Clays groundwater salinity

Leaching of clay and silt layers in the sandstone, saline groundwater under the sandstone and leaching of mineral deposits... 

Clay and shale are hydroaluminum silicates that by themselves do not add salts to the water that comes in contact with them. However, clay and shale often contain veins and nodules of gypsum, pyrite, and rock salt. Clay and shale are impermeable and form aquicludes rather than aquifers (sections 2.3 and 2.4), but because of the high solubility of gypsum and especially rock salt, groundwater in contact with clay and shale at the base of aquifers often gets saline and is of poor quality. 

Finally, seawater intrusion is also characterized by high 6 Ss ifa,e (>20%o) and S B (>39%o) values that are associated with low SO4/CI and B/Cl ratios below the marine ratios (0.05 and 8 X 10 , respectively). The relative depletion of sulfate and S enrichment is attributed to sulfate reduction along the salt-freshwater interface (Krouse and Mayer, 2000). The removal of dissolved boron is explained via adsorption onto clay minerals in which " B is adsorbed preferentially onto the clays and the residual saline groundwater becomes enriched in "B (Vengosh et al., 1994). 

The differentiation of soil permeability when soil becomes clogged with clay and mineral precipitation causes lateral flow of saline soil water and shallow groundwater towards low-lying areas. The final stage of the dryland cycle is salinization of adjacent streams and rivers. The chemical composition of the salinized river in the dryland environment reflects the net results of salt recycling between soil, subsoil, groundwater, secondary soil, soil solution, and surface water (Figure 10). 

The flow of water through a semi-permeable membrane (clay, shale) from water with a small concentration of dissolved solids to water with a greater concentration is called osmosis (e.g. Bredehoeft et al., 1982 Neuzil, 1986). The osmotically-induced flow of water occurs because of a difference in vapour pressure across the membrane (Hinch, 1980). The aqueous activity will be relatively small in water with a relatively large concentration of dissolved solids, because more water molecules are bonded on the dissolved ions (Hinch, 1980). In a sandstone-shale sequence with water of equal chemical concentration, the aqueous activity of the shale water will be less than that of the sandstone-water, because water molecules are adsorped on the large mineral surfaces of the shale (Hinch, 1980). As a consequence, the water salinity differences that may exist in sandstone-shale sequences in the intermediate and deep subsystems of burial-induced groundwater flow may actually be in osmotic equilibrium. 

These wells derive their water from the Plio-Pleistocene Dupi Tila aquifer which lies beneath the Madhupur Clay. The screened interval of the wells is unknown. The chemical analyses (Table 6) indicate that the groundwater is of low salinity (Na <50 mg L B <0.1 mg L (data not shown) SO4 0.6-35 mg L" ). Concentrations of most trace elements are low to very low. In particular, As concentrations are all <0.5 pg L and P concentrations are less than or equal to 0.1 mg L (data not shown). The low As concentrations are believed to have been confirmed by other more extensive, but unpublished, surveys of Dhaka city water. Dissolved iron concentrations are also low (<0.25 mg L ). Of the measured elements which have WHO health-based guideline values, only Mn has any exceedances one sample had a Mn con-... 

The various playa environments include a central salt crust or saline pan, composed of dry salt, commonly halite (Figure 10.3D, E), but in some cases trona (Eugster, 1970), gypsum (Stoertz and Ericksen, 1974) or other sulphates such as mirabilite, epsomite or bloedite. The saline mudflat is typically moist clay to silt with surface salt efflorescences and intrasediment (displacive) evaporite minerals. In some systems, these may be zoned on a broad scale, with more-soluble minerals towards the lowest central portion of the mudflat, caused by groundwater evaporation gradients (e.g. Saline Valley, California Hardie, 1968). 

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