, to counter some Earthly global warming. The Torres Strait Islanders will benefit economically as well as other social groups on the mainland of Papua New Guinea. Here we examine closely some of the technical aspects of a Papua New Guinea-Queensland (PNG-QLD) Undersea Freshwater Pipeline Macroproject (UFPM) installation. We offer a choice of two routings for the PNG-QLD UFPM (Case A) while at the same time making clear our informed preference.

Sorption and degradation of wastewater-associated non-steroidal anti-inflammatory drugs and antibiotics in soils

Presence of pharmaceuticals at trace levels in recycled water is an emerging issue impacting the beneficial reuse of treated wastewater, including practices such as irrigation and groundwater recharge in arid and semi-arid regions. To assess the environmental risks of irrigation with recycled water containing such micropollutants, in this study we evaluated sorption and degradation of five pharmaceuticals that are antibiotic and anti-inflammatory drugs in two soils collected from arid regions. Naproxen and trimethoprim showed moderate to strong sorption, while the sorption of diclofenac, ibuprofen and sulfamethoxazole was negligible in both soils. Under aerobic conditions, the studied compounds were susceptible to microbial degradation with half-lives varying from 4.8 to 69.3 d. Apart from sulfamethoxazole, the other compounds were relatively persistent under anaerobic conditions as indicated by a negligible loss over 84 d of incubation or half-lives >50 d. The degradation of the selected pharmaceuticals was influenced by microbial activities, oxygen status in the soil, soil type and compound characteristics. The poor sorption and relative persistence of diclofenac and ibuprofen under anaerobic conditions suggest that the two chemicals may pose a high leaching risk when using recycled for irrigation or groundwater replenishment.

Sustainable Sanitation as a Part of an IWRM in the Karst Area of Gunung Kidul: Community Acceptance and Opinion

To overcome the problem of water scarcity in a rural karst area located in Gunung Sewu, Yogyakarta, Indonesia, water from an underground cave (Bribin Cave) is pumped and distributed to the people. Since karst aquifers are often considered as highly vulnerable to contamination from human activities, potential problems caused by current sanitation and hygiene practices in the recharge region must be anticipated. About 90% of the people in the recharge area use pour flush syphon toilets with poorly designed septic tanks and the rest have simple pit latrines. However, the existing septic tanks are very poorly designed and are actually only improved infiltration pits. One possible option to reduce the risk of aquifer contamination is to promote sustainable sanitation, which combines hygienic aspects with recycling of nutrients from human feces and urine. Sustainable sanitation comprises different technologies including low cost technology, which can be afforded by the community and provide economic benefit to the low income inhabitants in Gunung Sewu. The technology proposed is urine diverting and composting toilets. This paper presents the existing condition of rural domestic wastewater treatment and the opinion/ acceptance of the community in karst area of Gunung Sewu towards urine diverting and composting toilets. The results of the survey shows that although the acceptance of using the composting toilet is not high (25% to 43%), the interest of the community in using the end product is relatively high. While the acceptance of composting fertilizer from feces (humanure) ranges from 37% to 57%, urine fertilizer attracts 42%-71% respondents' interest.

Effect of interannual climate oscillations on rates of submarine groundwater discharge

Submarine groundwater discharge (SGD) is an important component of the coastal hydrologic cycle, affecting mixing and biogeochemistry in the nearshore environment. El Niño–Southern Oscillation (ENSO) influences rates of precipitation and groundwater recharge in many regions, including barrier islands of the southeastern U.S. coast; however, the influence of ENSO on SGD is poorly understood for this region. Here we investigate the role of ENSO in controlling recharge and SGD at interannual time scales, using modeling results for both real and generic barrier island environments. Results of our 57 year simulations show that the freshwater component of seasonally averaged SGD as well as groundwater discharge velocity, water table elevation, and submarine groundwater recharge are significantly correlated with ENSO for a real barrier island (Hatteras Island, North Carolina) and, under certain conditions, for generics. These correlations persist for lag times as great as 5 months during winter, creating anomalies of up to 35% between El Niño and La Niña conditions and suggesting that both hydrologic cycling and biogeochemical cycling in these systems are significantly influenced by ENSO.

Chemical and isotopic indicators of groundwater evolution in the basal sands of a buried bedrock valley in the midwestern United States: Implications for recharge, rock-water interactions, and mixing

Buried bedrock valley aquifers can be found throughout Canada and the northern United States where glacial deposits have filled in previously exposed bedrock valleys. The Mahomet bedrock valley is an east-west–trending buried valley in central Illinois containing basal Pleistocene sands and gravels making up the Mahomet aquifer and the contemporaneous Sankoty Mahomet aquifer, which are the major sources of freshwater for east-central Illinois. The hydrochemical characteristics of the Mahomet and Sankoty Mahomet aquifers change significantly across the buried bedrock valley. To determine the geochemical processes controlling the chemistry of the water, possible groundwater mixing, and the regions of major recharge, over 80 samples from the Mahomet aquifer, the Sankoty Mahomet aquifer, and shallower aquifers were analyzed for their chemical and isotopic composition, including δ 18 O, δD, δ 13 C, δ 34 S, 14 C, and 3 H. Four geochemical regions were observed across the aquifers. The central and eastern region of the Mahomet aquifer had dilute chemistry and medium 14 C activities, suggesting relatively recent recharge from the surface. The northeastern Mahomet aquifer region had variable sulfate and δ 34 S values, medium chloride concentrations, and low 14 C activity, suggesting mixing with bedrock groundwater along with sulfate reduction. The western Mahomet aquifer region had the highest chloride, dissolved organic carbon, and methane concentrations and showed a continuous decrease in 14 C activity, suggesting seepage from bedrock units, strong reducing conditions, and isolation from surficial recharge. Characteristics of the Sankoty Mahomet aquifer indicated rapid freshwater recharge and mixing with western Mahomet aquifer water. The δD and δ 18 O values indicated little to no Pleistocene water in the Mahomet bedrock valley aquifer system, suggesting an age limit of ca. 11,000 yr B.P. for most of the groundwater. The tritium data indicated modern recharge in some shallower aquifers, but little to none in the Mahomet aquifer and Sankoty Mahomet aquifer, except near a river where stacked sands may have created a hydrologic window to the Mahomet aquifer. It appears that most of the Mahomet aquifer is well protected from surficial contamination. The approach used in this study enabled us to better understand and identify the processes that control the groundwater chemistry within the buried Pliestocene aquifer in central Illinois; processes that may be prevalent in other buried bedrock valley aquifers distributed throughout much of North America.

Quantifying Potential Recharge in Mantled Sinkholes Using ERT

Potential recharge through thick soils in mantled sinkholes was quantified using differential electrical resistivity tomography (ERT). Conversion of time series two-dimensional (2D) ERT profiles into 2D volumetric water content profiles using a numerically optimized form of Archie's law allowed us to monitor temporal changes in water content in soil profiles up to 9 m in depth. Combining Penman-Monteith daily potential evapotranspiration (PET) and daily precipitation data with potential recharge calculations for three sinkhole transects indicates that potential recharge occurred only during brief intervals over the study period and ranged from 19% to 31% of cumulative precipitation. Spatial analysis of ERT-derived water content showed that infiltration occurred both on sinkhole flanks and in sinkhole bottoms. Results also demonstrate that mantled sinkholes can act as regions of both rapid and slow recharge. Rapid recharge is likely the result of flow through macropores (such as root casts and thin gravel layers), while slow recharge is the result of unsaturated flow through fine-grained sediments. In addition to developing a new method for quantifying potential recharge at the field scale in unsaturated conditions, we show that mantled sinkholes are an important component of storage in a karst system.