Alamitos Barrier Improvement
Established in 1933, the Orange County Water District (OCWD) manages the Orange County Groundwater Basin (270 square miles) and six miles of the Santa Ana River, which together provide water for more than 2.5 million people in Orange County, California.
In coastal regions, freshwater meets saltwater in a transition zone that includes the mouth of the river (surface water) and the aquifer (groundwater). Excessive pumping from a coastal aquifer can reduce the seaward flow of freshwater to the extent that saltwater intrudes into the aquifer, rendering it useless for agriculture or consumption. As new inland wells are constructed, regional groundwater levels can be lowered, allowing for further saltwater migration.
Extensive pumping of water wells within the Orange County Basin in the 1930s and 1940s lowered groundwater levels to below sea level, resulting in saltwater intrusion in the shallow aquifers. Starting in the late 1940s, OCWD established and expanded a groundwater replenishment program to artificially recharge the basin with imported water. In the 1960s, a series of freshwater injection wells were constructed to form what is known as the Alamitos Barrier.
The Alamitos Barrier injection wells raise groundwater pressures to form a hydraulic underground pressure ridge to block inland flow of saline groundwater. If saline water breaches the seawater barrier injection wells, it threatens the quality of public production wells. Since 1965, the number of injection wells rose from 14 to 41. However, after 50 years of operation, growing demand for fresh water increased seawater intrusion, impacting potable water wells.
In 2015, Orange County Public Works received permits to construct 17 injection wells, four nested monitoring wells, and two shallow piezometers. To avoid construction-related complications that OCWD had previously experienced in similar geologic conditions, they turned to Geoscience to manage the project scope, schedule, and budget while providing, managing, and overseeing the construction contractors.
During the two-year project, Geoscience worked with the drilling contractor to develop and implement a drilling fluid system that successfully controlled drilling fluid properties and enabled the project to proceed without borehole stability problems.
The team conducted borehole drilling on a 24-hour-per-day schedule to reduce the risk of borehole collapse. For each injection well cluster, they drilled the deepest completion first, and then used the lithologic and geophysical logs collected from that borehole as the basis for the design of the shallower completions within that cluster.
The team constructed the injection wells with stainless steel well casings and wire wrapped well screens. Geoscience also assisted OCWD with the final well design considerations, including the placement of well screen intervals, artificial filter pack material, and inter-annular well seals to avoid mixing groundwater from otherwise isolated aquifers.
After construction, the team initially developed each well by swabbing and airlifting, and completed development by pumping and surging. Once clear water flowed with maximum well production and minimum sand production, the team performed aquifer pumping testing, including step drawdown and constant rate pumping tests. They then collected groundwater quality samples from each well to serve as a baseline water quality for each respective aquifer.
OCWD went into production with 17 functioning injection wells that met or exceeded OCWD expectations for injection potential.
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