Extreme groundwater flooding stimulates microbialmetabolic activity and biogeochemical turnover in a major UK aquifer

Microbial metabolic activity in most aquifers is considered to below and biogeochemical cycling rather constant, reflecting the stationarytemperature conditions and solute concentrations in groundwater.Substantialgroundwater recharge during groundwater flooding events may have thepotentialto critically alter the carbon and nutrient availability in highlyresponsiveunconfined aquifers. This study investigates the impacts of intensivegroundwater flooding in Spring 2014 on nutrient inputs into a shallow Chalkaquifer in the UK and its consequences for the microbial metabolicactivity andbiogeochemical turnover in groundwater.Groundwater levels during the flood event rose by > 10m within aweek, at some locations reaching the ground surface. The intensity andtimingof groundwater level responses varied in space, with carbon and nitrogenconcentrations in a network of monitoring boreholes increasing with furthervariability in response times to the observed pressure head changes.Quantificationof microbial metabolic activity (pioneering the first application of theResazurin/Resorufin smart tracer system in groundwater) in the observedgroundwater boreholes revealed distinct hotspots of metabolism andassociatedrespiration rates. Repeated observations saw baseline conditions beingrestoredat all boreholes within less than a year, with significant variability inthetailing of borehole concentrations and activities. Our results indicatethat inparticular for highly responsive unconfined aquifers with relatively shortvadoze zone transit times there could be significant event-basedvariability incarbon and nutrient recharge and related microbial metabolic activity andbiogeochemical turnover.