Bicarbonate, sulfate, and chloride water in a shallow, clastic-dominated coastal flow system, Argentina

Most of the cities southeast of Buenos Aires, Argentina, depend heavily on ground water for water supply. Whereas ground water quality is generally go od in the region, economic development along the coastal plain has been con strained by high salinities. Fifty-four webs were sampled for major ions in zones of recharge, transport and discharge in an area near La Plata, 50 km southeast of Buenos Aires. T he shallow, southwest to northeast coastal now system is >30 inn long but i s only 50 to 80 m thick. It consists of Plio-Pleistocene fluvial sand overl ain by Pleistocene eolian and fluvial silt and Holocene estuarine silty cla y. Hydrochemical endmembers include HCO3, SO4, and Cl water. Bicarbonate-type water includes high plain recharge water (Ca-Na-HCO3) that evolves through cation exchange and calcite dissolution to a high pH, pure Na-HCO3 endmembe r at the southwest edge of the coastal plain, Similar Na-HCO3 water is also found underlying recharge areas of the central coastal plain, and a lens o f Ca-HCO3 water is associated with a ridge of shell debris parallel to the coast. Mixed cation-Cl water near the coastline represents intruded sea water that has undergone cation exchange. Chemically similar water underlying the sou thwest coastal plain, however, can be shown isotopically to have formed fro m fairly dilute solutions concentrated many times by evapotranspiration. In the central coastal plain, Na-SO4-Cl water (up to 17,000 mg/L SO4) under lies hummocks and, at depth, forms plumes in the subsurface. The chemical c ontrols are dissolution of gypsum precipitated during pyrite oxidation, eva potranspiration, and calcite precipitation. This extreme diversity of water chemistry in a shallow, elastic now system is uncommon. It is made possible by (1) the presence of oxidized, originall y pyritic, but slightly permeable sediments that have been exposed to evolv ing geochemical and hydrodynamic conditions, and (2) highly focused recharg e that favors the development of fresh water bodies underlying recharge zon es and stagnant, evaporated water underlying other areas.