Freshwater-saline Water Interface Research Articles

Application of Cl/Br ratio to demarcate the fresh-saline water interface in coastal aquifers of northern Tamilnadu, Southern India

Geochemical parameters, chloride and bromide were used as tracers to demarcate the extent of saline water intrusion in the northern part of Tamilnadu coastal aquifer which served as a hub for major pumping well fields for supply of drinking water to Chennai city. 158 groundwater samples were collected during pre & post monsoon period to analyse the hydrochemical variations across this temporal stretch and two major water types were interpreted from the analytical data. During pre-monsoon period, Na–Cl is dominant followed by Na–HCO3 type and during post monsoon, the groundwater in the central and eastern part shift towards Na–HCO3 type from Na–Cl type. Bromide is absent in the freshwater samples, while the mixed water samples have varying bromide concentrations. In the study area, Br concentration in groundwater ranged from 0 to 22.1 mg/l while the seawater sample had Br concentration of 63 mg/l. The Cl/Br ratios of groundwater of the study area ranged from 0.01 to 188 while that of the seawater figured at 288. The groundwater of the eastern and north-eastern portion of the study area has Cl/Br ratio between 75 and 188 and this value has a similar signature to that of seawater. The continental part of the study area has Cl/Br ratio less than 50 which corresponds to that of the intermittent zone between the seawater and rainwater. Spatial distribution of Cl/Br ratio and their variation across distance from the coast indicate the remnants of seawater in the aquifer material upto 19–20 km inland and the ratio tends to decrease beyond 20 km distance. The study explores the applicability of the Cl/Br ratios for defining the extent and movement of he freshwater-saline water interface in a coastal region, which can be used by the water managers, administrators and planners for effective management and protection of available groundwater resources.

Numerical simulation of seawater intrusion to coastal aquifers and brine water/freshwater interaction in south coast of Laizhou Bay, China

Seawater intrusion and brine water/freshwater interaction have significantly affected agriculture, industry and public water supply at Laizhou Bay, Shandong Province, China. In this study, a two-dimensional SEAWAT model is developed to simulate the seawater intrusion to coastal aquifers and brine water/fresh water interaction in the south of Laizhou Bay. This model is applied to predict the seawater intrusion and brine water/freshwater interface development in the coming years. The model profile is perpendicular to the coastal line with two interfaces, freshwater-saline water interface near the shore and inland brine water-saline water-seawater interface. The hydrogeological parameters in the SEAWAT-2000 model are calibrated by the head and salinity measurements. The precipitation infiltration coefficient, boundary conditions and thicknesses of aquifers are studied in a sensitivity analysis. The predicted results indicate that equivalent freshwater head in shallow freshwater-saline water area will decline 2.0 m by the end of the forecasting period, caused by groundwater over-pumping for farmland irrigation. The groundwater head in the brine-saline water area will also decrease about 1.8 m by the end of forecasting period, caused by excessive brine mining. Salinity finally decreases below 105 g/L in the brine area, but increases in other areas and contaminates fresh groundwater resources.

Can bioturbation be responsible for thicker freshwater lenses than expected in littoral environments?

A study of hydrological and lithological characteristics that allow the development of freshwater lenses in littoral ridges underneath clay sediments of the coastal plain was undertaken, considering as a case of study the freshwater lenses located in the coastal sector of Samborombón Bay (Argentina). Geological characteristics were studied at a regional scale, as well as detailed sedimentological and ichnological aspects. Thickness of the lens and the freshwater–saline water interface were determined by salinity measurements in exploration wells located at different depths. Results obtained suggest that the infiltration of rainwater into the littoral ridges causes the formation of freshwater lenses that deepen below the clayey layers of the coastal-plain deposits. The key to answering this question are the crustacean burrows, which regionally occur in the fine sediments of the coastal plain. The burrows create an interconnected system of galleries and tunnels filled with sandy-silt material that favours seepage. Because the alternation of coastal-plain deposits and littoral ridges is a frequent situation throughout the world, the data provided by the present work do not represent just a simple case study; on the contrary, they show that biological structures promote bigger freshwater lenses in coastland environments, thus increasing water reserves.

Modeling Seawater Intrusion to Coastal Aquifers in South Coast of Laizhou Bay, China

In this study, a two-dimensional SEAWAT 2000 model is developed to simulate the seawater intrusion to coastal aquifers and brine water/fresh water interaction in the south of Laizhou Bay, Shandong Province, China and forecast the seawater intrusion and brine water/freshwater interface development in the coming years. The model profile is perpendicular to the coastal line, about 40 km long and 110 m in depth, and consists of two interfaces, freshwater-saline water interface and brine water-saline water-seawater interface. The parameters of aquifers in the SEAWAT-2000 model are calibrated by trial-error method repeatedly to fit the head and salinity measurements. Based on the historical groundwater and brine water exploration and natural precipitation condition, the prediction results indicate that equivalent freshwater head in shallow freshwater-saline water area will decrease year by year and decline 2.0 m in the forecasting period, caused by groundwater over-pumping for irrigating farmlands. The groundwater head in the brine-saline water area will also decrease about 1.8 m in forecasting period. A larger depression cone appears in the brine area, with smaller funnels in other areas. The salinity in the brine area finally drops below 105g/l. In the meanwhile, the salinity increases in other areas, damage fresh groundwater resources.

Chemolithoautotrophy supports macroinvertebrate food webs and affects diversity and stability in groundwater communities.

The prevailing paradigm in subterranean ecology is that below-ground food webs are simple, limited to one or two trophic levels, and composed of generalist species because of spatio-temporally patchy food resources and pervasive energy limitation. This paradigm is based on relatively few studies of easily accessible, air-filled caves. However, in some subterranean ecosystems, chemolithoautotrophy can subsidize or replace surface-based allochthonous inputs of photosynthetically derived organic matter (OM) as a basal food resource and promote niche specialization and evolution of higher trophic levels. Consequently, the current subterranean trophic paradigm fails to account for variation in resources, trophic specialization, and food chain length in some subterranean ecosystems. We reevaluated the subterranean food web paradigm by examining spatial variation in the isotopic composition of basal food resources and consumers, food web structure, stygobiont species diversity, and chromophoric organic matter (CDOM), across a geochemical gradient in a large and complex groundwater system, the Edwards Aquifer in Central Texas (USA). Mean δ13C values of stygobiont communities become increasingly more negative along the gradient of photosynthetic OM sources near the aquifer recharge zone to chemolithoautotrophic OM sources closer to the freshwater-saline water interface (FWSWI) between oxygenated freshwater and anoxic, sulfide-rich saline water. Stygobiont community species richness declined with increasing distance from the FWSWI. Bayesian mixing models were used to estimate the relative importance of photosynthetic OM and chemolithoautorophic OM for stygobiont communities at three biogeochemically distinct sites. The contribution of chemolithoautotrophic OM to consumers at these sites ranged between 25% and 69% of total OM utilized and comprised as much as 88% of the diet for one species. In addition, the food web adjacent to the FWSWI had greater trophic diversity when compared to the other two sites. Our results suggest that diverse OM sources and in situ, chemolithoautotrophic OM production can support complex groundwater food webs and increase species richness. Chemolithoautotrophy has been fundamental for the long-term maintenance of species diversity, trophic complexity, and community stability in this subterranean ecosystem, especially during periods of decreased photosynthetic production and groundwater recharge that have occurred over geologic time scales.

Environmental controls on organic matter production and transport across surface-subsurface and geochemical boundaries in the Edwards aquifer, Texas, USA

Karst aquifer phreatic zones are energy limited habitats supported by organic matter (OM) flow across physical and geochemical boundaries. Photosynthetic OM enters the Edwards Aquifer of Central Texas via streams sinking along its northeastern border. The southeastern boundary is marked by a rapid transition between oxygenated freshwaters and anoxic saline waters where OM is likely produced by chemolithoautotrophic microbes. Spatial and temporal heterogeneity in OM composition at these boundaries was investigated using isotopic and geochemical analyses. δ13C values for stream fine particulate OM (FPOM) (−33.34‰ to −11.47‰) decreased during regional drought between fall 2010 and spring 2012 (p<0.001), and were positively related to FPOM C:N ratios (r2=0.47, p<0.001), possibly due to an increasing contribution of periphyton. Along the freshwater-saline water interface (FWSWI), δ13CFPOMvalues (−7.23‰ to −58.18‰) correlated to δ13C values for dissolved inorganic carbon (δ13CDIC) (−0.55‰ to −7.91‰) (r2=0.33, p=0.005) and were depleted relative to δ13CDIC values by 28.44‰, similar to fractionation values attributed to chemolithoautotrophic carbon fixation pathways using DIC as the substrate. δ13CFPOMvalues also became enriched through time (p<0.001), and δ13CDIC values (r2=0.43, p<0.001) and δ13CFPOM values (r2=0.35, p=0.004) at FWSWI sites increased with distance along the southwest-northeast flowpath of the aquifer. Spatial variability in FWSWI δ13CDIC values is likely due to variable sources of acidity driving carbonate dissolution, and the temporal relationship is explained by changes to recharge and aquifer level that affected transport of chemolithoautotrophic OM across the FWSWI.Keywords: Carbon stable isotopes, spatial and temporal variability, chemolithoautotrophic production, allochthonous input, karst.DOI: 10.3986/ac.v42i2-3.666

Numerical Modeling of Saline Intrusion in Salar de Atacama

This paper presents the results of numerical simulations of groundwater circulation and solute transport at the Salar de Atacama through use of a numerical model to solve the two-dimensional problem of flow in an aquifer when considering the effects of variable density. The phenomena associated with solute transport are modeled by means of an advection-dispersion equation, and a linear relationship is assumed between fluid density and concentration of the dissolved solids. Simulations considered conditions of high groundwater evaporation, which depends on the depth of the phreatic surface. Results indicate that the discharge of groundwater occurs essentially in freshwater-saline water interface zones, where a number of lagunas begin. Different freshwater recharge scenarios were simulated, while it was verified that the effects of evaporation are important and minimize or buffer the variations in the phreatic surface and the discharges of groundwater that are the source of water supply for the lagunas.

Upconing and Saline Water Intrusion and the Need for Water Conservation in the Lower Part of Wadi Fatimah, Western Saudi Arabia

Shallow aquifers were exploited in western Saudi Arabia a long time ago in history. In the last five decades, and under the conditions of an increasing supply of water, many new wells were drilled at relatively deeper intervals than the hand-dug wells of the time. In the downstream part of Wadi Fatimah, and over time, more saline water has been brought up in a number of wells. Overexploitation of groundwater for industrial and agricultural purposes has led to the appearance of two phenomena responsible for the salinization-upconing and saline water intrusion. Upconing occurs in separate pockets while saline water encroachment affects the lower part of Wadi Fatimah and the area towards the Red Sea coast. This article deals with the identification and delineation of areas affected by each of the two phenomena. The methodologies used in this work are mainly hydrogeological and hydrochemical. Resistivity surveys helped in delineating the fresh water-saline water interface. Methods to control upconing and saline water intrusion are suggested to safeguard the aquifer from further contamination. Effective conservation measures are recommended for the protection of groundwater resources in Wadi Fatimah.

Discussion on ‘Delineation of saline intrusion in the Dungeness Shingle Aquifer’ by Okomeno Ulaya-Egbe Oteri

Mr R. A. Fox writes: Mr Oteri9s paper presents a limited review of the aquifer at Dungeness. However, the problem of saline intrustion is a complicated one and we would like to comment on several aspects where the content of the paper may be misleading. On page 48, the author notes that the lowest resistivity layer was interpreted as representing a saline water-saturated fine sand interface, which was accepted as the fresh water-saline water interface. He then goes on to note on page 49 that ’the depth of the fresh water-saline water interface beneath sea level in Dungeness is considerably smaller than that expected from the Ghyben-Herzberg principle‘. This states that, due to water density differences, the interface is about forty times deeper below mean sea level than the fresh water height is above sea level. A diagram to illustrate this point and the variations in interface level which apparently occur would be an interesting addition to the paper. The conclusion on page 50 infers that gravel-winning from the aquifer, already thinner than expected, raises the level of the saline water and encourages the intrusion of the saline wedge in the coastal fringe. However, comparing Figs 1 and 7 it is evident that the areas of thinner fresh water lens are more directly associated with supply well locations than gravel-winning areas. This tends to be confirmed whenFig. 7 contours are superimposed on Fig. 2. Ready Mixed Concrete (UK) Limited is one of the companies referred to in the introduction of

Hazards of Waste Disposal in Groundwater Basins: ABSTRACT

As problems of waste disposal become more critical, and environmental constraints more demanding, serious consideration is being given to isolated zones in groundwater basins as burial grounds for hazardous wastes. Proponents have suggested various geologic environments--thick impermeable clay or shale layers, heavily pumped subsidence basins, deep saline water bodies, etc.--as being safe disposal sites. However, several hydrogeologic constraints must be understood fully before a prospective site can be considered isolated. Unconsolidated deposits, even at great depth, are highly sensitive to changes in applied stress. Interstitial fluids may be squeezed from even relatively impermeable beds by modest stress changes. Compaction of the deposits below 2,500 ft has been induced in the San Joaquin Valley by groundwater pumping. As much as 30 ft of vertical shortening in 3,000 ft of water-bearing deposits has been measured. At Alpaugh, more than 4 ft of surface subsidence, principally due to the compaction of fine-grained beds at depth, has resulted from 250 ft of artesian head decline. Interstitial arsenic, presumably locked in clay interbeds for thousands of years of interaquifer circulation, now is being squeezed into the aquifers. Hydraulic stresses in a developing groundwater basin may affect beds considerably below the deepest pumping wells. Theoretically, seepage stresses, and thereby groundwater movement, ultimately would affect all beds down to basement or some underlying truly impermeable layer. Slow drainage continues from each interbed as long as excess pore pressures exist in that interbed. Injection of wastes into clay or shale interbeds would cause a sharp increase in pore pressures in these interbeds. Subsidence areas are usually poor candidates for waste-disposal sites. Generally, they have demonstrated their close communication with the groundwater system, their direct response to pressure changes, and their high compressibility. Heavily depleted artesian aquifers are not empty reservoirs ready for refilling. Rather, the groundwater systems are full and adjusting to the various stresses imposed on the basin. Compaction recorders show that pore pressures throughout the vertical section respond rapidly to pumping stresses. Horizontal and vertical ground movement, caused by horizontal and vertical groundwater flow, can be a serious threat to surface or buried structures. Subsidence affects probably 7,000 sq mi in California alone in amounts to 28 ft in a few places, and has caused the failure of thousands of deep and shallow water and oil wells. Horizontal shear stresses and surface tension cracks are a continued threat in a developing groundwater basin. Saline-water bodies in a basin are not necessarily permanently out of touch with the circulation system of the basin. Hydrologic stresses may force some of this saline water into the circulating groundwater system. Evidence suggests that upward seepage stresses may extend below the freshwater-saline water interface; also, in some areas, saline water may be moving upward into the freshwater zones. Few regions of a groundwater basin are isolated from the circulating flow system. Although injecting liquid wastes deep underground is sometimes expedient, the effects are complex and never ending. Long-lived radioactive wastes require isolation from the hydrosphere for periods of time ranging from 1,000 to several hundreds of thousands of years, but much waste material, to be safe, must be contained forever. And who can anticipate future natural and manmade stresses on the system? End_of_Article - Last_Page 1600------------