|Have you ever wondered what happens under your feet when you use too much groundwater? One of our Slurps didn't. In Los Angeles, 40% of our water supply comes from groundwater. In past times overpumping led to seawater intrusion into coastal fresh water supplies.
The Water Replenishment District supples water to a series of injection wells that form a barrier to keep our local supplies safe.
Battling Seawater Intrusion in the Central & West Coast Basins
WRD Technical Bulletin Volume 13, Fall 2007
By: Ted Johnson, Chief Hydrogeologist
Seawater intrusion is the movement of ocean water into
fresh groundwater, causing contamination of the groundwater
by salt. It is a natural process that can be made
worse by human activities. Virtually all coastal aquifers
around the world experience seawater intrusion to some
degree due to the density differences between salt water
and fresh water. If two equally tall columns are filled
with water, one with seawater and the other with fresh
water, the pressure at the bottom of the seawater column
will be greater than at the bottom of the fresh water column.
This is because the seawater has a higher mineral
content than the fresh water (more salt), making it denser
than the fresh water. If the two columns are connected at
their bases, the seawater will flow into the fresh water
column because of the pressure differences. The fresh
water column will have to be filled higher than the seawater
column to balance the pressures and the flows.
The same thing happens in nature along the coast where groundwater levels (a.k.a. heads, elevations, or potentiometric surfaces in a confined aquifer) approach sea level. The two "water columns" are equal in height and there is a natural flow of seawater inland into the fresh groundwater (Figure 1a). This intrusion is halted naturally when the groundwater levels rise and overcome the pressure of the seawater (like the taller column of fresh water). The relationship between the fresh water and seawater pressures is described by the Ghyben-Herzberg principle, which is based on the density differences between the two fluids. It gives an approximation to the depth to the seawater/fresh water interface and states that for every foot above sea level the fresh water head is, the depth to the seawater will be 40 times this amount below sea level. For example, if the potentiometric surface is 5 feet above sea level, the depth to the seawater will be approximately 200 feet below sea level. If groundwater levels drop a foot, then the interface will rise 40 feet.
In coastal areas where groundwater is used for potable or
agricultural purposes such as the Central and West Coast
Basins (CWCB), intrusion can be a serious problem resulting
in the shut down of wells or necessitating expensive
desalination treatment. In the early half of the 20th
century, groundwater extractions in the CWCB were double
natural replenishment, causing severe overdraft and
lowering of the groundwater elevations to over 100 feet
below sea level. This greatly increased the extent of the seawater intrusion inland (Figure 1b). Callison and Roth
(1967) reported that seawater intrusion was first noted in
Redondo Beach in 1912, Hermosa Beach in 1915, and El
Segundo in 1921. The California Department of Water
Resources (CDWR, 1962) estimated that up to 600,000
acre-feet (af) of seawater intruded into and contaminated
the Los Angeles coastal aquifers by the late 1950s.
To address this problem, in 1951 the Los Angeles County Flood Control District (LACFCD) used an abandoned water well in Manhattan Beach to inject potable water to test whether pressure could be built up in a confined aquifer to block the intrusion (Lipshie and Larson, 1995). The test worked, so LACFCD performed subsequent tests with additional wells to successfully create a pressure ridge or "wall" along the line of injection wells to overcome the pressure of the intruding seawater (Figure 1c). The results are well documented in a report by CDWR (1957).
|Figure 1 - Seawater Intrusion and Barrier Wells:
a) Natural condition where the potentiometric surface slopes towards the ocean and there is minimal intrusion;
b) Excessive pumping draws the potentiometric surface below sea level causing increased seawater intrusion; and
c) Injection well builds up pressure so that the potentiometric surface rises above sea level and blocks the intrusion.
Based on the success of the tests, the LACFCD eventually
constructed the West Coast Basin Barrier Project, the
Dominguez Gap Barrier Project, and the Alamitos Gap
Barrier Project. The locations of these projects are shown
on Figure 2. The barrier wells inject water into the aquifers
to build up a line of pressure equal to or exceeding the
protective elevations predicted by the Ghyben-Herzberg
principal, blocking the intrusion.
|Figure 2 - Location of Seawater Barrier Projects
The barrier projects have been successfully protecting the
fresh water aquifers in the CWCB for over 50 years. Currently,
both potable water and recycled municipal wastewater
treated by microfiltration, reverse osmosis, and advanced
oxidation in some cases (ultraviolet light and hydrogen
peroxide) are used. The water is injected into the
CWCB aquifers to depths up to 700 feet. Table 1 presents
some construction details for the barrier projects. The
LACFCD owns, operates, and maintains the barrier projects.
WRD purchases all of the water injected into the
barriers, except for about 2,500 af per year purchased by the Orange County Water District for a portion of the
Alamitos Barrier. The recycled water for the Alamitos
Barrier is treated by WRD, for the West Coast Barrier by
the West Basin Municipal Water District (WBMWD),
and for the Dominguez Gap Barrier by the City of Los
Angeles. Potable water is provided by WBMWD and
the City of Long Beach. The cost for the injection water
in 2007/2008 is approximately $14 million, and the
maintenance costs may approach $4 to $5 million.
Due to the aging infrastructure, rising water and maintenance
costs, and uncertainty of long-term potable water
availability, all six agencies participate in cooperative
efforts to optimize barrier performance, minimize costs,
and to ensure continued barrier effectiveness. Alternatives
to injection wells for seawater intrusion control
were reported in Johnson and Whitaker (2004). In addition,
if regional groundwater levels were to rise due to
reduced pumping, then barrier efforts and associated
costs would be reduced. Attempts are being made to
replace the groundwater pumped from wells along the
coast that is used only for industrial purposes with recycled
water. If successful, the reduced pumping will increase
groundwater levels along the coast and reduce the
amount of injection water needed to maintain a protective
|Overall Length (miles)
|Number of Injection
|Number of Observation Wells
|2008 Injection Amounts
(estimated acre feet)
In conclusion, injection wells have been successfully
used to battle seawater intrusion in the over-drafted
CWCB aquifers since the early 1950s. The precious
groundwater resource has been mostly spared from additional
salt contamination because of the barriers. However,
the rising cost of the barrier system and the aging
infrastructure requires alternatives to be developed. The
future will likely see innovative ways to protect the aquifers
from seawater intrusion while minimizing costs to
the overlying groundwater users.
- California Department of Water Resources, Bulletin No. 63, 1957.
Seawater Intrusion in California. Appendix B by the Los
Angeles County Flood Control District.
- California Department of Water Resources, Bulletin No. 104, 1962.
Planned Utilization of the Ground Water Basins of the
Coastal Plain of Los Angeles County, Appendix B - Safe
- Callison, J.C., and Roth, J.N., 1967, Construction Geology of the West
Coast Basin Barrier Project, Engineering Geology, Volume
4, No. 2.
- Johnson, T.A., and Whitaker, R., 2004, Saltwater Intrusion in the
Coastal Aquifers of Los Angeles County, California; Chapter
2 (pages 29-48) in Coastal Aquifer Management, edited by
Cheng, A.H., and Ouazar, D., Lewis Publishers
- Lipshie, S.R., and Larson, R.A., 1995, The West Coast Basin,
Dominguez Gap, and Alamitos Seawater-Intrusion Barrier
System, Los Angeles and Orange Counties, California. In
AEG News, Vol 38, No. 4, pgs. 25-29.