Relationships Between Groundwater Recharge Dates, Nitrate Levels, and Denitrification in a Central Wisconsin Watershed

Abstract

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.