Effect of Transient Hydrologic Events on Biogeochemical Processes

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0208386MillsIn field studies of groundwater in a range of hydrogeological settings, the rates of microbially mediated geochemical reactions have been shown to be far faster than hydrological transport rates. These relative rates lead to large changes in concentration of a variety of redox-sensitive chemical species over short distances within aquifers. In some systems, however, transient hydrological events (e.g., heavy precipitation on dry soil or floods in streams) can rapidly alter the geochemical environment in which the microbes exist. In that circumstance, the time constants associated with the physical events approach those of the microbially mediated processes. The proposed research addresses the question: How do hydrological processes and biogeochemical processes interact in the riparian-hyporheic zone of streams where time scales of the various processes are on the same order - hours to weeks? The proposed research will examine the effect of bank-storage events on reductive microbiological processes occurring in the riparian-hyporheic zone of a low-relief coastal plain stream. In particular, the concentration of nitrate-N in the groundwater feeding the stream is about 15 mg/liter, but the stream-water concentrations average only 1.6 mg/liter, suggesting high denitrification activity in the riparian- hyporheic zone. We envision the disruption of anaerobic processes during rain events in which rapid infiltration raises the water table and "flushes" chemicals (viz., nitrate) in the pore water out into the stream and perfuses the area with oxygenated water. We expect a similar disruption without flushing during storms in which a rapid rise in stream level pushes oxygenated water into the stream banks, thereby transiently extending the hyporheic zone. We will also examine the effect of flushing or bank-storage events on the denitrification and will document the rate of return of the functional abilities of the microbial communities to the pre-disturbance levels. Because these transient events have the potential to affect release of chemicals like nitrate from the groundwater into the surrounding surface waters, we will investigate how transient conditions in riparian soils affect the overall budget of biologically active chemicals. The proposed work includes detailed field observations, field manipulation experiments, laboratory batch and "mesocosm" experiments and mathematical modeling. We anticipate that we will learn how microbially mediated geochemical reactions and transient hydrological processes with time scales on the same order are linked. We will develop quantitative descriptions of these processes and investigate how frequently biogeochemical processes are "reset" by different hydrological events the synoptic data that we will examine how the transient effects of local processes influence regional nutrient fluxes.