IMPACT OF BIOFUELS PRODUCTION on NITROGEN FLUXES IN GROUNDWATER IN THE USA

Contamination of groundwater by nitrate is of concern because elevated concentrations in water can affect human and ecosystem health. Correlation between agricultural land use and high NO₃⁻ concentration in groundwater has been documented since the 1970’s. The Energy Independence and Security Act of 2007 calls for the production of 36 billion gallons of biofuels per year by 2022. This expansion will cause a significant change in agricultural land use including crop type shift, decrease in Conservation Reserve Program acreage , and changes in agricultural management practices. Questions have been raised as to the sustainability of water quality under the anticipated agricultural land use changes. This study selected 11 agricultural areas across the U.S.A. to illustrate the effects of changing land use on NO₃⁻ contamination in groundwater by using a 1-D vertical analytical model. Profiles of NO₃⁻, N₂ from denitrification, Cl⁻, and atmospheric age tracers were simulated by adjusting parameter values for recharge rate, N leaching rate, Cl⁻ leaching rate, denitrification rate and unsaturated zone travel time. The calibrated models for 11 sites were used to forecast N transport below the water table in response to scenarios of changing agricultural activities, which included the crop type shift due to corn production expansion, cellulosic biofuels feedstock (Switchgrass) production, and changes of recharge rates from irrigation rate changes. Model results showed that biofuels production will further increase N levels in shallow groundwater and the N contamination front move downward. For example, under historic N application rates, the N contamination front in year of 2006 defined as 10 mg/L (the EPA Maximum Contaminant Level) reached depths of 1.5 to 20 m below the water table at the 11 study sites. If the denitrification rates remain constant, the model predicts the N contamination front will shift downward by 3 to 5 m at these sites by the year 2022 in response to anticipated crop type shifts to achieve the biofuels goals. The rate of continued downward migration of the front after 2022 is dependent on the unsaturated zone travel time.