INTRODUCTION
This report is one in an annual series of reports that depicts 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 Houston-Galveston region comprises Harris, Galveston, Fort Bend, Waller, and Montgomery Counties and adjacent parts of Brazoria, Grimes, Walker, San Jacinto, Liberty, and Chambers Counties. The report was prepared in cooperation with the Harris-Galveston Coastal Subsidence District, the City of Houston, and the Fort Bend Subsidence District. For the Chicot and Evangeline aquifers, maps show approximate water-level altitudes in 2002, water-level changes from 2001 to 2002, approximate water-level changes from 1977 to 2002, lines of zero water-level change for intervals increasing in 5-year increments beginning with 1977–82, and approximate water-level changes from 1990 to 2002 (in the Fort Bend subregion [Fort Bend County and adjacent areas]) (figs. 1–10). For the Jasper aquifer, maps show approximate water-level altitudes in 2002 and water-level changes from 2001 to 2002 and 2000 to 2002 (figs. 11–13). The report also contains a map showing extensometer (well equipped with compaction monitor) site locations (fig. 14) and graphs showing measured compaction of subsurface material at selected sites from 1973 or later to 2001 (fig. 15).
The U.S. Geological Survey (USGS) has published annual reports of water-level altitudes and water-level changes for the Chicot and Evangeline aquifers in the Houston-Galveston region since 1979; and separate annual reports of same for the Fort Bend subregion since 1990. The USGS published its first water-level-altitude map for the Jasper aquifer in the greater Houston area (primarily Montgomery County) in 2001. This year (2002), the altitude and change maps for the three aquifers are in this report.
GEOHYDROLOGY
The Chicot aquifer (in Holocene- and Pleistocene-age sediments), Evangeline aquifer (in Pliocene- and Miocene-age sediments), and Jasper aquifer (in Miocene-age sediments) are the three primary aquifers in the Gulf Coast aquifer system. The lowermost Jasper aquifer is separated from the Evangeline aquifer by the Burkeville confining unit. The hydrogeologic units are laterally discontinuous fluvial-deltaic deposits of gravel, sand, silt, and clay that dip and thicken from northwest to southeast. The aquifers thus crop out in bands inland from and approximately parallel to the coast and become progressively more deeply buried and confined toward the coast. The Chicot outcrop, which comprises the youngest sediments, is the closest of the aquifer outcrops to the coast, followed farther inland by the Evangeline outcrop and then farthest inland by the Jasper outcrop.
The Chicot aquifer can be differentiated from the geologically similar Evangeline aquifer on the basis of hydraulic conductivity (Carr and others, 1985, p. 10). The Jasper aquifer can be differentiated from the Evangeline aquifer in the outcrops on the basis of water levels (higher in the Jasper than the Evangeline) and in the downdip parts of the aquifers on the basis of position relative to the Burkeville confining unit.
The water in the aquifers is fresh (less than 1,000 milligrams per liter dissolved solids concentration) in the region but becomes more saline in the downdip and deeply buried parts of the aquifers near the coast. In the natural ground-water-flow system, water recharges the aquifers in the unconfined outcrop areas, moves downward and coastward, and discharges upward as diffuse upward leakage in the confined downdip areas.
WATER-LEVEL MEASUREMENTS
Water-level measurements used to prepare these maps were obtained by steel tape, airline, and from reports of well operators. Most wells are pumped once daily, but some are pumped more frequently. Multiple measurements were made when wells were not being pumped; however, antecedent conditions and pumping status of nearby wells were not always known. Most measurements were made in January and February, the months when water levels usually are highest. For this year’s maps, 206 water-level measurements were used for the Chicot aquifer, 321 for the Evangeline aquifer, and 61 for the Jasper aquifer.
MEASURED COMPACTION
Compaction of subsurface material is measured continuously by 13 borehole extensometers at 11 sites (fig. 14). Compaction measured by the shallower of two extensometers at the Clear Lake site is not shown because it is similar to that measured by the deeper extensometer at the site. Graphs of long-term compaction for 12 extensometers are shown in figure 15.
Baker, E.T., Jr., 1979, Stratigraphic and hydrogeologic framework of part of the Coastal Plain of Texas: Texas Department of Water Resources Report 236, 43 p.
Carr, J.E., Meyer, W.R., Sandeen, W.M., and McLane, I.R., 1985, Digital models for simulation of ground-water hydrology of the Chicot and Evangeline aquifers along the Gulf Coast of Texas: Texas Department of Water Resources Report 289, 101 p.
Gabrysch, R.K., 1979, Approximate altitude of water levels in wells in the Chicot and Evangeline aquifers in the Houston area, Texas, spring 1977 and spring 1978: U.S. Geological Survey Open-File Report 79–334, 4 sheets.
Kasmarek, M.C., 1997, Water-level altitudes in wells completed in the Chicot and Evangeline aquifers, Fort Bend County and adjacent areas, Texas, January–February 1990: U.S. Geological Survey Open-File Report 97–784, 2 sheets.
University of Texas, Bureau of Economic Geology, 1968, Geologic atlas of Texas, Beaumont sheet: Austin, scale 1:250,000.
______1974, Geologic atlas of Texas, Austin sheet: Austin, scale 1:250,000.