Conventional Chloride Mass Balance Research Articles

Analysis of Saline Intrusion into a Coastal Aquifer: A Case History of Legacy Issues and Challenges to Water Security

Coastal aquifers are subject to seawater intrusion. Therefore, managing freshwater aquifers in coastal areas remain challenging. At present, determining safe yields from the coastal aquifers to prevent seawater intrusion is primarily based on the use of numerical simulation-optimization models or by the use of analytical models based on the Ghyben-Herzberg principle. This study examines the cause and effects of seawater intrusion into a coastal aquifer, Lincoln Basin in southern Eyre Peninsula, South Australia and shows that application of simple techniques would have prevented seawater intrusion. Three freshwater lenses, Lincoln A, B, and C of the Lincoln Basin, located about 13 km southwest of Port Lincoln township, have been developed as a town water supply source in 1960. The capacity of the basin has been assessed by three long-term pumping tests. Based on pump tests results, three areas were developed to supply 2×106 m3 per year distributed across three lenses as lens A : four wells to supply 0.84×106 m3, lens B: four wells to supply 0.5×106 m3 and lens C: four wells to supply 0.66 ×106 m3. Neither recharge to the freshwater lenses nor a water balance had been assessed, and a precautionary approach to groundwater extraction was not followed. The apparent driver for managing the basin was demand for the township. In this study, we assessed the recharge using two methods; water-table fluctuation (WTF) and the conventional chloride mass balance (CMB) method. Total recharge to the freshwater lenses is estimated at 1.6×106 m3 per year which is less than the average annual groundwater extraction from the basin during the 1961-1977 periods (average 2.14×106 m3). As a result mining of the groundwater storage has occurred in the basin leading to saline intrusion, upconing and lateral flow of brackish water into wellfield areas. The total volume extracted from the basin was 35×106 m3, which exceeded the average recharge over the 15 year period, 24×106 m3. Using analytical methods, the seawater/freshwater interface movement from its original position was estimated to be 35 m in lens A, 337 m in lens B and 188 m in lens C. For each pumping well at maximum discharge rate, the transient interface location directly underneath the well was calculated. This results in interface rises under pumping wells in lens A of 3.8 m, lens B of 0.5 m, and in lens C about 0.7 m. According to the risk-based groundwater allocation method, maximum extraction would have been as a proportion of 25% of the annual recharge. Thus, maximum annual abstraction limits for lens A, B and C would have been 210×103 m3, 72×103 m3 and 130×103 m3, totaling 412×103 m3.

Karst Aquifer Recharge: A Case History of over Simplification from the Uley South Basin, South Australia

The article “Karst aquifer recharge: Comments on ‘Characteristics of Point Recharge in Karst Aquifers’, by Adrian D. Werner, 2014, Water 6, doi:10.3390/w6123727” provides misrepresentation in some parts of Somaratne [1]. The description of Uley South Quaternary Limestone (QL) as unconsolidated or poorly consolidated aeolianite sediments with the presence of well-mixed groundwater in Uley South [2] appears unsubstantiated. Examination of 98 lithological descriptions with corresponding drillers’ logs show only two wells containing bands of unconsolidated sediments. In Uley South basin, about 70% of salinity profiles obtained by electrical conductivity (EC) logging from monitoring wells show stratification. The central and north central areas of the basin receive leakage from the Tertiary Sand (TS) aquifer thereby influencing QL groundwater characteristics, such as chemistry, age and isotope composition. The presence of conduit pathways is evident in salinity profiles taken away from TS water affected areas. Pumping tests derived aquifer parameters show strong heterogeneity, a typical characteristic of karst aquifers. Uley South QL aquifer recharge is derived from three sources; diffuse recharge, point recharge from sinkholes and continuous leakage of TS water. This limits application of recharge estimation methods, such as the conventional chloride mass balance (CMB) as the basic premise of the CMB is violated. The conventional CMB is not suitable for accounting chloride mass balance in groundwater systems displaying extreme range of chloride concentrations and complex mixing [3]. Over simplification of karst aquifer systems to suit application of the conventional CMB or 1-D unsaturated modelling as described in Werner [2], is not suitable use of these recharge estimation methods.

Pitfalls in application of the conventional chloride mass balance (CMB) in karst aquifers and use of the generalised CMB method

There is unanimity in the literature that chloride mass flux crossing the piezometric surface requires a steady-state for the saturated or unsaturated version of the conventional chloride mass balance (CMB) method to apply. Data indicate that chloride concentration in point recharge fluxes crossing the piezometric surface can remain at or near surface runoff chloride concentrations, rather than finding equilibrium with groundwater chloride. Preferential groundwater flows were observed through an interconnected network of highly permeable zones with groundwater mixing along flow paths. Measurements of salinity and chloride indicated that fresher water pockets exist at point recharge locations. A stable and measurable fresh water plume develops only when a large quantity of surface water enters the aquifer as a point recharge. In such circumstances, assumptions and boundary conditions of the conventional CMB method are not met, and the method requires modification to include both point and diffuse recharge mechanisms. This paper describes a generalised CMB that is applicable to groundwater basins with point recharge. In three case studies, point recharge flux is estimated to contribute 63, 85, and 98 % of total recharge. However, long-term average annual point recharge volumes are much smaller than the aquifer storage, at 1.5, 1.95, and 0.75 % and distributed across the basins at discrete locations. In the study basins, conventional CMB-estimated recharges are 46, 20, and 11 % of the recharge estimated using the generalised CMB, indicating the importance of accommodating point recharge into the CMB method. The generalised CMB method provides an alternative long-term net recharge estimation method for groundwater basins characterised by both point and diffuse recharge.

Characteristics of Point Recharge in Karst Aquifers

Karstic groundwater basins are characterized by both point and diffuse recharge. This paper describes the hydrologic characteristics of point recharge and their influence on recharge estimation for four groundwater basins. Point recharge is highly transient and may occur in relatively short-time periods, yet is capable of recharging a large volume of water, even from a single extreme rainfall event. Preferential groundwater flows are observed in karst aquifers with local fresher water pockets of low salinity that develop around point recharge sources. Measurable fresh water plumes develop only when a large quantity of surface water enters the aquifer as a point recharge. In fresh water plumes, the difference in chloride concentrations in diffuse and point recharge zones decreases as the plumes become enriched through mixing. The relative contributions to total recharge from point sources using the measured gap between groundwater and rainwater chloride in the chloride vs. δ18O plot is not necessarily indicative of sinkholes not directly recharging the aquifer. In karst aquifers, recharge estimation methods based on groundwater age distribution; average annual rainfall and basin average chloride in the conventional chloride mass balance (CMB) method are questionable due to theoretical limitations and key assumptions of these methods not being met. In point recharge dominant groundwater basins, application of: watertable fluctuation, numerical groundwater modelling, Darcy flow calculation or water budget methods are more suitable for recharge estimation as they are independent of the particular mode of recharge. The duality of the recharge mechanism in karst aquifers suggests that modification to the CMB method may be required to include both point and diffuse recharge components.

Three criteria reliability analyses for groundwater recharge estimations

There is a broad variety of methods to estimate groundwater recharge, but tools to assess the reliability of particular methods are not available. This paper introduces three distinct criteria to assess the reliability of recharge: applicability of the method used, availability of reliable data, and spatial coverage. Weightings of each criterion are assigned using key attributes present to total attributes required under each criterion. A reliability assessment was applied to two point-recharge and two diffuse-recharge dominated groundwater basins in South Australia. We show that the use of groundwater age based on chlorofluorocarbon and the conventional chloride mass balance method gives unreliable estimates of recharge in karstic aquifers, primarily due to inappropriate assumptions concerning the nature of recharge in these systems. Among the methods evaluated, watertable fluctuation, numerical groundwater modelling, Darcy flow calculation and the water budget method are applied to both point and diffuse recharge dominant groundwater basins. The reliability of these methods depends primarily on the quality of data and spatial coverage of the basin. Once the reliability level is known, water resources planners and managers assess the level of risk to aquifers, the environment, and the socio-economic development required for sustainable management of groundwater.

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.

A transient flux model for convective infiltration: Forward and inverse solutions for chloride mass balance studies

Forward and inverse solutions are provided for analysis of inert tracer profiles resulting from one‐dimensional convective transport under fluxes which vary with time and space separately. The approach is developed as an extension of conventional chloride mass balance techniques used to analyze vertical unsaturated aqueous phase transport over large timescales in arid environments. This generalized chloride mass balance (GCMB) approach allows incorporation of transient fluxes and boundary values of precipitation and chloride mass deposition and allows analysis of a tracer profile which does not remain constant with depth below the extraction zone, in terms of a purely convective water transport model. The conventional quasi–steady state chloride mass balance (CMB) can be derived from the transient GCMB model developed here. By specifying a link between precipitation and recharge, closed‐form forward and inverse solutions relating soil water chloride concentrations to transient boundary fluxes are obtained. This link is necessary for quantitative analysis of variable chloride profiles arising from climatic change. The GCMB can use transient chloride mass deposition rates, transient precipitation, and transient evapotranspiration rates. If two of these quantities are known or if the time frame is constrained such that a quantity can be treated as constant, then the inverse model can be used to determine the third. When mixing processes are limited, the GCMB can provide an alternative approach for estimating paleoprecipitation for performanceassessment modeling. The GCMB model is demonstrated with the following applications: (1) determination of time‐varying precipitation from a field chloride profile and (2) evaluation of transient changes in water extraction by evapotranspiration and transient recharge associated with a change in land use.

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