The Department's groundwater injection barriers are successful at accomplishing their designed task. However, efforts continue to improve their effectiveness. As demand for drinking water in California increases, so does the cost to provide the imported water needed by all three barrier systems. Additionally, as barrier wells reach their designed lifeexpectancy, increasing demands are placed on well maintenance and replacement budgets. The need for an effective barrier system remains paramount, so alternative methods to stem seawater intrusion are periodically revisited. Although candidate alternatives may have been examined in the past and found not to be feasible, significant technical advances often make different approaches attractive and justify reconsideration. Listed below are a number of alternative seawater barriers which have been proposed.
The construction of a slurry wall involves the cutting of a bentonite slurry stabilized trench. This is accomplished using a large backhoe excavator suplemented by a clam shell (a large tool operated by a crane for the purposes of digging). The ditch is then backfilled with a mixture of soil-bentonite, a mixture of cement-bentonite, or plastic concrete materials that, when stabilized, forms an impervious barrier to seepage. Because of the need to cut a trench using heavy equipment, this prodedure has limitations with respect to disturbances of existing utilities which are often located near the ground surface.
Grout curtains are constructed by drilling 2- to 6-inch diameter holes along a single line or multiple parallel lines. Grout is then injected into the holes under pressure to fill the surrounding soil pores or rock fractures. By placing these holes on a tight enough spacing ( typically 3 to 10 foot spacings ), a grout barrier of variable thickness is created. Grout curtains can be installed to most any depth and can be surgically injected to treat specific depth zones.
Air injection is used in the development of oil and gas fields and during tunneling to cutoff the flow of water. Compressed air injected into the groundwater attempts to cause a piezometric rise in water level that can be used to alter groundwater gradients and flow directions. Air entrained in soil pores causes an overall decrease in the permeability of the aquifer which could be used to reduce flow across specific zones.
This technology involves the injection of starved bacteria cultures into the pores of the aquifer media to develop a biological subsurface plug or bio-wall. This method uses microbial growth to fill in the void space found in all soils, thus decreasing it's permeability. The wall is maintained by periodic injection of nutrients to feed the bacteria in specific zones. This type of wall has only been evaluated by modeling and in the laboratory.