Chloride Mass Balance Technique Research Articles

Characterisation of groundwater flow and recharge in crystalline basement rocks in the Talensi District, Northern Ghana

A numerical groundwater flow model, calibrated under equilibrium conditions to characterise the spatial variation of key hydraulic parameters and groundwater flow pattern in a crystalline aquifer in the Upper East Region of Ghana has been developed. The development of groundwater flow model was to provide the baseline information to assist in assessing the impacts of population growth and climate change scenarios on large scale development of groundwater resources for various uses. The results revealed an apparent dominant northeast–southwest flow pattern influenced mainly by the hydraulic conductivity field, with estimated values ranging between 0.001 and 58 m/day, and local flow systems which are controlled mainly by local variations in the topography in the area. The estimated average groundwater recharge from the calibrated model and chloride mass balance technique: 2.00% and 2.07% of the annual precipitation respectively has a direct positive correlation with elevation. The low recharge rate is evident in the high evaporation rates as revealed by stable isotope (δ2H and δ1⁸O) analysis of sampled water sources. The study also reveals the aquifer is being partly recharged by the White Volta River. This gives an indication that the area holds good groundwater fortunes for potential commercial groundwater development as predicted by scenario analysis which suggests that the aquifer can sustain increased abstractions by more than 100% of the current rate provided the current recharge rate is maintained. However, for the aquifer to sustain large scale abstractions beyond 100% of the current rate under conditions of reduced vertical recharge by more than 40% of the current rate, deliberate efforts would have to be made to enhance artificial vertical recharge to augment the natural recharge.

Use of a modified chloride mass balance technique to assess the factors that influence groundwater recharge rates in a semi-arid agricultural region in China

The groundwater recharge potential in the West Liaohe Plain in northeast China was assessed using a modified chloride mass balance method to estimate the vertical infiltration of water through typical soil profiles at two study sites in the region: the Horqin Left Middle Banner (KZ) site and the Kailu County (KL) site. Samples of precipitation, groundwater, and soil water were collected at the KZ and KL sites to determine the effects of irrigation and soil texture on water infiltration in soil profiles. The results showed that the water infiltration rate at the KZ site is approximately 6–11 mm/a beneath farmland, with similar infiltration rates for mulched drip irrigation and flood irrigation methods, but the infiltration coefficient with a mulch cover was slightly higher than that of soils without a mulch cover. The infiltration rate in soils at the KL site was determined to be 25–41 mm/a beneath farmland and in this area the infiltration rate and infiltration coefficient in soils with a mulch cover were much higher than in soils without a mulch cover. Additionally, the infiltration rate and infiltration coefficient in sandy-textured soils at the KL site were higher than in clayey soils at the KZ site. The infiltration rate in bare land at the KL site with a similar soil texture to that of the farmland was determined to be approximately 16 mm/a, or approximately one-half the infiltration rate measured beneath the farmland. Water infiltration rates in cultivated soils were higher than those beneath non-cultivated areas; however, this does not mean that the actual groundwater recharge rate increased. The amount of irrigation water used is lower using mulched drip irrigation than in fields irrigated using flood irrigation, but the infiltration coefficient actually increased. This suggests that the mulched drip irrigation method can increase groundwater recharge in semi-arid regions such as the study area.

Characterising alluvial aquifers in a remote ephemeral catchment (Flinders River, Queensland) using a direct push tracer approach

The availability of reliable water supplies is a key factor limiting development in northern Australia. However, characterising groundwater resources in this remote part of Australia is challenging due to a lack of existing infrastructure and data. Here, direct push technology (DPT) was used to characterise shallow alluvial aquifers at two locations in the semiarid Flinders River catchment. DPT was used to evaluate the saturated thickness of the aquifer and estimate recharge rates by sampling for environmental tracers in groundwater (major ions, 2H, 18O, 3H and 14C). The alluvium at Fifteen Mile Reserve and Glendalough Station consisted of a mixture of permeable coarse sandy and gravely sediments and less permeable clays and silts. The alluvium was relatively thin (i.e. < 20 m) and, at the time of the investigation, was only partially saturated. Tritium (3H) concentrations in groundwater was ∼1 Tritium Unit (TU), corresponding to a mean residence time for groundwater of about 12 years. The lack of an evaporation signal for the 2H and 18O of groundwater suggests rapid localised recharge from overbank flood events as the primary recharge mechanism. Using the chloride mass balance technique (CMB) and lumped parameter models to interpret patterns in 3H in the aquifer, the mean annual recharge rate varied between 21 and 240 mm/yr. Whilst this recharge rate is relatively high for a semiarid climate, the alluvium is thin and heterogeneous hosting numerous alluvial aquifers with varied connectivity and limited storage capacity. Combining DPT and environmental tracers is a cost-effective strategy to characterise shallow groundwater resources in unconsolidated sedimentary aquifers in remote data sparse areas.

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.

Estimation of evapotranspiration losses in the vadose zone using stable isotopes and chloride mass balance method

This study used stable isotope and chloride data of porewater to evaluate water fluxes through the vadose zone and thereby estimate evapotranspiration water losses in parts of the Nabogo catchment of the White Volta Basin in Ghana. The overall objective was to develop a framework so that the recharge regime can be properly conceptualized for numerical hydrological modeling. Unsaturated zone material was sampled at regular intervals of 50 cm to a maximum depth of 300 cm in four different locations in the study area. Rainwater, groundwater and surface water were simultaneously sampled and analyzed for their δ18O and δ2H characteristics. Porewater, extracted from the unsaturated zone material was analyzed for the δ18O and δ2H and chloride content and profiles were created to gauge the isotopic evolution of precipitation and estimate evaporative losses at each interval of the sampling. The chloride mass balance technique was used to estimate the fraction of infiltrating water remaining at each interval of the sampling. Transpiration losses through the entire profile were then estimated. This study finds that the vertical infiltration of water through the vadose zone is dominated by piston flow and a mixture of piston and preferential flows. In the shallow subsurface (0.0–3.0 m), evaporative losses estimated from stable isotope data fall in the range of 29.3–52.4 % (322.3–576.4 mm/year) of the annual precipitation, with an average of 40 % (or 440 mm/year). Estimated vadose zone recharge at the maximum depth of sampling ranges between 11.1 and 185 mm/year with an average of 32.9 mm/year, representing 1.1, 18.5, and 3.29 % of the annual precipitation, respectively. Estimated transpiration losses within this interval range between 29.1 % (290 mm/year) and 69.5 % (695 mm/year), with an average of 54.7 % (547 mm/year) of the annual precipitation. Transpiration losses appear to increase down the profile and apparently account for a significant percentage of water losses in the vadose zone. A significant proportion of the original precipitation is lost within the upper 300 cm (3 m) of the vadose zone. The Water Table Fluctuations method was independently used to estimate saturated zone groundwater recharge and indicates that recharge rates range between 64.65 and 151.2 mm/year with an average of 102.5 mm/year which, respectively represent 5.9, 13.7, and 9.3 % of the average annual precipitation in the area. The apparently higher estimates from the water table fluctuations method may arise from uncertainties in the specific yield values used for the vadose zone material.

Groundwater recharge processes in the Nasia sub-catchment of the White Volta Basin: Analysis of porewater characteristics in the unsaturated zone

Vertical infiltration of precipitation has been examined in this study for the purpose of evaluating groundwater recharge processes in parts of the Nasia sub-catchment of the White Volta Basin. As recharge is an essential component in the detailed assessment of groundwater resources potential in a basin, evaluating its processes is vital in determining the spatial and temporal variability of the resource. Stable isotope data of precipitation, groundwater, surface water and porewater in the area suggest that the local precipitation is largely enriched compared to global meteoric water. This is consistent with the prevailing local conditions in the region and ties in with observations in other parts of the sub-region. The groundwater and porewater data indicate that prior to, and in the process of infiltration and final percolation into the saturated zone, rainwater undergoes evaporative enrichment such that the finally recharged water plots along an evaporation line with a much shallower gradient and intercept compared to the global meteoric water line and the local meteoric water line. The isotope data further suggest that through the shallow unsaturated zone, a significant fraction of the initial precipitation would have been evaporated by a depth of 3.0m. Evaporation rates in the range of 38–49% have been estimated for the depth range of 0–3.0m based on the porewater stable isotope data. Details of the procedures and implications of high evaporation rates within such shallower depths are presented and discussed. Groundwater recharge rates estimated from the chloride mass balance technique report values in the range of 73.26mm/yr (390Mm3/yr)–109.89mm/yr (585.27Mm3/yr), with an average of 94mm/yr (500.6Mm3/yr). These translate into 6.6–10.9% of annual precipitation. Based on the current population trends and per capita water demand of 50L per capita per day, this study finds that the estimated recharge rates exceed the demand 59 times. This suggests significant promise for developing groundwater resources for climate-proof livelihood support projects in the terrain. However, much more detailed hydrogeological assessments are required to constrain the effects of lateral and vertical lithological variations on the availability of the recharged groundwater for immediate abstraction and use. The study also observes that since evaporation is one of the main dynamic processes influencing the fraction of the initial precipitation that finally reaches the saturated zone, projected increases in temperature during the next decade may increase evaporation rates of infiltrating rainwater leading to reduced groundwater rates.

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

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

Recharge to groundwater in the Turi Basin, northern Chile: An evaluation based on tritium and chloride mass balance techniques

Summary The Turi Basin contains an unconfined phreatic aquifer within the Toconce Formation volcano-sedimentary sequence that passes eastwards under the Quaternary volcanic chain and outcrops in the Altiplano. The phreatic aquifer is floored by impermeable welded Miocene ignimbrites. Precipitation captured by the volcanic edifices infiltrates through high permeability soils to recharge the phreatic aquifer. The hydrochemistry of the groundwater in the phreatic aquifer shows little evidence of mixing with geothermal water and therefore approximates to a closed system. Tritium values demonstrate that recharge is occurring under current climatic conditions and takes around 40 years to pass 10+ km from recharge areas on the slopes of the volcanoes and the Altiplano to the center of the basin. The chloride mass balance technique has been used to quantify precipitation recharge at 15,500 m3 d−1. This is significantly less than the flow through the phreatic aquifer (58,000–86,000 m3 d−1) calculated from its physical properties and the difference is interpreted to be due to subsurface inflow through the Toconce Formation from the Altiplano. The results of this study have wider application throughout the Andean hydrological environment of northern Chile.

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.

Modelling the water balance of effluent-irrigated trees

Irrigation of effluent is an increasingly popular treatment option due to concern about nutrient additions to rivers and coastal waters. Since some studies have shown that irrigation with waste water can lead to contamination of groundwater resources, there is need for a model to predict the fate of irrigated water, salt, and nitrogen that can be applied to a variety of different soils, climates, and crops. We present the development of the water balance part of such a model, APSIM for Effluent, and carry out a comparison against data obtained from an effluent-irrigated plantation of Eucalyptus grandis. Over 10 months, modelled tree water use was within 1.5% of that obtained by sap-flux measurements. When compared over 5 years of the experiment, modelled drainage lay above that estimated by a water balance technique, which was known a priori to underestimate drainage, and was close to that estimated by the chloride mass balance technique. Simulated chloride accumulated in the soil was within the scatter of the observations, although it was consistently at the lower end of the range of the data. There was good agreement between the model predictions and measured chloride concentration distribution with depth in the soil. A considerable amount of water was lost as deep drainage, even for the treatment that aimed to add only enough effluent to replace that lost by evaporation. During 5 years, of the 3370 mm rainfall and 4480 mm effluent received by that treatment, 6710 mm was lost by the various evaporative routes, and 1080 mm was lost by deep drainage.

Geochemical Estimates of Paleorecharge in the Pasco Basin: Evaluation of the Chloride Mass Balance Technique

The Pasco Basin in southeastern Washington State provides a unique hydrogeologic setting for evaluating the chloride mass balance technique for estimating recharge. This basin was affected by late Pleistocene catastrophic floods when glacial dams in western Montana and northern Idaho were breached. It is estimated that multiple Missoula floods occurred between ∼13,000 and 15,000 years B.P. and reached a high water elevation of ∼350 m. These floods removed accumulated chloride from the sediment profile, effectively resetting the chloride mass balance clock at the beginning of the Holocene. The rate of chloride accumulation qCl in the sediments was determined by two methods and compared. The first method measured qCl by dividing the calculated natural fallout of 36Cl by a measured ratio of 36Cl/Cl in the pore water, while the second method used the total mass of chloride in the profile divided by the length of time that atmospheric chloride had accumulated since the last flood. Although the two methods are based on different approaches, they showed close agreement. In laboratory studies the sediment to water ratio for chloride extraction was sensitive to the grain size of the sediments; low extraction ratios in silt loam sediments led to significant underestimation of pore water chloride concentration. Br/Cl ratios were useful for distinguishing nonatmospheric (e.g., rock) sources of chloride. Field studies showed little spatial variability in estimated recharge at a given site within the basin but showed significant topographic control on recharge rates in this semiarid environment. An extension of the conventional chloride mass balance model was used to evaluate chloride profiles under transient, time‐varying annual precipitation conditions. This model was inverted to determine the paleorecharge history for a given soil chloride profile, and the parameters of the root extraction model required to estimate paleoprecipitation