Mean Recharge Rate Research Articles

Recharge Assessment in Greek Karst Systems: Methodological Considerations and Implications

The recharge of karst aquifers is significant for the effective management of groundwater resources, and its estimation should be tailored to accommodate their specific hydrogeological characteristics. This study provides a two-step methodological approach for the determination of karst aquifer recharge. Initially, mean annual recharge rates were estimated in the karst system of Ziria (Southern Greece) utilizing the APLIS and modified APLIS methods in order to decipher which was the most suitable version for recharge assessments. The results indicated similar mean recharge rate values at 42.7% and 41.4%, respectively, but significant differences in the spatial distribution. The modified methodology emerged as a more accurate and realistic approach, mainly due to the incorporation of permeability assessments. The final phase of the methodological approach involved the application of modified APLIS in two additional karst hydrosystems, Planitero and Xiromero, while a quantitative cross-comparison of the recharge rates was obtained for a deeper understanding of the factors controlling the groundwater recharge process. In Ziria, recharge rates exhibit a relatively uniform distribution throughout the area, with a median value of 46.7%. Conversely, in Planitero, High recharge rates (60–80%) occupy 56.8% of the surface, while in Xiromero, Moderate recharge rates (40–60%) dominate, representing 53.4% of the land coverage. These variations underscore the spatial heterogeneity of recharge within the karst systems, highlighting the importance of considering local geological and hydrological conditions in its assessments. The methodological approach of this study is flexible and can be adapted to different karst sites for the determination of recharge regimes, contributing to the alleviation of the groundwater depletion issue.

Calculation of the SPI, SPEI, and GRDI Indices for Historical Climatic Data from Doñana National Park: Forecasting Climatic Series (2030–2059) Using Two Climatic Scenarios RCP 4.5 and RCP 8.5 by IPCC

In this study, we utilized three different indices to assess drought conditions in the Doñana National Park (DNP) located in southern Spain. These indices included the Standardized Precipitation Index (SPI), which is based on precipitation statistics, the Standardized Precipitation Evapotranspiration Index (SPEI), which incorporates both precipitation and air temperature data, and the Groundwater Recharge Drought Index (GRDI), a newly developed index specifically designed to evaluate groundwater drought. The analysis covered the time period from 1985 to 2015, and future projections were made for the years 2030–2060 under different climate scenarios (RCP 4.5 and RCP 8.5). Our findings revealed a significant decrease in total precipitation of approximately 13–14% compared to historical records (1985–2015). Moreover, severely to extremely wet periods exhibited a reduction ranging from 25% to 38%. A key contribution of this study is the application of the GRDI index, which allowed us to assess groundwater recharge rates. We observed a decline in the simulated mean recharge rates during the 21st century when compared to the historical period spanning from 1950 to 2009. This decline can be attributed to increased evapotranspiration. The results of this research provide valuable insights for the Spanish water resources administration. The observed reductions in precipitation and groundwater recharge rates emphasize the need for appropriate mitigation measures. The findings will aid the administration in formulating an integrated water resources management strategy in the Doñana National Park and its surrounding basin. By understanding the projected changes in drought conditions, the administration can make informed decisions to ensure sustainable water resource management in the region.

Using 222Rn temporal and spatial distributions to estimate the groundwater discharge rate and associated nutrient fluxes into high salinity lakes in Badain Jaran Desert, Northwest China.

Groundwater is the main source of water and salt recharge for to lakes in arid regions. Quantifying the groundwater discharge and its nutrient input is important in the evolution of lake environments in the Badain Jaran Desert (BJD), Northwest China. In this study, ten BJD lakes were sampled for 222Rn in April and September 2021, and the 222Rn mass balance model was used to quantify the groundwater discharge rates and derived nutrient fluxes to these lakes. The results showed that the 222Rn activity and the groundwater recharge rate of lake water both present a positively correlated with lake water depth. The hot points of high 222Rn activity in the lake water were consistent with the locations of groundwater discharge areas. According to the 222Rn temporal and spatial distributions, the mean groundwater recharge rates for the ten lakes in April and September were 5.4 ± 0.6 and 7.7 ± 1 mm/d, respectively, and the annual mean groundwater discharge rates varied between 1.1 ± 0.2 and 14.6 ± 1.6 mm/d, with a mean of 7 ± 0.9 mm/d. Given that all the perennial lakes in the BJD have the same groundwater recharge rate as the mean recharge rate of the ten studied lakes, the annual mean groundwater recharge amount received by the lakes in the entire BJD is (5.6 ± 0.7) × 107 m3/a. According to the groundwater recharge amount, the inputs of dissolved inorganic nitrogen, dissolved inorganic phosphorus, dissolved inorganic silicon, total nitrogen, and total phosphorus to the BJD lakes from groundwater were (4.7 ± 0.6) × 105, (3.8 ± 0.5) × 104, (7.9 ± 1) × 105, (7.2 ± 0.9) × 105, (2.5 ± 0.3) × 104 kg/a, respectively. This study provides a reference for quantifying of groundwater discharge rates into salt lakes in other arid regions.

Groundwater recharge mechanisms on the Loess Plateau of China: New evidence for the significance of village ponds

Numerous studies have been conducted to understand the mechanism of groundwater recharge for water resources management. But the roles of wide-spread village ponds on the Chinese Loess Plateau on groundwater recharge remains uncertain. Here, to evaluate the groundwater recharge under village ponds, we measured environmental tracers (2H, 18O, Cl-) in rainwater, groundwater, and pore waters of the deep unsaturated zone under three land-cover types (cropland, a village pond, and two apple orchards with stand ages of 10 and 27 years). The results show that shallow groundwater is recharged by both diffusive recharge and focused recharge. Specifically, diffusive recharge occurs beneath farmlands, grasslands, orchards and similar land-cover types, but diminishes in old orchards due to deep root water uptake. The diffusive recharge beneath crop field had an infiltration velocity of 10.3–12.5 cm yr−1; the resulting recharge rate was 30–33 mm yr−1, accounting for 5–6% of the annual precipitation. However, focused recharge occurs mainly beneath village ponds with low-lying terrain, and had an infiltration velocity of 26 cm d−1 during the rainy season; the resulting mean recharge rate was 134 mm yr−1 in the catchment of the pond, accounting for 23.1% of the annual precipitation. Given that the water table is generally more than 30 m below the soil surface, it takes hundreds of years for rainwater to reach the water table via the diffusive recharge but only days to months via the focused recharge. Diffusive recharge contributed 55–68% to the total recharge while focused recharge contributed 32–45%. However, the conversion of croplands to apple orchards and the decrease in the number of village ponds are occurring, which diminishes diffusive recharge, and reduces focused recharge, respectively. These findings are critical to understanding hydrological processes and protecting groundwater resources. We should preserve, reclaim, and construct village ponds to ensure groundwater recharge, in addition to optimizing areal ratio of large farmland to apple orchards in the region for the sustainable utilization of groundwater resources.

Determination of vadose zone and saturated zone nitrate lag times using long-term groundwater monitoring data and statistical machine learning

Abstract. In this study, we explored the use of statistical machine learning and long-term groundwater nitrate monitoring data to estimate vadose zone and saturated zone lag times in an irrigated alluvial agricultural setting. Unlike most previous statistical machine learning studies that sought to predict groundwater nitrate concentrations within aquifers, the focus of this study was to leverage available groundwater nitrate concentrations and other environmental variables to determine mean regional vertical velocities (transport rates) of water and solutes in the vadose zone and saturated zone (3.50 and 3.75 m yr−1, respectively). The statistical machine learning results are consistent with two primary recharge processes in this western Nebraska aquifer, namely (1) diffuse recharge from irrigation and precipitation across the landscape and (2) focused recharge from leaking irrigation conveyance canals. The vadose zone mean velocity yielded a mean recharge rate (0.46 m yr−1) consistent with previous estimates from groundwater age dating in shallow wells (0.38 m yr−1). The saturated zone mean velocity yielded a recharge rate (1.31 m yr−1) that was more consistent with focused recharge from leaky irrigation canals, as indicated by previous results of groundwater age dating in intermediate-depth wells (1.22 m yr−1). Collectively, the statistical machine learning model results are consistent with previous observations of relatively high water fluxes and short transit times for water and nitrate in the primarily oxic aquifer. Partial dependence plots from the model indicate a sharp threshold in which high groundwater nitrate concentrations are mostly associated with total travel times of 7 years or less, possibly reflecting some combination of recent management practices and a tendency for nitrate concentrations to be higher in diffuse infiltration recharge than in canal leakage water. Limitations to the machine learning approach include the non-uniqueness of different transport rate combinations when comparing model performance and highlight the need to corroborate statistical model results with a robust conceptual model and complementary information such as groundwater age.

A review of radioactive isotopes and other residence time tracers in understanding groundwater recharge: Possibilities, challenges, and limitations

Documenting the location and magnitude of groundwater recharge is critical for understanding groundwater flow systems. Radioactive tracers, notably 14C, 3H, 36Cl, and the noble gases, together with other tracers whose concentrations vary over time, such as the chlorofluorocarbons or sulfur hexafluoride, are commonly used to estimate recharge rates. This review discusses some of the advantages and problems of using these tracers to estimate recharge rates. The suite of tracers allows recharge to be estimated over timescales ranging from a few years to several hundred thousand years, which allows both the long-term and modern behaviour of groundwater systems to be documented. All tracers record mean residence times and mean recharge rates rather than a specific age and date of recharge. The timescale over which recharge rates are averaged increases with the mean residence time. This is an advantage in providing representative recharge rates but presents a problem in comparing recharge rates derived from these tracers with those from other techniques, such as water table fluctuations or lysimeters. In addition to issues relating to the sampling and interpretation of specific tracers, macroscopic dispersion and mixing in groundwater flow systems limit how precisely groundwater residence times and recharge rates may be estimated. Additionally, many recharge studies have utilised existing infrastructure that may not be ideal for this purpose (e.g., wells with long screens that sample groundwater several kilometres from the recharge area). Ideal recharge studies would collect sufficient information to optimise the use of specific tracers and minimise the problems of mixing and dispersion.

Climate variability and vadose zone controls on damping of transient recharge

Increasing demand on groundwater resources motivates understanding of the controls on recharge dynamics so model predictions under current and future climate may improve. Here we address questions about the nonlinear behavior of flux variability in the vadose zone that may explain previously reported teleconnections between global-scale climate variability and fluctuations in groundwater levels. We use hundreds of HYDRUS-1D simulations in a sensitivity analysis approach to evaluate the damping depth of transient recharge over a range of periodic boundary conditions and vadose zone geometries and hydraulic parameters that are representative of aquifer systems of the conterminous United States (U.S). Although the models were parameterized based on U.S. aquifers, findings from this study are applicable elsewhere that have mean recharge rates between 3.65 and 730mmyr−1. We find that mean infiltration flux, period of time varying infiltration, and hydraulic conductivity are statistically significant predictors of damping depth. The resulting framework explains why some periodic infiltration fluxes associated with climate variability dampen with depth in the vadose zone, resulting in steady-state recharge, while other periodic surface fluxes do not dampen with depth, resulting in transient recharge. We find that transient recharge in response to the climate variability patterns could be detected at the depths of water levels in most U.S. aquifers. Our findings indicate that the damping behavior of transient infiltration fluxes is linear across soil layers for a range of texture combinations. The implications are that relatively simple, homogeneous models of the vadose zone may provide reasonable estimates of the damping depth of climate-varying transient recharge in some complex, layered vadose zone profiles.

δ18O and δ2H characteristics of rainwater, groundwater and springs in a mountainous region of Ghana: implication with respect to groundwater recharge and circulation

In this study, the characteristics of δ2H and δ18O in rainwater, groundwater and spring water have been used to understand the groundwater recharge dynamics in the Amedzofe area (one of the highest human habitation in Ghana). The local Meteoric Water Line (LMWL) using all the rainfall events gave an equation with the best fit line of δ2H = 7.32 δ18O + 8.20 (n = 113; r 2 = 0.94). Heavier rainfall events greater than 10, 20 and 30 mm further orient the slope closer to the Global Meteoric Water Line (GMWL). The data showed that the isotopic compositions of the rainwater in the area are mainly influenced by the rainfall amount with temperature having minimal effect. All the spring water and most of the groundwater plot around the GMWL, implying they are of meteoric origin. The Chloride Mass Balance (CMB) and δ18O shift methods suggest annual mean recharge rate of 160.43 and 125.79 mm/year representing 10.70 and 8.39% of annual rainfall, respectively. The estimated mean recharge rate for the spring was 317.51 mm/year and 300.68 mm/year representing 26.46 and 25.06% of annual rainfall for CMB and δ18O shift methods, respectively. Rainfall stable isotopes from Kpando and Amedzofe gave an altitude effect of 0.17/100 m and altitude of recharge of the springs from +724.00 to +788.61 m amsl. The stable isotopes data identified two distinct moisture sources, isotopically enriched local moisture (evaporated surface water bodies and evapo-transpiration water vapour flux from vegetation) and isotopically depleted moisture source from the Atlantic Ocean. The results further showed that significant groundwater recharge in the study area takes place in the months of March, June and August.

Estimating groundwater recharge based on mass balance evaluation of unsaturated zone in a coastal catchment characterized by tropical rainforest weather conditions

Estimation of recharge is necessary for evaluating water resources in the north Kelantan catchment of Malaysia due to rapid development and consequently over-drafting of available groundwater. In order to determine the amount of recharge, a study of natural salts (i.e., chloride and sulfate) was conducted to estimate groundwater recharge rates. Studies of the chloride mass balance (CMB) and sulfate mass balance (SMB) were carried out in the unsaturated zone under rainforest weather conditions with a relatively shallow groundwater table. Ten soil profiles in different parts of the catchment representing a range of soil textures were selected for the study. The CMB results showed that the mean recharge rate is 527.87 mm/year. The highest and lowest calculated recharge rates in the study area were 8 and 31 % of local rainfall. Times required to accumulate chloride/sulfate beneath the natural ecosystem highly depend on profile texture and the CMB age is maximized at KP94 and KT121 sites to be 15.01 and 14.15 years, respectively. The SMB method produced lower recharge rates than those based on chloride, yielding an average of 364.60 mm/year, with values ranging between 7 and 26 % of local rainfall and the SMB age is 21.01 years at KP94 site. Mobilization decreases with fine texture and low permeability and increases with coarse texture and high permeability. In this way, low permeability of surface geology not only controls the recharge rate but also causes high buffering in holding salts. Nevertheless, higher sulfate concentration in profile may be due to different dissolution or sorption processes which are required to be studied thoroughly. Based on two types of scenarios representing natural water movement in unsaturated zone using HYDRUS-1D, recharge rates were simulated to be 419 mm in profile with overlain fine texture soil and 636 mm in profile characterizing with single coarse texture for a total time of 213 days. Our study suggests the feasibility of using the mass balance methods in tropical rainforest weather conditions. The information may improve the accuracy of prediction of possible groundwater pollution and groundwater management in the future.

Groundwater Recharge Rates in Isolated and Riverine Wetlands: Influencing Factors

Isolated wetlands and riverine wetlands have been shown to have similar groundwater hydrology despite their difference in topography and surface water hydrology. The current study aimed to address the impact of topography and surface water hydrology on groundwater hydrologic behavior by comparing the groundwater recharge rates of several isolated and riverine wetlands in the Coastal Plain of South Carolina. Study sites contained an isolated wetland, a riverine wetland, and an upland that bisected the two wetland types. Shallow water tables and sandy soils, allowed a rapid response to precipitation to be clearly visible. Soil characteristics, water table fluctuations, and precipitation data from January 2012-September 2012 were evaluated and from that data mean recharge rates were calculated using an adapted version of the water table fluctuation method. During the study period, it was observed that the frequency of precipitation (storm events) and saturated zone soil type were more impactful on water table movement than topography, surface soil type, and surface water hydrology. One significant finding of this research is that the isolated wetlands in this study did, in fact, recharge groundwater, which implies that their presence increases the opportunity for groundwater replenishment.

Nonlinear Inversion of an Unconfined Aquifer: Simultaneous Estimation of Heterogeneous Hydraulic Conductivities, Recharge Rates, and Boundary Conditions

A new inverse method is developed to simultaneously estimate heterogeneous hydraulic conductivities, source/sink rates, and unknown boundary conditions for steady-state flow in an unconfined aquifer. Unlike objective function-based techniques, the new method does not optimize any data-model misfits. Instead, its formulation is developed by honoring physical flow principles as well as observation data at sampled locations. Under the Dupuit–Forchheimer assumption of negligible vertical flow, accuracy and stability of the new method are demonstrated using synthetic heterogeneous aquifer problems with increasingly complex flow: (1) aquifer domains without source/sink effects; (2) aquifer domains with a point sink (a pumping well operating under a constant discharge rate); (3) aquifer domains with constant or spatially variable recharge; (4) aquifer domains with constant or spatially variable recharge undergoing single-well pumping. For all problems, inversion yields stable solutions under increasing head measurement errors (up to $$\pm $$ 10 % of the total head variation in a problem), although accuracy of the estimated parameters degrades with the increasing errors. The inverse method is successfully tested on problems with high hydraulic conductivity contrasts—up to 10,000 times between the maximum and minimum values. In inverting several heterogeneous problems, if the aquifer is assumed homogeneous with a constant recharge rate, physically meaningful parameter estimates (i.e., equivalent conductivities and mean recharge rates) can be determined. Alternatively, if the inverse parameterization contains spurious parameters, inversion can identify such parameters, while the simultaneous estimation of non-spurious parameters is not affected. The method obviates the well-known issues associated with model “structure errors”, when inverse parameterization either simplifies or complexifies the true parameter field.

Nonstationary stochastic analysis of flow in a heterogeneous unconfined aquifer subject to spatially-random periodic recharge

The problem of two-dimensional groundwater flow in a heterogeneous unconfined aquifer under periodic random forcing in time is analyzed from a stochastic point of view. It is assumed that the periodic random forcing, namely random recharge, can be represented by a sinusoidal function in time. Analytical solutions are developed through the nonstationary spectral approach in conjunction with the principle of superposition. The results, namely the variances of hydraulic head and specific discharge, are expressed in terms of statistical properties of hydraulic parameters and recharge field. It can be concluded from the analytical results that the mean recharge rate and integral scale of spatially random recharge fluctuations play essential roles in enhancing the variability of hydraulic head and specific discharges.

Performance Evaluation of Injection Wells for Groundwater Recharge

The study was conducted to determine the performance of injection wells for groundwater recharge using surface runoff with respect to its groundwater recharging potential and impact on groundwater quality. The mean recharge rates of21.2 m3day' and 8.0 m3d-1were observed for injection wells installed below water table (saturated zone) and above water table (vadose zone), respectively. The results indicated that recharge rates increased with increase in recharge head and the incremental increase (increase in injection rate per meter increase in recharge head) was 20 m3d-'m-1 and 8.66 m3d-1m-1 for the well installed in the saturated and the vadose zone, respectively. It was also found that injection rate was more for the well constructed in the saturated zone than the injection well installed in the vadose zone since soil formation of the former had higher proportion of coarse sand mixed with gravel. The laboratory analysis of pre and post recharge groundwater samples has shown considerable change in chemical and biological qualities. The EC and pH values decreased for both the cases. The Na and K concentrations in groundwater increased after recharge at vadose zone recharge site. Similarly, Most Probable Number (MPN /1 00 m!) and Chemical Oxygen Demand (COD) for saturated and vadose zone recharge sites were found to be 26,65 and 110 and 175 ppm respectively whereas for pre recharge water samples corresponding values were 0 and 10.

Physically Based Simulation of Potential Effects of Carbon Dioxide–Altered Climates on Groundwater Recharge

Increasing concentrations of atmospheric CO2 will alter regional rainfall and evapotranspiration regimes that drive groundwater recharge. Improved methods of simulation and analysis are needed for assessing the potential sensitivities of soil–water–vegetation systems to climate change. This study demonstrates methods for generating climates and simulating soil–water and vegetation dynamics in response to current and double CO2 climate sequences. Climate change scenarios came from dynamic equilibrium (constant CO2) runs of a general circulation model (GCM). Based on historical climate and GCM output, a stochastic point weather generator produced realizations of the cross‐correlated daily climate variables. A numerical model of infiltration, variably saturated flow, and evapotranspiration produced temporal distributions of groundwater recharge rates for various soil–vegetation environments. Climate change scenarios were simulated for two climatic zones in Australia: subtropical (North Stradbroke Island, Queensland) and Mediterranean (Gnangara, Swan Coastal Plain, Western Australia) having summer‐ and winter‐dominated rainfall regimes, respectively. In these simulations, groundwater recharge values were affected by the dynamic growth and senescence of vegetation, as changes in temperature and rainfall regimes affected growth rates and leaf areas. The temperature regime dominated the hydrologic response in the Mediterranean climate, and the rainfall frequency–duration regime dominated in the subtropical climate. For the simulated Mediterranean climate change (14% rainfall increase), changes in mean recharge values ranged from −34% to +119%, while subtropical climate change (37% rainfall increase) caused increases from 74 to > 500%. Changes in mean recharge rate, interannual variability, and temporal persistence were related to the soil and vegetation characteristics. The model was useful for quantifying complex, nonlinear responses to climate change that require further exploration.

Groundwater and lake evolution in the Badain Jaran Desert ecosystem, Inner Mongolia

The amount and timing of aquifer recharge and the evolution of lakes and groundwater in the south-eastern Badain Jaran desert of Inner Mongolia, with high megadunes, has been investigated using stable isotopes and hydrochemistry. Unsaturated zone moisture profiles down to 22 m have recorded recharge over 1185 years. Small but finite amounts of recharge are recorded with mean recharge rates of 0.95-1.33 mm year−1, determined using a chloride mass balance technique. The unsaturated profile also acts as a unique archive of hydrological and climate change. Before 1300, it was relatively dry but distinct wet periods may be recognised during 1340-1450, 1500-1610 and 1710-1820. Since the mid 1800s, the climate shows a trend towards greater aridity. The interdune lakes are generally fresh but locally, hypersaline lakes are found in juxtaposition. This implies that in general, the lakes have low residence times and flow back into the dune system, but sedimentary obstruction locally prevents outflow and extreme evaporation occurs. The stable isotope records show that the lakes are fed by palaeowaters which on the basis of other proxy data must predate the Last Glacial Maximum. Their recharge source is problematic but most likely this derives from a diminishing water table extending some 30 m south to the Yabulai Mountains.

INCORPORATION OF CHLORIDE MASS BALANCE TO IMPROVE ESTIMATES OF EVAPOTRANSPIRATION, RECHARGE, AND RUNOFF

A method is presented whereby measured soil water chloride concentrations and longterm precipitation and airtemperature profiles are interpreted to provide temporal estimates of evapotranspiration, recharge, and runoff. Applying thechloride mass balance technique to soil water chloride profiles improves the boundary conditions associated with thelongterm mean recharge rate. Temporal estimates of evapotranspiration and runoff are calculated from precipitation andair temperature data. Next, these estimates and measured precipitation are used as inputs in an unsaturated groundwatermodel to estimate temporal recharge, which is subsequently compared to the longterm mean recharge rate calculated fromthe chloride profiles. Finally, the evapotranspiration and runoff components of the model are scaled such that the modeledrecharge rate is similar to the longterm mean recharge rate.<br><br>This method improves the chloride mass balance method, which up to now only provides longterm mean recharge.Additionally, the method allows initial estimates of evapotranspiration and runoff to be scaled such that the resultingestimates of evapotranspiration and runoff are consistent with both chloride mass balance and water mass balance. Althoughdirect methods to measure evapotranspiration, recharge, and runoff are attractive, they are not always reasonable due to theexpense of collecting data over long time periods. In contrast, this method obtains its required input from basic meteorologicaldata and soil cores collected at a single point in time.

Spatial and temporal variability of ground water recharge in central Australia: a tracer approach.

Two environmental tracer methods are applied to the Ti-Tree Basin in central Australia to shed light on the importance of recharge from floodouts of ephemeral rivers in this arid environment. Ground water carbon-14 concentrations from boreholes are used to estimate the average recharge rate over the interval between where the ground water sample first entered the saturated zone and the bore. Environmental chloride concentrations in ground water samples provide estimates of the recharge rate at the exact point in the landscape where the sample entered the saturated zone. The results of the two tracer approaches indicate that recharge rates around one of the rivers and an extensive floodplain are generally higher than rates of diffuse recharge that occurs in areas of lower topographic relief. Ground water 2H/1H and 18O/16O compositions are all depleted in the heavier isotopes (delta2H = -67 per thousand to -50 per thousand; delta18O = -9.2 per thousand to -5.7%o) compared with the long-term, amount-weighted mean isotopic composition of rainfall in the area (delta2H = -33.8 per thousand; delta18O = -6.3 per thousand). This indicates that recharge throughout the basin occurs only after intense rainfall events of at least 150 to 200 mm/month. Finally, a recharge map is developed to highlight the spatial extent of the two recharge mechanisms. Floodout recharge to the freshest ground water (TDS <1,000 mg/L) is approximately 1.9 mm/year compared with a mean recharge rate of approximately 0.2 mm/year to the remainder of the basin. These findings have important implications for management of the ground water resource.

Ground water/surface water interactions in Lake Naivasha, Kenya, using delta 18O, delta D, and 3H/3He age-dating.

We have analyzed a series of ground water samples from the Lake Naivasha region, Kenya, for their helium isotopic composition. Lake Naivasha is unique among the East Africa Rift Valley lakes in that it is fresh. It has long been thought that the low salinity of this lake is due, in part, to rapid water loss from the lake into the local ground water system. Our results show that the Olkaria geothermal waters, south of the lake, are devoid of tritium and, thus, are more than 50 years old. An important implication of these results is that even if Olkaria geothermal reservoir water originated from Lake Naivasha, it has been underground for a long time, (> 50 years) and is not derived from present-day Lake Naivasha water. This flow time is of the same order of magnitude as conservative major solutes, such as chloride, as determined through residence time calculations. On the north side of Lake Naivasha, deep wells (91 m) have water approximately 20 years old. Water from these wells has stable isotopic values resembling those of nearby rivers, and high-elevation eastern Rift water. This indicates that this water recharges from rains from high eastern Rift Valley escarpments. Many of the shallow wells on the south side of the lake have 3H/3He ages between four and 17 years. The young ages and the delta 18 O-enriched signature of the water from these wells indicate that they are recharged by a mixture of water from the lake, Rift flanks, and water from deep pumping wells that is recharged during irrigation. Water mixing ratio calculations using delta 18O and delta D isotopes show that about 50% to 70% of the southern ground water system is derived from the lake, while the Olkaria geothermal reservoir water shows that 40% to 50% of this water is originally lake water. Calculated mean recharge rates range from 0.10 to 1.59 m/yr with a mean of 0.52 +/- 0.40 m/yr. Estimated horizontal velocity from 3H/3He age dating between Lake Naivasha and a well about 3 km to the south is 75 m/yr, giving average horizontal hydraulic conductivity of 6 m/day.

Estimation of rainfall inputs and direct recharge to the deep unsaturated zone of southern Niger using the chloride profile method

An estimate of direct groundwater recharge below a region of natural woodland (tiger bush) has been made in south-west Niger using the solute profile technique. Data has been collected from a 77 m deep well drug within the study area covered by HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment), an international large-scale energy, water and carbon balance experiment carried out during the summer of 1992. During well construction samples were taken from the unsaturated zone at the following intervals: every 25 cm from 0–10 m, every 50 cm from 10–62.5 m, then every metre to the bottom of the well. Pore water was extracted from each sample either by centrifugation or elutriation and analysed for chloride; moisture contents of samples were obtained gravimetrically. These data have been used to produce depth profiles of pore water chloride concentration and moisture content throughout the unsaturated zone. From these profiles it has been possible to derive an estimate of historic direct recharge at the site. The chloride concentration of rainfall, which is required to make the estimate, was determined from the analysis of 123 rainfall samples collected from five EPSAT (vers une Estimation des Précipitation par Satellite au sahel) rain gauges in 1992. A mean recharge rate of 13 mm year −1 (range 10–19 mm) is estimated for the upper 70m of the profile, with a total residence time of 790 years (range 520–990 years). This is considered to be a representative estimate of the magnitude of direct recharge taking place below tiger bush areas.

Sources of recharge at Abu Delaig, Sudan

Various components of the water cycle in and around Abu Delaig, a small town in a semiarid region of Sudan, have been investigated with geochemical and isotopic techniques to determine the sources of groundwater recharge. Rain samples (total deposition during storms) contain significant concentrations of dissolved solids for a continental site (mean 4.6 mgl −1 for 1982–1985). The heaviest rains important for recharge tend to have the lowest chlorinities, and are also the most negative isotopically. Wadi floods generally have lower chlorinity than the rainfall indicating the proportionally lower amounts of dust in the more intense rainfall events. Shallow ground waters at Abu Delaig have relatively evolved compositions (higher Mg/Ca ratios) compared with rainfall and wadi floods, yet they also contain tritium and retain isotopic signatures similar to the local rainfall. They also have a distinctive chemistry compared with the deeper ground waters in the region and recharge from the former to the latter is considered to be insignificant. Ground waters in the unsaturated zone are saline and have isotopic compositions highly enriched in heavy isotopes compared with rainfall, indicating strong evaporation. The chloride balance indicates that mean recharge rates are 0.2–1.3 mm year −1 through the interfluve areas. It is concluded that the only significant replenishable resources at Abu Delaig are from wadi recharge during floods and that direct regional recharge is insignificant. Fluctuations in the water table, however, lead to solutes from the lower unsaturated zone contributing to the chemistry of the shallow ground waters. Elsewhere it is possible that wadi recharge may be a possible route for deeper replenishment, but this needs to be demonstrated by dedicated experiments.