EFFECT OF GROUNDWATER SPRINGS ON NO3 CONCENTRATIONS DURING SUMMER IN CATSKILL MOUNTAIN STREAMS

Groundwater and stream water data collected at three headwater catchme nts in the Neversink River watershed indicate that base flow is sustai ned by groundwater from two sources: a shallow flow system within the till and soil and a deep flow system within bedrock fractures a;nd bed ding planes that discharges as perennial springs. Data from eight well s finished near the till/bedrock interface indicate that saturated con ditions are not maintained in the shallow flow system during most summ ers. In contrast, the discharge of a perennial spring remained constan t during two summer rainstorms, providing evidence that the deep flow system is disconnected from the shallow flow system in summer. Dischar ge from perennial springs was the principal source of streamflow in a headwater reach during low flow. Mean NO3- concentrations were 20-25 m u mol L-1 in five perennial springs during the summer but only 5-10 mu mol L-1 in shallow groundwater. Thus the deep flow system does not re flect typical NO3- concentrations in the soil during summer. A hydrolo gic budget at a headwater drainage reveals that March and late fall ar e the.principal groundwater recharge periods. Residence time modeling based on analyses of O-18 and S-35 indicates that groundwater in the d eep flow system is 6-22 months old. These data indicate that summer ba se flow largely originates from previous dormant seasons when availabl e soil NO3- is greater. In these Catskill watersheds, high base flow c oncentrations of NO3- during summer do not provide sufficient evidence that the atmospheric N deposition rate exceeds the demand of terrestr ial vegetation.