MODELING BASIN SCALE NITRATE TRANSPORT CONSIDERING ALLUVIAL AND BIOGEOCHEMICAL HETEROGENEITY

Nitrate is a pervasive contaminant in groundwater basins. Sources of nitrate include fertilizer, confined animal feeding operations, and septic systems. Given rapid urbanization, increase in fertilizer use, and industrialization of farming since the 1940's, future impact of nitrate contamination is not well understood.

To better understand long-term basin-scale nitrate transport, we are developing high-resolution nitrate transport models that include consideration of alluvial and microbiological heterogeneity. Alluvial heterogeneity is characterized by combining transition-probability/Markov and Gaussian random field geostatistical methods. The geostatistical approach considers numerous (but low quality) driller's logs as "soft data" to help constrain the hydrogeologic model. The geostatistical model also serves as a template for identifying potential zones of denitrification, where microbial activity may convert nitrate to nitrogen gas.

Unless denitrification occurs, nitrate transport behaves like a conservative tracer. Under certain conditions of low dissolved oxygen and no organic carbon, "autotrophic" denitrification can occur with presence of an electron donor such as pyrite. Conditions for denitrification will vary tremendously within a groundwater basin. At our dairy field site, we are identifying zones of active denitrification and obtaining samples for laboratory analysis to quantify changes in populations of denitrifying bacteria for parameterization of Monod kinetic denitrification rate models. Isotopically-determined groundwater ages help calibrate the flow and transport models and scale-up laboratory and field results to the basin scale. Preliminary modeling results indicate that without denitrification or decreased nitrate loading, nitrate will eventually contaminate deeper aquifers used for municipal water supplies.