Groundwater Ambient Monitoring & Assessment (GAMA)

Domestic Groundwater Well Sampling Groundwater Ambient Monitoring & Assessment Program

Northern Sacramento Valley; view is eastward toward Lassen Peak.

In 2012, the Groundwater Ambient Monitoring & Assessment Priority Basin Project (GAMA-PBP) began water-quality assessments of shallow aquifers, the groundwater resources typically used for private domestic and small system drinking-water supplies. These groundwater resources typically are shallower than the groundwater resources used for public drinking-water supplies.

GAMA-PBP is designed to identify where and to what extent water quality inside groundwater aquifers meets benchmarks that have been established for drinking water. Results of GAMA-PBP sampling are used to determine how groundwater quality is changing and to help explain patterns in chemical concentrations at the basin scale.

What We Want to Learn

For these studies, the USGS is sampling domestic wells that we expect to draw water from the shallow part of the aquifer. As in the deep aquifer study, the new study is defining the percentage of the water source with water quality above drinking water benchmarks.

This is important for several reasons:

  • It provides information about which communities may need groundwater treatment systems.
  • Shallow and deep groundwater slowly mix, so knowing more about the shallow system can help interpret trends in the entire groundwater system.
  • Regulatory programs are in place to protect groundwater quality; understanding the complete resource can help determine if management systems are working.

Northern Sacramento Valley Area Groundwater Quality

location map of the study area

Sampling Underway Winter/Spring 2018/2019

The Northern Sacramento Valley domestic-supply aquifer study includes two study areas — the Redding area and the Red Bluff area. The areas are separated geologically by the Red Bluff arch, a buried geologic structure that creates a groundwater divide (California Department of Water Resources, 2014)[PDF, 43MB]. Water that recharges the aquifer north of the arch flows into the Redding area aquifers and recharge water south of the arch flows into the aquifers of the North Sacramento Valley area.

In 2007-2008, GAMA-PBP sampled the public-supply aquifer zone (about 200-400 feet deep) in the Northern Sacramento Valley. That part of the aquifer typically supplies water used for public drinking water.

Results of that study can be viewed here to get a sense of water quality in public-supply aquifers (note that the public-supply aquifer study area is different from the new domestic-supply aquifer study area). In general, <1% of the deep aquifer area contains concentrations of organic constituents above human health benchmarks. About 2% has concentrations of inorganic constituents above the benchmarks. Groundwater quality results presented by GAMA-PBP represent the raw (untreated) resource. Groundwater delivered for public-supply is treated; therefore, people are not drinking this water directly from the ground.

Pie charts showing the concentrations of inorganic and organic constituents measured in the Northern Sacramento Valley public-supply aquifer study

Overview of water-quality results presented in USGS Fact Sheet 2011-3004.

Groundwater Well Sampling Area

What to Expect as a Participating Well Owner

Volunteer well owners who participate in the study will receive a free chemical analysis of their well water.

Before releasing data or reports to the public, the State Water Resources Control Board (SWRCB) and USGS will provide results to individual well owners. Well owner information will be kept confidential.

If you choose to participate, your first step will be a conversation with a USGS scientist about the following details:

  • Well construction: For example, well depth and depth to perforation. If you don't have a well completion report, we can look it up at the California Department of Water Resources archives.
  • Sampling access: Is there a place to collect a raw groundwater sample from your well? Typically this is a spigot or threaded plug between the well and any treatment system. A USGS scientist can help determine this with you at your well.
  • Pipe access: Can we measure the depth to water?
  • Water flow: Can the pump be run continuously for about an hour?

If your well can be used in the study, we will make an appointment to visit your well with our field team and mobile laboratory to conduct sampling.


How and Why

Well Q&A


The program is designed to give water managers in each region information about the quality of groundwater being used for drinking water supplies and to give volunteer well owners detailed analytical results about their wells. The data will be used to determine what factors are affecting water quality and how it changes over time. Water managers can then use this when planning for public supply treatment systems and when evaluating contaminant control measures.
You will learn more about your well, the depth to water in your well, and about the quality of the raw groundwater pumped from your well. If an elevated concentration of a contaminant is detected, then we will provide you with information on how to follow up.
There is no charge to participate in the study and results are provided for free. The program is funded by the State of California.

There are several things we need:

  1. Well Construction Details
    For example, well depth and depth to perforation. If you don't have a well completion report, we can look it up at the California Department of Water Resources archives.

  2. Sampling Access
    A place we could sample water from, typically a spigot or threaded plug between the well and any treatment system.

  3. Pipe Access
    Can we measure the depth to water?

  4. Water Flow
    Can the pump be run continuously for about one hour?
At the appointment time, a team of USGS scientists will arrive in our mobile laboratory, measure the water level in your well, let the pump run for about an hour, and then collect at most 60 water samples in prepared bottles. After the team leaves, the bottles are shipped to our National Water Quality Laboratory overnight.
The well must be available to pump continuously for about an hour.
Yes. Approximately 6 months after all of the wells are sampled for the study, we will send you a 'Well Owner Report' with the sampling results. We will include our contact information and a USGS scientist will be available to discuss the results and answer questions.
The sampling results will be used to provide information about groundwater quality in your area and statewide. This information is served through an online database at and through USGS publications available at
When a test result is over a Maximum Contaminant Level (MCL), the State recommends that well owners not drink the water, and talk to a water treatment professional. When results are above a health-based benchmark but below an MCL, or if no MCL has been established, the State recommends that well owners consider including these substances in any future water quality analyses they do on their own well. The health-based benchmarks represent the amount of a constituent in drinking water that if consumed by one million people over their lifetimes, would result in one person contracting cancer as a result. These benchmarks are conservative and designed for use by public water supply systems. More information is available here.

What Well Owners are Saying

  • "Thank you for the drillers log, I thought my well was more shallow. I must be thinking of the pump depth."
  • "I am drinking bottled water now and I shared my results with my neighbors. I am so glad I ran into you that day. I would still be drinking that water unknowingly. I always felt the water was actually really good because it tastes so good."
  • "Thank you! I will talk to our well guy to compare it to the pump depth."

Sacramento Metro

The Sacramento Metropolitan Shallow Groundwater Study consists of three groundwater basins with a relatively high density of domestic wells. A total of 49 wells were sampled from August through November 2017 with well depths from about 60—250 feet deep. Sampling results were mailed to well owners in May 2018 and the public data release is currently in preparation.

In 2004 and 2005, GAMA-PBP sampled the deep aquifer zone (about 250—500 feet deep) in the southern Sacramento and northern San Joaquin valleys. That part of the aquifer supplies water to public drinking water districts. Results of that study can be viewed here and here.

map of the Sacramento metro area, shaded by the density of domestic wells per square mile


The Mojave Shallow Groundwater Study focused on two areas — the floodplain aquifer and the regional aquifer surrounding it. Both areas are tapped by private domestic wells with depths from about 90—600 feet deep. A total of 59 wells were sampled (some of which were monitoring wells) from January through May 2018 with well depths from about 50—250 feet deep. Sampling results are currently being prepared for release.

In 2008, GAMA-PBP sampled the deep aquifer zone (about 200-1,000 feet deep) in the Mojave Desert. That part of the aquifer supplies water to public drinking water districts. Results of that study can be viewed here. Additional water quality results for selected water-quality constituents for sampling activities completed during 2000—12 as part of the Mojave Regional Groundwater Study conducted by the USGS can be found on the Mojave Regional Groundwater Studies Data Map.

In Cooperation With

California State WAter Resources Control Board logo

California State Water Resources Control Board

Contact Us

To learn more about GAMA-PBP shallow aquifer sampling, please contact:

Jennifer Shelton
(916) 278-3068

George Bennett
(916) 278-3099

Water Quality Results Map

See the Data

GAMA-PBP Publications

California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project: Shallow Aquifer Assessment: U.S. Geological Survey Fact Sheet 2012-3136

Priority Basin Project: Shallow Aquifer Assessment

Other Resources

State Water Resources Control Board: GAMA - Domestic Well Project Site

State Water Resources Control Board: Domestic Well Owner Guide (PDF)

Valley Water: Private Well Owner Guide

This short list is a small fraction of the public information available; the USGS does not endorse any commercial products or services.