An in-situ study of the occurrence and rate of denitrification in a shallow unconfined sand aquifer

Abstract

Denitrification in shallow groundwater flow systems has been inferred from the observation of declining nitrate concentrations below the water table, and corresponding decline in dissolved oxygen concentrations. To provide direct evidence of denitrification within the saturated zone, and to determine the rate of denitrification, an in-situ injection experiment was conducted using a specially designed injection-withdrawal-sampling drive point. Nitrate and a conservative tracer (bromide) were added to natural groundwater and injected at 3 m depth into a shallow, unconfined sand aquifer. The relative changes in concentration were then observed with time. After 356 h the concentration of nitrate-N in the injected water declined from the initial 13 g m −3 to less than 0.1 g m −3. The decrease in nitrate concentration was much greater than the corresponding decrease in the concentration of bromide, confirming a preferential loss of nitrate. The loss of nitrate was preceded by a decline in dissolved oxygen concentration to less than 0.1 g m −3, and coincided with an increase in bicarbonate concentration of 142 g m −3. The production of bicarbonate observed in the injection experiment, 2.59 mmole HCO − 3 per mmole nitrate denitrified, agreed with that calculated using an equilibrium geochemical model of the denitrification process. An increase in the population of denitrifying organisms from 1 to 23 organisms per gram of soil was detected in core samples collected at the depth of injection 169 h after the start of the experiment. The measured rate of denitrification ranged from 0.0078 to 0.13 g m −3 NO − 3/1bN h −1, and is in reasonable agreement with published rates for saturated soils. The organic carbon source required for denitrification is either dissolved organic carbon or soil organic carbon. Soil organic carbon, at 0.08–0.16% by weight, is adequate to denitrify large amounts of nitrate.