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 μg N kg⁻¹ d⁻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 NO₃⁻-bearing groundwater.

In the NO₃⁻-free aquifer zone, a lag phase of denitrification after NO₃⁻ 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 NO₃⁻ injections. It was also demonstrated that the microbial community in the NO₃⁻-free zone just below the NO₃⁻-bearing zone can be adapted to denitrification by NO₃⁻ injections into wells for an extended period. In situ denitrification rates were 30 to 65 times higher after pre-conditioning with NO₃⁻. Results from this study suggest that such pre-conditioning is crucial for the measurement of Dr(in situ) in deeper aquifer material from the NO₃⁻-free groundwater zone and thus for the prediction of D_cum(365) and SRC from D_r(in situ).