Spatial patterns of nitrate, chloride, sulfate, and fluoride concentrations in the Woodbine Aquifer of north-central Texas.

A geographic information system was used to map and analyze nitrate, chloride, sulfate, and fluoride concentrations in 110 wells tapping the Woodbine Aquifer. The study area, covering nine counties in north-central Texas, includes large percentages of both urban and agricultural land uses. Land use maps were compared with solute concentration data, and statistics were applied to detect associations between solutes, well depth, and land use. Anthropogenic sources such as fertilizer applications and natural sources such as gypsum, lignite, and clay deposits controlled nitrate, chloride, and sulfate concentrations, each inversely correlated with well depth. However, only one nitrate observation--from a shallow well in the aquifer's outcrop zone--surpassed the maximum contaminant level (MCL) of 44.3 mg L(-1). By comparison, nearly half of the sulfate and several of the chloride observations surpassed the MCL of 250 mg L(-1) for each of those ions. Volcanic ash deposits influenced fluoride concentrations, which directly correlated with well depth. There were no statistically significant associations between solute concentrations and land use. Low recharge rates and confining layers have mitigated anthropogenic impacts on solute levels in the aquifer.