Abstract
Montgomery County is in the northern part of the
Houston, Texas, metropolitan area, the fourth most populous
metropolitan area in the United States. As populations have
increased since the 1980s, groundwater has become an
important resource for public-water supply and industry in the
rapidly growing area of Montgomery County. Groundwater
availability from the Gulf Coast aquifer system is a primary
concern for water managers and community planners in
Montgomery County and requires a better understanding of
the rate of recharge to the system. The Gulf Coast aquifer
system in Montgomery County consists of the Chicot,
Evangeline, and Jasper aquifers, the Burkeville confining
unit, and underlying Catahoula confining system. The
individual sand and clay sequences of the aquifers composing
the Gulf Coast aquifer system are not laterally or vertically
continuous on a regional scale; however, on a local scale,
individual sand and clay lenses can extend over several
miles. The U.S. Geological Survey, in cooperation with the
Lone Star Groundwater Conservation District, collected
groundwater-quality samples from selected wells within or
near Montgomery County in 2008 and analyzed these samples
for concentrations of chlorofluorocarbons (CFCs), sulfur
hexafluoride (SF6), tritium (3H), helium-3/tritium (3He/3H),
helium-4 (4He), and dissolved gases (DG) that include argon,
carbon dioxide, methane, nitrogen and oxygen. Groundwater
ages, or apparent age, representing residence times since
time of recharge, were determined by using the assumption
of a piston-flow transport model. Most of the environmental
tracer data indicated the groundwater was recharged prior
to the 1950s, limiting the usefulness of CFCs, SF6, and 3H
concentrations as tracers. In many cases, no tracer was usable
at a well for the purpose of estimating an apparent age. Wells
not usable for estimating an apparent age were resampled
in 2011 and analyzed for concentrations of major ions and
carbon-14 (14C). At six of these wells, additional 4He and DG
samples were collected and analyzed.
Recharge rates estimated from environmental tracer
data are dependent upon several hydrogeologic variables and
have inherent uncertainties. By using the recharge estimates
derived from samples collected from 14 wells completed in
the Chicot aquifer for which apparent groundwater ages could
be determined, recharge to the Chicot aquifer ranged from 0.2
to 7.2 inches (in.) per year (yr). Based on data from one well,
estimated recharge to the unconfined zone of the Evangeline
aquifer (outcrop) was 0.1 in./yr. Based on data collected from
eight wells, estimated rates of recharge to the confined zone of
the Evangeline aquifer ranged from less than 0.1 to 2.8 in./yr.
Based on data from one well, estimated recharge to the
unconfined zone of the Jasper aquifer (outcrop) was 0.5 in./yr.
Based on data collected from nine wells, estimated rates of
recharge to the confined zone of the Jasper aquifer ranged
from less than 0.1 to 0.1 in./yr. The complexity of the
hydrogeology in the area, uncertainty in the conceptual model,
and numerical assumptions required in the determination
of the recharge rates all pose limitations and need to be
considered when evaluating these data on a countywide or
regional scale. The estimated recharge rates calculated for
this study are specific to each well location and should not
be extrapolated or inferred as a countywide average. Local
variations in the hydrogeology and surficial conditions can
affect the recharge rate at a local scale.
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