This report depicts 2000–2004 water-level changes in the Chicot and Evangeline aquifers in the Houston-Galveston region (figs. 1 and 2). 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 and the Fort Bend Subsidence District.
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 similar annual reports for the Fort Bend subregion since 1990. Beginning with 2002, the separate annual reports were combined into one report.
The Chicot aquifer (in Holocene- and Pleistocene-age sediments) and the Evangeline aquifer (in Pliocene- and Miocene-age sediments) are the two primary aquifers in the Houston-Galveston region and are part of the Gulf Coast aquifer system. 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 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.
The Chicot aquifer can be differentiated from the geologically similar Evangeline aquifer on the basis of hydraulic conductivity (Carr and others, 1985, p. 10). A weak hydraulic connection between land surface and the Chicot aquifer and between the Chicot and Evangeline aquifers allows vertical movement of water into and between the aquifers; the aquifer system thus is characterized as “leaky” (Gabrysch and Coplin, 1990, p. 2).
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 vertically as diffuse upward leakage in the confined downdip areas.
Water-level measurements used as the basis for these change 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.
Note: This change map was generated by contouring the differences between 2000 water-level data (Coplin and Santos, 2000) and 2004 water-level data (Kasmarek and Lanning-Rush, 2004).
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.
Coplin, L.S., and Santos, H.X., 2000, Water-level altitudes 2000, water-level changes 1977–2000 and 1999ñ2000, and compaction 1973ñ99 in the Chicot and Evangeline aquifers, Houston-Galveston region, Texas: U.S. Geological Survey Open-File Report 00–094, 8 sheets.
Gabrysch, R.K., and Coplin, L.S., 1990, Land-surface subsidence resulting from ground-water withdrawals in the Houston-Galveston region, Texas, through 1987: Harris-Galveston Coastal Subsidence District Report of Investigations 90–01, 53 p.
Kasmarek, M.C., and Lanning-Rush, Jennifer, 2004, Water-level altitudes 2004 and water-level changes in the Chicot, Evangeline, and Jasper aquifers and compaction 1973–2003 in the Chicot and Evangeline aquifers, Houston-Galveston region, Texas: U.S. Geological Survey Open-File Report 2004–1084, 13 sheets.
University of Texas, Bureau of Economic Geology, 1968, Geologic atlas of Texas, Beaumont sheet: Austin, scale 1:250,000.