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.