Regional Groundwater Sampling Design

Scientists at the USGS with expertise in groundwater and geochemistry design a unique water-quality sampling plan for each study area using information from the salinity mapping and geological context efforts and historical information about subsurface flows and potential areas of contamination.

Conceptual Model

Image showing how zone of useable groundwater will be sampled along three 2-d planesThe basic conceptual model underlying the sampling plan design is represented as a block diagram in which the upper and lower bounding planes of the block correspond to the depth interval or zone containing protected groundwater

State water-quality laws and regulations provide for the protection of groundwater resources that are or may be useable for purposes including drinking water, irrigation, and industrial supplies, and SWRCB policy currently targets 3,000 mg/LTDS as a general threshold for protection (SWRCB, 1988). Federal laws target the protection of groundwater resources for drinking water supplies and define these as resources containing less than 10,000 mg/LTDS. The same laws provide for an exemption to regulations governing injection of waste into aquifers if a) the zone does not currently serve as a drinking water sources, b) the zone cannot serve as a source in the future because it produces hydrocarbons, is too deep to be economically or technically practical, it is so contaminated that treatment would be impractical, or c) the groundwater contains between 3,000 mg/L and 10,000 mg/LTDS and is not reasonably expected to serve a public water system (summarized from 40 CFR 144.7).


For a given oil field, we use the salinity mapping tools to identify where protected groundwater is. Typically, the upper boundary of that zone would be the water table. In simple cases, the lower boundary of that zone would be a two-dimensional surface defined by the depths at which groundwater salinity increases to 10,000 mg/LTDS.

The goal of the water-quality monitoring is to determine if there is fluid movement from zones where oil and gas production activity is taking place into the zone of protected groundwater.

To accomplish that, we identify various potential flow paths through the oil field that extend from the upgradient boundary through areas of oilfield infrastructure and to the downgradient edge of the field and potentially the edge of the buffer zone. The strategy then is to collect groundwater samples from shallow, intermediate, and deep wells at various points along the flow paths.

Example of Conceptual Model Applied to Fruitvale Oil Field

As oil and gas extraction in most fields in California spans from several decades up to more than a century, and as groundwater withdrawn from a well may have entered an aquifer decades to thousands of years before present, the effects of historical practices and natural hydrogeologic setting on groundwater-quality need to be considered in regional monitoring of groundwater-quality near oil fields.

Available TDS data from water well samples within a 5km buffer of the Fruitvale oil field reveal two things. First, only a few of the samples had TDS values greater than 2,000 mg/L (shown by red dots on the map), so we know there is chiefly fresh water overlying this oil field. Second, there are numerous water supply wells within the oil field boundary and adjacent to it (California Department of Water Resources, 1961; California Department of Water Resources, 2015; California State Water Resources Control Board, 2015).

In the late 1950s, studies by the California Department of Water Resources and the USGS identified a zone of degraded shallow groundwater-quality in the central part of the oil field (in red on the map) where TDS concentrations were over 1,000 mg/L. That zone corresponds to the highest density of oil and gas wells in the field, as well as the area where produced water disposal in unlined sumps occurred historically (indicated by the brown line on the map).

map showing that the highest density of oil, gas, and dry-gas wells is greater than 150 wells per section (from California Department of Resources, Division of Oil, Gas, and Geothermal Resources, 2015, and Davis, United States Geological Survey) to the north-east of the oil field boundary, and that the highest total dissolved solids were not found in the area where groundwater-quality was found to be severly degredated in 1959-1960 next to the Kern River.
Fruitvale legend for the map above.

Map of available groundwater salinity data, measured at water supply wells, within a 3 mile buffer of the Fruitvale oil field. The density of oil and gas wells is mapped by section with the darker colors having higher well densities (California State Water Resources Control Board; Davis and others, 2018; California Department of Water Resources, 2015; California Department of Water Resources, 1961; DOGGR, 2015; U.S. Geological Survey, 2015).

However, water well samples collected from supply wells screened deeper in the aquifer during the 1980s to 2010s generally had TDS concentration less than 1,000 mg/L, and often less than 500 mg/L. These observations may indicate that the degraded water from the pre-1960s has been diluted with fresher recent recharge or has not yet reached the depths of supply wells. Groundwater chemistry, isotopic, and groundwater-age dating data collected by the regional monitoring may help to explain why legacy surface-disposal effects are not generally observed in deeper groundwater used for supply and to distinguish constituents from these legacy surface sources from those potentially moving upwards from oil development zones, if present.

To identify the top of the zone where oil and gas development has been taking place, information about well construction was extracted from theDOGGR well records and recorded in a database. The top perforations of the oil well casings were then used to define the top of that zone. In this oil field, most of the top perforations are between 3,000 ft and 4,000 ft below land surface, but some oil wells have perforations less than 3,000 ft.

map of fruitvale oil field showing that the wells with the greatest depth to top perforation are generally from the north-west to the south-east corner of the field, wells with depths from 3001 to 4000 feet are in the center, and the shallowest wells appear to be in the north-east corner. wells with no data appear throughout the results.

Top perforation depth (in ft) of oil wells at Fruitvale Oil Field (DOGGR, 2015).

Three flowpaths across the system spanning a range of oil infrastructure densities were identified.

Along those flow paths, three target sampling depths were identified based on lithology.

conceptual image showing a blue line representing the flowpath and three groups of three wells each along the flowpath. Each group of wells has a shallow well, an intermediate-depth well, and a deep well.

We attempt to arrange sampling of shallow, intermediate, and deep wells along the flowpaths.

map of fruitvale oil field showing groundwater contours and three flowpaths along flow gradient

Map of water table contours at Fruitvale oil field (California Department of Water Resources, written commun., 2014). Blue arrows indicate flow paths selected for sampling.

Once the ideal sampling locations have been defined in 3D, COGG team members search for existing wells that could be used to access those points. In some cases, new wells may need to be drilled in subsequent monitoring efforts to fully answer questions about zonal isolation.

map showing that 14 public-supply wells and 6 oil wells in and surrounding the fruitvale oil field boundary have been sampled prior to February 2017

Existing wells that were sampled in the Fruitvale study area between July 2016 and February 2017.

Example of Conceptual Model Applied to Lost Hills Oil Field

The Lost Hills oil field on the southwest edge of the San Joaquin Valley has a different physical setting from the Fruitvale oil field on the Valley's east side.

The zone of protected water is only about 1,000 ft deep, and the area is a good example of a field that has both protected and unusable groundwater; the focus of groundwater sampling efforts is thus on those fresher zones and in the buffer area adjacent to the field.

The density of oil and gas wells is very high in the Lost Hills oil field, where the central portion contains more than 150 wells per square mile. In addition, many wells are fairly shallow, having top perforations of well casings less than 1,000 ft deep (DOGGR, 2015).

map that shows fruitvale in the eastern central valley and lost hills on the western side.

Map of oil fields with Lost Hills field highlighted in pink and Fruitvale highlighted in blue. (DOGGR, 2015)

Map of Lost Hills oil field showing shallower wells (0-250 feet) in the northern and central part of the oil field and deeper wells further south.

Map of depth and density of oil wells in the Lost Hills oil field (DOGGR, 2015; Davis and others, 2018).

Map of oil field boundaries showing 3-mile buffers overlapping and arrows indicating generalized direction of groundwater flow to the north-east. Priority areas for sampling are drawn on the north-east side of the oil fields and groundwater wells that might be sampled are plotted within and nearby to those priority areas.
Legend for map above.

Map of Lost Hills and North and South Belridge oil fields showing groundwater wells in relation to several key features, including areas of high wastewater disposal volumes, dense oil and gas development areas, and groundwater flow direction (data from DOGGR, 2015; Department of Water Resources, 2015).



California Department of Water Resources, 1961, Effects of waste water disposal, Fruitvale Oil Field, Kern County: Sacramento , 28 p. [Also available at ].


California Department of Water Resources, 2015, Water quality data reports: Digital spatial data with water quality data for groundwater wells; data received June 2015 via email.

Division of Oil, Gas, and Geothermal Resources, 2015, DOGGR Well Finder: Digital spatial data for oil wells in California; California Department of Conservation, accessed February 28, 2015 at:

Shimabukuro, D.H., Haugen, E.A., Battistella, C., Treguboff, E.W., and Kale, J.M., 2015, Using oil and gas well log records to understand possible connections between wastewater injection zones and usable groundwater aquifers in California, in American Geophysical Union Fall 2015 Meeting, San Francisco, Calif., December 14-18, 2015: American Geophysical Union, unpaginated.

U.S. Geological Survey, 2015, National Water Information System - Web interface: U.S. Geological Survey water database, DOI: 10.5066/F7P55KJN


California State Water Resources Control Board, 2017, GeoTracker Web Map: accessed September 1, 2017 at


Davis, Tracy A., Landon, Matthew K., and Bennett, George L. V, 2018, Prioritization of oil and gas fields for regional groundwater monitoring based on a preliminary assessment of petroleum resource development and proximity to California’s groundwater resources, U.S. Geological Survey, Scientific Investigations Report, SIR 2018-5065, DOI: 10.3133/sir20185065