Monitoring denitrification-driven microbial activity in floodplain aquifer sediments using spectral induced polarization (SIP)

Cora Strobel1, Olaf Cirpka1, Adrian Mellage1
1 Universität Tübingen

V 16.2 in Freie Themen

23.03.2022, 10:30-10:45, HS 1

Nitrate, stemming from nitrogen fertilizers applied over agricultural landscapes, persists as a groundwater contaminant of concern despite its natural attenuation potential by groundwater microbial communities. The co-occurrence of nitrate (electron acceptor), microbes and an electron donor is a necessary prerequisite for denitrification to take place. Thus, whether and where the microbes experience the necessary conditions to reduce nitrate is governed by the physico-chemical heterogeneity of aquifers, hindering our ability to monitor natural attenuation in the subsurface. Geophysical methods, such as spectral induced polarization (SIP), provide a potentially powerful alternative to monitor reactive transport phenomena, by detecting the changes in subsurface charging properties induced by the reactions of interest. Here, we present results from a flow-through column experiment packed with natural aquifer sediment from the Ammer floodplain aquifer near Tübingen, Germany. We biostimulated the sediment pack via a continuous nitrate and lactate injection followed by a static period during which no nitrate was injected. Concurrently, we collected pore water (along the column length) and breakthrough curve samples for geochemical analysis while also measuring time-lapse SIP responses. Concentration time series highlighted that bulk nitrate turnover occurred in the upper half of the columns, closest to the inlet. Measured SIP responses showed the appearance of a distinct secondary peak in polarization during biostimulation, that was otherwise absent during the injection of bromide, a conservative tracer. We quantified this additional contribution to subsurface charge storage via a double Cole-Cole model, accounting for the superposition of a static background contribution of charge storage from the sediment and a temporally-variant ‘secondary’ contribution. The increase in this secondary polarization response during biostimulation and subsequent decrease during the static phase, matched the expected timing of increasing and later decreasing microbial activity based on the geochemical data. In addition, a comparison with signals recorded in a control column, experiencing an identical flow regime but with no added nitrate, highlighted a clear absence of the secondary signal, providing further evidence for a microbial-contribution. The peak frequency of the secondary peak indicates a dominant polarization length-scale of 1 µm, the size range of imaged bacterial cells in our sediment samples. Our findings provide a framework for decoupling parallel geophysical signal contributions during reactive transport in natural porous media, enabling the non-invasive detection of the location and timing of elevated microbial activity. Furthermore, they poise time-lapse SIP as an in situ approach to extract real-time information of reaction kinetics in experimental systems, with implications for field investigations that aim to delineate microbial degradation hotspots.



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