San Diego Formation Aquifer Storage and Recovery Study, Phase 1: Executive Summary
Introduction and Purpose
The Phase 1 work identifies cost-effective and regionally beneficial project concepts for further detailed analysis in Phases 2 and 3.
This report summarizes the first phase of a planned three-phase feasibility study conducted by the San Diego County Water Authority (Authority).
The purpose of the three-phase study is to investigate the feasibility of utilizing the San Diego Formation for the storage of surface water supplies ("conjunctive use"). The primary goals of the first phase of the study are to identify cost-effective and regionally beneficial storage concept alternatives, and potential well sites for detailed investigation in Phases 2 and 3.
The San Diego Formation Study is a component of the Authority’s Regional Water Facilities Master Plan.
The Authority is the regional wholesaler of imported water to its 23 member agencies. Together, the Authority and its member agencies supply water to approximately 97 percent of the County’s 2.8 million residents. The Authority is currently developing a comprehensive Regional Water Facilities Master Plan (Master Plan) to identify the best combination of imported and local water facilities for meeting the region’s long term needs for water supply, quality, and reliability. This groundwater storage and recovery study of the San Diego Formation and a similar study of the Lower San Luis Rey River are being conducted in coordination with the broader master planning effort. The results of both of these studies will be weighed in the Master Plan against other storage and supply options.
The San Diego Formation
The San Diego Formation Task Force ("Task Force") funded a study in 1995 to determine the extent and possible thickness of the San Diego Formation.
The San Diego Formation is part of a thick wedge of sediment that was deposited along the coast in the San Diego Bay area in southwestern San Diego County. Based upon the results of the Task Force Study (completed in 1996), the San Diego Formation is believed to be at least 1000 feet thick in an area that extends from the U.S./Mexico border in the south, to Mission Bay in the North, and is bounded on the east by the La Nacion and Rose Canyon Fault Zones, located approximately two miles inland. Because it has been shown that the sediment wedge thickens to the west toward the bay and the Pacific Ocean, it is assumed that the San Diego Formation does also. However, the western-most extent of the formation has generally not been determined. The general location of the San Diego Formation and the project concepts (discussed later) are shown in Figure ES-1.
The gross storage potential of the San Diego Formation is high. However, the "usable" storage is constrained by practical considerations.
If the entire volume of sediments contained within the San Diego Formation could be fully drained, it has been estimated that they would yield roughly 2-million acre-feet or more of water. In practice, however, most of this volume cannot be drained without inducing land subsidence and/or eventually, seawater intrusion. In addition, portions of the San Diego Formation can not be accessed due to siting issues and/or economics. It has been estimated by the Study Team that a useable storage capacity of between 40,000 and 90,000 acre-feet per year could ultimately be developed if all the project concepts identified in Phase1 were implemented.
Semi-Arid Climate Limits Natural Recharge and Current Annual Production.
Preliminary investigations suggest that there is relatively little storage in the San Diego Formation that could be developed without over-drafting the basin and perhaps inducing seawater intrusion. The Authority’s study has therefore focused on identifying and evaluating artificial recharge projects to increased the average annual yield of the aquifer(s) within the San Diego Formation.
The San Diego Formation complex geology reduces predictability of aquifer geometry and well performance.
The San Diego Formation is characterized by extreme vertical and horizontal heterogeneity (non-uniformity) with regard to both the composition and texture of sediments, and the hydraulic character of sediment layers. This heterogeneity is in part due to the inherent nature of sedimentary deposits and in part due to the history of faulting in the region. This complexity has limited our current understanding of the stratigraphy of the San Diego Formation and makes it more difficult to accurately predict groundwater flow and well performance in unexplored areas of the formation. Consequently, project concepts planned with the current knowledge base must first include site-specific field investigations.
Study Approach
The purpose of the Phase 1 effort is to identify cost-effective, regionally beneficial project concepts, and potential well-field locations that can be further evaluated in Phases 2 and 3.
The current lack of understanding of the hydrogeology of the San Diego Formation makes it difficult, if not impossible, to develop a predictive model that can be used to site and design a groundwater recharge and storage project in unexplored areas. Therefore, Phase 1 of the study has been devoted primarily to the identification of cost-effective, regionally beneficial project concepts, and potential well field sites. As a result, the initial exploration required for development of project designs and cost evaluations will be limited to those areas where a project has the highest likelihood of being constructed. At the same time our knowledge and predictive capability regarding the San Diego Formation will be improved.
Project Concepts
San Diego Formation storage projects can be planned and operated either as constant-yield projects or as purely storage projects. Storage projects may be operated for carry-over, seasonal, emergency purposes, or a combination of purposes.
The Study Team has identified are two broad categories of recharge projects for the San Diego Formation: projects that are designed to operate on a constant or near constant basis (with recharge), providing a constant water supply, and projects that use Aquifer Storage and Recovery (ASR) wells to inject water during periods of availability for later extraction when needed. Recharge projects can be operated to provide carry-over (dry-year), seasonal, or emergency storage, or a combination of these, as shown in Figure ES-2. San Diego Formation hydrogeology, groundwater quality, and other considerations will determine the actual project design and operation mode that would be ultimately implemented at a particular site.
Once ASR well capacity is developed, the wells can be operated to respond to the most pressing storage need at the time.
The water supply to the San Diego region is subject to changing projections of water supply reliability, delivery capacity, treatment capacity, and other variables. A groundwater project planned originally for carry-over use could be re-operated to provide seasonal storage should the need arise. However, for an ASR project developed in an area where the native groundwater quality is high in salinity, only that water that is stored can be extracted later without advanced treatment.
Five ASR project concepts and two constant-yield project concepts were developed and evaluated.
Storage concepts were, in part, developed based on hydrogeology encountered elsewhere in the formation, and on preliminary information regarding land use, regulatory, contamination, and other constraints. Constant yield project concepts were developed assuming the availability of surplus surface or recycled water for recharge. Proposed project concepts are identified in Tables ES-1 and ES-2.
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Table ES-1: San Diego Formation ASR
Project Concepts
|
|
Project
|
No. of Wells
|
Annual Extraction Capacity
(AFY)
|
Remarks
|
|
Point Loma
|
17
|
11,000 to 24,000
|
Many potential well sites, well
location limited primarily by 1,500-foot minimum spacing to avoid well
interference. Injection with potable water. Stored water placed back into
water network during dry periods. |
|
Downtown
|
18
|
8,000 to 22,000
|
Highly urbanized so well location
dictated primarily by land use. If land use not consideration, a grid of
1,500 ft would allow 10 more wells resulting in 5,000 to 12,000 AFY additional
capacity. Injection with potable water. Stored water placed back into water
network during dry periods. |
|
Mid-Bay
|
19
|
11,000 to 24,000
|
Many wells located in Zone B with
unknown hydrogeologic characteristics. Most well sites limited by 1,500-ft
spacing. Injection with potable water. Stored water placed back into water
network during dry periods. |
|
Otay River Area
|
9
|
4,000 to 11,000
|
This project is located in the
same area as the proposed "Demin. III" project. The two projects are mutually
exclusive. Injection with potable water. Stored water placed back into
water network during dry periods. |
|
Tijuana Valley
|
7
|
3,000 to 9,000
|
The number of wells in this area
is limited by the hydraulic capacity of existing potable water pipelines.
An additional main is needed for additional hydraulic capacity for more
wells. This would result in an additional 2,000 to 7000 AFY of storage
capacity. Injection with potable water. Stored water placed back into water
network during dry periods. |
|
TOTAL
|
70
|
37,000 to 90,000
|
This is a reasonable expectation
of annual ASR extraction capacity in the SD Formation. If operated for
carry-over storage, these projects would develop the capacity to extract
between 37,000 and 90,000 AFY of previously injected water. |
|
Table ES-2: San Diego Fm. Constant
Yield Project Concepts
|
|
Project
|
No. of Wells
|
Annual Yield (AFY)
|
Remarks
|
|
Demin III
|
13
|
6,000 to 10,000
|
Includes artificial recharge with
reclaimed water (for salinity intrusion barrier and to add to basin safe
yield) and a 6 to 10 mgd RO plant. Cost-effectiveness problematic if additional
treatment is needed of reclaimed water prior to injection. |
|
Tijuana Valley II
|
6
|
3,000 to 5,000
|
Reclaimed water recharge upstream
of extraction wells. Groundwater treatment with 3-5 mgd RO plant. Cost-effectiveness
problematic if additional treatment is needed of reclaimed water prior
to injection. |
Benefits and Costs
Regional benefits of increased water storage capacity exceed the projected
costs of San Diego Formation ASR projects.
Storage benefits accrue from two areas, direct and indirect benefits.
First, in the short-term, incentive programs offered by the Metropolitan
Water District of Southern California (MWD) could provide direct monetary
benefits for seasonal and carry-over storage operations. In addition, other
surplus reclaimed or surface water (e.g. spot transfers or excess local
runoff) may become available periodically at reduced cost.
Second, the availability of increased locally-controlled storage could
provide indirect monetary benefits related to increased water supply reliability,
and in the long-term may allow the deferral of projects to expand water
delivery and treatment capacities.
The Phase 1 cost/benefit analysis indicates that the regional benefits
of increased water storage capacity exceed the projected costs of San Diego
Formation ASR project concepts. Based on assumptions reflecting current
benefit conditions, ASR projects concepts that are operated for carry-over
storage appear to offer the highest returns. ASR project concepts operated
solely for seasonal storage do not appear to be as cost-effective at the
present time. However, these findings are based upon present assumptions
regarding hydrogeology and the future benefits of seasonal operation which
may change as the study proceeds through Phases 2 and 3.
The benefit-cost ratios calculated by the Study for the ASR projects
are shown in Table ES-3. Note that the table shows a range of ratios.
The ranges reflect some of the hydrogeologic uncertainty that currently
exists regarding the Formation. The Phase 2 hydrogeologic investigations
will allow for more certain assessment of the proposed project concepts,
operations, and their associated costs and benefits.
Table ES-3: Benefit/Cost Ratios for ASR Projects
|
Project
|
Seasonal Storage Benefit
Cost Ratio
|
Carry-Over Storage Benefit
Cost Ratio
|
Modified Carry-Over Benefit
Cost Ratio
|
|
Downtown
|
0.3 to 0.5
|
1.0 to 2.6
|
0.9 to 1.7
|
|
Point Loma
|
0.3 to 0.5
|
1.4 to 2.3
|
1.0 to 1.6
|
|
Mid-Bay
|
0.3 to 0.6
|
1.4 to 2.7
|
1.0 to 1.8
|
|
Otay River Valley
|
0.3 to 0.6
|
1.0 to 2.5
|
0.8 to 1.8
|
|
Tijuana River Valley I
|
0.3 to 0.6
|
1.0 to 2.6
|
0.8 to 1.8
|
San Diego Fm. projects designed and
operated for constant yield may be cost-effective depending on the resolution
of certain key regulatory and institutional issues.
Constant yield projects provide benefits by providing a new
source of water, reducing a local agency’s need to buy imported water.
Some of these projects are also eligible to receive financial credits from
MWD and from the United States Bureau of Reclamation. The analysis indicates
that while these constant yield projects may prove to be cost-effective,
the development of these projects will be dependent on the resolution of
key issues including the availability of reclaimed water as a source of
artificial recharge and the regulations governing reclaimed water treatment
prior to injection. Historically, the development of similar projects (i.e.
those projects designed to produce a local water supply) has been characterized
by a strong local agency participation. The final determination of the
regional benefit of these yield projects (designed to serve a local agency
(or agencies)) will likely depend on how the costs and benefits are ultimately
distributed.
Even with these incentives, the cost-effectiveness of projects that
include recharge with recycled water may be problematic if additional treatment
is needed of reclaimed water prior to injection.
The benefit-cost ratios calculated by the Phase I study for the constant
yield projects are shown in Table ES-4. As before, the exhibit shows
a range of ratios. The Phase 2 hydrogeologic investigations will allow
for more certain assessment of project feasibility.
Table ES-4: Summary of Benefit/Cost Ratios for Constant-Yield Projects
|
Project Name
|
Local Yield Benefit to
Cost Ratio
|
|
Demin III
|
w/o reclaimed water treatment
|
1.0
|
|
w/ reclaimed water treatment
|
0.7 to 0.9
|
|
Tijuana River Valley II
|
w/o reclaimed water treatment
|
1.0
|
|
w/ reclaimed water treatment
|
0.7 to 0.9
|
All of the projects would provide non-economic water management benefits.
Each of the projects is consistent with a California water ethic that
recognizes the importance of maximizing the development of practical local
water supply and storage projects. The Authority’s visible commitment to
such projects benefits the political climate surrounding statewide efforts
to manage California’s water resources.
Conclusions
The hydrogeology of the San Diego Fm
the usable and more cost-effective storage in the formation has been approximated
to be on the order of 40,000 to 90,000 acre-feet per year.
Although theoretically very large, the San Diego Formation practical
storage volume is limited to more modest numbers (Figure ES-3).
However, even at these lowered numbers, the Formation may still prove to
be an important resource for the region. This preliminary estimate is a
rough approximation of the storage that, at this time, is economically
reasonable to develop. Available storage and yield may increase depending
upon economics and the results of future field investigations.
There is a reasonable likelihood that cost-effective increments of regionally
beneficial storage capacity can be developed.
Based on the economic analyses, several of the ASR projects appear
capable of creating regional storage benefits at costs that are competitive
with other leading options for developing new storage capacity in San Diego
County.
The evaluation of storage benefits justifies continued investigation
of the San Diego Formation.
Projects that use the resources of the San Diego Formation to create
new increments of storage capacity seem to provide the highest regional
benefit and offer the highest economic use of the aquifer based on current
benefit assessments which exceed costs at an average ratio of 1.5 to 1.9.
The scalability of the projects provides flexibility for their implementation.
The project concepts presented in this report each include multiple
well groups. These projects can be implemented one well group at a time
without any loss of project economy. Sites that indicate acceptable well
performance can then be scaled up on a timeline consistent with the region’s
needs for new storage, while poorer sites can be deferred or abandoned
in favor of other sites.
Project implementation should be accompanied by the formation of a locally
approved groundwater management plan.
In order to better manage groundwater resources and protect any investments
in the development of groundwater resources and storage, the Authority
and other interested water agencies should cooperatively adopt a Groundwater
Management Plan as part of the project implementation process. The development
of groundwater management plans is provided for by California statute (CA
Water Code Sections 10750 to 10755, previously AB 3030) or http://www.leginfo.ca.gov/cgi-bin/calawquery?codesection=wat&codebody=
Recommendations
The Phase 1 conclusions provide suitable justification for proceeding
with the Phase 2 hydrogeologic investigations.
Phase 2 of the Study will provide focused hydrogeologic testing of
a select number of project sites. In this way, Phase 2 will serve both
to test assumptions regarding hydrogeology used to develop specific project
concepts and costs identified in Phase 1, as well as to advance the general
hydrogeologic knowledge and understanding of the Formation as a whole.
The study team recommends that four projects warrant advancement to
Phase 2, and that they be pursued incrementally in the order of their ranking.
Based on their cost-effectiveness and their potential benefits to
the region as a whole, the study team recommends that the Authority advance
the following four projects to Phase 2 and that they be pursued incrementally
in the order of preference listed.
Mid-Bay ASR Project: Develops between 11,000 and 24,000 AF of annual
extraction capacity at full implementation. Carry-over storage benefit
to cost ratio = 1.4-2.7.
Point Loma ASR Project: Develops between 11,000 and 24,000 AF of
annual extraction capacity at full implementation. Carry-over storage benefit
to cost ratio = 1.4-2.3.
Tijuana Valley I ASR Project: Develops between 3,000 and 9,000 AF
of annual extraction capacity at full implementation. Carry-over storage
benefit to cost ratio = 1.0-2.6.
Downtown ASR Project: Develops between 8,000 and 22,000 AF of annual
extraction capacity at full implementation. Carry-over storage benefit
to cost ratio = 1.0-2.6.
Cost estimates for the various Phase 2 hydrogeologic investigations
are provided in Table ES-5. Given the expense, investigations could
proceed one project at a time. Note that some of the investigation items
are optional.
Table ES-5: Phase 2 Component Cost Estimates
|
Component
|
Basic Program
|
Optional Seismic Reflection
|
Optional Coring
|
| Common Support Tasks |
$105,000
|
--
|
--
|
| Mid-Bay (With 2 Wells) |
$282,000
|
$180,000
|
$77,000
|
| Point Loma (With 3 Wells) |
$389,000
|
$98,000
|
$77,000
|
| Downtown
(With 3 Wells) |
$379,000
|
$52,000
|
$77,000
|
| Tijuana Valley I (With
2 Wells) |
$400,000
|
$171,000
|
$99,000
|
For more information about the San Diego County Water Authority's San
Diego Formation aquifer studies contact: Dan Diehr, Senior Water Resources
Specialist at ddiehr@sdcwa.org
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