University of California

Presentations 2016

Green, Christopher

Presentation Title
Efficient data-driven estimation of nitrate transport and reactions in groundwater using a vertical flux model
US Geological Survey
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Nitrate contamination in shallow groundwater extends across large regions of agricultural land around the world. Predictions of vertical nitrate migration are difficult because the effects of input histories can appear similar to the effects of reactions. Furthermore, nitrate reactive transport at large scales can be difficult to capture with detailed numerical models. Towards addressing these issues, we developed a simple modeling framework called the Vertical Flux Model (VFM) to analyze nitrate input histories and future concentrations in groundwater based on well information and measured tracer concentrations including dissolved oxygen, nitrate, atmospheric tracers of groundwater age, and dissolved gases. Application of the method yielded information about reaction rates and eventual depth of migration of nitrate at 14 detailed study sites across the US. Under the current N application rates and hydrogeochemical conditions, downward migration of the nitrate front will continue at seven sites with low denitrification rates (zero order rate constant less than 0.2 mg/L/yr). Comparison of results among the 14 sites suggests that rates of oxygen reduction and denitrification are correlated, and that denitrification rates tend to exceed oxygen reduction rates on an electron equivalent basis. Preliminary results from regional VFMs in California and Wisconsin confirm the correlation of rates, suggesting that that oxygen reduction rates, which are relatively easy to obtain, can serve as a proxy for denitrification rates in some regional studies. The applications of the VFM indicate that the method gives an efficient and flexible means of characterizing current and future nitrate fate and vertical transport in groundwater.

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