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Additional Water
for the San Joaquin River Agreement,
2000 – 2010 Supplemental EIS/EIR
5. GROUNDWATER
RESOURCES
Index
5.1 Affected Environment
This section of the Supplemental EIS/EIR (SEIS/EIR)
is a condensed summary of Section 3.3 and Appendix
B of the Meeting Flow Objectives for the San
Joaquin River Agreement, 1999-2010, Final
Environmental Impact Statement/Environmental Impact
Report dated January 28, 1999 (Final EIS/EIR,
Reclamation 1999). The Final EIS/EIR included a
detailed overview of the regional groundwater resources
within the San Joaquin Groundwater Basin and area-specific
information on the groundwater basins underlying
the service areas of the San Joaquin River Group
Authority members willing to provide up to 47,000
acre-feet of additional water that may be needed
to achieve full Vernalis Adaptive Management Plan
(VAMP) Target Flows during double-step years. The
additional water would supplement VAMP test flows,
which otherwise may not be achieved during certain
years due to the San Joaquin River Agreement cap
of 110,000 acre-feet.
5.1.1 Introduction
This summary includes information on historical
perspective and recent groundwater conditions for
the San Joaquin River Region. It covers the following
basins that include the service areas that would
be providing the additional water:
- Turlock Groundwater Basin
- Modesto Groundwater Basin
- Merced Groundwater Basin
Detailed characteristics of each basin are included
in the Final EIS/EIR as follows: hydrogeology (Section
3.3.4.1), groundwater storage and production (Section
3.3.4.2), groundwater levels (Section 3.3.4.3),
groundwater quality (Section 3.3.4.4), land subsidence
(Section 3.3.4.5), agricultural subsurface drainage
(Section 3.3.4.6), and seepage-induced waterlogging
of farmlands (Section 3.3.4.7). In addition, Appendix
B of the Final EIS/EIR includes detailed information
on groundwater conditions.
5.1.2 Historical Perspective
and Recent Conditions
Groundwater resources of the San Joaquin Valley
are described in the Final EIS/EIR with regard to
regional hydrogeology, groundwater storage and production,
groundwater levels, and groundwater quality. The
Final EIS/EIR presentation of groundwater quality
covers those constituents of concern that affect
agricultural productivity and others that are noted
to be in high concentrations and known to affect
human health and wildlife, including total dissolved
solids (TDS), boron, nitrates, arsenic, selenium,
and dibromochloropropane.
In addition, three other issues are historically
related to groundwater conditions:
- Agricultural subsurface drainage has been affected
by the presence of perched shallow groundwater
conditions in parts of the San Joaquin Valley;
- Seepage-induced waterlogging of farmlands has
occurred due to the movement of water from streams
into adjacent shallow groundwater aquifers;
- Land subsidence may be caused by one or a combination
of the following mechanisms: compaction of aquifer
sediments from groundwater overdrafts with subsequent
lowered hydraulic head in the aquifer; compaction
of sediments in petroleum reservoir rocks from
oil and gas exploration and extraction; hydrocompaction
(the compaction of moisture-deficient sediments
following the first application of water); compaction
of peat soils following land drainage; and tectonic
subsidence (Bertoldi et al. 1991).
Historically, the greatest occurrence of land subsidence
in the San Joaquin Valley has resulted from groundwater
overdrafts and lowered hydraulic head, and was the
only type of land subsidence discussed in the Final
EIS/EIR.
5.1.3 Overview of the Central
Valley Regional Aquifer System
The Central Valley regional aquifer system is a
400-mile-long, northwest-trending asymmetric trough
averaging 50 miles in width. The significant water-producing
geologic units are the unconsolidated to semiconsolidated
nonmarine sediments that range from the Oligocene
and Miocene ages (13 million to 25 million years
old) too recent. The west side of the aquifer system
is bounded by pre-Tertiary and Tertiary semiconsolidated
to consolidated marine sedimentary rocks of the
Coast Ranges. These faulted and folded sediments
extend eastward beneath most of the Central Valley
and any water contained in the sediments is usually
saline. The east side of the valley is underlain
by pre-Tertiary igneous and metamorphic rocks of
the Sierra Nevada. Only small quantities of water
are extracted from the joints and cracks of these
basement rocks.
5.1.4 Groundwater Resources
of the San Joaquin River Region
The southern two-thirds of the Central Valley regional
aquifer system extends from just south of the Delta
to just south of Bakersfield, covering over 13,500
square miles, and is referred to as the San Joaquin
Valley Basin (DWR 1975). The Department of Water
Resources (DWR) further divides this basin into
subbasins. Subbasins in the northern half of the
San Joaquin Valley Basin, lying within the San Joaquin
River Region, include the Tracy, Eastern San Joaquin
County, Modesto, Turlock, Merced, Chowchilla, Madera,
and Delta-Mendota subbasins (DWR 1994).
5.1.5 Summary of Groundwater
Conditions
Each of the San Joaquin River Region basins that
include service areas that would be providing the
additional water are summarized in Table 5-1. The
areas include:
- Turlock Groundwater Basin
- Modesto Groundwater Basin
- Merced Groundwater Basin
A detailed description of the groundwater resources
in each of the basins is contained in Appendix B
of the Final EIS/EIR.
Table 5-1
Summary of Groundwater Conditions in the San Joaquin
River Region
| |
Groundwater Basin
or Service Area
|
|
Groundwater Conditions
|
Turlock Groundwater
Basin
|
Modesto Basin
|
Merced Groundwater
Basin
|
| Elevation/Levels |
Depth to groundwater ranges
from 6 to over 100 feet bgs
Water table declined approximately
5 feet
|
Depth to groundwater ranges
from less than 5 feet to over 100 feet
bgs
Water table declined 15 feet for period
from 1970-1990
|
Depth to groundwater ranges
from less than 1 foot to over 100 feet
bgs
Water table declined up to 40 feet
for period from 1980-1992
|
| Water Quality |
Water hardness moderate
to very hard |
Generally acceptable |
Numerous constituents detected |
|
|
High TDS in wells deeper
than 350 feet |
Relatively low |
High TDS in wells deeper
than 350 feet |
|
|
Localized and some levels
above MCL |
Localized and some levels
above MCL |
Below current MCL |
|
|
Below current MCL |
Below current MCL |
Below current MCL |
|
|
Elevated naturally occurring
concentrations |
Elevated naturally occurring
concentrations |
Elevated naturally occurring
concentrations |
|
|
High naturally occurring
uranium |
High naturally occurring
uranium |
No data available |
|
|
Localized and some levels
above MCL |
Localized and some levels
above MCL |
Levels detected at or below
MCL |
|
|
Isolated occurrences principally
the result of leaking USTs � none in public
water supply |
Isolated occurrences principally
the result of leaking USTs � some in public
water supply |
Localized and some levels
above MCLs |
| Subsidence |
Not a problem |
Not a problem |
Not a problem |
| Agricultural Subsurface
Drainage |
Drainage pumping |
Drainage pumping |
Drainage pumping |
Notes:
bgs = below ground surface
MCL = maximum contaminant level
TDS = total dissolved solids
UST = underground storage tank
5.1.5.1 Turlock Groundwater Basin
Water level data shows that water levels have declined
between 1971 and 1991 (TID 1997) in response to
pumping that was taking place outside the Turlock
Irrigation District (TID). The largest water level
declines have occurred within the eastern part of
the basin, where declines are as much as 90 feet.
Water levels have declined approximately 5 feet
throughout the western part of the basin.
These declines are largely the result of pumping
from outside of the TID resulting in annual overdrafting
in the Turlock Groundwater Basin at an annual estimated
rate of 70,000 to 85,000 acre-feet (TID 1997). DWR
estimates that the typical groundwater production
rate in the basin is 452,000 acre-feet per year
(DWR 1998). On a local level, groundwater extraction
rates vary throughout the basin (based on factors
such as location of municipalities, depth to groundwater,
and crop water needs).
Water levels will probably continue to decline
within the TID service area as a result of the overdrafting.
The rate of groundwater decline will vary throughout
the area depending on conditions such as groundwater
extraction rates, underflow to groundwater depressions
located outside TID, and recharge from sources including
irrigation seepage, precipitation, groundwater inflow,
and artificial recharge.
5.1.5.2 Modesto Groundwater Basin
Long-term water-level monitoring conducted by DWR
indicates that the Modesto Groundwater Basin has
experienced groundwater declines of 15.3 feet in
the period from 1970 to 1990 (HCI 1992). This decline
represents depletion of storage of 404,000 acre-feet.
The average annual water budget for the period from
1952 to 1991 indicates the average annual overdraft
in the basin was 2,300 acre-feet per year (MID 1996).
The overdraft was indicated by water level declines
of approximately 0.5 foot per year (HCI 1993).
The average annual overdraft in the Modesto Groundwater
Basin is estimated at 15,000 acre-feet (MID 1996).
DWR estimates that the typical groundwater production
rate in the basin is 229,000 acre-feet per year
(DWR 1998). On a local level, groundwater extraction
rates vary throughout the basin (based on factors
such as location of municipalities, depth to groundwater,
and crop water needs).
The rate of groundwater decline within the Modesto
Irrigation District (MID) service area will vary
throughout the area depending on conditions such
as groundwater extraction rates, underflow to groundwater
depressions located outside MID, and recharge from
sources including irrigation seepage, precipitation,
groundwater inflow and artificial recharge.
5.1.5.3 Merced Groundwater Basin
Long-term water-level monitoring indicates that
the Merced Groundwater Basin has experienced groundwater
declines of 40 feet in the period from 1960 to 1992.
This decline represents depletion of storage of
404,000 acre-feet.
The average annual overdraft in the Merced Groundwater
Basin is estimated at 20,000 acre-feet (Merced ID
1997). DWR estimates that the typical groundwater
production rate in the basin is 555,000 acre-feet
per year (DWR 1998). Based on factors such as location
of municipalities, depth to groundwater, and crop
water needs, on a local level, groundwater extraction
rates vary throughout the basin.
Projected agricultural and municipal groundwater
usage for the Merced Irrigation District (Merced
ID) service area is approximately 638,000 acre-feet
per year (Merced ID 1997). Of this volume, Merced
ID pumps approximately 10,000 acre-feet per year.
Total agricultural groundwater demand in the basin
is projected to decrease by 12 percent over the
next 40 years, from 601,800 acre-feet per year in
1996 to 529,584 acre-feet per year in 2036 (Merced
ID 1997). Total groundwater demand for municipal
uses in the basin is projected to increase by approximately
33 percent by 2030, from approximately 40,000 acre-feet
per year in 1996 to 121,000 acre-feet per year in
2036 (Merced ID 1997). No estimates were given for
the increase in demand for industrial uses. Given
the projected usage of groundwater, overdrafting
of the Merced Groundwater Basin would continue to
increase.
Water levels will probably continue to decline
within the Merced ID service area as a result of
the overdrafting. The rate of groundwater decline
will vary throughout the area depending on conditions
such as groundwater extraction rates, underflow
to groundwater depressions located outside Merced
ID, and recharge from sources including irrigation
seepage, precipitation, groundwater inflow, and
artificial recharge.
In the Final EIS/EIR, Appendix G discussed the
California Environmental Quality Act requirement
of a mitigation monitoring or reporting program
for any significant impacts identified in the Final
EIS/EIR. To mitigate the potentially significant
impacts to groundwater conditions, the Merced ID
has implemented a conjunctive use project that will
work towards the goals of stabilizing the groundwater
at 1992 levels. Chapter 21 discusses the current
progress of this project in more detail.
5.2 ENVIRONMENTAL CONSEQUENCES
AND MITIGATION MEASURES
This section describes changes to groundwater conditions
associated with the Proposed Project (i.e., release
of up to 47,000 acre-feet of additional flow from
carry over storage to achieve the full VAMP test
flows), as compared to the No Action Alternative
(i.e., the existing conditions with VAMP test flows).
5.2.1 Key Impact Issues and
Evaluation Criteria
With respect to groundwater, the primary issue
is the extent to which the additional water releases
from the Project reservoirs (up to 47,000 acre-feet)
would affect groundwater storage in the Project
Area.
As described in Section 2.1, the No Action Alternative
includes the VAMP flows up to 110,000 acre-feet.
The following discussions will only deal with the
environmental consequences of additional flows of
up to 47,000 acre-feet. No change in groundwater
pumping is anticipated as a consequence of the Proposed
Project/Action.
5.2.2 Environmental Impacts
and Mitigation
The potential impact of the Proposed Project/Action
manifests as reduced storage in New Don Pedro and/or
New Exchequer Reservoirs until such time that the
storage is recovered through reductions in releases
from the reservoirs that would otherwise be in excess
of instream flow requirements. The Proposed Project/Action
is not anticipated to affect the diversion or delivery
of water to any water user within the affected basins;
therefore, no change to the timing or occurrence
of return flows is anticipated.
In the Supplemental Hydrologic Analysis (Appendix
A), the model results indicate that Proposed Project
releases would only occur during approximately 10
percent of the years during the modeled period of
1922 through 1992 (71 years). The modeled supplemental
flows would range from 2,000 to 46,000 acre-feet.
During the few years when supplemental water would
be required, a direct release to the lower Tuolumne
River (i.e., MID and TID water from New Don Pedro
Reservoir) or the lower Merced River (i.e., Merced
ID water from Lake McClure could occur. As a result,
the river stage downstream of the release would
be raised with the corresponding potential for groundwater
recharge where the river stage is higher than the
adjacent groundwater levels.
The release also results in a one-to-one decrease
in reservoir storage. In most modeled instances,
the additional water released would be recovered
in reservoir storage the following year by a reduction
in releases that would otherwise be in excess of
minimum Tuolumne or Merced river flow requirements.
In a couple of instances, the modeled recoveries
did not occur until one or more years later.
The modeled results indicate that release of Proposed
Project water would only take place in water years
where excess carry-over storage is present. Deliveries
to surface water customers would not be diminished
during these years, and no additional groundwater
pumping would occur as a result. As such, the Proposed
Project would not have a negative impact on groundwater
supply.
Potential groundwater impacts for water resources
projects typically could include changes to groundwater
overdrafting, groundwater levels, groundwater quality,
subsidence, subsurface drainage, and waterlogging.
The Proposed Project would not result in these groundwater
impacts because groundwater would not be pumped
directly to account for Proposed Project water or
indirectly to replace Project water.
Significance determinations for the impact analysis
are based on the quantity of groundwater involved
directly or indirectly and on the existing overdraft
conditions in the basin. Within the San Joaquin
River Region, approximately 2.6 million acre-feet
of groundwater are extracted in a typical year (DWR
1998). The majority of the groundwater is pumped
by a combination of private agricultural interests
and municipalities. Because the supplemental releases
would not result in increased groundwater pumping,
many of the environmental impact and mitigation
measures do not apply. The following sections are
organized by river and by basin where the supplemental
water would be released.
5.2.2.1 Proposed Action on the Tuolumne River
Modesto Groundwater Basin
No Action. The No Action Alternative represents
existing conditions, including reliance on groundwater
in portions of the Project Area. The No Action Alternative
includes the VAMP 31-day pulse flows during April
and May, flow releases in accordance with various
operating plans, settlement agreements, and Federal
Energy Regulatory Commission requirements now in
place for the San Joaquin River Basin. Because No
Action represents the baseline, no impacts are identified
for its implementation.
Proposed Action. The maximum annual surface
water release for the supplemental water for the
VAMP flows from MID would be all or a portion of
47,000 acre-feet in conjunction with releases from
TID. Of this volume, none of the water is projected
to come from groundwater. The available water would
come from carry-over storage from either New Don
Pedro Reservoir or Lake McClure that would otherwise
be released under discretionary operations of the
two reservoirs. This would result in no impact on
groundwater levels in the Modesto Groundwater Basin.
This would also mean that there would be no impacts
with respect to groundwater overdrafting, subsidence,
water quality, or agricultural subsurface drainage.
Turlock Groundwater Basin
No Action. The No Action Alternative represents
existing conditions as described above for the Modesto
Groundwater Basin. Because No Action represents
the baseline, no impacts are identified for its
implementation.
Proposed Action. The maximum annual surface
water release for the supplemental water for the
VAMP flows from TID would be all or a portion of
47,000 acre-feet in conjunction with MID. Of this
volume, none of this water is projected to come
from groundwater because the supplemental water
for the VAMP flows would take place following a
combination of Wet or Above Normal years when there
would be no demand for additional groundwater pumping.
As such, the Proposed Action would have no impact
on water levels within the Turlock Groundwater Basin.
This would also mean that there would be no impacts
with respect to groundwater overdrafting, subsidence,
water quality or agricultural subsurface drainage.
5.2.2.2 Proposed Action on the Merced River
No Action. The No Action Alternative represents
existing conditions, including reliance on groundwater
in portions of the Project Area. The No Action Alternative
includes the VAMP 31-day pulse flows during April
and May, flow releases in accordance with various
operating plans, settlement agreements, and Federal
Energy Regulatory Commission requirements now in
place for the San Joaquin River Basin. Because No
Action represents the baseline, no impacts are identified
for its implementation.
Proposed Action. The maximum annual surface
water release for the supplemental water for the
VAMP flows from Merced ID is 47,000 acre-feet. Of
this volume, none of this water is projected to
come from groundwater The available water would
come from carry-over storage from either New Don
Pedro Reservoir or Lake McClure that would otherwise
be released under discretionary operations of the
two reservoirs. As such, the Proposed Action would
not have a negative impact on water levels, groundwater
overdrafting, subsidence, water quality, or agricultural
subsurface drainage within the Merced Groundwater
Basin.
5.2.3 Impact Summary and Mitigation
of Impacts
- The following summarizes impacts of the Proposed
Action in comparison to No Action by groundwater
basin in the Project Area.
5.2.3.1 Modesto Groundwater Basin
- The maximum surface water release for the supplemental
water for the VAMP flows could come from MID (up
to 47,000 acre-feet) in conjunction with water
from TID. All of this water would come from carry-over
storage in New Don Pedro Reservoir; none of the
water would come from groundwater. As such, there
would be no impacts to groundwater overdrafting,
groundwater levels, water quality, subsidence,
or agricultural subsurface drainage within the
Modesto Groundwater Basin as a result of this
action. Accordingly, no mitigation is necessary.
5.2.3.2 Turlock Groundwater Basin
- The maximum surface water release for the supplemental
water for the VAMP flows could come from TID (up
to 47,000 acre-feet) in conjunction with water
from MID. This water would come from carry-over
storage in New Don Pedro Reservoir; none of the
water would come from groundwater. As such, there
would no impacts to groundwater overdrafting,
groundwater levels, water quality, subsidence,
or agricultural subsurface drainage within the
Turlock Groundwater Basin as a result of this
action. Accordingly, no mitigation is necessary.
5.2.3.3 Merced Groundwater Basin
- The maximum surface water release for the supplemental
water for the VAMP flows could come from Merced
ID (up to 47,000 acre-feet). This water would
come from carryover storage in Lake McClure. None
of this water is expected to come from groundwater.
As such, there would no impacts to groundwater
overdrafting, groundwater levels, water quality,
subsidence, or agricultural subsurface drainage
within the Merced Groundwater Basin as a result
of this action. Accordingly, no mitigation is
necessary.
Draft SES/EIR
CH5 |
December 20, 2000
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