Mobilization of Authigenic Nitrate in an Arid Environment

Mobilization of Authigenic Nitrate In An Arid Environment

At an industrial locality in arid northern Arizona, a multi-acre process water impoundment has created an artificial area of localized groundwater recharge. Water dissipating through this zone of recharge has manifested itself in a small series of localized seeps and a down-gradient, subsurface plume with a characteristic geochemical signature. The subsurface plume occurs in an alluvial, water-table aquifer and has an orientation that somewhat mimics the topography of an overlying wash. Repeated groundwater sampling events from a series of strategically-located monitoring wells has allowed for a coherent subsurface conceptual model to be developed. Similarly, the chemistry of the source water within the impoundment has been closely characterized. Numerous researchers working in New Mexico and Arizona have noted the occurrence of naturally-occurring (authigenic) nitrate in arid environments produced by strong storm events when followed by rapid evaporation. The subsequent transfer of atmospheric nitrogen into a nitrate residuum in the soil is a viable explanation for the occurrence detailed in this presentation. The entire drainage basin encompassing this small wash is devoid of any history of agricultural activities, other than stock grazing. The potential for artificial contributions of nitrate to the subsurface flow system has been completely ruled out.

Geophysical investigative techniques, in concert with monitoring well data, analytical results and hydrogeologic information have allowed for a thorough characterization of the source-plume relationships observed in the field. This presentation will pose a compelling argument explaining the occurrence of an otherwise inexplicable nitrate plume mobilized from a source that does not contain nitrate.

Mobilization of Authigenic Nitrate in an Arid Environment
Matthew J. Rhoades, CPG, Golder Associates Inc., 44 Union Blvd., Ste. 300, Lakewood, CO 80228 Mobilization of Authigenic Nitrate In An Arid Environment At an industrial locality in arid northern Arizona, a multi-acre process water impoundment has created an artificial area of localized groundwater recharge. Water dissipating through this zone of recharge has manifested itself in a small series of localized seeps and a down-gradient, subsurface plume with a characteristic geochemical signature. The subsurface plume occurs in an alluvial, water-table aquifer and has an orientation that somewhat mimics the topography of an overlying wash. Repeated groundwater sampling events from a series of strategically-located monitoring wells has allowed for a coherent subsurface conceptual model to be developed. Similarly, the chemistry of the source water within the impoundment has been closely characterized. Numerous researchers working in New Mexico and Arizona have noted the occurrence of naturally-occurring (authigenic) nitrate in arid environments produced by strong storm events when followed by rapid evaporation. The subsequent transfer of atmospheric nitrogen into a nitrate residuum in the soil is a viable explanation for the occurrence detailed in this presentation. The entire drainage basin encompassing this small wash is devoid of any history of agricultural activities, other than stock grazing. The potential for artificial contributions of nitrate to the subsurface flow system has been completely ruled out. Geophysical investigative techniques, in concert with monitoring well data, analytical results and hydrogeologic information have allowed for a thorough characterization of the source-plume relationships observed in the field. This presentation will pose a compelling argument explaining the occurrence of an otherwise inexplicable nitrate plume mobilized from a source that does not contain nitrate.