Predicting the denitrification capacity of sandy aquifers from in situ measurements using push-pull 15N tracer tests

Abstract

Knowledge about the spatial variability of in situ denitrification rates ( D r(in situ)) and their relation to the denitrification capacity in nitrate-contaminated aquifers is crucial to predict the development of groundwater quality. Therefore, 28 push-pull 15N tracer tests for the measurement of in situ denitrification rates were conducted in two sandy Pleistocene aquifers in northern Germany. The 15N analysis of denitrification-derived 15N-labelled N2 and N2O dissolved in water samples collected during the push-pull 15N tracer tests was performed using isotope ratio mass spectrometry (IRMS) in the lab and additionally for some tracer tests online in the field with a quadrupole membrane inlet mass spectrometer (MIMS) in order to test the feasibility of on-site real-time 15N analysis. Aquifer material from the same locations and depths as the push-pull injection points was incubated, and the initial and cumulative denitrification after 1 year of incubation ( D cum(365)) as well as the stock of reduced compounds (SRC) was compared with in situ measurements of denitrification. This was done to derive transfer functions suitable to predict D cum(365) and SRC from D r(in situ). D r(in situ) ranged from 0 to 51.5 Î1/4g N kgg'1 dg'1. Denitrification rates derived from on-site isotope analysis using MIMS satisfactorily coincided with laboratory analysis by conventional IRMS, thus proving the feasibility of in situ analysis. D r(in situ) was significantly higher in the sulfidic zone of both aquifers compared to the zone of non-sulfidic aquifer material. Overall, regressions between the D cum(365) and SRC of the tested aquifer material with D r(in situ) exhibited only a modest linear correlation for the full data set. However, the predictability of D cum(365) and SRC from D r(in situ) data clearly increased for aquifer samples from the zone of NO3g'-bearing groundwater. In the NO3g'-free aquifer zone, a lag phase of denitrification after NO3g' injections was observed, which confounded the relationship between reactive compounds and in situ denitrification activity. This finding was attributed to adaptation processes in the microbial community after NO3g' injections. It was also demonstrated that the microbial community in the NO3g'-free zone just below the NO3g'-bearing zone can be adapted to denitrification by NO3g injections into wells for an extended period. In situ denitrification rates were 30 to 65 times higher after pre-conditioning with NO3g. Results from this study suggest that such pre-conditioning is crucial for the measurement of D r (in situ) in deeper aquifer material from the NO3g'-free groundwater zone and thus for the prediction of D cum(365) and SRC from D r(in situ).