Groundwater Nitrate Sources and Contamination in the Central Valley.
by Katherine Ransom and Thomas Harter In California’s Central Valley, many communities depend significantly or entirely on groundwater as their drinking water supply.
Other sources in the Central Valley are estimated to leach 20-25 thousand tons N to groundwater (urban areas: 10, municipal wastewater and food processing percolation basins: 4, dairy lagoons and animal holding areas: 6, and septic leach fields: 3).
Crop production has continued to increase steadily over the past 70 years (Harter et al., 2012; Tomich et al., 2016; Harter et al., 2017).
Land use, nitrate leaching, and domestic groundwater The amount of nitrate that leaches to groundwater (nitrogen loading rate) can be highly variable between different crop or land use types and among an individual crop or land use.
We performed a Central Valley analysis of domestic well nitrate data to relate groundwater nitrate to surrounding land uses and to estimate the amount of nitrogen loading from 15 crop and land use groups.
Results of this “fingerprinting” study indicate that multiple nearby sources have likely contributed to an individual well’s nitrate concentration; it also shows some regional patterns in groundwater nitrate sources: manure sources are often more dominant in private wells located in dairy regions such as Hilmar, while fertilizer sources are more dominant in the citrus crop regions of Orosi and Woodlake.
Among the 146 mapped variables, groundwater chemistry related to denitrification or very old groundwater, historical nitrogen application amounts in agriculture, groundwater age, well distance to rivers, and amount of natural land use surrounding wells (among others) were rated as the most important to determine a location’s nitrate concentration.
Ransom, K. M., A. M. Bell, Q. E. Barber, G. Kourakos, and T. Harter, 2017a.
Nitrogen sources and loading to groundwater, Addressing Nitrate in California’s Drinking Water with a Focus on Tulare Lake Basin and Salinas Valley Groundwater.