Assessment of Subsidence Risk Associated with Brackish Groundwater Development in the Coastal Lowlands Aquifer, Houston, Texas, USA
2020
Abstract. Significant undeveloped brackish groundwater resources
exist within the Coastal Lowlands Aquifer System (Gulf Coast Aquifer System)
near Houston, Texas, USA. As the development of these frontier resources is
imminent, an improved understanding of the impact development may have on
the availability of the resource and land subsidence is needed. In this
region, land subsidence is caused by the depressurization of the aquifer and
compaction of the many clay lenses in the subsurface. The Gulf Coast Aquifer
System in the study area includes three primary water bearing units (from
shallow to deep): the Chicot (Pleistocene and Pliocene) and Evangeline
(Pliocene and Miocene) aquifers, and the Jasper aquifer (Miocene). Although
there has been much research and data supporting the causal relation between
water-level decline and subsidence in the areas of fresh groundwater
development, little data exists to inform on the potential subsidence
impacts upon deeper brackish groundwater development. Data were compiled,
and multiple hydrologic parameters were utilized to improve the
understanding of the brackish resources within the study area. Geophysical
logs were compiled and analysed to refine the aquifer stratigraphy,
determine the binary classification of sand and clay, and estimate the
groundwater salinity. These data were used to develop a MODFLOW groundwater
flow model to estimate the risk of compaction and land subsidence upon the
development of brackish zones within the Jasper aquifer. Compiled data
detailing the total clay thickness, clay bed thickness, and clay bed
location were input into the model along with a hypothetical stress to
predict compaction within the Jasper aquifer across the study area while
incorporating the observed heterogeneity in clay properties. Using the
results from the model simulations and two other risk performance measures
(depth of burial and surface flood risk), the total subsidence normalized
risk score was estimated. The results of this study confirm the potential
for compaction in the Jasper aquifer and for land subsidence to occur upon
development. Areas with the highest risk are located in the up-dip, inland
areas, near where the aquifer becomes fresh and is currently used for
municipal supply. The results will inform water managers and planners in the
Houston area on the future availability of brackish groundwater resources.
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