A Tracer Test to Determine the Fate of Nitrate in Shallow Groundwater

Abstract

AbstractThe effectiveness of shallow groundwater areas to serve as a sink for NO3 is affected by many biological and physical properties. However, the direct impact of these properties on the fate of NO3 in shallow groundwater is not well understood, especially where the soils are intermittently saturated. This study was conducted to assess in situ reaction and transport of NO3‐N in an intermittent shallow groundwater system. Tracer experiments were conducted within an imposed constant flowing shallow groundwater. A constant‐head, single injection well technique was adapted for this study using multilevel soil water samplers placed at 14 locations around the center injection well. The use of Br as a tracer for NO3‐N in these constant‐flow experiments provided the means to assess in situ NO3‐N removal both with and without added C. In experiments without added C, an average NO3 removal rate of 0.33 g N m−2 d−1 was estimated. In a second experiment with dextrose added as an added C source, an average NO3 loss rate 1.06 g N m−2 d−1 was observed. The observed response to added dextrose indicates that the N removal processes were primarily microbial in origin, i.e., the NO3 was denitrifled or immobilized into microbial biomass.