Benchmarks for Aquifer Assessments

The GAMA program conducts groundwater-quality sampling statewide and then compares the observed concentrations of constituents to benchmark concentrations to flag cases where source water may be above or close to drinking-water standards.

Selection of Benchmarks for Evaluating Groundwater Quality

Groundwater-quality data are compared to regulatory and advisory benchmarks established by the state and federal agencies for protecting human health and the results are classified as "high," "moderate," and "low."

  • "High" means concentrations above either a regulatory health-based benchmark (an MCL or action level) or secondary MCLs. For constituents with both recommended and upper SMCLs, the upper value is used.
  • The "Moderate" category was created to identify constituents that might be present in elevated concentrations and worth additional analyses/ trend monitoring. The category is defined as anything less than "High," but greater than:
    • 1/2 the high/ moderate boundary for inorganic constituents
    • 1/10 of the high/ moderate boundary for organic constituents
    • unless a constituent has a SWRCB-DDW notification level, in which the category boundary is the notification level

In the past, the GAMA-PBP has also used non-regulatory health-based benchmarks in the same classification system as described above. The purpose for these was to provide scientific information to emerging contaminant study groups and to provide for "early warning" to water-resource managers of where and what concentrations of these unregulated chemicals were present in California's groundwater systems. Since that time, the focus of GAMA's assessment products has shifted to only include regulatory benchmarks. As a result, summary pie charts may not be comparable to each other.

Benchmarks and Change--Contaminants of Emerging Concern Example

In some cases, changes are required to keep pace with new understandings about emerging contaminants of concern. Many federal and state agencies have processes for evaluating these "contaminants of emerging concern" or "emerging contaminants" (for example, Murnyak and others, 2011) that include increased environmental and post-treatment monitoring to better document exposure and new toxicological studies to better document effects. The GAMA program responds to California's need for additional information on specific classes of emerging contaminants by both expanding the suite of chemicals analyzed in groundwater samples and by conducting contaminant-specific assessments. The story presented below describes how the GAMA program responded to concern over the fumigant 1,2,3-Trichloropropane (1,2,3-TCP) and Per- and Polyfluoroalkyl Substances (PFAS).


Red dots showing groundwater samples above MCL for 1,2,3-TCP and blue dots below the MCL
Groundwater samples containing concentrations of 1,2,3-TCP over the California MCL of 0.005 ug/L (red dots) and sampled where concentrations were detectable but below the MCL (blue dots).

Before 2014, the fumigant 1,2,3-Trichloropropane (1,2,3-TCP) was not regulated and thus excluded from water-supply management considerations. In 1999, 1,2,3-TCP was found in groundwater at a superfund site in Southern California and the State began the process of issuing notifications, defining public health goals, and ultimately establishing an MCL of 0.005 µg/L in 2017 (Bell and others, 2019). Because 1,2,3-TCP was also broadly used as a soil fumigant, the USGS began analyzing for the compound in its groundwater-quality studies.

GAMA conducted a systematic assessment of where and why 1,2,3-TCP could pose a risk to public drinking-water supplies (Burow and others, 2019) and found

  • widespread detections in groundwater across California due to nonpoint agricultural sources
  • high concentrations found in shallow, oxic groundwater beneath orchards and vineyards
  • preferentially found in areas where coarse-grained sediments allow rapid recharge
  • in the five sub-basins of the southeast San Joaquin Valley, GAMA has documented that about 20% of the aquifer system supplying domestic wells has 1,2,3-TCP over the MCL


PFAS is a class of chemicals used in fire retardant foam and fabric treatments, nonstick cookware, and food wrappers. They are widespread in the environment, do not easily degrade, and are linked to numerous health risks.

The GAMA program began analyzing for 24 PFAS compounds in 2019 and an additional 4 in 2020. When USGS analytical labs do not have the capability, the program will use contract labs and carry out the full laboratory evaluation of that lab's data quality.

A detailed description of how PFAS is being addressed in GAMA and preliminary findings is presented in the fact sheet "Sampling for Per- and Polyfluoroalkyl Substances (PFAS) by the Groundwater Ambient Monitoring and Assessment Priority Basin Project" by Robert Kent