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 i n the context of a layered aquifer. A pattern of greater NO3-N concent rations during periods of greater discharge was generally followed by declining concentrations during hydrograph recession. The coincident t emporal pattern of NO3-N and discharge appears hydrologically controll ed because both NO3-N and SO4 exhibited similar patterns even though t here are important differences in their behavior and reactivity in bio logically controlled systems. Chemical concentration patterns in strea m flow arose from flow through a two-layer geologic system in which NO 3-N and SO4 concentrations were different between layers. The substant ially lower concentrations observed in the deeper water were attribute d to recharge from areas with forest and mixed land uses. Discharge fr om this deeper layer supported the lowest flows and the lowest chemica l concentrations observed in stream flow. Increases in stream flow res ulted from greater discharge from the shallower groundwater layer. Add ing discharge from the shallower groundwater resulted in higher chemic al concentrations in stream flow. Nitrate-nitrogen concentration estim ated with a mixing cell model, in which concentration changes result f rom dilution only, closely matched measured concentration.