Comparison of Denitrification Activity Measurements in Groundwater Using Cores and Natural-Gradient Tracer Tests

Abstract

The transport of many solutes in groundwater is dependent upon the relative rates of physical flow and microbial metabolism. Quantifying rates of microbial processes under subsurface conditions is difficult and is most commonly approximated using laboratory studies with aquifer materials. In this study, we measured in situ rates of denitrification in a nitrate-contaminated aquifer using small-scale, natural-gradient tracer tests and compared the results with rates obtained from laboratory incubations with aquifer core material. Activity was measured using the acetylene block technique. For the tracer tests, co-injection of acetylene and bromide into the aquifer produced a 30 μM increase in nitrous oxide after 10 m of transport (23−30 days). An advection−dispersion transport model was modified to include an acetylene-dependent nitrous oxide production term and used to simulate the tracer breakthrough curves. The model required a 4-day lag period and a relatively low sensitivity to acetylene to match the narrow nitrous oxide breakthrough curves. Estimates of in situ denitrification rates were 0.60 and 1.51 nmol of N2O produced cm-3 aquifer day-1 for two successive tests. Aquifer core material collected from the tracer test site and incubated as mixed slurries in flasks and as intact cores yielded rates that were 1.2−26 times higher than the tracer test rate estimates. Results with the coring-dependent techniques were variable and subject to the small-scale heterogeneity within the aquifer, while the tracer tests integrated the heterogeneity along a flow path, giving a rate estimate that is more applicable to transport at the scale of the aquifer.