COASTAL LOWLANDS AQUIFER SYSTEM
INTRODUCTION
The coastal lowlands aquifer system consists of mostly Miocene and younger unconsolidated deposits that lie above and coastward of the Vicksburg-Jackson confining unit; the deposits extend to land surface (figs. 53 and 54). The aquifer system is in the Coastal Plain Physiographic Province and is in all or parts of 51 counties in Texas. It extends eastward into parts of the Coastal Plain of Louisiana and Mississippi and is further discussed in Chapter F of this Atlas. A small part of the system extends into southern Alabama and the western part of Panhandle Florida where it is called the sand and gravel aquifer (Chapter G of this Atlas). In Texas, the aquifer system underlies about 35,000 square miles of the level, low-lying coastal plain whose surface rises gradually toward the north and northwest.
The major rivers that flow through the area and empty into the Gulf are, from west to east, the Rio Grande, the Nueces, the Frio, the San Antonio, the Guadalupe, the Colorado, the Brazos, the Navasota, the Trinity, the Neches, the Angelina, and the Sabine. Average annual precipitation ranges from about 22 inches in the Rio Grande Valley in the southwest to about 56 inches at the Louisiana border in the east. The coast-ward-dipping sediments reach thicknesses of thousands of feet and contain waters that range from freshwater to brine. The coastal lowlands aquifer system yields large amounts of water for public, agricultural, and industrial needs.
HYDROGEOLOGY
The deposits that compose the coastal lowlands aquifer system range in age from Oligocene to Holocene (fig. 55). The lithology is generally sand, silt, and clay and reflects three depositional environments-continental (alluvial plain), transitional (delta, lagoon, and beach), and marine (continental shelf). The gradual subsidence of the depositional basin and rise of the land surface caused the deposits to thicken Gulf-ward, which resulted in a wedge-shaped configuration of the hydrogeologic units as seen in the cross section shown in figure 54. Coarser grained nonmarine deposits updip grade laterally into finer-grained material that was deposited in marine environments. Numerous oscillations of ancient shorelines resulted in a complex, overlapping mixture of sand, silt, and clay. This heterogeneity has made it difficult for investigators to subdivide the thick deposits into individual hydrogeologic units, and various schemes of subdivision are found in the literature.
HYDROGEOLOGIC FRAMEWORK
Different names have been used for the aquifers and confining units of the coastal lowlands aquifer system. The term "Gulf Coast aquifer" has been used to refer to and describe the composite sands, silts, and clays of the aquifer system. The "Chicot aquifer" and "Evangeline aquifer" are commonly used hydrogeologic-unit designations for subdivisions of the upper, mostly sandy part of the deposits. In a recently completed regional study that was part of the U.S. Geological Survey's Regional Aquifer-System Analysis (RASA) program, the deposits were subdivided into five permeable zones and two confining units. An informal letter designation has been assigned to each subdivision. The basis of this seven-unit subdivision was primarily differences in permeability, but included an evaluation of depths of water-producing zones and the resultant vertical differences in hydraulic head at large pumping centers in Houston, Tex., and Baton Rouge, La. Comparison of the subdivisions used in this Atlas with names of hydro-geologic units used in Texas is shown in figure 55.
Some of the boundaries of the aquifer system are geographic and some coincide with permeability contrasts. The landward boundary, or updip limit of the aquifer system, is in outcrop areas where the aquifer system feathers out at point of contact with the underlying Vicksburg-Jackson confining unit (figs. 53 and 54). The Gulfward boundary is near the coastline where the ground water becomes increasingly saline; the upper boundary is the land surface.
The base of the aquifer system is either its contact with the top of the Vicksburg-Jackson confining unit or the approximate depth at which the water in the system has a dissolved-solids concentration of more than 10,000 milligrams per liter. The altitude of the base of the aquifer system is shown in figure 56. The base ranges from a few hundred feet above sea level near the updip limit, to as much as 6,000 feet below sea level in areas about midway between the updip limit and the coastline.
GROUND-WATER HYDRAULICS
The aquifer system is recharged by the infiltration of precipitation that falls on topographically high aquifer outcrop areas. Natural discharge occurs by evapotranspiration, loss of water to streams as base flow, and upward leakage to shallow aquifers in low-lying coastal areas or in the Gulf of Mexico. Recharge and discharge in areas with little or no pumpage are generally between 0 and 1 inch per year. Additional recharge occurs where water levels are lowered by pumping because the vertical hydraulic head gradient is increased. In places where head gradients might become reversed, water might move from former discharge areas along streams into the aquifers.
With the exception of shallow zones in the outcrop, the water in the coastal lowlands aquifer system is under confined conditions. In the shallow zones, the specific yield for sandy deposits ranges generally between 10 and 30 percent; for confined aquifers, the storage coefficient is estimated to range between 1x10-4 and 1x10-3. The storage coefficient is the volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. In an unconfined aquifer, the storage coefficient is virtually equal to the specific yield. The storage coefficient is an important factor that determines the size and rate of development of cones of depression that result from ground-water withdrawals.
The productivity of the aquifer system is directly related to the total thickness of the sands in the aquifer system that contain freshwater. This aggregate sand thickness is shown in figure 57. Values range from zero at the updip limit of the aquifer system to as much as 2,000 feet in the east. The transmissivity of the sands is a measure of the ease with which water will move through them. Transmissivity can be calculated by multiplying the average hydraulic conductivity of the sands times the thickness of the sands that contain freshwater. Transmissivity, storage coefficient, and recharge rate control the rate of well yields and the size and shape of the cones of depression that result on an aquifer's potentiometric surface because of pumping.
The average hydraulic conductivity of the sands was estimated from a digital computer model. East of the San Antonio River, the average hydraulic conductivity is about 21 feet per day; west of the river, it is about 17 feet per day. By using these values and the freshwater sand thickness as shown in figure 57, an estimate of the transmissivity can be computed and mapped, as shown in figure 58. Values of transmissivity range from less than 5,000 to nearly 35,000 feet squared per day.
GROUND-WATER DEVELOPMENT
For the coastal lowlands aquifer system in general, ground-water pumpage was relatively small and constant from the early 1900's until the late 1930's. Pumping rates increased sharply during the 1940's and 1950's until about 1960, when about 800 million gallons per day was withdrawn. Withdrawal rates increased relatively slowly thereafter, and, during 1985, 1,090 million gallons per day was withdrawn.
Withdrawals during 1985 were largely from the east-central area; the largest pumpage was in the Houston area of Harris County. Harris County accounted for 35 percent of the total withdrawals, and the combined withdrawals from Harris and Wharton Counties were 50 percent of the total (table1). Ten counties in the east-central area accounted for 82 percent of total withdrawals; the largest usage was divided about equally between public supply and agriculture (table 1).
During 1982, some of the greatest pumpage from the aquifer system was in the coastal area of rice irrigation centered in Jackson and Wharton Counties and including parts of Colorado, Lavaca, Victoria, and Matagorda Counties. About 322 million gallons per day was withdrawn from permeable zone A, the uppermost permeable zone of the aquifer system in this area. Because the permeable zone crops out near this area, recharge in the outcrop area provided a source to quickly balance the large withdrawals. Thus, drawdowns were not large (generally less than 50 feet), but the increase in recharge rates over predevelopment rates was large. Recharge rates were increased by as much as 4 to 6 inches per year in the rice irrigation area, as indicated by the model simulation results shown in figure 59.
Another area that was pumped intensively during 1982 is centered in the city of Houston and includes Harris and all or parts of Chambers, Galveston, Brazoria, Fort Bend, Waller, Montgomery, and Liberty Counties. Withdrawals from permeable zones A, B, and C, the three uppermost water-yielding zones in the aquifer system, were mostly for public and industrial supplies and were about 260, 260, and 165 million gallons per day, respectively. As a result of the intense pumping, the potentiometric surface was lowered in all three zones. The lowering was least severe in zone A, the shallowest zone, where water levels declined to a maximum of 150 feet below sea level. The effect on the potentiometric surface was more severe in the deeper zones because their outcrop recharge areas were far updip from the pumping centers and a substantial amount of water was removed from aquifer storage. In Houston, the 1982 potentiometric surface declined to more than 250 feet below sea level in zone B (fig. 60) and more than 350 feet below sea level in zone C (fig. 61). Maps of the distribution of the change in the predevelopment to 1982 potentiometric surfaces show a decline of more than 300 feet in zone B (fig. 62), and more than 400 feet in zone C (fig. 63).
The large ground-water withdrawals in Harris County and adjacent areas have reduced the artesian pressure sufficiently to cause water from clay beds in the permeable zones to flow into the sands. As the water flows out of the compressible clays, they become irreversibly compacted, which causes permanent subsidence of the land surface. The land has subsided several feet in parts of the area (fig. 64); more than 9 feet of subsidence has been recorded in areas east of the Houston city limits. The subsidence has increased the risk of flood damage to residential and commercial properties (fig. 65) and has activated faults which have caused structural damage.
With the creation of the Harris-Galveston Coastal Subsid-ence District in 1975, the reduction of ground-water pumpage and increased reliance on surface-water supplies have been emphasized. Pumping rates have been substantially reduced in much of southeastern Harris County and in Galveston County; this has caused a recovery of water levels and a cessation or sharp decrease in the rate of land-surface subsidence in that area. The subsidence that has already occurred, however, is virtually irreversible.
FRESH GROUND-WATER WITHDRAWALS
Withdrawals of freshwater from the coastal lowlands aquifer system in Texas totaled about 1,090 million gallons per day-during 1985 (fig. 66). About 476 million gallons per day was withdrawn for public supply, and about 447 million gallons per day was withdrawn for agricultural purposes. Withdrawals for industrial, mining, and thermoelectric-power uses were about 114 million gallons per day. About 53 million gallons per day was withdrawn for domestic and commercial uses.
POTENTIAL FOR DEVELOPMENT
Problems associated with ground-water pumpage, such as land subsidence and saltwater encroachment, have caused pumping to be curtailed in some areas. The Texas Water Development Board has made projections of ground-water use to 2030. The tentative projections undergo revision and updating as technical and socioeconomic factors change. For the 10 counties that withdrew the largest amounts of water from the coastal lowlands aquifer system during 1985, State officials project a large decline in pumpage for 6 counties and an increase in 4 counties by 2030 (table 2). For the 10 counties, the total projected pumpage in 2030 is 39 percent less than that of 1985.
Although overall use of ground water might decline, some areas can sustain additional development. Pumping from water-yielding zones in geologically older rocks that are farther inland will minimize land subsidence and saltwater encroachment. Pumping in areas that have more abundant precipitation, and thus greater recharge potential, is less likely to cause continuous, steep water-level declines and such problems as stream base-flow depletion and greater pumping lifts.
