Ground-water chemistry in the Valle de Yabucoa alluvial aquifer, southeastern Puerto Rico

The Valle de Yabucoa alluvial aquifer is located along the southeastern coast of Puerto Rico and is the public and industrial water supply for the town of Yabucoa, The aquifer consists of more than 100 meters of alluvium overlying relatively impermeable bedrock. Water in the aquifer has iron and manganese concentrations as high as 28 and 3.6 milligrams per liter, respectively. The largest contributor of ferrous iron to the ground water appears to be an iron oxide phase (probably maghemite, gamma-Fe2O3, an alteration of magnetite). Ilmenite, (Fe,Mn)TiO3, is probably the sole significant source of manganese. Although the elevated iron and manganese concentrations in the aquifer pose no health threat, they frequently exceed the secondary standards for drinking water established by the U.S. Environmental Protection Agency of 0.3 milligram per liter for iron and 0.05 milligram per liter for manganese. In addition to the iron and manganese problems, the ground water in the aquifer locally contains concentrations of dissolved solids that exceed the secondary drinking water standard of 500 milligrams per liter. The high concentrations of dissolved solids have caused some water-supply wells to be taken out of service.