Groundwater & location maps
OCWD’s basin model is a computer representation of the groundwater basin. It encompasses the entire groundwater basin and extends approximately three miles into Los Angeles County to provide for more accurate model results than if the model boundary stopped at the county line. Each of the basin’s three major aquifer systems (Shallow, Principal, and Deep) is represented by a separate corresponding “layer” in the model.
The District’s surface water recharge system is composed of more than two dozen facilities covering over 1,000 wetted acres and a total storage capacity of approximately 26,000 acre-feet. OCWD carefully tracks the amount of water being recharged in each facility on a daily basis.
OCWD collects groundwater elevation and quality data from nearly 700 wells, including over 400 District-owned monitoring wells. All entities that operate large-capacity wells must equip their wells with meters and report their production totals every six months. Approximately 200 large-capacity public and private supply wells account for 97 percent of all groundwater pumping. The remaining pumping comes from private wells that pump relatively small amounts of groundwater.
OCWD owns approximately 2,400 acres of land in the Prado Basin which includes 465 acres of constructed wetlands, a system comprised of 50 shallow ponds. The Prado Wetlands are designed to remove nitrogen and other chemicals from the Santa Ana River before the water flows downstream into Orange County where it is diverted into OCWD’s surface water recharge system where it infiltrates into the groundwater basin’s aquifers.
Although most large-capacity water supply wells pump groundwater from aquifers that are hundreds of feet below ground surface, some areas are underlain by shallow groundwater that is less than 40 feet below ground surface. The movement of shallow groundwater down into the deeper, drinking water aquifers is slowed by intervening low-permeability clays and silts.
Groundwater contour maps are based on groundwater level measurements of production and monitoring wells screened within the principal aquifers (approximately 300 to 1,200 feet deep). Contour maps were historically prepared based on November data. Beginning in 2005, contour maps were prepared based on June data to coincide with the end of the water year (July 1 – June 30).
The purpose of the North Basin Groundwater Protection Program (NBGPP) is to develop a remedial strategy to prevent VOC-contaminated groundwater in the cities of Fullerton and Anaheim from further spreading in the Shallow Aquifer and migrating vertically into the Principal Aquifer. The District is working with regulatory agencies and stakeholders to evaluate and develop effective remedies to address the contamination under the National Contingency Plan (NCP) process.
Seawater intrusion into coastal aquifers is a continuing management challenge for OCWD. OCWD measures chloride concentrations in groundwater to monitor seawater intrusion. Chloride concentrations are monitored twice a year at coastal-area monitoring wells, and chloride contour maps are prepared every two years to delineate the extent of seawater intrusion and determine areas where it is migrating inland or being pushed seaward.
The Talbert Seawater Barrier is composed of a series of 36 well sites that are used to inject water into multiple aquifer zones for seawater intrusion control as well as basin replenishment. The injection raises groundwater pressures within the aquifers along the barrier alignment and thus forms a hydraulic barrier to seawater that would otherwise migrate inland toward areas of groundwater pumping.
The Alamitos Seawater Intrusion Barrier was constructed in 1965 to protect the Central Basin of Los Angeles County and the Orange County Groundwater Basin from seawater intrusion through the Alamitos Gap. Since the barrier alignment lies in both Los Angeles and Orange Counties, the barrier facilities are jointly owned by the Los Angeles County Flood Control District (LACFCD) and OCWD and include 43 injection wells and 177 active monitoring well sites.
Over the last several years, elevated chloride concentrations above the secondary drinking water standard (250 mg/L) have been observed inland of the barrier within Orange County, both downgradient of the north-south barrier alignment and near the southeast portion of the barrier. This demonstrates that seawater intrusion is occurring through and south of the barrier. To address deficiencies along the north-south reach of the barrier, 17 injection wells at eight locations are planned to be constructed to augment injection capacity, along with four monitoring wells, and two piezometers which is all part of the Alamitos Barrier Improvement Project.