Local Recharge Research Articles

Effects of variations in hydrogeological parameters on water-table mounding in sandy loam and loamy sand soils beneath stormwater infiltration basins

The two-dimensional variably-saturated numerical model HYDRUS-2D, previously calibrated to recharge events from an infiltration basin, was used to predict water-table mounding under hypothetical basin design scenarios, and the primary factors that affect water-table mounding were evaluated. Infiltration basins are often utilized in urban environments to recharge stormwater to the aquifer. As a result of localized recharge beneath these basins, mound formation may reduce the thickness of the unsaturated zone available to filter pollutants and may reduce the infiltration rate of the basin. Understanding the effects of various physical factors on water-table mound formation is important for infiltration basin siting. For sandy loam and loamy sand subsurface materials, mound heights increased as the thickness of both the unsaturated and saturated zones decreased. Mound heights increased as the initial soil moisture, basin size and ponding depth increased. A thin sedimentation layer on the basin floor delayed mound formation, but only slightly decreased the maximum mound height. This analysis could be used in future selection of infiltration basin locations; however, the analysis is limited to conditions that represent only a select range of basin design conditions and parameters typical of a glacial till environment in Wisconsin, USA.

Understanding wetland sub-surface hydrology using geologic and isotopic signatures

Abstract. This paper attempts to utilize hydrogeology and isotope composition of groundwater to understand the present hydrological processes prevalent in a freshwater wetland, source of wetland groundwater, surface water/groundwater interaction and mixing of groundwater of various depth zones in the aquifer. This study considers East Calcutta Wetlands (ECW) – a freshwater peri-urban inland wetland ecosystem located at the lower part of the deltaic alluvial plain of South Bengal Basin and east of Kolkata city. This wetland is well known over the world for its resource recovery systems, developed by local people through ages, using wastewater of the city. Geological investigations reveal that the sub-surface geology is completely blanketed by the Quaternary sediments comprising a succession of silty clay, sand of various grades and sand mixed with occasional gravels and thin intercalations of silty clay. At few places the top silty clay layer is absent due to scouring action of past channels. In these areas sand is present throughout the geological column and the areas are vulnerable to groundwater pollution. Groundwater mainly flows from east to west and is being over-extracted to the tune of 65×103 m3/day. δ18O and δD values of shallow and deep groundwater are similar indicating resemblance in hydrostratigraphy and climate of the recharge areas. Groundwater originates mainly from monsoonal rain with some evaporation prior to or during infiltration and partly from bottom of ponds, canals and infiltration of groundwater withdrawn for irrigation. Relatively high tritium content of the shallow groundwater indicates local recharge, while the deep groundwater with very low tritium is recharged mainly from distant areas. At places the deep aquifer has relatively high tritium, indicating mixing of groundwater of shallow and deep aquifers. Metals such as copper, lead, arsenic, cadmium, aluminium, nickel and chromium are also present in groundwater of various depths. Therefore, aquifers of wetland and surrounding urban areas which are heavily dependent on groundwater are vulnerable to pollution. In the area south of ECW isotope data indicates no interaction between shallow and deep aquifer and hence this area may be a better location to treat sewage water than within ECW. To reduce the threat of pollution in ECW's aquifer, surface water-groundwater interaction should be minimized by regulating tubewell operation time, introducing treated surface water supply system and artificial recharging of the aquifer.

Extremely rapid and localized recharge to a fractured rock aquifer

The factors that govern recharge to fractured rock aquifers with overlying soil are poorly understood. The objective of this study is to determine if recharge to a fractured crystalline aquifer in a humid climate is governed by heterogeneous fracture networks and/or overlying soil characteristics. A 10 km2 study area is instrumented with a network of 15 bedrock wells completed to observe conditions in the shallow bedrock aquifer. Hydrogeological characterization, detailed observation of the 2007 snowmelt freshet and numerical simulations are used. Snowmelt is a valuable natural tracer because it has a distinct thermal and isotopic signature and is applied simultaneously and evenly to the entire ground surface. Results indicate that soil thickness and bedrock transmissivity are both highly heterogeneous at the site scale but that much of the study area is underlain by silty sand with a thickness of >1 m. Cold, δ2H depleted snowmelt locally recharged the bedrock aquifer to depths of at least 20 m within two days. Since recharge is typically quantified and discussed as an annual flux, the snowmelt event is extremely rapid. However, hydraulic, isotopic, and thermal data also indicate that most wells did not rapidly recharge. Numerical simulations indicate that soil thickness and hydraulic conductivity are critical parameters that controls whether the underlying bedrock aquifer rapidly recharges. The vertical fracture aperture, number of vertical fractures, the amount of snowmelt, water-table gradient, depth to water table and pressure–saturation relations are all less significant controls on the rate and amount of rapid recharge. Both field results and numerical simulations indicate that the rapid recharge process is localized to areas where the soil is very thin, such as the fringes of outcrops. Outcrops are exposed in <0.1% of the study area indicating that the process of rapid recharge is also extremely localized. This poorly documented hydrogeologic phenomenon has broad implications for groundwater management and protection, as well as our understanding of recharge processes in fractured rock aquifers.

Modeling the Usefulness of Spatial Correlation Analysis on Karst Systems

Cross-correlation analyses on field data collected in karst aquifer systems can be used to develop a conceptual understanding of the aquifer. This includes the use of many data sets from the same aquifer to develop an understanding of how properties vary spatially. We focus on a method for characterizing the distribution of recharge, which is becoming increasingly important in regions where urban development encroaches on these important sources of water. Spatially varying precipitation data and cross-correlation analysis provide a means of spatially characterizing recharge locations on a karst aquifer. Our work expands on the numerical experiments conducted by Padilla and Pulido-Bosch (1995) using the numerical ground water model MODFLOW to introduce spatially varying parameters. The numerical experiments include conduit-controlled, matrix-controlled, and mixed karst systems with more than one precipitation time series input. The results show that spatially varying parameters can be inferred based on the cross-correlation of precipitation data and spring discharge. Simulations were completed using aquifer parameters derived from studies of the Barton Springs segment of the Edwards Aquifer. The simulations indicate that spatial variability within an aquifer can be inferred using cross-correlation analysis. A field study using these methods is summarized for Barton Springs near Austin, Texas.

On the use of meteorological data to assess the evaporation from a bare soil

Summary The main goal of this study was to evaluate different methods for the estimate of the evaporation from a bare soil. The starting point was the experimental data set of a field survey carried out for a period of 16 months in the area of Parco Lambro, in Milano (Italy), where meteorological and soil measurements were collected to estimate the local recharge to the phreatic aquifer. The evaporation from the nearly bare soil was assessed with two different approaches. A first, indirect approach was based on the evaluation of the actual evaporation as a fraction of the potential evaporation, which was computed with both Penman’s and Hamon’s equations. As a second approach, the Bowen ratio method, that directly estimates the actual evaporation from atmospheric data, was applied. Attention was mainly focused to the effects of the configuration of the measurement instruments (particularly the measurement heights of air temperature and humidity) on the estimate of the potential and actual evaporation and to the numerical stability of each method, in relation to the measurement uncertainty. In particular, numerical problems related to the intrinsic instability of the Bowen ratio equation were deeply investigated and an original method, that identifies the data leading to unphysical results on the basis of the error propagation, was proposed. The obtained results were discussed in order to evaluate the suitability of each approach, taking into account several aspects. Penman’s and Hamon’s methods resulted to be quite stable with respect to the measurement uncertainty and the potential evaporation estimates were not affected by the configuration of the field instruments. Conversely, the Bowen ratio estimates were affected by large uncertainties due to both the intrinsic numerical instability of the method and to the sensitivity to the heights of measurement; nevertheless, the application of a suitable criterion for the data rejection allowed for a significant reduction of uncertainties on the estimates of the actual evaporation.

Interbasin flow in the Great Basin with special reference to the southern Funeral Mountains and the source of Furnace Creek springs, Death Valley, California, U.S.

summary Interbasin flow in the Great Basin has been established by scientific studies during the past century. While not occurring uniformly between all basins, its occurrence is common and is a function of the hydraulic gradient between basins and hydraulic conductivity of the intervening rocks. The Furnace Creek springs in Death Valley, California are an example of large volume springs that are widely accepted as being the discharge points of regional interbasin flow. The flow path has been interpreted historically to be through consolidated Paleozoic carbonate rocks in the southern Funeral Mountains. This work reviews the preponderance of evidence supporting the concept of interbasin flow in the Death Valley region and the Great Basin and addresses the conceptual model of pluvial and recent recharge [Nelson, S.T., Anderson, K., Mayo, A.L., 2004. Testing the interbasin flow hypothesis at Death Valley, California. EOS 85, 349; Anderson, K., Nelson, S., Mayo, A., Tingey, D., 2006. Interbasin flow revisited: the contribution of local recharge to high-discharge springs, Death Valley, California. Journal of Hydrology 323, 276–302] as the source of the Furnace Creek springs. We find that there is insufficient modern recharge and insufficient storage potential and permeability within the basin-fill units in the Furnace Creek basin for these to serve as a local aquifer. Further, the lack of high sulfate content in the spring waters argues against significant flow through basin-fill sediments and instead suggests flow through underlying consolidated carbonate rocks. The maximum temperature of the spring discharge appears to require deep circulation through consolidated rocks; the Tertiary basin fill is of insufficient thickness

Oxygen, hydrogen, and helium isotopes for investigating groundwater systems of the Cape Verde Islands, West Africa

Stable isotopes (δ18O, δ2H), tritium (3H), and helium isotopes (3He, 4He) were used for evaluating groundwater recharge sources, flow paths, and residence times of three watersheds in the Cape Verde Islands (West Africa). Stable isotopes indicate the predominance of high-elevation precipitation that undergoes little evaporation prior to groundwater recharge. In contrast to other active oceanic hotspots, environmental tracers show that deep geothermal circulation does not strongly affect groundwater. Low tritium concentrations at seven groundwater sites indicate groundwater residence times of more than 50 years. Higher tritium values at other sites suggest some recent recharge. High 4He and 3He/4He ratios precluded 3H/3He dating at six sites. These high 3He/4He ratios (R/Ra values of up to 8.3) are consistent with reported mantle derived helium of oceanic island basalts in Cape Verde and provided end-member constraints for improved dating at seven other locations. Tritium and 3H/3He dating shows that Sao Nicolau Island’s Ribeira Faja Basin has groundwater residence times of more than 50 years, whereas Fogo Island’s Mosteiros Basin and Santo Antao Island’s Ribeira Paul Basin contain a mixture of young and old groundwater. Young ages at selected sites within these two basins indicate local recharge and potential groundwater susceptibility to surface contamination and/or salt-water intrusion.

Alluvial aquifer recharge enhanced by a natural dam: feasibility assessment based on multidisciplinary characterization (Khuzestan, Southwest Iran)

In the Kushkak Valley (Khuzestan, Southwest Iran) an anticlinal structure has partially impounded an ephemeral stream. This natural impounded area has been chosen for an artificial recharge site due to its current rate of recharge, capability to store water and favorable situation for the construction of man-made barriers to stream flow. The aquifer to be recharged is the Kushkak unconfined aquifer which consists of medium to coarse-grained alluvial deposits that overlie consolidated conglomerate rock. In this semi-arid area with infrequent relatively heavy falls of rain, alluvial aquifer recharge can be an important process that sustains shallow, over-exploited groundwater bodies. In this investigation a multidisciplinary approach including: hydrometerological studies, and a detailed hydrogeochemical survey, have been carried out. Other essential prerequisite parameters for the scheme were also taken into account to determine the suitability of this location for groundwater artificial recharge. The assessment has brought out that (1) the proposed reservoir will conserve a major part of the water being lost, (2) annual runoff of about 0.27 MCM can be injected into the aquifer through recharge from impounded water, (3) hydrochemical data from surface water and from the Kushkak aquifer water demonstrates that dilution and change in compositional trend in the groundwater proximal to the impounded alluvial bed areas would be expected based on the infiltration capacity of this site, and (4) cost–benefit ratio of the project is 1:2 and it is assumed to recover the investment within six years.

Groundwater recharge/discharge in semi-arid regions interpreted from isotope and chloride concentrations in north White Nile Rift, Sudan

Deuterium, oxygen-18 and chloride were analyzed for 84 samples from deep and shallow wells, precipitation and the river White Nile to investigate groundwater recharge/discharge relations in the semi-arid central Sudan. Spatial and vertical variation in isotopic signature and chloride concentration in the groundwater show similar patterns and indicate local recharge and evaporative discharge. Progressive decrease in isotopic composition along the regional groundwater flow path demonstrates aquifer continuity down the NW–SE recharge-discharge path. Isotope-heavy recharged water progressively mixes with lighter older groundwater formed during cooler and humid conditions in the late Pleistocene. However, evaporative fractionation in the flow path’s final reach in the southeast re-enriches the isotopic composition and suggests evaporative loss of groundwater as the plausible discharge mechanism. Chloride concentration increases down the gradient from the recharge area and reaches its peak in the discharge zones indicating: lack of recharge from direct infiltration down the gradient, evaporation and prolonged rock/water interaction. Head differences and increased isotopic concentration in the vicinity of the White Nile suggest recharge from the river from subsurface flow. Reduced chloride content and relatively heavier isotopic composition in the deep groundwater beneath the wadi of Khor Abu Habil indicate recharge from the streambed into the deep aquifer.

Impact of local recharge on arsenic concentrations in shallow aquifers inferred from the electromagnetic conductivity of soils in Araihazar, Bangladesh

The high‐degree of spatial variability of dissolved As levels in shallow aquifers of the Bengal Basin has been well documented but the underlying mechanisms remain poorly understood. We compare here As concentrations measured in groundwater pumped from 4700 wells &lt;22 m (75 ft) deep across a 25 km2 area of Bangladesh with variations in the nature of surface soils inferred from 18,500 measurements of frequency domain electromagnetic induction. A set of 14 hand auger cores recovered from the same area indicate that a combination of grain size and the conductivity of soil water dominate the electromagnetic signal. The relationship between pairs of individual EM conductivity and dissolved As measurements within a distance of 50 m is significant but highly scattered (r2 = 0.12; n = 614). Concentrations of As tend to be lower in shallow aquifers underlying sandy soils and higher below finer‐grained and high conductivity soils. Variations in EM conductivity account for nearly half the variance of the rate of increase of As concentration with depth, however, when the data are averaged over a distance of 50 m (r2 = 0.50; n = 145). The association is interpreted as an indication that groundwater recharge through permeable sandy soils prevents As concentrations from rising in shallow reducing groundwater.

Modeling of water flow and heat transport in the vadose zone: Numerical demonstration of variability of local groundwater recharge in response to monsoon rainfall in Korea

The rainfall of Korea in the summer monsoon period occupies more than 50% of the annual precipitation in most areas, and thus groundwater recharge to shallow aquifers is dominantly controlled by the amount and the pattern of monsoon precipitation. This paper presents two numerical models that demonstrate linear relationships between precipitation and recharge. First, a simple heat transport model employing a lumped parameter approach is presented for estimating two lumped parameters related to water flux and thermal diffusivity in the vadose zone. The model determines the parameters by a simple optimization process that minimizes the root-mean-square error between simulated and measured temperatures. The model is applied to 22-year time series data of soil temperatures measured at a synoptic station of Korea. The impact of monsoon precipitation on the thermal regime is clearly reflected in the simulated results by illustrating a linear relationship between precipitation and the water flux in the vadose zone. Secondly, an infiltration model is presented for analyzing variability of precipitation recharge in relation to the monsoon rainfall. The model simulates the unsaturated flow from time series data of precipitation and pan evaporation, assuming immediate removal of surface ponding, a linear relationship between the evaporation rate and the soil water content, and a static water table. Numerical simulations were performed for three soil textural groups by using 20-year meteorological data. The results demonstrate that the annual recharge is linearly proportional to the annual precipitation with varying degrees of the correlation coefficient depending on soil types. Sensitivity analyses show that the uncertainties in evaporation-related model parameters significantly affect the model results with controlling tradeoff between recharge and evaporation estimates.

Our future resources, groundwater

for site investigation, and how projects can benefit from early and ongoing peer reviews. The second case study explains difficulties in working on unique problems without an effective conceptual hydrogeologic model, the need to always be concerned about the quality of chemical data, and the need to be alert to behaviors that are beyond typical experience. Sudicky et al. (2008) present large-scale surface-subsurface hydrodynamic model that was applied to a 17 km 2 subcatchment of the Laurel Creek Watershed in Southern Ontario, Canada. A steady-state condition was achieved in the model by calibrating the subsurface flow field to 16 observation wells where long-term hydraulic head data were available. The model was then subjected to several hundred hours of rainfall data and the resulting discharge hydrographs were compared with the measured hydrographs. The calculated subsurface hydraulic head distribution and surficial rainfall-runoff responses, respectively, were shown to agree moderately well with those observed in the system during this period. These results suggested that the model is capable of reproducing surface and subsurface hydrodynamic processes at the subcatchment scale although the results could be better through improved parameterization of the subcatchment and the manner in which the model simulates evapotranspiration processes. Koo and Kim (2008) perform numerical simulations to demonstrate variability in local groundwater recharge in response to monsoon rainfall in Korea. Monsoon in the summer provides more than 50% of the total precipitation in Korea, thus important for recharge to shallow aquifers. This paper specifically examines impact of monsoon precipitation on the water flux and thermal diffusivity in the vadose zone and variability of precipitation recharge in relation to the monsoon rainfall by applying heat transport and infiltration models to 22-year time series soil temperature and meteorological data. The results demonstrate that the annual recharge is linearly proportional to the annual precipitation with varying degrees of the correlation coefficients depending on soil types. Nordstrom (2008) reports results of recent investigation to determine the pre-mining groundwater quality at a molybdenum mine in northern New Mexico, USA. The pre-mining groundwater quality was inferred by studying a

Demonstrating fractal scaling of baseflow residence time distributions using a fully‐coupled groundwater and land surface model

The influence of the vadose zone, land surface processes, and macrodispersion on the shape and scaling behavior of residence time distributions of baseflow is studied using a fully coupled watershed model in conjunction with a Lagrangian, particle‐tracking approach. Numerical experiments are used to simulate groundwater flow paths from recharge locations along the hillslope to the streambed. These experiments are designed to isolate the influences of topography, vadose zone/land surface processes, and macrodispersion on subsurface transport of tagged parcels of water. The results of these simulations agree with previous observations that such distributions exhibit a power law form and fractal behavior, which can be identified from plots of the residence time distribution and the power spectra. It is shown that vadose zone/land surface processes significantly affect both the residence time distributions and their spectra.

Evaluation of groundwater in a three-aquifer system in Ramtha area, Jordan: recharge mechanisms, hydraulic relationship and geochemical evolution

The groundwater wells in the Ramtha region of Jordan are tapping three aquifers: the upper, intermediate and deep aquifers. The upper aquifer groundwater is tritiated and its stable isotopic composition varies over a wide range. This signifies short residence times and local recharge from an elevation around 600 m above sea level. The groundwater of the upper aquifer has an elevated level of \( {\text{NO}}^{ - }_{{\text{3}}} \), which is attributed to anthropogenic sources. The intermediate and deep aquifers are untritiated and have long residence times. The stable isotope results signify a recharge elevation for the intermediate aquifer higher than that for the upper aquifer. Stable isotopes in groundwater from both aquifers clustered along the eastern meteoric water line and demonstrate association with the dominant climate of Jordan. The groundwater of the intermediate aquifer is classified as Ca2+- \( {\text{HCO}}^{ - }_{{\text{3}}} \), which reflects circulation through a carbonate aquifer. There is evidence that leakage from the upper aquifer has influenced the isotopic and chemical makeup of the groundwater in an intermediate aquifer well. The groundwater of the deep aquifer has the highest temperature in the basin and its isotopic composition is much more depleted than both the upper and intermediate aquifers and plots on the global meteoric water line.

Hydrological and geochemical processes controlling variations in Na +–Mg 2+–Cl −–SO 42− groundwater brines, south-eastern Australia

Evaporative discharge of regional groundwater from more than 50 playa lakes at the 400 km 2 Raak Plain groundwater discharge complex forms a very large reservoir of hypersaline Na +–Mg 2+–Cl −–SO 4 2− type subsurface brine, but very little salt precipitates. The main processes controlling the formation and evolution of brine compositions are: (a) evapo-concentration of the regional groundwater, (b) mixing between the high – and low – TDS end-members and (c) subsurface gypsum precipitation. Uniformity in these processes across the entire groundwater discharge complex, coupled with the chemical similarity between the regional groundwater and local recharge sources, has led to remarkably similar brine chemistries and evaporite compositions across Raak Plain. The similarity between the source waters and the ultimate brine compositions indicates that solutes are generally conserved within the system. The greatest variation in groundwater chemistry at Raak Plain was found to occur within the playa lakes, particularly within the top 1–2 m of the subsurface brine body, rather than amongst the different lakes. This small-scale variability is a result of 1) advective and diffusive mixing between the hypersaline shallow (< 3 m below ground surface) brines and the less saline underlying groundwaters, 2) the transient precipitation and re-dissolution of surface salt efflorescences, and 3) differential diffusion rates of the various solutes over long time scales (> 20 kyr).

Groundwater recharge and discharge processes in the Jakarta groundwater basin, Indonesia

Proper management of groundwater resources requires knowledge of the processes of recharge and discharge associated with a groundwater basin. Such processes have been identified in the Jakarta groundwater basin, Indonesia using a theory that describes the simultaneous transfer of heat and fluid in a porous medium. Temperature-depth profiles in monitoring wells are used to determine the geothermal gradient. To examine the rules of groundwater flow in the distortion of the isotherms in this area, several methods are compared. Subsurface temperature distribution is strongly affected by heat advection due to groundwater flow. Under natural flow conditions, the recharge area is assumed to occur in the hills and uplands, which are located on the periphery of the Jakarta basin, and the discharge area is located in the central and northern part of the Jakarta groundwater basin. A transition area, which could act as local recharge and discharge areas, occupies the middle of the lowland. Subsurface temperatures show good correlation with the groundwater flow conditions, and the data yield important information on the location of recharge and discharge areas.

Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh

BackgroundThe origin of the spatial variability of dissolved As concentrations in shallow aquifers of the Bengal Basin remains poorly understood. To address this, we compare here transects of simultaneously-collected groundwater and aquifer solids perpendicular to the banks of the Hooghly River in Chakdaha, India, and the Old Brahmaputra River in Araihazar, Bangladesh.ResultsVariations in surface geomorphology mapped by electromagnetic conductivity indicate that permeable sandy soils are associated with underlying aquifers that are moderately reducing to a depth of 10–30 m, as indicated by acid-leachable Fe(II)/Fe ratios <0.6 in the solid phase and concentrations of dissolved sulfate >5 mg L-1. More reducing aquifers are typically capped with finer-grained soils. The patterns suggest that vertical recharge through permeable soils is associated with a flux of oxidants on the banks of the Hooghly River and, further inland, in both Chakdaha and Araihazar. Moderately reducing conditions maintained by local recharge are generally associated with low As concentrations in Araihazar, but not systematically so in Chakdaha. Unlike Araihazar, there is also little correspondence in Chakdaha between dissolved As concentrations in groundwater and the P-extractable As content of aquifer particles, averaging 191 ± 122 ug As/L, 1.1 ± 1.5 mg As kg-1 (n = 43) and 108 ± 31 ug As/L, 3.1 ± 6.5 mg As kg-1 (n = 60), respectively. We tentatively attribute these differences to a combination of younger floodplain sediments, and therefore possibly more than one mechanism of As release, as well as less reducing conditions in Chakdaha compared to Araihazar.ConclusionSystematic dating of groundwater and sediment, combined with detailed mapping of the composition of aquifer solids and groundwater, will be needed to identify the various mechanisms underlying the complex distribution of As in aquifers of the Bengal Basin.

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.

Influences of the unsaturated, saturated, and riparian zones on the transport of nitrate near the Merced River, California, USA

Transport and transformation of nitrate was evaluated along a 1-km groundwater transect from an almond orchard to the Merced River, California, USA, within an irrigated agricultural setting. As indicated by measurements of pore-water nitrate and modeling using the root zone water quality model, about 63% of the applied nitrogen was transported through a 6.5-m unsaturated zone. Transport times from recharge locations to the edge of a riparian zone ranged from approximately 6 months to greater than 100 years. This allowed for partial denitrification in horizons having mildly reducing conditions, and essentially no denitrification in horizons with oxidizing conditions. Transport times across a 50–100-m-wide riparian zone of less than a year to over 6 years and more strongly reducing conditions resulted in greater rates of denitrification. Isotopic measurements and concentrations of excess N2 in water were indicative of denitrification with the highest rates below the Merced River. Discharge of water and nitrate into the river was dependent on gradients driven by irrigation or river stage. The results suggest that the assimilative capacity for nitrate of the groundwater system, and particularly the riverbed, is limiting the nitrate load to the Merced River in the study area.

Snow impact on groundwater recharge in Table Mountain Group aquifer systems with a case study of the Kommissiekraal River catchment South Africa

Snowmelt in the mountainous areas of the Table Mountain Group (TMG) in South Africa is believed to be one of sources of groundwater recharge in some winter seasons. This paper provides a scientific assessment of snow impact on groundwater recharge in Table Mountain Group Aquifer Systems for the first time. Snowfall periodically occurs on the highest mountain ranges of about 1 000 to 1 200 m above mean sea level (a.m.s.l) in the TMG area. Snow over the mountainous catchments is often observed on the gentle side of the slope, which is substantially affected by wind and vegetation. Based on climatic analysis, recharge processes and Landsat 7 Enhanced Thematic Mapper (ETM) images, the recharge areas influenced by snowmelt in the TMG are identified as those catchments that are located above 1 000 m a.m.s.l. Physical processes within the snowpack are very complex involving mass and energy balances as well as heat and mass transport. Snowmelt rate was calculated using a variable degree-day melt factor determined as a function of snowpack density and vegetation cover. The hourly snowmelt rates estimated with different new snow density models. Groundwater recharge from snowmelt is affected by snowmelt mechanisms and local recharge conditions. The recharge rate is constrained by characteristics of the fractures rather than snowmelt rate. Recharge is also discounted due to prevailing interflow occurring in favourite geomorphological locations. This hypothesis is confirmed by an infiltration experiment in which up to 13.6% of the infiltrating water can recharge the aquifer. The estimated snowmelt recharge in the Kommissiekraal River catchment in the Villiersdorp of South Africa ranges from 14.1 &#61548;·s-1 to 15.0 &#61548;·s-1. Water SA Vol.31 (3) 2005: pp.275-282