Da. Burns et al., EFFECT OF GROUNDWATER SPRINGS ON NO3 CONCENTRATIONS DURING SUMMER IN CATSKILL MOUNTAIN STREAMS, Water resources research, 34(8), 1998, pp. 1987-1996
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.