Spatial and Temporal Variation in Groundwater Nitrate Removal in a Riparian Forest

We quantified nitrate (NO − 3 ) removal rates from groundwater in a red maple ( Acer rubrum L.) riparian forest subjected to NO − 3 dosing. The site was in Southern New England on soils classified as sandy mixed mesic Haplaquept soils and contained somewhat poorly (SPD) and poorly drained (PD) soils. The specific objectives were to examine groundwater NO − 3 removal rates within a riparian forest with respect to: (i) soil drainage class; (ii) depth below water table; and (iii) time of year. We created 16 experimental dosing/monitoring stations at two depths along three soil drainage class transects (SPD, SPD/PD, PD). We added solution containing NO − 3 and Br − continuously for 11 mo to a dosing well at each station. Groundwater was monitored at sampling wells 0.6 m downgradient of the dosing well. Nitrate removal rates were determined by coupling changes in the NO − 3 /Br − ratio with groundwater flux estimates from each experimental station. Although located just 20 m downgradient, the PD transects had substantially higher NO − 3 removal rates and lower dissolved oxygen than the SPD transects. Both the SPD/PD and PD transects had considerable NO − 3 ‐N removal capacity throughout the upper 1.5 m of the groundwater (16–46 µg kg −1 d −1 ). Rates were not significantly influenced by temperature. The scale of variation in removal rates suggests that high resolution soil and groundwater maps may be needed when riparian forests are to be used for water quality management.