Chloride Mass Balance Research Articles

Groundwater recharge mechanism in an integrated tableland of the Loess Plateau, northern China: insights from environmental tracers

Assessing groundwater recharge characteristics (recharge rate, history, mechanisms (piston and preferential flow)) and groundwater age in arid and semi-arid environments remains a difficult but important research frontier. Such assessments are particularly important when the unsaturated zone (UZ) is thick and the recharge rate is limited. This study combined evaluations of the thick UZ with those of the saturated zone and used multiple tracers, such as Cl, NO3, Br, 2H, 18O, 13C, 3H and 14C, to study groundwater recharge characteristics in an integrated loess tableland in the Loess Plateau, China, where precipitation infiltration is the only recharge source for shallow groundwater. The results indicate that diffuse recharge beneath crops, as the main land use of the study area, is 55–71 mm yr−1 based on the chloride mass balance of soil profiles. The length of time required for annual precipitation to reach the water table is 160–400 yrs. The groundwater is all pre-modern water and paleowater, with corrected 14C age ranging from 136 to 23,412 yrs. Most of the water that eventually becomes recharge originally infiltrated in July–September. The Cl and NO3 contents in the upper UZ are considerably higher than those in the deep UZ and shallow groundwater because of recent human activities. The shallow groundwater has not been in hydraulic equilibrium with present near-surface boundary conditions. The homogeneous material of the UZ and relatively old groundwater age imply that piston flow is the dominant recharge mechanism for the shallow groundwater in the tableland.

Estimation of groundwater recharge in sedimentary rock aquifer systems in the Oti basin of Gushiegu District, Northern Ghana

Sustainable development and the management of groundwater resources for optimal socio-economic development constitutes one of the most effective strategies for mitigating the effects of climate change in rural areas where poverty is a critical cause of environmental damage. This research assessed groundwater recharge and its spatial and temporal variations in Gushiegu District in the Northern Region of Ghana, where groundwater is the main source of water supply for most uses. Isotopic data of precipitation and groundwater were used to infer the origin of groundwater and the possible relationship between groundwater and surface water in the partially metamorphosed sedimentary aquifer system in the study area. Though the data do not significantly establish strong relation between groundwater and surface water, the study suggests that groundwater in the area is of meteoric origin. However, the data also indicate significant enrichment of the heavy isotopes (18O and 2H) in groundwater relative to rainwater in the area. The Chloride Mass Balance (CMB) and Water Table Fluctuations (WTF) techniques were used to quantitatively estimate the groundwater recharge in the area. The results suggest groundwater recharge in a range of 13.9 mm/y - 218 mm/y, with an average of 89 mm/yr, representing about 1.4%–21.8% (average 8.9%) of the annual precipitation in the area. There is no clearly defined trend in the temporal variations of groundwater recharge in the area, but the spatial variations are discussed in relation to the underlying lithologies. The results suggest that the fraction of precipitation that reaches the saturated zone as groundwater recharge is largely controlled by the vertical hydraulic conductivities of the material of the unsaturated zone. The vertical hydraulic conductivity coupled with humidity variations in the area modulates the vertical infiltration and percolation of precipitation.

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.

Comparison of groundwater recharge estimation techniques in an alluvial aquifer system with an intermittent/ephemeral stream (Queensland, Australia)

This study demonstrates the importance of the conceptual hydrogeological model for the estimation of groundwater recharge rates in an alluvial system interconnected with an ephemeral or intermittent stream in south-east Queensland, Australia. The losing/gaining condition of these streams is typically subject to temporal and spatial variability, and knowledge of these hydrological processes is critical for the interpretation of recharge estimates. Recharge rate estimates of 76–182 mm/year were determined using the water budget method. The water budget method provides useful broad approximations of recharge and discharge fluxes. The chloride mass balance (CMB) method and the tritium method were used on 17 and 13 sites respectively, yielding recharge rates of 1–43 mm/year (CMB) and 4–553 mm/year (tritium method). However, the conceptual hydrogeological model confirms that the results from the CMB method at some sites are not applicable in this setting because of overland flow and channel leakage. The tritium method was appropriate here and could be applied to other alluvial systems, provided that channel leakage and diffuse infiltration of rainfall can be accurately estimated. The water-table fluctuation (WTF) method was also applied to data from 16 bores; recharge estimates ranged from 0 to 721 mm/year. The WTF method was not suitable where bank storage processes occurred.

Recharge mechanism and processes controlling groundwater chemistry in a Precambrian sedimentary terrain: a case study from Central India

Unplanned and unsustainable extraction has created stress on groundwater resources in many parts of India. The stress symptoms are more pronounced in hard rock areas, where the aquifer potentials are comparatively low. Present research targeted an area of 3000 km2 in the interstream region between the Kharun and Seonath rivers, which is one such region in Central India. In spite of being a water-stressed area, so far, little is understood about processes of recharge, amount of recharge and processes controlling chemical quality, which are key inputs for groundwater management in the area. This study presents an appraisal of recharge mechanism, recharge rate and prevailing water–rock interactions in the study area. Stable isotope composition of groundwater when compared to that of rainfall indicates monsoon rainfall as the primary source of groundwater recharge. Winter rains, which are characteristically enriched in heavier isotopes, do not contribute notably to groundwater recharge. Recharge is rapid with minor or no evaporative enrichment before recharge. Further, analysis of stable isotopes show that ‘macropore recharge’ is dominant in limestone or calcareous shale, covering more than 70% of the study area. Also apparent is the vertical connectivity amongst the aquifers. However, active intermixing of surface water and groundwater is not a predominant process. Annual groundwater recharge from rainfall, as derived from chloride mass balance, is 105.26 million cubic metre. Groundwater is predominantly of bicarbonate type, irrespective of its hydrostratigraphic (lithology) setting. Dissolution of carbonates and gypsum (occurring as veins), weathering of feldspar and ion exchange of clay minerals are amongst the most likely processes controlling the regional groundwater chemistry.

Exploration of groundwater flowpaths and effective recharge in the Amargosa Desert, Nevada, using multivariate statistical analysis and elevation-dependent chloride mass-balance method

Estimation of groundwater flowpaths and effective recharge was performed in the vicinity of Amargosa Desert, Nevada, USA, using multivariate statistical methods (MSMs) and a simple elevation-dependent chloride mass-balance method modified by estimating a minimum effective recharge elevation (MERE). MSMs were applied to major ion data to explain the relationships among different groundwater chemical species, define hydrochemical facies needed to determine groundwater flowpaths and evolution, and identify the influence of surface and geologic features on groundwater. For each defined watershed, an average groundwater chloride concentration is used and the chloride deposition rate is assessed in the subject watersheds over all land elevations above the estimated MERE by assuming two constant spatial and temporal concentrations of chloride in precipitation which is a function of elevation. The MERE for the Amargosa Desert was appraised as 1200 m above mean sea level (m-amsl), based upon the elevation of the orifice of Cane Spring (1237 m-amsl), one of the lowest elevation, permanent springs, on the Nevada National Security Site. This minimum elevation is the cutoff altitude for our calculations at which precipitation is considered to make a significant contribution to recharge. Elevation-dependent precipitation was evaluated relating elevation data from the online mapping software ACME Labs Mapper 2.1 and precipitation data from the PRISM Climate Group’s PRISM. These analyses provide further insight into the hydrology of the region and demonstrate the strong impact surface and geologic features can have on surface water infiltration and groundwater in arid regions. Specifically, evidence is presented of past focused recharge around the ephemeral Fortymile Wash and interaction between geologic faults and groundwater infiltration and flowpaths. In addition, groundwater flow, based upon water chemistry, appears to be segmented by major faults. Groundwater recharge in the vicinity of the Amargosa Desert has been estimated around 15,280 acre-ft/year.

Differences in groundwater and chloride residence times in saline groundwater: The Barwon River Catchment of Southeast Australia

The residence times of groundwater and chloride and the processes contributing to the development of saline (total dissolved solids (TDS) up to 45,379 mg/L) groundwater within the Barwon River Catchment of southeast Australia were investigated using major ion, stable isotope (δ18O, δ2H, and δ13C) and radioactive isotope (3H, 14C, 36Cl) geochemistry. The elevated groundwater salinity in the region is primarily due to evapotranspiration and recycling of solutes in saline lakes with minor contributions from weathering of halite, silicate and calcite minerals. Groundwater residence times estimated from 14C vary from modern to ~20 ka; for groundwater with lower 14C activities, the estimated residence times vary significantly depending on the assumed flow model and the 14C activity of recharge. Chloride residence times downgradient of Lake Murdeduke (a saline through-flow lake in the centre of the catchment) are greater than the corresponding groundwater residence times due to the recycling of Cl within the lake. Precise estimates of chloride residence time could not be determined using 36Cl due to R36Cl in precipitation being lower than that of groundwater. This is most likely due to R36Cl values in rainfall having been higher in the past than they are at present due to climate variability. δ18O, δ2H, and δ13C values also suggest that the region has experienced increasingly more evaporative conditions with time. The results of this study demonstrate that, while Cl is a useful tracer of hydrological processes, it must be applied carefully in arid and semi-arid regions of the world. In particular, recharge rates calculated using chloride mass balance may be underestimated where recycling of Cl has occurred.

Estimation of spatial distribution of groundwater recharge from stream baseflow and groundwater chloride

In this study groundwater chloride concentration and baseflow are used to estimate the spatial variability of recharge. Total recharge over the entire watershed is estimated using the baseflow method, and then the spatial variability of recharge is approximated using groundwater chloride concentration. The efficacy of the method is demonstrated using data from a rural watershed in Alberta, Canada. By using the combination of two well established methods of estimating recharge, baseflow and chloride mass balance, there is no need to estimate wet and dry deposition rate of chloride. The presented method is tested by using a steady-state groundwater flow model. The groundwater model showed higher agreement between modeled vs observed heads when spatially variable recharge forced the upper boundary of the model (root mean square error reduced from 13.5m to 8m). In addition, we demonstrate a unique method for parameterizing hydraulic conductivity of a fluvial aquifer using a sand fraction transfer function. This new method reduces the dimensionality of the parameter estimation problem and provides a consistency check on the spatially varying recharge estimates.

Groundwater recharge in an arid grassland as indicated by soil chloride profile and multiple tracers

AbstractPrevious studies have shown that shallow groundwater in arid regions is often not in equilibrium with near‐surface boundary conditions due to human activities and climate change. This is especially the case where the unsaturated zone is thick and recharge rate is limited. Under this nonequilibrium condition, the unsaturated zone solute profile plays an important role in estimating recent diffuse recharge in arid environments. This paper combines evaluation of the thick unsaturated zone with the saturated zone to investigate the groundwater recharge of a grassland in the arid western Ordos Basin, NW China, using the soil chloride profiles and multiple tracers (2H, 18O, 13C, 14C, and water chemistry) of groundwater. Whereas conventional water balance and Darcy flux measurements usually involve large errors in recharge estimations for arid areas, chloride mass balance has been widely and generally successfully used. The results show that the present diffuse recharge beneath the grassland is 0.11–0.32 mm/year, based on the chloride mass balance of seven soil profiles. The chloride accumulation age is approximately 2,500 years at a depth of 13 m in the unsaturated zone. The average Cl content in soil moisture in the upper 13 m of the unsaturated zone ranges from 2,842 to 7,856 mg/L, whereas the shallow groundwater Cl content ranges from 95 to 351 mg/L. The corrected 14C age of shallow groundwater ranges from 4,327 to 29,708 years. Stable isotopes show that the shallow groundwater is unrelated to modern precipitation. The shallow groundwater was recharged during the cold and wet phases of the Late Pleistocene and Holocene humid phase based on palaeoclimate, and consequently, the groundwater resources are nonrenewable. Due to the limited recharge rate and thick unsaturated zone, the present shallow groundwater has not been in hydraulic equilibrium with near‐surface boundary conditions in the past 2,500 years.

The potential vulnerability of the Namib and Nama Aquifers due to low recharge levels in the area surrounding the Naukluft Mountains, SW Namibia

The Naukluft Mountains in the Namib Desert are a high rainfall-high discharge area. It sees increased stream-, spring-flow as well as waterfalls during the rainy season. The mountains are a major resource for additional recharge to the Namib and Nama aquifers that are adjacent to the mountains. This paper aimed to highlight the potential vulnerability of the aquifers that surround the Naukluft Mountain area; if the strategic importance of the Naukluft Karst Aquifer (NKA) for bulk water supply becomes necessary. Chloride Mass Balance Method (CMBM) was applied to estimate rainfall available for recharge as well as actual recharge thereof. This was applied using chloride concentration in precipitation, borehole and spring samples collected from the study area. Groundwater flow patterns were mapped from hydraulic head values. A 2D digital elevation model was developed using Arc-GIS. Results highlighted the influence of the NKA on regional groundwater flow. This paper found that groundwater flow was controlled by structural dip and elevation. Groundwater was observed to flow predominantly from the NKA to the south west towards the Namib Aquifer in two distinct flow patterns that separate at the center of the NKA. A distinct groundwater divide was defined between the two flow patterns. A minor flow pattern from the northern parts of the NKA to the north east towards the Nama Aquifer was validated. Due to the substantial water losses, the NKA is not a typical karst aquifer. While the project area receives an average rainfall of 170.36 mm/a, it was estimated that 1–14.24% (maximum 24.43 mm/a) rainfall was available for recharge to the NKA. Actual recharge to the NKA was estimated to be less than 1–18.21% (maximum 4.45 mm/a) reflecting the vast losses incurred by the NKA via discharge. This paper concluded that groundwater resources of the NKA were potentially finite. The possibility of developing the aquifer for bulk water supply would therefore drastically lower recharge to surrounding aquifers that sustain local populations because all received rainfall will be utilized to maximise recharge to the NKA instead of surrounding aquifers.

Inter-Basin Groundwater Transfer and Multiple Approach Recharge Estimation of the Upper Awash Aquifer System

Multiple approaches have been used to estimate groundwater recharge in the Upper Awash river basin. The amount of recharge reaching the Upper Awash aquifer system from the Blue Nile sub-basins is also estimated. Water Balance, Chloride Mass Balance and HYDRUS 1D infiltration model are used to estimate recharge. A total of 29 sites were selected for the HYDRUS 1D multiple “at point” recharge simulations. Base Flow Separation (BFS) methods, using both River Analysis Package software Version 3.0.3 and Excel-based Time Plot program are also used as a proxy for recharge. Besides, overlay analysis in Processing MODFLOW, ArcGIS, and SURFER environments has been done to thoroughly consider spatial heterogeneity between any two point estimates and appreciate the effect of lineament density, topography, slope and major urbanized land on pattern of spatial distribution of recharge. Because of differences inherent in the assumptions and datasets used, the various methods employed give wide range of differences in recharge estimates. Recharge estimated for the Upper Awash basin ranges from 51.5 mm/year to 157 mm/year and for the two southern left-bank sub-basins of the Middle Blue Nile basin (Mugher and Jema) ranges from 86 mm/year to 239 mm/year. Consequently, annual average volumetric recharge in the Upper Awash and annual groundwater flux from portion of the Blue Nile sub basins to the Upper Awash aquifer system are estimated to be 983 Mm3 and 365 Mm3 respectively. The significant flux joining the Upper Awash groundwater system from part of the Middle Blue Nile basin, which is almost 37% of the total annual recharge to the Upper Awash basin makes this part of the Middle Blue Nile basin an important recharge zone for the Upper Awash groundwater. Estimating recharge using integrated approaches was found to be useful to identify range of plausible recharge rates in the two basins. Besides, the new methodological approach of superimposing recharge governing factors on interpolation of point recharge estimates helps to produce physical based spatial distribution of recharge.

Groundwater Recharge Quantification from Historical Rainfall Records and Salinity Profiling in the RAMSAR Listed Verlorenvlei Catchment, South Africa

The Verlorenvlei RAMSAR listed wetlands in the Sandveld of South Africa are threatened by the effects of salinization coupled with low rainfall. The lack of surface water throughout most of the year suggests that the wetlands may be fed by deeper groundwater sources. The Verlorenvlei River feeds into the wetlands and has four major tributaries which will form the basis of this study. A number of recharge techniques have been implemented in the Sandveld, but little research has characterised the groundwater of the tributaries of the Verlorenvlei River and their contribution to the wetlands. The Chloride Mass Balance technique and stable isotopes will be used to estimate the recharge occurring in these sub-catchments, with tritium and radiocarbon providing time constraints. These results can be compared to longer-term rainfall records to predict future fluctuations in groundwater availability. Eight rainfall collection points have been established throughout the catchment, with corresponding deep boreholes to be sampled on a regular basis. Preliminary results show that the shallow groundwater is more saline than the corresponding deep sources, with the exception of the fresh Krom Antonies tributary which is thought to be the main input source for the Verlorenvlei River.

Groundwater flow estimation using temperature-depth profiles in a complex environment and a changing climate

Obtaining reliable estimates of vertical groundwater flows remains a challenge but is of critical importance to the management of groundwater resources. When large scale land clearing or groundwater extraction occurs, methods based on water table fluctuations or water chemistry are unreliable. As an alternative, a number of methods based on temperature-depth (T-z) profiles are available to provide vertical groundwater flow estimates from which recharge rates may be calculated. However, methods that invoke steady state assumptions have been shown to be inappropriate for sites that have experienced land surface warming. Analytical solutions that account for surface warming are available, but they typically include unrealistic or restrictive assumptions (e.g. no flow initial conditions or linear surface warming). Here, we use a new analytical solution and associated computer program (FAST) that provides flexible initial and boundary conditions to estimate fluxes using T-z profiles from the Willunga Super Science Site, a complex, but densely instrumented groundwater catchment in South Australia. T-z profiles from seven wells (ranging from high elevation to near sea level) were utilised, in addition to mean annual air temperatures at nearby weather stations to estimate boundary conditions, and thermal properties were estimated from down borehole geophysics. Temperature based flux estimates were 5 to 23mmy−1, which are similar to those estimated using chloride mass balance. This study illustrates that T-z profiles can be studied to estimate recharge in environments where more commonly applied methods fail.

Hydrogeological characterization of the Kosi Bay Lakes system, north-eastern South Africa

This paper reports a hydrogeological characterization of a groundwater-dependent lakes system, the Lakes Kosi Bay catchment located in the north-eastern coast of South Africa. Conventional hydrogeological techniques including water balance analysis, hydrogeochemical and environmental isotope methodologies were applied in the study. Three distinct aquifer systems are recognized in the area, namely the unconfined Holocene cover sands, the Kosi Bay and Port Durnford Formations, and the Umkhwelane and Uloa Formations. Groundwater recharge to the topmost aquifer, which is connected to the lakes, is estimated using the chloride mass balance method to be 12 % of the mean annual precipitation (MAP = 939 mm/a). Evaporation rate from the lakes and evapotranspiration from the catchment are 1310 and 1135 mm/a, respectively. The mean annual water balance components indicate a positive balance between inputs and outputs. Groundwater flows from the west through the lakes (flow-through lakes) to the Indian Ocean. Groundwater in the shallow Holocene aquifer and streams has similar hydrochemical and isotopic signature indicating strong interconnection. These groundwater and streams have a hydrochemistry dominated mainly by a Na–Cl–HCO3 water type, a very low salinity (ranging from 88 to 400 µS/cm) and a negative isotopic signature (negative δD and δ18O values). The lakes are characterized by Na–Cl hydrochemical water type, have salinity that ranges from 1014 to 25,300 µS/cm, and positive stable isotopic signature (positive δD and δ18O signature) indicating strong evaporation effects. The study further reveals a salinity and isotopic series among the lakes from south to north, towards the estuary.

Geochemical Recharge Estimation and the Effects of a Declining Water Table

Core Ideas Intensive groundwater development has led to steep water‐level declines. Geochemically, vadose zone water in drained zone is identical to groundwater. Geochemical estimates of recharge rates not possible through this drained zone. With steep water‐level declines, care is needed for geochemical recharge estimates. In this area, focused recharge likely dominates over diffuse recharge. The High Plains aquifer (HPA) is one of the largest aquifers in the world and is critical for agricultural production in the United States. In Kansas, irrigation using the HPA has resulted in steep groundwater declines. Unexpected water‐level response to pumping recovery in the northwestern Kansas portion of the HPA was recently observed, indicating a previously unknown source of recharge. The goal of this research was to investigate water movement and vadose zone chemical inventories to constrain recharge pathways and rates to the aquifer. Detailed geochemical profiles at a cored site indicated displaced chloride, bromide, nitrate–nitrogen (NO3–N), and sulfate reservoirs—likely mobilized by the onset of on‐site irrigation—believed to have commenced in 1984. Water in the vadose zone beneath the predevelopment water table (36–63 m below ground surface) is remnant groundwater left as the water table declined. Thus, assumptions of a chloride mass balance are not valid in this zone. These results indicate irrigation return flow has yet to travel through the vadose zone to recharge the aquifer at this location. Calculated chemical transit times through the profile to the predevelopment water table exceed the start of irrigation at the site, and groundwater radiocarbon ages on the order of 3000 yr BP provide further evidence against substantial irrigation return to the aquifer. These data establish evidence for complicated flow paths through the vadose zone to the aquifer and argue against broad, diffuse recharge to the aquifer. Future studies in the area should investigate focused recharge pathways to identify recharge sources.

Estimation of Groundwater Recharge Using Tracers and Numerical Modeling in the North China Plain

Water resource shortage has been a serious problem since the 1980s in the North China Plain (NCP), resulting in plenty of environmental problems. Estimating the groundwater recharge rate accurately is vital for managing groundwater effectively. This study applied several methods, including chloride mass-balance, tracers (bromide and tritium) and numerical modeling (Hydrus-1D), to estimate groundwater recharge at three representative sites of the NCP: Zhengding (ZD), Luancheng (LC) and Hengshui (HS). The chloride concentration of the soil profile in the ZD site showed that the mean recharge was 3.84 mm/year with the residence time of 105 years for soil water transferring through the vadose area of 45.0 m in depth in the preferential flow model mainly. Considering the influence of preferential flow on the soil water movement in the field scale, the traditional methods (e.g., peak method of bromide and tritium tracers based on piston flow described in the literature) could be unsuitable to estimate groundwater recharge in the LC and HS sites, especially in areas with low recharge rates. Therefore, multi-region and mass balance methods were applied in this study. The results of this investigation showed that the mean values of recharge were 124.3 and 18.0 mm/year in the LC and HS sites, respectively, in 2010. Owing to complexity and uncertainty on the surface resulting from the measuring of evapotranspiration, the upper boundary of 1.4 m (under the ground where most of the plant roots did not reach) was chosen for the numerical modeling of Hydrus-1D, and the result showed that the mean recharge was 225 mm/year from 2003 to 2007, consistent with the result of tracers in the previous literature. The result also showed that the positive relation of groundwater recharge and the sum of irrigation and rainfall was presented in the spatial and temporal scale. Additionally, human activities promoted the recharge rate, and recharge rates increased with greater depths in the LC site generally. However, both cases did not appear clearly in the HS site, showing that the low penetrability of soil controlled the recharge rate in this site.

Groundwater recharge assessment in an upland sandstone aquifer of southern California

The Chloride Mass Balance (CMB) method was used to obtain long-term recharge values for the Santa Susana Field Laboratory (SSFL) site, which features a groundwater flow system beneath an upland ridge formed of sandstone and shale beds in the Simi Hills, Ventura County, southern California. This application relied on the availability of on-site measurements of bulk atmospheric chloride deposition comprised of dry fallout and wet concentration, a large number of groundwater samples (∼1490) collected over three decades from 206 wells spanning a depth range from 10 to 360m, and measurements of chloride in surface runoff during rain events. The use of the CMB method is suited to the assessment of recharge for the study area because the mean chloride values in groundwater show minimal spatial trends, indicating no sources other than atmospheric. In addition, the Cl/Br ratio was used to exclude wells with possible anthropogenic chloride. The site-wide average recharge ranges between 1.8 and 9.5% of the mean annual precipitation (455mm) with a mean value of 4.2%. The measured surface runoff varies from 2.3 to 10.2% with mean value of 6.1% (28 mm) and, therefore, the volume of water lost to evapotranspiration is between 95.9 and 80.3% with a mean value of 89.6% (408mm). The long-term recharge calculated using the CMB method is consistent with tritium distribution based on a subset of groundwater monitoring wells and with an analysis of steady flow in the groundwater mound beneath the SSFL. Furthermore, the recharge value matches those in the literature for sandstone aquifers in arid and semi-arid climates. This recharge estimate has important relevance for site characterization in terms of constraining the volumetric groundwater flow rates and water balance and understanding the mechanisms of transport towards the water table. Moreover, this is the first application of the CMB in an upland area of California. Hence, the method is demonstrated to be robust and applicable to many upland bedrock areas in southern California and similar regions around the world, and can be used to quantify groundwater flow rates and supplies relevant to water resource management.

Integrating soil water and tracer balances, numerical modelling and GIS tools to estimate regional groundwater recharge: Application to the Alcadozo Aquifer System (SE Spain)

Groundwater recharge is one of the key variables for aquifer management and also one of the most difficult to be evaluated with acceptable accuracy. This is especially relevant in semiarid areas, where the processes involved in recharge are widely variable. Uncertainty should be estimated to know how reliable recharge estimations are. Groundwater recharge has been calculated in the Alcadozo Aquifer System, under steady state conditions, at regional (aquifer) and sub-regional (spring catchment) scales applying different methods. The regional distribution of long-term average recharge values has been estimated with the chloride mass balance method using data from four rain stations and 40 groundwater samples covering almost the whole aquifer surface. A remarkable spatial variability has been found. Average annual recharge rates ranges from 20 to 243mmyear−1 across the aquifer, with an estimated coefficient of variation between 0.16 and 0.38. The average recharge/precipitation ratio decreases from 34% in the NW to 6% in the SE, following the topographic slope. At spring-catchment scale, recharge has been estimated by modelling the soil water balance with the code Visual Balan 2.0. The results, calibrated with discharge data of the two main springs Liétor and Ayna, are 35.5 and 50mmyear−1 respectively, with estimated coefficients of variation of 0.49 and 0.36. A sensitivity analysis showed that soil parameters influence the most the uncertainty of recharge estimations. Recharge values estimated with both methods and at two temporal and spatial scales are consistent, considering the regional variability obtained with the chloride method and the respective confidence intervals. Evaluating the uncertainties of each method eased to compare their relative results and to check their agreement, which provided confidence to the values obtained. Thus, the use of independent methods together with their uncertainties is strongly recommended to constrain the magnitude and to provide reliance to recharge estimations.

(美国)新墨西哥州中南部沙加缅度山脉地下水补给的量和位置

The Sacramento Mountains and the adjacent Roswell Artesian Basin, in south-central New Mexico (USA), comprise a regional hydrologic system, wherein recharge in the mountains ultimately supplies water to the confined basin aquifer. Geologic, hydrologic, geochemical, and climatologic data were used to delineate the area of recharge in the southern Sacramento Mountains. The water-table fluctuation and chloride mass-balance methods were used to quantify recharge over a range of spatial and temporal scales. Extrapolation of the quantitative recharge estimates to the entire Sacramento Mountains region allowed comparison with previous recharge estimates for the northern Sacramento Mountains and the Roswell Artesian Basin. Recharge in the Sacramento Mountains is estimated to range from 159.86 × 106 to 209.42 × 106 m3/year. Both the location of recharge and range in estimates is consistent with previous work that suggests that ~75 % of the recharge to the confined aquifer in the Roswell Artesian Basin has moved downgradient through the Yeso Formation from distal recharge areas in the Sacramento Mountains. A smaller recharge component is derived from infiltration of streamflow beneath the major drainages that cross the Pecos Slope, but in the southern Sacramento Mountains much of this water is ultimately derived from spring discharge. Direct recharge across the Pecos Slope between the mountains and the confined basin aquifer is much smaller than either of the other two components.

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.