Identifying Groundwater Nitrate Sources and Sinks

Effective management of nitrate-impacted aquifers requires identifying the sources of nitrate to the aquifer, and understanding the transport and fate of nitrate within the aquifer. Source identification is crucial to effective source mitigation; conversion of residential septic systems to sewer in a rural community will be ineffective if the primary source of nitrate contamination is agricultural operations. Determining the source of nitrate in an aquifer, however, can be difficult Nitrate occurs naturally and has a number of anthropogenic sources, including synthetic fertilizers, animal manure, septic systems, and municipal wastewater. More than one source may be present in a recharge area, and the sources may be diffuse, distributed, or localized. As a result, groundwater nitrate contamination is often widely distributed with limited concentration range, making traditional plume-tracking methods of placing wells upgradient and downgradient of a suspected source ineffective. Characterizing nitrate transport is also essential to effective management. Since nitrate travels without significant attenuation in oxygen-rich groundwaters, natural tracers of groundwater flow can help distinguish between ongoing and historic sources of nitrate, and can be used to identify appropriate wells for assessing the impact of land use or management changes on groundwater quality. The most significant process for degradation of nitrate in groundwater is denitrification, the microbial conversion of nitrate to nitrogen gas. Demonstrating that denitrification is occurring can lead to acceptance of monitored natural attenuation as a viable remediation plan, and can reconcile measured and modeled nitrate concentrations.