Understanding Nitrate Accumulation in the Shallow Vadose Zone of Spanish Springs Valley, Nevada

Recent studies indicate that elevated nitrate accumulation has been occurring within the arid southwest, and this research focuses on investigating the occurrence of elevated soil nitrate in the mesic regions of northern Nevada, possible mechanics of nitrate accumulation and finally, remediation potential. Soil profiles in the Spanish Springs Valley, Nevada have indicated high concentration of nitrate 2--3 meters below the soil surface. The main vegetation of the area is sagebrush (Artemesia tridentata ) and rabbitbrush (Chrysothamnus nauseosus) with groundwater depths varying from very deep (∼ 28--30 m) to very shallow (∼3 m). This research investigated the interaction of vegetative nitrate uptake, soil organic carbon, soil nitrate, and root growth with the elevated soil nitrate at these locations, as well as a possible nitrate accumulation mitigation technique, and the modeling of nitrate migration and microbial uptake within these locations. Soil and vegetative samples were collected from 4 different locations (three sites with deep groundwater and one with shallow groundwater), and previous data collected from monitor well installations were examined and analyzed. Laboratory experiments involving the migration and uptake of nitrate within carbon-augmented soil columns were conducted, and the modeling of these processes using Hydrus 1D was completed. The analyses indicate that the higher C:N in conjunction with shallow groundwater supports a higher root density with greater denitrification potential, precise irrigation of soil containing elevated nitrate layers can mitigate the transport of nitrate through microbial uptake, and Hydrus 1D modeling can, with accurate soil hydraulic properties, model the migration and uptake of nitrate within the soil columns. Also indicated from this research is the absence of distinction between sites in relation to the sagebrush (Artemesia tridentata) and rabbitbrush (Chrysothamnus nauseosus) plant leaf nitrate concentrations, as well as the capacity for microbial uptake from un-augmented soil. Conclusions indicate that nitrate levels in soil are directly influenced by groundwater levels which also appear to directly influence the C:N in the soil and the potential denitrification, and that soil within the Spanish springs Valley area contains sufficient carbon to promote the microbial uptake of almost half the nitrate contained within these elevated nitrate horizons when leaching may be initiated.