Relationships Between Groundwater Recharge Dates, Nitrate Levels, and Denitrification in a Central Wisconsin Watershed
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Date
2004-01Author
Guldan, Nathan M.
Publisher
University of Wisconsin-Stevens Point, College of Natural Resources
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Show full item recordAbstract
Stream water quality is strongly affected by groundwater discharge in baseflow
dominated stream systems. However, during baseflow periods, stream nitrate (NO3-)
concentrations tend to be lower than groundwater NO3- concentrations in adjacent
agricultural recharge areas. Two factors contributing to this difference are denitrification
and lag time (time between groundwater recharge and discharge). In this study we
collected groundwater at the tail end of its flow path at the groundwater/surface water
interface. Samples were collected from minipiezometers installed approximately 70 cm
below the streambed in permanent discharging areas of the baseflow dominated
Tomorrow/Waupaca River Watershed. The effects of 1) denitrification and 2) lag time
on basin-scale NO3- concentrations were examined. Denitrification removed up to 373
μmoVL NO3- from groundwater before it discharged to streams with the overall
denitrification efficiency (% of NO3- in recharge water converted to N2 gas before
discharge) in discharging groundwater estimated at 59%. Measured NO3- concentrations
were nearly zero in all groundwater recharged prior to 1970 and varied up to
approximately 600 μmoVL in groundwater recharged after 1970, but total NO3- (NO3- +
denitrified N) measurements suggested that the onset of groundwater NO3- contamination
actually occurred prior to 1970. Denitrification efficiency was high (86%) in older
groundwater (> 32 yrs), which contributes about one-third of the discharge to the TWR,
while in younger groundwater, denitrification efficiency was dramatically lower (47%).
Our results suggest that conventional baseflow monitoring approaches can provide little
useful information about the influence of current land use and land management practices
on baseflow water quality. By allying water quality and lag time measurements with a
basin-scale network of minipiezometers, this study provided insight into how spatial and·
temporal patterns of land use have affected stream water quality over several decades.