Origin of dissolved groundwater sulphate in coastal plain sediments of the Rio de la Plata, Eastern Argentina

Groundwater of the coastal plain of the Rio de La Plata, Argentina, contains up to 17 g L super(-1) SO sub(4) and 37 g L super(-1) TDS. Some of this SO sub(4) is from paleo-seawater intrusion; however, SO sub(4):Cl ratios can be >2:1, and most of the SO sub(4) must, therefore, have another source. Three possible sources were investigated: gypsum, organic matter, and iron sulphides. Dissolved SO sub(4) showed delta super(34)S values from -7 to 0ppt, typical values for S from iron sulphides or organic matter, but distinct from that of seawater (+22ppt). To test whether the SO sub(4) was derived from oxidation of reduced S, four 4-m cores were taken from marine sediments of the coastal plain. Two were taken from higher, drier areas where the highest dissolved SO sub(4) values were encountered, and two were taken from lower, wetter areas that had much lower SO sub(4) concentrations. Pore water SO sub(4), Cl and alkalinity were determined; solids were analyzed for SO sub(4), sulphide-S and organic-S. Sulphide-S was the dominant form of reduced S, averaging about 0.5% S in the lower interval (2.5-4 m) of the cores. Sulphide was absent in the upper 2.5 m in both topographically higher and lower areas. Sulphate was present in the entire unit in the higher, drier areas, but almost absent in lower areas. Organic-S was insignificant. Our model for the origin of dissolved SO sub(4) is: fine-grained pyrite was oxidized during hotter or drier periods. Some resulting dissolved SO sub(4) was precipitated as gypsum. Iron from the pyrite precipitated as FeOOH. Lower, wetter areas formed over time where recharging groundwater dissolved most of the gypsum. In higher areas with low hydraulic gradients and high net evapotranspiration, SO sub(4) remained as gypsum and in the dissolved phase.