Predicting long-term denitrification capacity of sandy aquifers from incubation experiments and sediment properties

Abstract

Abstract. Knowledge about the spatial variability of denitrification rates and the lifetime of denitrification in nitrate-contaminated aquifers is crucial to predict the development of groundwater quality. Therefore, regression models were derived to estimate the measured denitrification capacity of incubated aquifer sediments from initial denitrification rates and several sediment parameters, namely total sulphur, total organic carbon, extractable sulfate, extractable dissolved organic carbon, hot water soluble organic carbon and potassium permanganate labile organic carbon. For this purpose, we incubated aquifer material from two sandy Pleistocene aquifers in Northern Germany under anaerobic conditions in the laboratory using the 15N tracer technique. The measured long-term denitrification capacities ranged from 0.18 to 56.2 mg N kg−1 yr−1. The laboratory incubations exhibited high differences between non-sulphidic and sulphidic aquifer material in both aquifers with respect to all investigated sediment parameters. Denitrification rates and the estimated lifetime of denitrification were higher in the sulphidic samples. Denitrification capacity measured during one year of incubation (Dcap) was predictable from sediment variables within a range of uncertainty of 0.5 to 2 (calculated Dcap/measured Dcap) for aquifer material with a Dcap > 20 mg N kg−1 yr−1. Predictions were poor for samples with lower Dcap like samples from the NO3−-bearing groundwater zone, which includes the non-sulphidic samples, from the upper part of both aquifers where Dcap is not sufficient to protect groundwater from anthropogenic NO3− input. Calculation of Dcap from initial denitrification rates was only successful for samples from the NO3−-bearing zone, whereas a lag-phase of denitrification in samples from deeper zones of NO3− free groundwater caused imprecise predictions. Our results thus show that Dcap of sandy Pleistocene aquifers can be predicted using a combination of short-term incubation and analysis of sediment parameters. Moreover, the protective lifetime of denitrification sufficient to remove NO3− from groundwater in the investigated aquifers is limited which demonstrates the need to minimize anthropogenic NO3− input.