Groundwater Report - Executive Summary
San Diego County Water Authority's June 1997 Groundwater Report
Groundwater Occurrence. An aquifer is any unit of rock or sediment that is capable of both storing water and transmitting water to wells and springs. In the Authority’s service area, the principle aquifer materials are alluvium, semi-consolidated sediment, and to a lesser degree, consolidated sediment, residuum, and fractured crystalline rock.
A significant majority of the municipal groundwater supply in the Authority's service area is currently produced from shallow alluvial aquifers. Some of the better known and more productive alluvial aquifers (or groundwater basins) that are found in the major watersheds within Authority's service area include:
- Lower Ysidora, Chappo, and Upper Ysidora basins (Santa Margarita River Basin),
- Mission, Bonsall, Pala, and Pauma basins (San Luis Rey River Basin),
- San Dieguito, San Pasqual, and Santa Maria Valley basins (San Dieguito River Basin),
- Mission Valley and Santee/El Monte basins (San Diego River Basin),
- Lower Sweetwater and Middle Sweetwater basins (Sweetwater River Basin), and
- Lower Tijuana River Valley basin (Tijuana River Basin).
Total groundwater stored within these alluvial aquifers is estimated to exceed 700,000 acre-feet.
In addition to the shallow alluvial aquifers listed above, another deeper aquifer has been identified and recently delinated in the southwester portion of the county. This aquifer, commonly known as the San Diego Formation, is comprised of a thick accumulation of older, semi-consolidated alluvial sediments. The San Diego Formation aquifer is estimated to have stored groundwater in excess of 960,000 acre-feet in the Sweetwater Authority service area alone. It is thought that "with appropriate engineering design, as much as 120,000 to 240,000 acre-feet of this storage capacity may be usable without adverse impact to water quality and/or overlying structures" (Boyle Engineering, 1994).
San Diego Formation. Based upon the results of a recently completed gravity survey, sections of the San Diego Formation, believed to be sufficiently thick for municipal water supply production, have been delineated. These sections are thought to extend roughly north from the International Border with Mexico to the San Diego River and Mission Bay, and west from approximately I-805 (south of I-8) to the San Diego Bay and the Pacific Ocean in the City of San Diego. However, the gravity survey has not produced much in the way of stratigraphic information and much of the geology of the San Diego Formation remains unexplored, particularly in the north sections. A recent Report of Investigation, completed for the San Diego Formation Task Force by Dr. David Huntley at the San Diego State University, has identified two "zones" within the San Diego area that are suitable for exploration. "Zone A includes those areas where it is likely that the San Diego Formation is in excess of 1000 feet" thick, and "Zone B" "includes those areas that represent a greater exploration risk, but where the San Diego Formation could be of sufficient thickness to develop." A third area identified as "Zone C" was also delineated within the San Diego Formation study area. Zone C includes areas where it appears unlikely that the San Diego Formation is in excess of 1000 feet.
Sweetwater Authority (which overlies the central portion of the San Diego Formation aquifer) is currently proceeding with project development and the exploration of the San Diego Formation. The San Diego County Water Authority has recently completed Phase 1 of a planned three-phase groundwater storage and recovery feasibility study.
Other Aquifers. Groundwater also exists in fractured granitic bedrock, residuum, and consolidated sediments within the Authority’s service area. While well yields in these aquifers are often adequate to support individual homes or other small-scale uses, long-term yields tend to be inadequate for the economic development of municipal water supplies. Sometimes, productive municipal wells are screened in fractured bedrock that underlies shallower alluvial basins. In these cases, it is likely that the water stored in the alluvium replaces water extracted from fractures in bedrock that are in hydraulic connection with the alluvium.
Chapter 2 of the Groundwater Report summarizes in more detail "local groundwater occurrence and aquifer types," "current groundwater use and potential of principle aquifers," and "groundwater development constraints and opportunities," and presents an "overview of groundwater development and management strategies."
Groundwater Project Constraints and Opportunities. The ability of the member agencies to optimize groundwater and local surface water resources may be impacted by a number of issues, uncertainties, or constraints, including:
- water quality issues,
- water rights and groundwater basin management issues,
- institutional issues,
- regulatory considerations,
- hydrogeologic information availability and needs,
- economic uncertainties, and
- financial and funding issues.
The generally poor quality of local groundwater resources represents a key economic constraint to additional groundwater development, particularly in the case where concentrations of dissolved minerals are significantly greater than concentrations associated with imported water and reclaimed water supplies. Groundwater quality complies with drinking water standards in only a few aquifers within the Authority service area. Oftentimes extracted groundwater can be blended with imported water supplies to lower dissolved mineral concentrations to levels acceptable for potable uses. Otherwise, membrane treatment is required to remove dissolved solids (salts) to acceptable levels. Many times, high concentrations of nitrates, iron, and manganese are also present in local groundwater resources and may affect the beneficial uses of the groundwater.
On the other hand, opportunity now exists for groundwater development which was previously hindered by institutional and water rights constraints and concerns. New opportunities have in part resulted from the enactment of groundwater management legislation by the State of California (AB 3030), and from rulings by the California courts. Recent court rulings protect the rights of entities that have introduced "foreign" water into an aquifer as well as the rights of the overlying landowner who is pumping water to which he has a right.
In addition, a number of regional, state, and federal loan and grant programs are available for providing financial assistance to local agencies interested in developing groundwater resources. Programs have been available to assist in funding capital facilities, and to provide other financial incentives for project operation. Key among those programs have been the Authority’s Financial Assistance Program and MWD’s Groundwater Recovery Program (currently part of MWD's Local Resource Program).
Groundwater Development Strategies. It is useful to recognize that a wide variety of groundwater development strategies may be appropriate within the Water Authority service area (link to map of service area) Appropriate strategies will be dependent upon the following criteria:
- existing quality and use of groundwater,
- availability and quality of supplemental recharge water,
- method of recharge,
- frequency of groundwater recharge,
- frequency of groundwater withdrawal, and
- planned use and treatment needs of extracted groundwater.
Project Summary. Several Authority member agencies and other water agencies have either implemented groundwater projects or are planning or evaluating potential projects within the Authority’s service area. Chapters 3, 4, 5, and 6 of the Groundwater Report describes in various levels of detail "existing" and "planned" projects, "projects under study," and other "project concepts." These four categories of projects are identified below:
Existing Projects. Existing projects include projects where (1) member agencies have installed operational groundwater withdrawal facilities, and (2) the member agencies utilize the facilities for all or a portion of their water supply.
Planned Projects. The "planned projects" category include projects which member agencies have identified and evaluated through the completion of detailed feasibility studies. The feasibility studies have identified required facilities, costs, and implementation strategies. After completion of the feasibility studies, member agencies may or may not have decided to proceed with implementation of projects within the "planned project" category.
Projects Under Study. The "projects under study" category includes groundwater management alternatives or groundwater production strategies currently being evaluated by member agencies. Formal feasibility studies have not been completed for projects under this category, but member agencies are currently proceeding with feasibility studies, hydrogeologic investigations, or other evaluations.
Project Concepts. The "project concepts" category which includes potential projects which have been independently identified by Authority member agencies or in cooperation with Authority staff, but for which detailed investigations have not begun.
Existing Groundwater Development. Authority member agencies currently develop an annual average of approximately 26,200 acre-feet per year (AFY) of groundwater from local aquifers. Existing groundwater projects and associated annual production rates within the San Diego region are summarized in Chapter 3 of the report and are identified below.
- Approximately 8,500 AFY of groundwater production (actual annual production values may vary depending on hydrologic factors and base water demands) by Camp Pendleton from the San Mateo, San Onofre, Las Flores, and Santa Margarita River alluvial aquifers. Camp
- Pendleton groundwater basins are currently recharged with approximately 3,400 acre-feet per year of secondary-treated reclaimed water as well as rainfall runoff captured in constructed recharge basins. A portion of the recharge water that is contributed by the percolation of secondary effluent will be discontinued in the near future in order to comply with Regional Water Quality Control Board mandates.
- 2,200 AFY of potable water production from the City of Oceanside’s Mission Basin Brackish Groundwater Demineralization Program,
- Approximately 2,700 AFY of pumping (actual values vary, but have ranged from 2,020 AFY in 1990-1991 to 2,780 AFY in 1992-1993) by the Yuima MWD from the Pauma groundwater basin in the upper San Luis Rey River basin.
- An approximate annual average of 9,000 AFY of pumping (actual values vary, but have ranged from 4,000 AFY in 1992-1993 to 14,000 AFY in 1990-1991) by Vista Irrigation District from the Warner basin for use in recharging Lake Henshaw (and subsequent downstream potable use),
- 200 AFY of potable production (current production rate) by Ramona MWD from three deep wells in the Santa Maria basin,
- 1,000 AFY of potable water production (current production rate) from the El Monte basin by Lakeside Water District, which withdraws and treats groundwater to remove iron and manganese,
- 350 AFY (current pumping rate) of groundwater extraction by Riverview Water District from the El Monte basin,
- 250 AFY (current pumping rate) of potable groundwater production by Helix Water District within the El Monte basin, and
- An approximately annual average of 2,000 AFY of potable groundwater production (actual value vary, but have ranged from 1,232 AFY in 1992-1993 to 2,025 AFY in 1990-1991) from the San Diego Formation by Sweetwater Authority (National City Well Field).
Planned Groundwater Projects. In addition to the above existing groundwater projects, several Authority member agencies have completed feasibility studies and comprehensive facilities plans for groundwater development projects. Authority member agencies are currently proceeding toward implementation of the following three planned projects which are described in more detail in Chapter 4 of the report:
- Expansion of the existing City of Oceanside groundwater demineralization facility to a production capacity of 6.37 mgd (will result in an additional 4,900 AFY of potable water supply over and above the existing project capacity).
- Extraction and demineralization of brackish groundwater from the San Diego Formation and the Lower Sweetwater River alluvial basin by
- Sweetwater Authority. The project will yield approximately 3,600 AFY of new supply when completed in 1999.
- The goal of the Tia Juana Valley County Water District is to ultimately 2,500 AFY (or more) of groundwater from the San Diego Formation in the lower Tijuana River Valley.
Feasibility studies and implementation plans have also been developed for three additional "planned" groundwater projects within the Authority’s service area, but implementation of the planned projects is currently on hold pending resolution of a variety of issues. In the case of the 8,500 AFY recharge and recovery project considered by Padre Dam MWD in the Santee/El Monte basin, it has been postponed indefinitely. However, Padre Dam MWD is participating in a Groundwater Management Planning Study: Santee/El Monte Basin. Projects for which feasibility studies have been completed and that are currently on hold include:
- The 7,000 AFY seasonal recharge and recovery project proposed by Fallbrook Public Utility District (in conjunction with Camp Pendleton) in the lower Santa Margarita River basin,
- The 8,500 AFY seasonal reclaimed water recharge and recovery project investigated by Padre Dam MWD and Helix Water District in the Santee/El Monte basin, and
- A series of potential groundwater extraction projects for the Middle Sweetwater River groundwater basin proposed by Otay Water District for developing from 750 to 1,500 AFY of potable supply.
Groundwater Projects Under Study. Several agencies are currently proceeding with studies to determine the feasibility of proposed groundwater project concepts within the Authority’s service area. The "projects under study" are summarized in Chapter 5 of the report. Included among these projects are:
- Camp Pendleton and Tri-Cities Water District are evaluating the potential for up to 2,000 AFY of additional groundwater development through the conjunctive use of the San Mateo basin,
- Rainbow MWD is evaluating the potential for developing 3,000 AFY of potable supply by implementing a groundwater extraction and demineralization project in the Bonsall basin,
- Valley Center MWD is evaluating the potential for developing irrigation water supplies by withdrawing 600 AFY of groundwater from the lower Moosa Canyon basin and 400 AFY from the upper Moosa Canyon basin,
- The City of San Diego is evaluating the potential for implementing a 3,800 AFY groundwater recharge and recovery program in the San Pasqual basin,
- The San Dieguito Basin Task Force (composed of nine water supply and wastewater agencies) is currently evaluating the feasibility of groundwater management and a 4,000 to 8,000 AFY conjunctive use project in the lower San Dieguito basin,
- Lakeside Water District is evaluating the potential for expanding its El Monte basin groundwater withdrawal and treatment facilities from 1000 AFY to 1,200 acre-feet per year in 1997, and then possibly to 2,400 AFY thereafter, and
- Riverview Water District is considering increasing its groundwater production from 350 AFY to 1,000 AFY in the El Monte basin.
Note: Other studies not included in the Authority's 1997 Groundwater Report, that are currently (1999) ongoing include:
The Authority's Groundwater Storage and Recovery Feasibility Studies in the San Diego Formation and the Lower San Luis Rey River Valley (Mission and Bonsall Basins).
Other Project Concepts. Numerous groundwater development opportunities exist within the Water Authority service area in addition to the above-noted projects. Feasibility or hydrogeologic investigations necessary for the development of these concepts, however, have not been initiated. Some of these project concepts are identified in Chapter 6 of the report. They include:
- Expanding groundwater development in the City of Oceanside’s Mission basin from the planned 6.37 mgd demineralization project to 20 mgd or greater. The concept would likely require groundwater recharge and would ultimately yield an additional 15,300 AFY of supply (over and above the proposed 6.37 mgd project). This is one of several concepts that is being evaluated by the San Diego County Water
- Authority in its Lower San Luis Rey River Valley Groundwater Storage and Recovery Feasibility Study.
- Expanding the Ramona MWD groundwater production program to the central and western portions of the Santa Maria basin.
- Padre Dam MWD’s potential implementation of a brackish groundwater demineralization program in the Santee basin to produce approximately 3,600 AFY of potable supply.
- A potential (identified by Water Authority staff) for a 1,600 AFY groundwater demineralization project in Mission Valley.
- Developing up to 1,000 AFY of additional potable water production for Otay Water District through purchasing groundwater from private well owners in the Middle Sweetwater River basin.
- Expanding the proposed Sweetwater Authority groundwater demineralization program to a production capacity of 8 mgd (increasing the project yield by approximately 4,000 AFY) by recharging the San Diego Formation with excess local runoff and/or reclaimed water.
- Development of the remaining production and storage potential of the San Diego Formation outside the Sweetwater Authority and Tia Juana River Valley.
Three potential approaches to exploit the San Diego Formation as a groundwater aquifer are apparent. First the simple extraction and treatment of groundwater (both brackish and high quality groundwater). Second, groundwater extraction combined with artificial recharge (either to replenish the aquifer or to act as a salt-water injection barrier, or both). Third, seasonal injection and extraction of discounted imported water or local surface-water runoff utilizing the same wells (ASR wells). This alternative does not risk inducing sea-water intrusion.
Analysis. Each of the above "planned projects", "projects under study", and "project concepts" detailed in Chapters 4, 5, and 6 is evaluated by Authority staff in Chapter 7 to estimate the potential for implementation within the year 2015 planning period. Institutional, regulatory, environmental, economic, funding, hydrogeological, facilities, scheduling, and water rights issues where considered in the analysis. Table EX-1 summarizes the Authority staff’s analysis of the likelihood of member agencies implementing the planned or potential groundwater projects based upon current conditions.
As shown in the table, four projects are considered to be "extremely likely" to be implemented by the year 2015. Further, the implementation of six additional projects is considered "probable" within the same time frame. Finally, Authority staff consider it "possible" that as many as twelve additional projects (for a total of 22 projects) could be implemented by the year 2015.
Based upon current trends and a survey of member agency plans and activities, Authority staff believe that it is "extremely likely" that the 26,200 AFY existing groundwater production within the Authority service area will increase by 11,200 AFY by the year 2015 to an annual average of approximately 37,400 AFY.
The investigations also suggest that it is "probable" that total annual-groundwater production within the Authority’s service area will increase by 41,550 AFY (from 26,200 AFY to 67,750 AFY) by the year 2015. It is additionally concluded that the "possible" total increase in groundwater production within the Authority’s service area by the year 2015 is approximately 65,450 AFY (an increase from 26,200 AFY to 91,650 AFY) if appropriate actions are taken to overcome possible constraints to project development.
As part of the project analysis, critical issues that may affect project implementation were identified and evaluated. Future groundwater production would in large part be dependent upon: availability of capital financing, availability of reclaimed water and other surface water supplies for aquifer recharge, and the economics of groundwater projects compared to other sources of "new water." The report also concludes that it is likely member agencies may require Authority assistance for resolving funding/financing obstacles as well as possible institutional problems that are likely to arise.
REFERENCES
1. Boyle Engineering, 1994. Groundwater Studies of the San Diego Formation. February, 1994. Boyle Engineering Corporation. A study completed for the Sweetwater Authority.
For more information about the Authority's Groundwater Report or to obtain a copy of the June 1997 Report, contact Dan Diehr, Senior Water Resources Specialist at ddiehr@sdcwa.org
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