Natural contaminants, arsenic and uranium, in one-fifth of California's groundwater

Unlike other regional assessments, the USGS incorporated area and population as indicators of groundwater quality, an approach that captures a more accurate regional picture and is more applicable at multiple scales. Measurements of contaminants were weighted according to the population depending on the groundwater, and the size of the affected area. The study used data from 11,000 public supply wells in California, including 2,400 sampled by USGS, to evaluate nearly 200 natural and human-made contaminants in untreated groundwater.

The study, conducted as part of the California State Water Board's Groundwater Ambient Monitoring and Assessment Program, found that natural contaminants, such as arsenic and uranium, occur at high concentrations in about 20 percent of the groundwater resources used for supply. Human-made contaminants, such as nitrate and solvents, occur at high concentrations in about 5 percent of the resource. "High" concentrations are defined as above the Environmental Protection Agency's or California's State Water Board's Maximum Contaminant Levels or other non-regulatory health-based levels for chemical constituents or elements not having MCLs.

The USGS evaluated the quality of groundwater that occurs in place; those supplies are typically blended or treated prior to delivery to consumers in order to ensure that the water is safe to drink.

"Other studies typically evaluate a portion of the resource and attempt to extrapolate the results to other areas," said Dr. Kenneth Belitz, the USGS scientist who led the study. "The GAMA study covers just about the entire resource used for public supply, and evaluates unregulated contaminants, such as manganese, as well as regulated contaminants." Manganese, an unregulated contaminant, occurs at concentrations above non-regulatory human health benchmarks in about 5 percent of the resource used for drinking water supply.

Human-made contaminants are not as prevalent at high concentrations as natural contaminants, but they do occur in locations where large numbers of people rely upon groundwater. For example, organic solvents are present in parts of the San Fernando, San Gabriel, and Santa Ana Basins, where millions of people depend on groundwater for part of their drinking supply. The presence of solvents at high concentrations in these basins has long been known, and the problem is actively managed by local and regional agencies.

High concentrations of nitrate in groundwater are generally associated with the application of fertilizers, animal manure, and septic systems. In California, the occurrence of high concentrations of nitrate in groundwater used for public supply is primarily due to activities that occurred in the past. Consequently, high concentrations of nitrate can be found in areas where the land use in the past was agricultural but currently is urban, and not just in areas that are currently agricultural. Examples include the Livermore Valley and parts of the Santa Ana Basin.

In addition, there are areas where the land use is currently agricultural, but high concentrations of nitrate are not currently prevalent at the depth zone used for public supply. Concentrations of nitrate might become high in the future as groundwater moves from the land surface downward toward the depth zone used for public supply. Examples include parts of the San Joaquin and Sacramento Valley.