Groundwater and Hydrogeology || Diablo Water

Diablo Water District Well Utilization Project Phase 2 and Future Phase 3

The California Department of Water Resources (DWR) delineates groundwater

basins throughout California through California’s Groundwater Bulletin 118.

The proposed project is located in the San Joaquin Valley Groundwater Basin,

Tracy Subbasin (Groundwater Basin Number 5-22.15), in the northwestern

portion of the subbasin. Review of hydrographs for the Tracy Subbasin indicate that except for seasonal variation resulting from recharge and pumping, the majority of the water levels in wells have remained relatively stable over at least the last 10 years (California Department of Water Resources 2006). However, there is a lack of significant historical level data in the project area, and DWD recognizes the need for continued groundwater level monitoring in the DWD district. A survey was conducted of all wells within a 0.5-mile radius of the existing Glen Park well, and the results indicated that the majority of these wells are shallow and typically less than 100 feet (Luhdorff & Scalmanini Consulting Engineers 2007). Similarly, shallow wells are expected to be located in the

vicinity of the Phase 2 and future Phase 3 project sites.

The Tracy Subbasin is comprised of continental deposits of Late Tertiary to

Quaternary age. These deposits include the Tulare Formation, Older Alluvium,

Flood Basin Deposits, and Younger Alluvium (California Department of Water Resources 2006). The cumulative thickness of these deposits increases from a few hundred feet near the Coast Range foothills on the west to about 3,000 feet along the eastern margin the basin (California Department of Water Resources 2006).

Detailed hydrogeologic studies pertaining to the eastern Contra Costa County are

relatively limited. Luhdorff & Scalmanini Consulting Engineers (LSCE)

conducted a search of water well drillers reports on file at DWR for a report on

local and regional hydrogeological conditions for several east county agencies

including DWD. Well reports that were reviewed were in the vicinity of

approximately 2 miles west of Oakley, through the Delta Islands just east of the

county line, and south through Brentwood to about 2 miles south of Byron.

Between 400 and 500 well logs were collected and classified into depth zones of

100-foot intervals. The majority of these wells were found to be less than

300 feet deep (Luhdorff & Scalmanini Consulting Engineers 1999).

At present, there is limited available data on land subsidence in eastern Contra Costa County. However, as an element of its AB 3030 Groundwater

Management Plan, DWD will assess its operations and pumping for the potential

to induce land subsidence. This would include reviewing available monitoring

data in the county and early identification of impacts to groundwater levels that

might forewarn of subsidence.

Surface Water Quality

Physical and chemical characteristics of the watershed, hydrologic and climatic

factors, and urban and agricultural discharges affect the water quality of Marsh

Creek (City of Brentwood 1998). Based on the State Water Board’s 303(d) list,

Marsh Creek’s water quality from Marsh Creek Reservoir to the San Joaquin

River is impaired for mercury and metals (California State Water Resources

Control Board 2006).

In addition, data collected upstream of the Brentwood WWTP’s discharge (which

is upstream of the proposed project) indicates maximum concentrations of

bromoform, carbon tetrachloride, dibromochloromethane,

bromodichloromethane, bis(2-ethylhexyl)phthalate, aluminum, barium,

chromium (VI), cyanide, iron, manganese, chloride, electrical conductivity (EC),

sulfate, and total dissolved solids (TDS) would exceed their applicable criterion.

Although the Brentwood WWTP’s discharges affect the water quality of Marsh

Creek downstream of the plant, this effect was determined to be less than

significant in a previous CEQA document (City of Brentwood 1998).

Groundwater Quality

Groundwater quality has constrained groundwater development in some parts of

eastern Contra Costa County. According to DWR Bulletin 118, the northern part

of the Tracy Subbasin is characterized as a sodium water type with a combination

of bicarbonate, chloride, and mixed bicarbonate-chloride water type (California

Department of Water Resources 2006). TDS, an indication of salt content, was

tested in San Joaquin County and Contra Costa County. TDS ranged from 50 to3,520 mg/L and average 463 mg/L (California Department of Water Resources


DWD’s project wells are evaluated in terms of suitability for municipal supply.

Under DPH requirements, the wells must meet all state drinking water standards.

DWD has found that hardness in groundwater may affect customer satisfaction

and has established a blending target to mitigate the impact to aesthetic quality.

Otherwise, the District seeks to develop sources that meet all DPH drinking

water standards.

Water Supply

The primary source water for DWD comes from the Central Valley Project

(CVP) purchased from the CCWD. In addition, to surface water, DWD also

pumps groundwater. Figure 3-1 includes the DWD service area, including the

existing Glen Park well along with other wells in the area. The CVP water is

conveyed through the Contra Costa Canal and treated at the RBWTP in Oakley.

Current and buildout (year 2040) DWD water supplies are summarized in Table

3-2 for normal and single-dry years; and in Table 3-3 for multiple dry years.

Table 3-2. DWD Water Supply for Normal and Single Dry Years

Norma Year or Single

Dry Year



Max Day =

2 x Average Day

Annual Supply =

365 x Average Day

mgd mgd mg af


Surface Water 7.5 15 2,738 8,400

Ground Water 1 2 365 1,120

Total 8.5 17 3,103 9,520

Year 2040 (Buildout)

Surface Water 15 30 5,475 16,800

Ground Water 2.5 5 913 2,800

Total 17.5 35 6,388 19,600

Source: Urban Water Management Plan (Diablo Water District 2005).

Notes: mgd = million gallons a day; mg = million gallons; af = acre-feet.

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