Storage and mobilization of natural and septic nitrate in thick unsaturated zones, California

•Natural and NO3 from septic sources are stored in unsaturated zone in arid areas.•Denitrification in the unsaturated zone is about 5–10%.•Managed aquifer recharge can mobilize natural and septic NO3 from the unsaturated zone.•Storage in layered unsaturated zones delays arrival of septic NO3 at water table.•Management of housing density may delay arrival of septic NO3 at water table. Mobilization of natural and septic nitrate from the unsaturated zone as a result of managed aquifer recharge has degraded water quality from public-supply wells near Yucca Valley in the western Mojave Desert, California. The effect of nitrate storage and potential for denitrification in the unsaturated zone to mitigate increasing nitrate concentrations were investigated. Storage of water extractable nitrate in unsaturated alluvium up to 160meters (m) thick, ranged from 420 to 6600kilograms per hectare (kg/ha) as nitrogen (N) beneath undeveloped sites, from 6100 to 9200kg/ha as N beneath unsewered sites. Nitrate reducing and denitrifying bacteria were less abundant under undeveloped sites and more abundant under unsewered sites; however, δ15N–NO3, and δ18O–NO3 data show only about 5–10% denitrification of septic nitrate in most samples—although as much as 40% denitrification occurred in some parts the unsaturated zone and near the top of the water table. Storage of nitrate in thick unsaturated zones and dilution with low-nitrate groundwater are the primary attenuation mechanisms for nitrate from septic discharges in the study area. Numerical simulations of unsaturated flow, using the computer program TOUGH2, showed septic effluent movement through the unsaturated zone increased as the number and density of the septic tanks increased, and decreased with increased layering, and increased slope of layers, within the unsaturated zone. Managing housing density can delay arrival of septic discharges at the water table, especially in layered unsaturated alluvium, allowing time for development of strategies to address future water-quality issues.