An Introduction to the Central and West Coast Groundwater Basins

The WRD is pleased to present the first in a series of Technical Bulletins designed to provide useful information on the groundwater resources of the Central and West Coast Basins (CWCB). Volume 1 presents an introduction to the general hydrogeology of the basins. Future issues will cover topics such as seawater intrusion, computer modeling, well drilling, water quality, aquifer properties, and replenishment facts. The WRD welcomes any comments on its Technical Bulletins or suggestions for future topics.

Geology

The CWCB are two groundwater basins in the Coastal Plain of Los Angeles County. They are comprised of Quaternaryage sediments (less than 1.8 million years old) of gravel, sand, silt, and clay that were deposited from the erosion of nearby hills and mountains, and from historic beaches and shallow ocean floors that covered the area in the past. Underlying these Quaternary sediments are basement rocks such as the Pliocene Pico Formation that generally do not provide sufficient quantities of groundwater for pumping. Separating the Central Basin from the West Coast Basin is the Newport-Inglewood Uplift (NIU), a series of discontinuous faults and folds that form a prominent line of northwest trending hills including the Baldwin Hills, Dominguez Hills, and Signal Hill.

The Central Basin (CB) covers approximately 270 square miles and is bounded on the north by the Hollywood Basin and the Elysian, Repetto, Merced, and Puente hills, to the east by the Los Angeles County/Orange County line, and to the south and west by the NIU. The California Department of Water Resources (1961) divided the Central Basin into four sections; the Los Angeles Forebay, the Montebello Forebay, the Whittier Area, and the Pressure Area (Figure 1).

Map of Central and West Coast Basins
Figure 1 – Map of Central and West Coast Basins

The two forebays represent areas of unconfined aquifers (water table aquifers) that allow percolation of surface water down into the deeper aquifers to replenish the basins. The Whittier Area and Pressure Area are confined aquifer systems that receive relatively minimal recharge from surface water. They are replenished from the up-gradient forebay areas and adjacent groundwater basins.

The West Coast Basin (WCB) covers approximately 140 square miles and is bounded on the north by the Baldwin Hills and the Ballona Escarpment (a bluff just south of the Ballona Creek), on the east by the NIU, to the south by San Pedro Bay and the Palos Verdes Hills, and to the west by the Santa Monica Bay. Aquifers in the West Coast Basin are generally confined and receive the majority of their natural recharge from adjacent groundwater basins or from the Pacific Ocean (seawater intrusion).

aquifers of Central and West Coast Basins
Figure 2 – Generalized Cross Section going East / West through Central and West Coast Basins showing Aquifers

Groundwater Occurrence and Movement

Groundwater pumped from the CWCB currently provides approximately 36% of the total water supplies used by the overlying population of four million people in 43 cities of southern Los Angeles County.

Groundwater Flow Directions Los Angeles
Figure 3 – General Groundwater Flow Directions

Groundwater occurs in the pore spaces of the sediments in the basins. Where these sediments are thick and transmissive enough to supply sufficient quantities of water to wells for beneficial use, they are termed “aquifers”. The name “aquitard” is given to the less permeable silt and clay layers that separate the aquifers. The major aquifers identified in the CWCB are shown on Figure 2 and include the following, from shallowest to deepest: A) the Gaspur and Semiperched aquifers of the Holocene Alluvium Formation; B) the Exposition,

Artesia, Gage, and Gardena aquifers of the Upper Pleistocene Lakewood Formation; C) the Hollydale, Jefferson, Lynwood, and Silverado aquifers of the Lower Pleistocene Upper San Pedro Formation; and D) the Sunnyside Aquifer of the Lower Pleistocene Lower San Pedro Formation. Aquifer depths can reach more than 2,000 feet in the CB and 1,500 feet in the WCB, although production wells generally do not need to be drilled this deep to tap sufficient water. The general direction of groundwater flow is shown on Figure 3. Both the NIU and the Charnock Fault (in the WCB) are partial barriers to groundwater flow, causing differences in water levels on opposite sides of each fault system.

Most of the groundwater in the CWCB remains at an elevation below sea level due to historic over pumping, so the importance of maintaining the seawater barrier wells to keep out the intruding saltwater is critical.

The Groundwater Budget

Based on recent modeling of the CWCB by the U.S. Geological Survey (2003), an average groundwater budget was developed for the 30-year base period between water years 1970/71 and 1999/00 (Table 1). This budget shows the average annual inflows (recharge) and outflows (discharge) to the basins in acre-feet per year. Natural inflows come from the conservation of rainfall or snow melt. Artificial inflows come from imported and recycled water purchased by WRD. The major areas for inflow include the Rio Hondo and San Gabriel River spreading grounds in the Montebello Forebay, the seawater barrier injection wells along the coast, areal recharge from precipitation falling on the basin floor and hillside runoff, groundwater underflow from adjacent basins, and continued seawater intrusion in certain areas. The major outflow is from groundwater extracted from the nearly 550 production wells located throughout both basins.

This average water budget provides a useful picture of the long-term ins and outs of the CWCB, but will change over time as hydrological, cultural factors and water sources evolve. It will be updated in the future as needed.

CENTRAL BASIN
WEST COAST BASIN
TOTAL
Inflow Spreading РStormflow
48,825
0
48,825
Spreading – Imported
39,305
0
39,305
Spreading – Recycled
34,770
0
34,770
Areal Recharge
32,300
15,600
47,900
Barrier Injection
5,300
29,300
34,600
Net Underflow
44,680
3,800
48,480
Seawater Intrusion
0
7,100
7,100
Total
205,180
55,800
260,980
Outflow Pumping
197,500
53,090
250,590
Net (Inflow – Outflow)
7,680
2,710
10,390

Table 1 -Average Annual Water Balance 1970/71 -1999/00 (afy)

For more information on any of these topics, please contact the Water Replenishment District of SouthernCalifornia.
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