Abstract
The Marine Corps Air Ground Combat Center
(MCAGCC) Twentynine Palms, California, overlies the
Surprise Spring, Deadman, Mesquite, and Mainside subbasins
of the Morongo groundwater basin in the southern Mojave
Desert. Historically, the MCAGCC has relied on groundwater
pumped from the Surprise Spring subbasin to provide all of its
potable water supply. Groundwater pumpage in the Surprise
Spring subbasin has caused groundwater levels in the subbasin
to decline by as much as 190 feet (ft) from 1953 through 2007.
Groundwater from the other subbasins contains relatively
high concentrations of fluoride, arsenic, and (or) dissolved
solids, making it unsuitable for potable uses without treatment.
The potable groundwater supply in Surprise Spring subbasin
is diminishing because of pumping-induced overdraft and
because of more restrictive Federal drinking-water standards
on arsenic concentrations. The U.S. Geological Survey, in
cooperation with the MCAGCC, completed this study to better
understand groundwater resources in the area and to help
establish a long-term strategy for regional water-resource
development.
The Surprise Spring, Deadman, Mesquite, and Mainside
subbasins are filled with sedimentary deposits of Tertiary age,
alluvial fan deposits of Quaternary-Tertiary age, and younger
alluvial and playa deposits of Quaternary age. Combined, this
sedimentary sequence reaches a maximum thickness of more
than 16,000 ft in the Deadman and Mesquite subbasins. The
sedimentary deposits of Tertiary age yield a small amount
of water to wells, and this water commonly contains high
concentrations of fluoride, arsenic, and dissolved solids. The
alluvial fan deposits form the principal water-bearing unit in
the study area and have a combined thickness of 250 to more
than 1,000 ft. The younger alluvial and playa deposits are
unsaturated throughout most of the study area. Lithologic and
downhole geophysical logs were used to divide the Quaternary/
Tertiary alluvial fan deposits into two aquifers (referred
to as the upper and the middle aquifers) and the Tertiary sedimentary
deposits into a single aquifer (referred to as the lower aquifer). In general, wells perforated in the upper aquifer yield
more water than wells perforated in the middle and lower
aquifers. The study area is dominated by extensive faulting
and moderate to intense folding that has displaced or deformed
the pre-Tertiary basement complex as well as the overlying
Tertiary and Quaternary deposits. Many of these faults act as
barriers to the lateral movement of groundwater flow and form
many of the boundaries of the groundwater subbasins.
The principal recharge to the study area is groundwater
underflow across the western and southern boundaries that
originates as runoff in the surrounding mountains. Groundwater
discharges naturally from the study area as spring flow, as
groundwater underflow to downstream basins, and as water
vapor to the atmosphere by transpiration of phreatophytes
and direct evaporation from moist soil. The annual volume
of water that naturally recharged to or discharged from the
groundwater flow system in the study area during predevelopment
conditions was estimated to be 1,010 acre-feet per year
(acre-ft/yr). About 90 percent of this recharge originated as
runoff from the Little San Bernardino and the Pinto Mountains
to the south, and the remainder originated as runoff from the
San Bernardino Mountains to the west. Evapotranspiration by
phreatophytes near Mesquite Lake (dry) was the primary form
of predevelopment groundwater discharge. From 1953 through
2007, approximately 139,400 acre-feet (acre-ft) of groundwater
was pumped by the MCAGCC from the Surprise Spring
subbasin.
A regional-scale numerical groundwater flow model was
developed using MODFLOW–2000 for the Surprise Spring,
Deadman, Mesquite, and Mainside subbasins. The aquifer
system was simulated by using three model layers representing
the upper, middle, and lower aquifers. Measured groundwater
levels for predevelopment conditions (before 1953) and for the
period 1953 through 2007 were used to calibrate the groundwater-
flow model. The simulated steady-state (predevelopment)
recharge was about 980 acre-ft/yr; about 90 percent of
the recharge was in the Mesquite subbasin. Most of the simulated
steady-state discharge occurred as evapotranspiration at
the Mesquite Lake (dry). A total of about 145,450 acre-ft of
groundwater was simulated as being pumped from the model domain during the transient simulation period (1953–2007);
about 139,400 acre-ft of the total was extracted from the Surprise
Spring subbasin. The transient simulation indicates that
almost all of the groundwater pumped in the Surprise Spring
subbasin comes from groundwater storage, which is consistent
with the measured long-term declines in groundwater levels.
the measured long-term declines in groundwater levels.
The calibrated groundwater model was used to evaluate
the potential effects on water levels and aquifer conditions in
the Surprise Spring, Deadman, Mesquite, and Mainside subbasins
for water-management strategies being considered by the
MCAGCC to meet the projected water demand at the base for
2008–2017. One of the main objectives of the water-management
strategies is to reduce pumpage from the Surprise Spring
subbasin. Reducing groundwater pumpage in the Surprise
Spring subbasin by about 38 percent (about 1,345 acre-ft/yr)
substantially decreased or reversed simulated hydraulic-head
declines in the subbasin. Redistributing about 15 percent of
the 2007 groundwater pumpage (about 550 acre-ft/yr) from the
Surprise Spring to the Mainside subbasin resulted in more than
60 ft of simulated declines in hydraulic head in the Mainside
subbasin by 2017; however, redistributing about 22 percent
of the 2007 pumpage (about 800 acre-ft/yr) from the Surprise
Spring subbasin to the Deadman subbasin resulted in 5–10 ft
of simulated hydraulic-head decline in the Deadman subbasin.
The water-management scenarios simulated for this study
demonstrate how the calibrated regional model can be utilized
to evaluate the hydrologic effects of a water-management
strategy.