Abstract
Most of the subsidence in the Houston–Galveston region
has occurred as a direct result of groundwater withdrawals
for municipal supply, industrial use, and irrigation that
depressured and dewatered the Chicot and Evangeline
aquifers causing compaction of the clay layers of the aquifer
sediments. This report, prepared by the U.S. Geological
Survey, in cooperation with the Harris–Galveston Subsidence
District, City of Houston, Fort Bend Subsidence District, and
Lone Star Groundwater Conservation District, is one in an
annual series of reports depicting water-level altitudes and
water-level changes in the Chicot, Evangeline, and Jasper
aquifers and compaction in the Chicot and Evangeline aquifers
in the Houston–Galveston region. The report contains maps
showing 2011 water-level altitudes for the Chicot, Evangeline,
and Jasper aquifers; maps showing 1-year (2010–11) water-level-altitude changes for each aquifer; maps showing 5-year
(2006–11) water-level-altitude changes for each aquifer; maps
showing long-term (1990–2011 and 1977–2011) water-level-altitude
changes for the Chicot and Evangeline aquifers;
a map showing long-term (2000–11) water-level-altitude
change for the Jasper aquifer; a map showing locations of
borehole extensometer sites; and graphs showing measured
compaction of subsurface material at the extensometers from
1973, or later, through 2010. Tables listing the data used to
construct each aquifer-data map and the compaction graphs
are included.
Water levels in the Chicot, Evangeline, and Jasper
aquifers were measured during December 2010–February
2011. In 2011, water-level-altitude contours for the Chicot
aquifer ranged from 200 feet below North American Vertical
Datum of 1988 (hereinafter, datum) in a small area in
southwestern Harris County to 200 feet above datum in central
to southwestern Montgomery County. Water-level-altitude
changes in the Chicot aquifer ranged from a 40-foot decline
to a 33-foot rise (2010–11), from a 10-foot decline to an
80-foot rise (2006–11), from a 140-foot decline to a 100-foot
rise (1990–2011), and from a 120-foot decline to a 200-foot
rise (1977–2011). In 2011, water-level-altitude contours for
the Evangeline aquifer ranged from 300 feet below datum
in north-central Harris County to 200 feet above datum at
the boundary of Waller, Montgomery, and Grimes Counties.
Water-level-altitude changes in the Evangeline aquifer ranged
from a 43-foot decline to a 73-foot rise (2010–11), from a
40-foot decline to a 160-foot rise (2006–11), from a 200-foot
decline to a 240-foot rise (1990–2011), and from a 340-foot
decline to a 260-foot rise (1977–2011). In 2011, water-level-altitude
contours for the Jasper aquifer ranged from 200 feet
below datum in south-central Montgomery County to 250 feet
above datum in east-central Grimes County. Water-level-altitude
changes in the Jasper aquifer ranged from a 45-foot
decline to a 29-foot rise (2010–11), from a 90-foot decline
to a 10-foot rise (2006–11), and from a 190-foot decline to
no change (2000–11). Compaction of subsurface materials
(mostly in the clay layers) composing the Chicot and
Evangeline aquifers was recorded continuously at 13 borehole
extensometers at 11 sites. For the period of record beginning
in 1973, or later, and ending in December 2010, cumulative
clay compaction data measured by 12 extensometers
ranged from 0.100 foot at the Texas City–Moses Lake site
to 3.544 foot at the Addicks site. The rate of compaction
varies from site to site because of differences in groundwater
withdrawals near each site and differences among sites in the
clay-to-sand ratio in the subsurface materials. Therefore, it is
not possible to extrapolate or infer a rate of clay compaction
for an area based on the rate of compaction measured at a
nearby extensometer.
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First posted September 15, 2011
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