SummaryGroundwater describes water in soil and sub-soil substrates (e.g., aquifers) that is replenished across various time-frames by surface water that percolates to these underground reservoirs. For this water to be useable to meet human needs (e.g., drinking, irrigation) it must meet the same kinds of water quality requirements as surface water. One indicator of groundwater quality is nitrate concentration. General Information about this IndicatorWhat is it?: Groundwater describes water in soil and sub-soil substrates (e.g., aquifers) that is replenished across various time-frames by surface water that percolates to these underground reservoirs. For this water to be useable to meet human needs (e.g., drinking, irrigation) it must meet the same kinds of water quality requirements as surface water. One indicator of groundwater quality is nitrate concentration. Why is it important?: Groundwater is the primary source of drinking water for many communities in California. Groundwater resources are also under threat from over-use and contamination from surface water and soil contamination. Degradation of groundwater quality jeopardizes use of this resource for drinking water. California’s Drought Contingency Plan (DWR, 2010) depends on groundwater as part of its “Conjunctive Management and Groundwater Storage” and “Recharge Area Protection” strategies. In order for these measures to function as part of the overall plan, then groundwater quality must be high enough to support human use. Nitrates are the primary (most extensive) contaminant in groundwater originating from human activities. Nitrates from fertilizer application in agricultural and urban areas can leach into groundwater and will penetrate and spread according to the underlying geology. Other contaminants can also affect groundwater, including organic chemicals originating from past and current industrial and commercial activity. This contamination may spread underground in “plumes”, which are areas of increasing concentration as contaminated groundwater naturally moves underground, or the chemicals themselves diffuse through the ground and/or water. Various agencies track these contaminants in groundwater and in drinking water wells originating from groundwater as a way of understanding risk to communities from drinking water contamination. What can Influence or Stress Condition?: Groundwater naturally varies in quality based on underlying geology and interaction with percolating surface water. Groundwater contamination by any chemical will decrease or increase due to penetration of less or more-contaminated water, respectively. Groundwater concentrations of nitrate increase due to leaching of nitrate from various agricultural and urban activities, such as: surface application of fertilizer, confined animal feeding operations, and septic tanks. In mining and urban areas, commercial and industrial activities can result in inorganic and organic chemicals leaching into local and regional groundwater. In areas where these resources are particular valuable or threatened, wells may be used to extract and treat contaminated water, usually at great expense. In other areas, introduction of captured storm-water or surface water could be used to dilute contaminants in groundwater. Target or Desired Condition: The desired target condition is for groundwater to be free of artificial contaminants. The undesired condition is for groundwater to violate drinking water standards set by environmental regulatory or health agencies, or to pose a risk of violation. 1) Nitrates: Desired condition is for groundwater to have nitrate concentrations at or below naturally-occurring background concentrations. According to Harter et al. (2012), background nitrates concentrations in the Tulare Lake Basin are 9 mg/L nitrate. The state and federal drinking water standard (maximum concentration) for nitrate is 45 mg/L nitrate (equivalent to 10 mg/L NO3 nitrogen). This is the undesired condition. http://water.epa.gov/drink/contaminants/basicinformation/nitrate.cfm Additional Details: Citations Barlow, P.M., and Leake, S.A., 2012, Streamflow depletion by wells—Understanding and managing the effects of groundwater pumping on streamflow: U.S. Geological Survey Circular 1376, 84 p. (Also available at http://pubs.usgs.gov/circ/1376/. ) Department of Water Resources (DWR) 2010. California Drought Contingency Plan. Report to the Governor, 102 p. Harter, T., J. R. Lund, J. Darby, G. E. Fogg, R. Howitt, K. K. Jessoe, G. S. Pettygrove, J. F. Quinn, J. H. Viers, D. B. Boyle, H. E. Canada, N. DeLaMora, K. N. Dzurella, A. Fryjoff-Hung, A. D. Hollander, K. L. Honeycutt, M. W. Jenkins, V. B. Jensen, A. M. King, G. Kourakos, D. Liptzin, E. M. Lopez, M. M. Mayzelle, A. McNally, J. Medellin-Azuara, and T. S. Rosenstock. 2012. Addressing Nitrate in California's Drinking Water with a Focus on Tulare Lake Basin and Salinas Valley Groundwater. Report for the State Water Resources Control Board Report to the Legislature. Center for Watershed Sciences, University of California, Davis. 78 p. http://groundwaternitrate.ucdavis.edu. SWRCB. 2013. Communities that rely on a contaminated groundwater source for drinking water. Report by the State Water Resources Control Board to the Legislature. 181 p. Indicator Preparation InformationData Sources: Nitrate data were obtained from the State Water Resources Control Board http://www.swrcb.ca.gov/gama/ Data Transformations: Scoring of nitrate concentrations were based on the following criteria: Nitrate concentrations 45 mg/L received a score of 0, regardless of magnitude.