Impact Groundwater Quality Research Articles

Increased fermentation activity and persistent methanogenesis in a model aquifer system following source removal of an ethanol blend release

The increased probability of groundwater contamination by ethanol-blended fuel calls for improved understanding of how remediation efforts affect the fate and transport of constituents of concern, including the generation and fate of fermentation byproducts. A pilot-scale (8 m3) model aquifer was used to investigate changes in the concentrations of ethanol and its metabolites (methane and volatile fatty acids) after removal of the contamination source. Following the shut-off of a continuous release of a dissolved ethanol blend (10% v:v ethanol, 50 mg/L benzene, and 50 mg/L toluene), fermentation activity was surprisingly stimulated and the concentrations of ethanol metabolites increased. A microcosm experiment showed that this result was due to a decrease in the dissolved ethanol concentration below its toxicity threshold (∼2000 mg/L for this system). Methane generation (>1.5 mg/L of dissolved methane) persisted for more than 100 days after the disappearance of ethanol, despite clean air-saturated water flowing continuously through the tank at a relative high seepage velocity (0.76 m/day). Quantitative real-time PCR showed that functional genes associated with methane metabolism (mcrA for methanogenesis and pmoA for methanotrophy) also persisted in the aquifer material. Persistent methanogenesis was apparently due to the anaerobic degradation of soil-bound organic carbon (e.g., biomass grown on ethanol and other substrates). Overall, this study reflects the complex plume dynamics following source removal, and suggests that monitoring for increases in the concentration of ethanol metabolites that impact groundwater quality should be considered.

Spatial Distribution, Potential Sources, and Risk Assessment of Trace Metals of Groundwater in the North China Plain

ABSTRACTDiversion of water for irrigation from the Yellow River has impacted groundwater quality in the North China Plain (NCP). In this study, by using geochemical and isotope methods, groundwater origin, the spatial distribution of trace metals, pollution sources, and health risks were evaluated. Groundwater is recharged from surface water. The primary pollution components in surface water were B, Al, Se, Zn, Mn, and Ni—with concentrations exceeding standards by 100%, 84.4%, 64.4%, 31.1%, 20%, and 2.2%, respectively. In groundwater, exceeding standard rates for those elements were 100%, 100%, 61%, 25.7%, 39%, and 4.9%, respectively. The spatial distribution of polluted metals in surface water and groundwater was correlated, indicating an irrigation-influenced spatial distribution of trace metals in the groundwater. The trace metals were introduced via anthropogenic and geogenic activities. Zinc poses the most serious non-carcinogenic hazard for local residents. The ingestion pathway is much more likely to lead to zinc toxicity than the dermal absorption pathway. The carcinogenic Cd and Pb could result in an increased cancer risk for individuals exposed to the water. Non-carcinogenic hazard and carcinogenic risk attributable to groundwater is serious in the regions traversed by the rivers in the study area.

Impacts of Shallow Geothermal Energy Production on Redox Processes and Microbial Communities

Shallow geothermal systems are increasingly being used to store or harvest thermal energy for heating or cooling purposes. This technology causes temperature perturbations exceeding the natural variations in aquifers, which may impact groundwater quality. Here, we report the results of laboratory experiments on the effect of temperature variations (5-80 °C) on redox processes and associated microbial communities in anoxic unconsolidated subsurface sediments. Both hydrochemical and microbiological data showed that a temperature increase from 11 °C (in situ) to 25 °C caused a shift from iron-reducing to sulfate-reducing and methanogenic conditions. Bioenergetic calculations could explain this shift. A further temperature increase (>45 °C) resulted in the emergence of a thermophilic microbial community specialized in fermentation and sulfate reduction. Two distinct maxima in sulfate reduction rates, of similar orders of magnitude (5 × 10(-10) M s(-1)), were observed at 40 and 70 °C. Thermophilic sulfate reduction, however, had a higher activation energy (100-160 kJ mol(-1)) than mesophilic sulfate reduction (30-60 kJ mol(-1)), which might be due to a trade-off between enzyme stability and activity with thermostable enzymes being less efficient catalysts that require higher activation energies. These results reveal that while sulfate-reducing functionality can withstand a substantial temperature rise, other key biochemical processes appear more temperature sensitive.

Reconnaissance of Groundwater Quality Impact

Groundwater is the primary source of drinking water in areas with decentralized water supplies. Lately this resource is being increasingly contaminated due to both natural and anthropogenic activities. A literature review of groundwater quality impact due to natural and anthropogenic activities has been conducted. This review covers a broad spectrum of organic and inorganic pollutants that impacted groundwater quality across the world. The literature review includes extensive coverage of peer reviewed journal articles, reports, books and periodicals published in 2012. Particular emphasis is placed on published material that deals with groundwater quality impact but is not readily accessible. This review article provides general findings of groundwater monitoring activities that demonstrated actual as well as potential threat to groundwater quality. For the readers' convenience, this review article is presented in distinct sections highlighting groundwater quality impact from both organic and inorganic pollutants.

Biosolid and distillery effluent amendments to Irish short rotation coppiced willow plantations: Impacts on groundwater quality and soil

The impact of landspreading organic byproduct (OB) on the quality of groundwater (GW) and soil underlying short rotation coppiced willow plantations was assessed. Organic byproduct, namely biosolid (BS) and distillery effluent (DE), was spread on six plots (each with a plot area of 0.059ha) at OB treatment-rates of 100%, 50% and 0%. The OB treatment rate was defined by the maximum permissible soil-P load defined in Irish regulation. Groundwater was sampled each month and tested for pH, electrical conductivity (EC), NO3, PO43, total soluble phosphorous (TSP), K and six heavy metals (HMs) Cu, Cd, Cr, Pb, Ni, and Zn. Results were assessed in relation to Irish GW threshold values (GTVs), to assess overall quality using a “well bottom” approach. The background concentration of GW K and GW P (established in early 2008, prior to all OB spreading) were above permissible Irish statutory limits, in all cases. There was evidence of percolation of K and TSP to GW; K and PO43 concentrations breached GTVs across all plots. There was also evidence of percolation of Cu through soil profile, though Cu GW concentration did not breach GTV, and no risk to GW quality is necessarily implied. A potential (though small) risk of P- and K-percolation could impact GW quality in commercially scaled-up plantations; however, risk to GW quality from NO3, Cu, Cd, Cr, Pb, Ni, and Zn leaching in soil are minimal. Sites considered for energy crop should be assessed for soil conditions, and vulnerability to leaching, before OB amendment programs are initiated. The study was conducted over two years; the longer term impact of applications of BS and DE on soil organic-matter content, salinity and pH (and subsequent loss through soil profile) should be assessed. The implication of scaling up test-bed sized applications to larger commercial plantation scales needs to be considered.Capsule: Amendments of organic byproduct to short rotation coppice willow may lead to leaching of nutrients to groundwater in certain circumstances.

Monitoring CO2 Intrusion and Associated Geochemical Transformations in a Shallow Groundwater System Using Complex Electrical Methods

The risk of CO(2) leakage from a properly permitted deep geologic storage facility is expected to be very low. However, if leakage occurs it could potentially impact potable groundwater quality. Dissolved CO(2) in groundwater decreases pH, which can mobilize naturally occurring trace metals commonly contained in aquifer sediments. Observing such processes requires adequate monitoring strategies. Here, we use laboratory and field experiments to explore the sensitivity of time-lapse complex resistivity responses for remotely monitoring dissolved CO(2) distribution and geochemical transformations that may impact groundwater quality. Results show that electrical resistivity and phase responses correlate well with dissolved CO(2) injection processes. Specifically, resistivity initially decreases due to increase of bicarbonate and dissolved species. As pH continues to decrease, the resistivity rebounds toward initial conditions due to the transition of bicarbonate into nondissociated carbonic acid, which reduces the total concentration of dissociated species and thus the water conductivity. An electrical phase decrease is also observed, which is interpreted to be driven by the decrease of surface charge density as well as potential mineral dissolution and ion exchange. Both laboratory and field experiments demonstrate the potential of field complex resistivity method for remotely monitoring changes in groundwater quality due to CO(2) leakage.

Gravel pit lake ecosystems reduce nitrate and phosphate concentrations in the outflowing groundwater

Gravel excavation often bears conflicts with the use of drinking water as under-water-table mining can directly impact groundwater quality downstream of the open gravel pit lake due to exposure of the groundwater aquifer to the atmosphere and to human activities. To assess this potential impact of GPLs on groundwater, we assessed the mass balance for nitrate (NO3) and phosphate (PO4) and whole-ecosystem metabolism of five post-excavation GPLs in Austria. GPLs differed in both age and residence time of lake water. We found that GPLs significantly reduced the concentration of NO3 and PO4 as groundwater passes through the lake ecosystem, which in most cases acted as a net sink for these nutrients. Groundwater-derived nutrients enhanced both epilithic and pelagic net primary production in the GPLs, which ultimately leads to biomass accrual. Our data also suggest that this biomass accrual may induce, at least in part, clogging of the GPLs and their successive hydrodynamic isolation from the adjacent groundwater. Despite continuous biomass build-up and elevated concentrations of dissolved organic carbon (DOC) in the lake water compared to the inflowing groundwater, DOC export into the outflowing groundwater remained low. Our data suggest that GPLs could contribute to groundwater amelioration where agricultural land use increases nutrient concentrations in the groundwater given a proper management of these man-made ecosystems.

Environmentally Sustainable Nitrogen Fertilizer Management for ‘Tifton 85’ Hay Production in Sandy Soils

ABSTRACTNitrogen is a critical element for sustainable forage production but N surpluses in sandy soils could result in nitrate leaching and water contamination. Our objective was to identify a N application rate that optimizes yield and quality of ‘Tifton 85’ bermudagrass (Cynodon Rich. spp.) with minimal N leaching losses in a typical Florida sandy soil. Four N application rates (30, 50, 70, and 90 kg N ha−1 per harvest) and an unfertilized (0 N) control in a Tifton 85 bermudagrass hay field were evaluated. Suction lysimeters were installed in each plot at 30 and 100 cm below the soil surface to measure soil water nitrate concentration. Nitrogen application at the recommended rate of 90 kg N ha−1 per harvest produced the greatest total dry matter yield (DMY) (∼22 Mg ha−1 yr−1), but a modeled optimum DMY of ∼18 Mg ha−1 yr−1 occurred at 57 kg N ha−1 per harvest. Nitrogen application increased herbage nutritive value, but the difference was not significant above 30 kg N ha−1 per harvest. Treatments with N rates ≤ 50 kg N ha−1 per harvest had leachate nitrate concentration below the maximum contaminant limit (MCL) of 10 mg L−1. However, N rates ≥ 70 kg N ha−1 per harvest had leachate nitrate concentration that exceeded the MCL, suggesting a risk of impacting groundwater quality. The results demonstrate that although Tifton 85 bermudagrass is an efficient N remover, there is potential for N leaching in sandy soils. Therefore, site‐specific evaluation of appropriate N management is critical.

Agro-ecological nitrogen management in soils vulnerable to nitrate leaching: a case study in the Lower Suwannee Watershed

Environmental benefits associated with reduced rates of nitrogen (N) application, while maintaining economically optimum yields have economic and social benefits. Although N is an indispensable plant nutrient, residual soil N could leach out to contaminate groundwater and surface water resources, particularly in sandy soils. A 2-year field study was conducted in an established bermudagrass (Cynodon dactylon) pasture in the Lower Suwannee Watershed, Florida, to evaluate N application rates on forage yield, forage quality, and nitrate (NO3-N) leaching in rapidly permeable upland sandy soils. Four N application rates (30, 50, 70, and 90 kg N ha−1 harvest−1) corresponding to 0.33, 0.55, 0.77 and IX, respectively, of recommended N rate (90 kg N ha−1 harvest−1) for bermudagrass hay production in Florida were evaluated vis-a-vis an unfertilized (0 N) control. Suction cups were installed near the center of each plot at two depths (30 and 100 cm) to monitor NO3-N leaching. The grass was harvested at 28 days intervals to determine dry matter yield, N uptake, and herbage nutritive value. Nitrogen application at the recommended rate produced the greatest total dry matter yield (~18.4 Mg ha−1 year−1), but a modeled economically optimum N rate of ~57 kg N ha−1 harvest−1 (~60% of the recommended N rate) projected an average dry matter yield of ~17.3 Mg ha−1 year−1, which represents >90% of the observed maximum yield. Nitrogen application increased nutritive quality of the grass, but increases in N application rate above 30 kg N ha−1 did not result in significant increases in in vitro digestible organic matter concentration, and tissue crude protein was not significant above 50 kg N ha−1. Across the sampling period, treatments with N rates ≤50 kg N ha−1 harvest−1 had leachate NO3-N concentration below the maximum contaminant limit of <10 mg l−1. Conversely, applying N at rates ≥70 kg N ha−1 harvest−1 resulted in leachate N concentration that exceeded the maximum contaminant limit, and suggest high risk of impacting groundwater quality, if such rates are applied to soils with coarse (sand) textures. The study demonstrates that recommendation of a single N application rate may not be appropriate under all agro-climatic conditions and, thus, a site-specific evaluation of best N management strategy is critical.

Estrogen receptor mediated activity in bankside groundwater, with flood suspended particulate matter and floodplain soil – An approach combining tracer substance, bioassay and target analysis

Bankside groundwater is widely used as drinking water resource and, therefore, contamination has to be avoided. In the European Union groundwater protection is explicit subject to Water Framework Directive. While groundwater pollution may originate from different sources, this study investigated on impacts via flood events. Groundwater was sampled with increasing distance to the river Rhine near Karlsruhe, Germany. Samples were HPLC–MS–MS analyzed for the river contaminant carbamazepine to indicate river water infiltration, giving permanent presence in 250 m distance to the river (14–47 μg L −1). Following a flood event, concentrations of about 16–20 μg L −1 could also be detected in a distance of 750 m to the river. Furthermore, estrogenic activity as determined with the Yeast Estrogen Screen assay was determined to increase up to a 17β-ethinylestradiol equivalent concentration (E-EQ) = 2.9 ng L −1 near the river, while activity was initially measured following the flood with up to E-EQ = 2.6 ng L −1 in 750 m distance. Detections were delayed with increasing distance to the river indicating river water expansion into the aquifer. Flood suspended matter and floodplain soil were fractionated and analyzed for estrogenic activity in parallel giving up to 1.4 ng g −1 and up to 0.7 ng g −1, respectively. Target analysis focusing on known estrogenic active substances only explained <1% of measured activities. Nevertheless, river water infiltration was shown deep into bankside groundwater, thus, impacting groundwater quality. Therefore, flood events have to be in the focus when aiming for groundwater and drinking water protection as well as for implementation of Water Framework Directive.

Sources of elevated salinity in the Mississippi River Alluvial Aquifer, south-central Louisiana, USA

Abstract Salinization is a process impacting groundwater quality and availability across much of southern Louisiana, USA. However, a broad divergence of opinion exists regarding the causes of this elevated salinity: updip-migrating marine waters from the Gulf of Mexico, saline fluids migrating up fault planes, movement of water from salt domes, and/or remnant seawater from the last major marine transgression. The Mississippi River Alluvial Aquifer (MRAA) in south-central Louisiana is recharged by the Mississippi River, and there are discharge zones to the west and east. Recharge waters from the Mississippi River are fresh, but Cl− levels in the western portions of the aquifer are as high as 1000 mg/L. The aquifer is an important source of water for several municipalities and industries, but prior to this study the source(s) of the elevated salinity or whether the salinization can be remediated had not been determined. The low Br/Cl ratios in the groundwaters are consistent with a saline endmember produced by subsurface dissolution of salt domes, not a marine source. The H and O isotopic systematics of the aquifer waters indicate meteoric sources for the H2O, not marine waters or diagenetically-altered deep brines. The westward salinization of aquifer water represents a broad regional process, instead of contamination by point sources. Mapping of spatial variations in salinity has permitted the identification of specific salt domes whose subsurface dissolution is producing waters of elevated salinity in the aquifer. These include the Bayou Choctaw and St. Gabriel domes, and possibly the Bayou Blue dome. Salinization is a natural, on-going process, and the potential for remediation or control is slight, if not non-existent.

Long-term reclaimed water application effects on phosphorus leaching potential in rapid infiltration basins

Rapid infiltration basins (RIBs) are effective tools for wastewater treatment and groundwater recharge, but continuous application of wastewater can increase soil P concentrations and subsequently impact groundwater quality. The objectives of this study were to (1) investigate the effects of reclaimed water infiltration rate and "age" of RIBs on soil P concentrations at various depths, and (2) estimate the degree (percentage) of sorption equilibrium reached between effluent P and soil attained during reclaimed water application to different RIBs. The study was conducted in four contrasting cells of a RIB system with up to a 25 year history of secondary wastewater application. Soil samples were collected from 0 to 300 cm depth at 30 cm intervals and analyzed for water extractable phosphorus (WEP) and oxalate extractable P, Al, and Fe concentrations. Water extractable P and P saturation ratio (PSR) values were generally greater in the cells receiving reclaimed water compared to control soils, suggesting that reclaimed water P application can increase soil P concentrations and the risk of P movement to greater depths. Differences between treatment and control samples were more evident in cells with longer histories of reclaimed water application due to greater P loading. Data also indicated considerable spatial variability in WEP concentrations and PSR values, especially within cells from RIBs characterized by fast infiltration rates. This occurs because wastewater-P flows through surface soils much faster than the minimum time required for sorption equilibrium to occur. Studies should be conducted to investigate soil P saturation at deeper depths to assess possible groundwater contamination.

Reduction in Metolachlor and Degradate Concentrations in Shallow Groundwater through Cover Crop Use

Pesticide use during crop production has the potential to adversely impact groundwater quality. In southern Florida, climatic and hydrogeologic conditions and agronomic practices indicate that contamination risks are high. In the current study, dissipation of the widely used herbicide, metolachlor, and levels of the compound and selected degradates in shallow groundwater beneath six 0.15-ha plots in sweet corn (Zea mays) production were evaluated over a two-year period. During fallow periods (May to October), plots were either left bare or cover cropped with sunn hemp (Crotalaria juncea L.). Metolachlor was broadcast applied at label recommended rates prior to planting sweet corn each year. Groundwater monitoring wells hydraulically upgradient and downgradient, and within each plot were sampled biweekly. Results showed that metolachlor dissipation was rapid, as evidenced by the detection of relatively high levels of the metolachlor ethane sulfonic degradate (MESA) in groundwater beneath plots and a rapid metolachlor DT(50) (9-14 days) in a companion laboratory soil incubation. Other degradates detected included hydroxymetolachlor in soil and in groundwater metolachlor oxanilic acid (MOA) and a product tentatively identified as 2-chloro-N-(2-acetyl-6-methylphenyl-N-(2-methoxy-1-methylethyl) acetamide, a photo-oxidation product. Metolachlor and MESA levels, up to 16 and 2.4 times higher in groundwater beneath the noncover cropped plots when compared to those of the cover cropped plots, indicate that cover cropping results in more rapid dissipation and/or reduced leaching. The study demonstrated that integration of cover crops into agronomic systems in the region may yield water quality benefits by reducing herbicide inputs to groundwater.

Land clearing, climate variability, and water resources increase in semiarid southwest Niger: A review

The water table in southwestern Niger has been rising continuously for the past decades (4 m rise from 1963 to 2007), despite a ∼23% deficit in monsoonal rainfall from 1970 to 1998. This paradoxical phenomenon has been linked with a change in land use from natural savannah to millet crops that have expanded in area sixfold since 1950 and have caused soil crusting on slopes that has, in turn, enhanced Hortonian runoff. Runoff concentrates in closed ponds and then recharges the aquifer; therefore, higher runoff increases aquifer recharge. At the local scale (2 km2), a physically based, distributed hydrological model showed that land clearing increased runoff threefold, whereas the rainfall deficit decreased runoff by a factor of 2. At a larger scale (500 km2, 1950–1992 period), historical aerial photographs showed a 2.5‐fold increase in the density of gullies, in response to an 80% decrease in perennial vegetation. At the scale of the entire study area (5000 km2), analytical modeling of groundwater radioisotope data (3H and 14C) showed that the recharge rate prior to land clearing (1950s) was about 2 mm a−1; postclearing recharge, estimated from groundwater level fluctuations and constrained by subsurface geophysical surveys, was estimated to be 25 ± 7 mm a−1. This order of magnitude increase in groundwater fluxes has also impacted groundwater quality near ponds, as shown by a rising trend in groundwater nitrate concentrations of natural origin (75% of δ15N values in the range +4 to +8‰). In this well‐documented region of semiarid Africa, the indirect impacts of land use change on water quantity and quality are much greater than the direct influence of climate variability.

Aldicarb and carbofuran transport in a Hapludalf influenced by differential antecedent soil water content and irrigation delay

Pesticide use in agroecosystems can adversely impact groundwater quality via chemical leaching through soils. Few studies have investigated the effects of antecedent soil water content (SWC) and timing of initial irrigation (TII) after chemical application on pesticide transport and degradation. The objectives of this study were to investigate the effects of antecedent soil water content (wet vs dry) and timing of initial irrigation (0 h Delay vs 24 h Delay) on aldicarb [( EZ)-2-methyl-2-(methylthio)propionaldehyde O-methylcarbamoyloxime] and carbofuran [2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate] transport and degradation parameters at a field site with Menfro silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalf) soils. Aldicarb and carbofuran were applied to plots near field capacity (wet) or near permanent wilting point (dry). Half of the dry and wet plots received irrigation water immediately after chemical application and the remaining plots were irrigated after a 24 h Delay. The transport and degradation parameters were estimated using the method of moments. Statistical significance determined for SWC included averages across TII levels, and significance determined for TII included averages across SWC levels. For the dry treatment, aldicarb was detected 0.10 m deeper ( P < 0.01) on two of the four sampling dates and carbofuran was detected at least 0.10 m deeper ( P < 0.05) on all of the sampling dates compared to the wet treatment. Pore water velocity was found to be higher ( P < 0.10) in the dry vs wet treatments on three of four dates for aldicarb and two of four dates for carbofuran. Retardation coefficients for both pesticides showed similar evidence of reduced values for the dry vs wet treatments. These results indicate deeper pesticide movement in the initially dry treatment. For aldicarb and carbofuran, estimated values of the degradation rate were approximately 40–49% lower in the initially dry plots compared to the initially wet plots, respectively. When the initial irrigation was delayed for 24 h, irrespective of antecedent moisture conditions, a 30% reduction in aldicarb degradation occurred. This study illustrates the deeper transport of pesticides and their increased persistence when applied to initially dry soils.

Contamination of groundwater by outdoor highway deicing agent storage

This research quantifies the impact of outdoor highway deicing agent storage on groundwater quality. Data and theory realize the objective at a well characterized salt/premix storage facility on a glacial drumlin comprised of clayey sand till. Tritium and tritiogenic helium were observed in 17 monitoring wells in 2003, while chloride concentrations were measured in 43 monitoring wells from 1998 through 2004. The 3He/ 3H ratios confirm an analytical model of drumlin hydraulics (Ostendorf, D.W., DeGroot, D.J., Shelburne, W.M., and Mitchell, T.J., 2004. Hydraulic head in a clayey sand till over multiple timescales. Can. Geotech. J. 41, 89-105.), and support advective travel time estimates from the monitoring well screens back to the water table. An advective balance of recharge, precipitation, and surface runoff routes the water table Cl − concentrations inwards to the outdoor storage pile maintained at the site from the 1960s to the mid 1980s. Concentrations as high as 320 meq Cl −/L were observed in groundwater, although the deicing agent contamination had not yet reached the bottom of the drumlin in the study area. The travel time simulations yield a 200 meq Cl −/L water table isopleth in 1985 under the prior outdoor storage pile. The recharge concentration model matches the radial decrease of Cl − water table concentrations from the pile, and implies that 4400 kg of Cl − leached into the groundwater in 1985. This is about 0.3% of the deicing agent Cl − stored at the site each year. These results suggest that outdoor storage of highway deicing agents significantly impacted groundwater quality near the pile. The groundwater quality began to recover after source removal however: the leached Cl − flux dropped to 2,300 kg in 1992, more than 5 years after elimination of the outdoor storage pile.

Assessing Background Ground Water Chemistry beneath a New Unsewered Subdivision

Previous site-specific studies designed to assess the impacts of unsewered subdivisions on ground water quality have relied on upgradient monitoring wells or very limited background data to characterize conditions prior to development. In this study, an extensive monitoring program was designed to document ground water conditions prior to construction of a rural subdivision in south-central Wisconsin. Previous agricultural land use has impacted ground water quality; concentrations of chloride, nitrate-nitrogen, and atrazine ranged from below the level of detection to 296 mg/L, 36 mg/L, and 0.8 microg/L, respectively, and were highly variable from well to well and through time. Seasonal variations in recharge, surface topography, aquifer heterogeneities, surficial loading patterns, and well casing depth explain observed variations in ground water chemistry. This variability would not have been detected if background conditions were determined from only a few monitoring wells or inferred from wells located upgradient of the subdivision site. This project demonstrates the importance of characterizing both ground water quality and chemical variability prior to land-use change to detect any changes once homes are constructed.

Prediction of Groundwater Nitrate Contamination after Closure of an Unlined Sheep Feedlot

Nitrate contamination of groundwater by a sheep feedlot in Hawke9s Bay, New Zealand led to closure of the feedlot in 1998. However, knowledge of the processes controlling how long the contamination will remain and an analysis of whether the current land use (vineyard) will also impact groundwater quality are required to assess long-term groundwater quality issues after feedlot operations cease. To determine the fate of NO 3 following land use changes and the rate of reduction of contamination that may be expected under natural conditions following such changes, we compared the chemical concentrations of NO 3 –N, Cl, and alkalinity (HCO 3 ) in the groundwater and rivers from surveys conducted in 1994 and 1995 with sampling conducted in 2001. Profile sampling of total N and C of the 3 –N concentrations were under the feedlot (>140 g m −3 NO 3 –N) and down gradient. In 2001, 3 yr after the feedlot closed, the Cl concentrations had increased in down-gradient wells but remained similar to the 1994 survey in other wells. There has been a decrease in NO 3 –N concentrations in most wells, compared with the peak NO 3 –N concentrations recorded in the 1995 survey, but an increase compared with 1994. Alkalinity concentrations in wells located within the influence of the feedlot are approximately 150 g m −3 lower than in surrounding wells. This indicates that nitrification reactions are affecting the HCO 3 concentrations in the feedlot-influenced wells. However, the HCO 3 concentrations of some of these wells are increasing, indicating that nitrification could be slowing down and the aquifer is beginning to recover. SPASMO modeling indicates that NO 3 contamination from the site will continue for the next 3 to 5 yr. The impact of NO 3 leaching due to current land use practices is likely to be much less than the feedlot. The model predicts there will be an improvement in groundwater quality in the next 3 to 5 yr as NO 3 from the feedlot eventually leaves the vadose zone profile and mixes into the unconfined aquifer.

Prediction of Groundwater Nitrate Contamination after Closure of an Unlined Sheep Feedlot

Nitrate contamination of groundwater by a sheep feedlot in Hawke's Bay, New Zealand led to closure of the feedlot in 1998. However, knowledge of the processes controlling how long the contamination will remain and an analysis of whether the current land use (vineyard) will also impact groundwater quality are required to assess long‐term groundwater quality issues after feedlot operations cease. To determine the fate of NO3 following land use changes and the rate of reduction of contamination that may be expected under natural conditions following such changes, we compared the chemical concentrations of NO3–N, Cl, and alkalinity (HCO3) in the groundwater and rivers from surveys conducted in 1994 and 1995 with sampling conducted in 2001. Profile sampling of total N and C of the &lt;2‐mm size fraction in the vadose zone from two sites was used in a one‐dimensional soil–plant–atmosphere system model (SPASMO) program to predict N loading in future years and predict how long it would take to improve the groundwater quality. Groundwater sampling in 1994 and 1995 determined that the highest NO3–N concentrations were under the feedlot (&gt;140 g m−3 NO3–N) and down gradient. In 2001, 3 yr after the feedlot closed, the Cl concentrations had increased in down‐gradient wells but remained similar to the 1994 survey in other wells. There has been a decrease in NO3–N concentrations in most wells, compared with the peak NO3–N concentrations recorded in the 1995 survey, but an increase compared with 1994. Alkalinity concentrations in wells located within the influence of the feedlot are approximately 150 g m−3 lower than in surrounding wells. This indicates that nitrification reactions are affecting the HCO3 concentrations in the feedlot‐influenced wells. However, the HCO3 concentrations of some of these wells are increasing, indicating that nitrification could be slowing down and the aquifer is beginning to recover. SPASMO modeling indicates that NO3 contamination from the site will continue for the next 3 to 5 yr. The impact of NO3 leaching due to current land use practices is likely to be much less than the feedlot. The model predicts there will be an improvement in groundwater quality in the next 3 to 5 yr as NO3 from the feedlot eventually leaves the vadose zone profile and mixes into the unconfined aquifer.

Isotopic and geochemical evolution of ground and surface waters in a karst dominated geological setting: a case study from Belize, Central America

Analysis of stable isotopes and major ions in groundwater and surface waters in Belize, Central America was carried out to identify processes that may affect drinking water quality. Belize has a subtropical rainforest/savannah climate with a varied landscape composed predominantly of carbonate rocks and clastic sediments. Stable oxygen (δ 18O) and hydrogen (δD) isotope ratios for surface and groundwater have a similar range and show high d-excess (10–40.8‰). The high d-excess in water samples suggest secondary continental vapor flux mixing with incoming vapor from the Caribbean Sea. Model calculations indicate that moisture derived from continental evaporation contributes 13% to overhead vapor load. In surface and groundwater, concentrations of dissolved inorganic carbon (DIC) ranged from 5.4 to 112.9 mg C/l and δ 13C DIC ranged from −7.4 to −17.4‰. SO 4 2, Ca 2+ and Mg 2+ in the water samples ranged from 2–163, 2–6593 and 2–90 mg/l, respectively. The DIC and δ 13C DIC indicate both open and closed system carbonate evolution. Combined δ 13C DIC and Ca 2+, Mg 2+, and SO 4 2− suggest additional groundwater evolution by gypsum dissolution and calcite precipitation. The high SO 4 2−content of some water samples indicates regional geologic control on water quality. Similarity in the range of δ 18O, δD and δ 13C DIC for surface waters and groundwater used for drinking water supply is probably due to high hydraulic conductivities of the karstic aquifers. The results of this study indicate rapid recharge of groundwater aquifers, groundwater influence on surface water chemistry and the potential of surface water to impact groundwater quality and vise versa.