HYDROLOGIC CONTROLS IN NITRATE, SULFATE, AND CHLORIDE CONCENTRATIONS

Nitrate-nitrogen, SO4, and Cl concentrations and discharge rates from a small, upland watershed in east central Pennsylvania were analyzed in the context of a layered aquifer. A pattern of greater NO3-N concentrations during periods of greater discharge was generally followed by declining concentrations during hydrograph recession. The coincident temporal pattern of NO3-N and discharge appears hydrologically controlled because both NO3-N and SO4 exhibited similar patterns even though there are important differences in their behavior and reactivity in biologically controlled systems. Chemical concentration patterns in stream flow arose from flow through a two-layer geologic system in which NO3-N and SO4 concentrations were different between layers. The substantially lower concentrations observed in the deeper water were attributed to recharge from areas with forest and mixed land uses. Discharge from this deeper layer supported the lowest flows and the lowest chemical concentrations observed in stream flow. Increases in stream flow resulted from greater discharge from the shallower groundwater layer. Adding discharge from the shallower groundwater resulted in higher chemical concentrations in stream flow. Nitrate-nitrogen concentration estimated with a mixing cell model, in which concentration changes result from dilution only, closely matched measured concentration.