Chloride Mass Balance Approach Research Articles

Groundwater deeper than 500 m contributes less than 0.1% of global river discharge

Groundwater is one of the largest reservoirs of water on Earth but has relatively small fluxes compared to its volume. This behavior is exaggerated at depths below 500 m, where the majority of groundwater exists and where residence times of millions to even a billion years have been documented. However, the extent of interactions between deep groundwater (>500 m) and the rest of the terrestrial water cycle at a global scale are unclear because of challenges in detecting their contributions to streamflow. Here, we use a chloride mass balance approach to quantify the contribution of deep groundwater to global streamflow. Deep groundwater likely contributes <0.1% to global streamflow and is only weakly and sporadically connected to the rest of the water cycle on geological timescales. Despite this weak connection to streamflow, we found that deep groundwaters are important to the global chloride cycle, providing ~7% of the flux of chloride to the ocean.

Estimating Evapotranspiration Using Chloride Mass Balance in a New Mexico Paired Basin Study 2009-2019

A paired basin study in the upper Santa Fe River watershed following forest thinning and prescribed burns successfully measured water budget components in a treated and an untreated (control) basin. The paired basin study was established to investigate questions that have arisen regarding changes in water yield from forest treatments. Precipitation, stream flow, soil moisture, and chloride concentrations in precipitation and stream flow were measured to quantify the water budget components. The results from eleven years of data collection and analysis have a high degree of confidence with respect to measuring the water budget components based on the mass balance of water and chloride. The differences in the geologic structure and topography between the two paired basins appeared to impact the water budgets more than the forest treatments, except during periods when winter precipitation and snowmelt represented a significant component of inflow. Although this paired basin study was not able to portray a simple relationship between forest thinning and water yield, the chloride concentration methodology used to estimate evapotranspiration (ET) was successful. These detailed observations of chloride deposition and transport characteristics may be relevant for other researchers working in forested basins with substantial ET. ET rates were estimated by examining the cycle of chloride entering and exiting each basin over six integration periods. ET was estimated to be about 90% to 94% of precipitation in the treated basin and 77% to 86% in the control basin. The higher ET in the treated basin both before and after forest treatments may be due to the much greater area of west-facing hillslopes in the treated basin, which receive warm afternoon sun, and the greater area of rock cover in the control basin. Variation in the chloride concentration of collected precipitation samples from different sites indicates that horizontal precipitation of chloride in the tree canopy is an important consideration when using the chloride mass balance approach to calculate water budget components.

Assessing recharge process in plain catchments using isotopic and hydrochemical techniques

In order to increase the sustainability and conservation of water resources, a joint approach of isotopic and hydrochemical techniques was attempted to identify and quantify water sources and favourable recharge areas that would explain the integral functioning of a typically plain catchment. Chloride and δ18O and δ2H were measured in rain, groundwater, and pond samples. Chloride mass balance approach was applied to calculate the recharge and mixed water calculation was carried out using conservative tracers. Also, the local meteoric water line was calculated. Depleted isotopic and slightly progressive chloride contents are observed and they are grouped around to the local meteoric water line in the upper part and in the west of the lower catchment. In the East lower area 20% of recharge is calculated; isotopes are more enriched and lower concentration of chloride are observed. Depleted isotopic fingerprint and low salinity concentration for Ballenera pond were identified in a well close to it. This work demonstrates that in very flat zones, small changes in the hydrologic fluxes can impact on the water table of surface waters and groundwaters, leading to establish very local recharge zones that hardly can be recognized in large scale or esatfrequency of measurement, but can quiet easily be identified when chemical conservative tracers and water stable isotopes are used. Also, isotopic data demonstrate the importance of shallow wetlands as certain recharge areas to the regional aquifer, becoming areas of high groundwater vulnerability.

A new modified chloride mass balance approach based on aquifer hydraulic properties and other sources of chloride to assess rainfall recharge in brackish aquifers

AbstractA reasonable estimation of aquifer recharge can assist in the sustainable management of groundwater resources. The methodology to calculate aquifer recharge presents many challenges due to geological complexity and the existence of different water qualities in the aquifer. The classic chloride mass balance (CMB) method has been used widely for aquifer recharge estimation but the assumptions in the equations neglect other sources of chloride than rainfall; chloride accumulation due to evaporation and the hydraulic properties of the recharged aquifer. This research aims to introduce modifications and derive a new CMB equation that covers the shortcomings in the earlier classic CMB approach. The newly developed approach was verified mathematically to identify the sensitive parameters that affect the results of aquifer recharge. Also, it was verified by independent data and method (groundwater modelling) and showed that it is approaching reality about recharge estimation than the earlier classic method which underestimates aquifer recharge especially for brackish aquifers. This article applies the new modified method to sedimentary aquifers of brackish nature and carbonate aquifers predominantly with fresh water and brackish water. In particular, numerical modelling showed that the optimum percentage of aquifer recharge from rainfall for brackish aquifers is between 10% and 20%. Application of the classic method in brackish aquifers in Kuwait estimates the aquifer recharge to be 0.5%–3% due to rainfall, while the modified method yields aquifer recharge of 9.3%–20% from total rainfall which agrees with the results of numerical modelling approach. The application of the classic CMB method for the carbonate aquifers containing fresh to brackish groundwater in the West Bank, gave recharge percentage of 25% from rainfall while the modified method indicated the recharge percentage is 32% from rainfall. The results of the study obtained by the modified CMB technique are proved promising.

Groundwater recharge and potential exploitation in the Densu basin, Southwestern Ghana

Recharge estimation is important to the effective development and management of groundwater resources, since it forms the basis for assigning abstraction rates. This study applied the water table fluctuation method and chloride mass balance approach to estimate groundwater recharge in the Densu basin, identified as the most problematic and vulnerable catchment in Ghana, using chloride and water level data from 10 monitoring wells. In general, the results revealed seasonal and spatial variation in groundwater recharge in the basin, with a north-south decreasing trend. The chloride mass balance approach and water level fluctuation method revealed average groundwater recharge of 70 mm and 62 mm/year respectively. With the total groundwater abstraction in the basin estimated to be only 8% of the annual recharge rate, the potential for groundwater use in the basin is great, granted adequate water quality assessment supports the useable quantity available in the basin.

Irrigation return flows in a mediterranean aquifer inferred from combined chloride and stable isotopes mass balances

The supply of irrigation water often overcomes crop evapotranspiration, and the resulting return flow may infiltrate and significantly contribute to an aquifer water budget. Despite its crucial importance for water resource management, the proportion of irrigation water that contributes to groundwater recharge, namely the return flow coefficient, often remains difficult to assess. Here, a chloride mass balance is combined with an isotopic mixing model (δ18O and δD) to quantify return flow coefficients, in the Crau alluvial-type aquifer (Southern France), characterized by a long-term traditional practice of flood irrigation. Local groundwater compositions are interpreted in terms of average recharge along different flow paths. The high isotopic contrast between irrigation water and regional precipitation allows the partitioning of recharge between rainfall infiltration and irrigation return flows. Isotopic mixing proportions are then used to decipher the chloride concentration of groundwater purely recharged by return flow. This allows an original application of the chloride mass balance approach to estimate return flow coefficients, which doesn't rely on any atmospheric chloride survey. Values around 0.53 ± 0.16 were found for well defined stream lines averaging the functioning of the upstream aquifer, which leads to a return flow rate of 1190 ± 140 mm yr−1. These results are consistent with a local daily time series of recharge fluxes derived from the water-table fluctuation method over the 2003-2009 period, and in line with the spatial average previously quantified over the whole aquifer. This study confirms the ability of geochemical tracers to provide recharge rates fully independent from flux measurements. They can be further used to assess the irrigation efficiency in other similar systems, or to monitor the variations of irrigation return flow, which will result from the future modifications of land use, irrigation practices and climate.

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.

Spatial distribution of chloride and nitrate within an unsaturated dune sand of a cold-arid desert: Implications for paleoenvironmental records

Abstract Based on chloride mass balance approach, we used two tracers (Cl − and NO 3 − ) to study the interstitial water in four profiles of an unsaturated dune sand. The aim was to reconstruct the recharge history and use this to infer the long-term related climate changes and vegetation transitions in a cold-arid desert region of northwestern China. The estimated recharge rate of 0.21 to 0.59 mm year − 1 represented approximately 0.2 to 0.7% of the annual precipitation in the western Badain Jaran desert. The orientation of a megadune's slopes appeared to greatly affect recharge variability. Below the near-surface peaks, the total cumulative NO 3 –N inventories ranged from 61 to 121 kg ha − 1 . The near-surface peaks in NO 3 –N, with larger NO 3 –N/Cl − molar ratios, reflected active modern N-fixation by cyanobacteria, and the nitrate enrichment deeper in the unsaturated moisture zone indicated drier periods with less vegetation, limited nitrate fixation, or a combination of the two.

Bedrock infiltration and mountain block recharge accounting using chloride mass balance

AbstractMountain front catchment net groundwater recharge (NR) represents the upper end of mountain block recharge (MBR) groundwater flow paths. Using environmental chloride in precipitation, streamflow and groundwater, we apply chloride mass balance (CMB) to estimate NR at multiple catchment scales within the 27 km2 Dry Creek Experimental Watershed (DCEW) on the Boise Front, southwestern Idaho. The estimate for average annual precipitation partitioning to NR is approximately 14% for DCEW. In contrast, as much as 44% of annual precipitation routes to NR in ephemeral headwater catchments. NR in headwater catchments is likely routed to downgradient springs, baseflow, and MBR, while downgradient streamflow losses contribute further to MBR. A key assumption in the CMB approach is that the change in stored chloride during the study period is zero. We found that this assumption is violated in some individual years, but that a 5‐year integration period is sufficient. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Soil chloride and deep drainage responses to land clearing for cropping at seven sites in central Queensland, northern Australia

Summary Soil cores were taken at seven paired sites (native vegetation and adjacent dryland cropping on cracking clay soils) which had been cropped for 10–65 years in the Fitzroy Basin in central Queensland, northern Australia. Levels of soil chloride (Cl − ) and nitrate nitrogen (NO 3 –N) were determined in 0.3 m increments to a depth of 5 m where possible. The amounts of Cl − in the soil (0–1.5 m depth) under native vegetation were generally high (10–23 t ha −1 at six of the seven sites). The amounts of Cl − that had leached below 1.5 m depth during dryland cropping varied from 2.2 to 16.8 t ha −1 or 19–91% of the original totals at 0–1.5 m. Leaching of salt from the crop rooting zone in combination with higher rates of deep drainage can lead to outbreaks of soil salinisation but can also increase the soil plant available water capacity (PAWC). NO 3 –N had also been leached below crop rooting depth at three sites. Such leaching not only contaminates the groundwater but also wastes crop nutrients. The transient chloride mass balance approach was used to determine mean annual rates of deep drainage below crop rooting depth (1.5 m). At all seven sites annual deep drainage was low under native vegetation (0.2–1.7 mm yr −1 ) but increased under dryland cropping (1.6–27.5 mm yr −1 ). Drainage losses showed an inverse relationship with plant available water content (PAWC). Drainage losses waste the limited supply of water available for dryland cropping but can be reduced by practising opportunity cropping or by growing ley (temporary) pastures in rotation with annual crops.

A 2000 year geochemical record of palaeoclimate and hydrology derived from dune sand moisture

Chloride concentrations in the interstitial water of the unsaturated zone in dune sands of the Badain Jaran desert, northwestern China have been used to examine the recharge history over the last 2000 years using a chloride mass balance approach. The estimated 0.9 mm yr − 1 recharge represents approximately 1% of annual precipitation in the western part of the desert and this low but finite infiltration allows preservation of this record. The 30 m deep profile yields a total archive equivalent to 2050 years with well-preserved climatic events of 30–50 years duration. Periods of relatively dry years occurred during AD 100–250, 350–600, 870–1220, 1330–1450, 1570–1720, 1780s and 1900–1970. The unsaturated zone profile provides a robust climate record with a resolution of +/− 20 yr comparable favourably to tree ring and ice core records in this region.

Species‐Level Impacts on Chaparral Root Zone Hydrology

A set of four unconfined (earthen‐walled) lysimeters in the San Dimas Experimental Forest near Los Angeles, CA, provides a unique opportunity to examine the effect of species‐level vegetation on root zone hydrology under highly controlled experimental conditions. The 28.1‐m2 by 2.1‐m deep lysimeters were specifically designed to minimize variation of all environmental conditions except vegetation. Monocultures of buckwheat (Eriogonum fasciculatum Benth. var. foliolosum Nutt.), chamise (Adenstomata fasciculatum Hooke &amp; Arn.), scrub oak (Quercus dumosa Nutt.), and Coulter pine (Pinus coulteri D. Don.) were planted in 1946. In 1997 two sets of soil cores were sampled from each lysimeter to evaluate species level effects resulting from 50 yr of growth. The cores were analyzed for water content, soil water potential, and pore water chloride. The chloride mass balance approach was used to estimate long‐term average fluxes and residence times of water in the lysimeters. One core from each lysimeter was also analyzed for pore water stable isotope (δ2H) composition. Only small differences were observed between the four species in terms of the shapes of the depth profiles and the absolute values of the hydrologic characteristics and fluxes. However, some differences were statistically significant. For example, differences in some characteristics were found between the chamise and the other species. We also observed differences between the lowest aboveground biomass group (buckwheat and chamise) versus the highest aboveground biomass group (oak and pine). These results suggest that although species‐ and biomass‐level differences in root zone hydrologic behavior appear to be minor, there are detectible differences in how these vegetation types affect the water budget in chaparral ecosystems.

Separation of physical loading from photosynthesis/respiration processes in rivers by mass balance

Diurnal variations in physical and chemical concentrations, including nutrients, are observed in river ecosystems. Understanding these cycles and separating the effects of physical loading (from point and non-point sources) and biogeochemical processes are necessary for water management and the TMDL process. A chloride mass balance approach is used to separate the relative influences of physical loading and biogeochemical processes in the Bow River through Calgary, Canada, which has a significant influence on the river water chemistry. Sampling campaigns were conducted in December 2005, when minimal photosynthesis and respiration occur, and in July 2006, when river discharge is high and some photosynthesis and respiration activity is present. Samples in each campaign were collected at point source input and output along the river reach through the city every hour for a 24-hour period, allowing for time of travel. The two wastewater treatment facilities within the city contributed the majority of physical mass loading to the river, with temporal variations in effluent discharge, chloride, and nutrient concentrations. Wastewater effluent chloride to nutrient (as well as other parameter relationships) concentrations also varied diurnally. An hourly chloride mass balance was achieved, within 0.5% (average, S.D.=4.4) for December and 7.7% (average, S.D.=4.2) for July, between estimated cumulative sum values from all inputs and measured values at the river output downgradient of the city, allowing for the investigation of other parameter conservativeness. Some slight diurnal variations associated with photosynthesis and respiration were observed even with limited productivity in the river. Nitrate mass fluxes appeared to be most strongly influenced by photosynthesis and respiration processes, with phosphate being less influenced. Ammonia mass fluxes appeared to be most strongly influenced by wastewater effluent loading. Physical loading can mask or enhance biogeochemical diurnal fluctuations, creating errors in river process interpretations. Chloride was a useful tracer in the mass balance to distinguish between and assist in separating physical loading and biogeochemical processes in the river.

Deep arid system hydrodynamics 1. Equilibrium states and response times in thick desert vadose zones

Quantifying moisture fluxes through deep desert soils remains difficult because of the small magnitude of the fluxes and the lack of a comprehensive model to describe flow and transport through such dry material. A particular challenge for such a model is reproducing both observed matric potential and chloride profiles. We propose a conceptual model for flow in desert vadose zones that includes isothermal and nonisothermal vapor transport and the role of desert vegetation in supporting a net upward moisture flux below the root zone. Numerical simulations incorporating this conceptual model match typical matric potential and chloride profiles. The modeling approach thereby reconciles the paradox between the recognized importance of plants, upward driving forces, and vapor flow processes in desert vadose zones and the inadequacy of the downward‐only liquid flow assumption of the conventional chloride mass balance approach. Our work shows that water transport in thick desert vadose zones at steady state is usually dominated by upward vapor flow and that long response times, of the order of 104–105 years, are required to equilibrate to existing arid surface conditions. Simulation results indicate that most thick desert vadose zones have been locked in slow drying transients that began in response to a climate shift and establishment of desert vegetation many thousands of years ago.

Deep arid system hydrodynamics 2. Application to paleohydrologic reconstruction using vadose zone profiles from the northern Mojave Desert

Site‐specific numerical modeling of four sites in two arid alluvial basins within the Nevada Test Site employs a conceptual model of deep arid system hydrodynamics that includes vapor transport, the role of xeric vegetation, and long‐term surface boundary transients. Surface boundary sequences, spanning 110 kyr, that best reproduce measured chloride concentration and matric potential profiles from four deep (230–460 m) boreholes concur with independent paleohydrologic and paleoecological records from the region. Simulations constrain a pluvial period associated with infiltration of 2–5 mm yr−1 at 14–13 ka and denote a shift linked to the establishment of desert vegetation at 13–9.5 ka. Retrodicted moisture flux histories inferred from modeling results differ significantly from those determined using the conventional chloride mass balance approach that assumes only downward advection. The modeling approach developed here represents a significant advance in the use of deep vadose zone profile data from arid regions to recover detailed paleohydrologic and current hydrologic information.

Geochemical evolution and recharge of the shallow aquifers at Tulul al Ashaqif, NE Jordan

Shallow aquifers in the Tulul Ashqaf area in northeast Jordan represent areas where relatively high recharge is occurring in an arid environment. The significance of this phenomenon in terms of potential for water resource development in the area is related to the amount of recharge that is occurring as well as factors that may affect the water quality within the aquifers. Modeling of the geochemical evolution of these waters gives insight into these issues. Chemical data from rainwater, surficial salts, crushed basalt samples, runoff, and groundwaters were collected, and the chloride mass balance approach was used by integrating these various data into geochemical mass balance models using NETPATH. Variations between rainwater chemistry, the surficial salts and basalt, and runoff exist, suggesting various processes and sources, and allows determination of the input into the groundwater. Geochemical modeling shows that the majority of chloride added to the groundwater after runoff events was salt from surface deposits, and that the rainwater had undergone only small amounts of evaporation. Isotopic investigations of 18O/16O and 2H/1H of the waters confirm this result. The results indicate that most of the water that falls over the study area percolates to the shallow groundwater with little evaporation. Less than 25% of the water evaporates prior to recharge. Water stored in the aquifer is affected by water–rock interaction and little evidence suggests that evaporation is a factor in the chemical evolution of the waters of the shallow aquifers, except in areas where salt efflorescence occurs.

Uncertainties in estimating water fluxes and residence times using environmental tracers in an arid unsaturated zone

Environmental tracers are used widely to evaluate flow processes and estimate fluxes and ages of pore water in arid regions. The purpose of this study was to evaluate uncertainties in water flux and age on the basis of data from environmental tracers, including meteoric Cl, 36Cl, 3H, δ2H, and δ18O in porous media. Representative profiles of environmental tracers from drainage and interdrainage areas at a site in the Chihuahuan Desert of Texas were evaluated. The chloride mass balance approach (CMB) was used to evaluate water fluxes and ages. The long residence times indicated by the Cl data in interdrainage areas (55,000 to 105,000 years to 25 m depth) were generally corroborated by residence times estimated from radioactive decay of 36Cl (39,000 ±13,000 to 59,000 ±14,400 years). Uncertainties in the CMB approach include uncertainties in transport processes, Cl input, and Cl output. Although the CMB approach assumes one‐dimensional, downward piston flow, water potential and stable isotope data in interdrainage areas suggest net upward water movement. Cl data indicate that drying of the profiles may have persisted throughout the Holocene (∼10,000 years). Therefore the downward flow assumption may only be applicable in the older, deeper sections of the profiles. Cl diffusion is significant near the surface where Cl concentration gradients are steep. Anion exclusion may affect calculated water fluxes based on Cl in clay‐rich zones. Although it is difficult to quantify uncertainties in diffusion and anion exclusion processes, they act in concert and result in overestimation of water flux and underestimation of age by the CMB approach. Therefore, in interdrainage areas the CMB approach provides an upper bound on actual water fluxes and a lower bound on actual ages. Error bars on these bounding estimates were evaluated on the basis of uncertainties in Cl input (∼±35%) and in Cl output (±3%) that result in ±38% uncertainty in water flux and −24 to 56% uncertainty in water age in interdrainage areas. In drainage areas it is much more difficult to apply the CMB approach because of preferential flow, large uncertainties in Cl input as a result of run‐on, reduced sensitivity of Cl to water flux, and analytical uncertainties in Cl measurements. Although preferential flow was shown by 3H data, mixing calculations suggest that 36Cl/Cl ratios cannot be used to evaluate preferential flow when Cl concentrations in the matrix exceed 10 to 100 g m−3, as is found in the playa and the fissure. Neglecting Cl input from run‐on results in underestimation of water flux by about an order of magnitude. Therefore the apparent CMB water flux, which ignores preferential flow and run‐on, represents a lower bound on the actual water flux in contrast to an upper bound for interdrainage areas. These results have important implications for waste disposal in arid regions because they suggest that water fluxes estimated using the CMB approach are conservatively high in interdrainage areas characterized by porous media.

Estimating contamination potential at waste-disposal sites using a natural tracer

Chloride is a conservative, natural tracer found in precipitation, soil water, and groundwater. The chloride mass-balance approach, long used to estimate groundwater recharge, also provides a downward flux of moisture and solute at sites where there is a potential for groundwater contamination. The flux is obtained by dividing the product of the mean annual precipitation and total annual chloride input (via precipitation and dust) by the mean soil-water chloride content. Chlorideversusdepth profiles can also be used to determine optimum depth of waste burial to minimize deterioration of waste containers. The method has been applied to three sites in arid alluvial-basin settings in New Mexico, U.S.A.: a proposed landfill, a battery recycling plant, and a hazardous-waste disposal facility. It is concluded that the method is reliable, economical, and practical. Furthermore, it can be applied at any stage in the development of a site. The chloride method should apply in any recharge area where the base of the root zone is separated from the water table by at least 3 m or so and chloride in soil water comes only from precipitation and dust.