Harris County, in the West Gulf Coastal Plain in southeastern Texas, has one of the heaviest concentrations of ground-water withdrawal in the United States. Large quantities of water are pumped to meet the requirements of the rapidly growing population, for industry, and for rice irrigation. The water is pumped from artesian wells which tap a thick series of sands ranging in age from Miocene (?) to Pleistocene.
The water-bearing sands, many of which contained slightly saline water, are interbedded with clays. Subsequent artesian circulation has flushed the sands, probably to the limit determined by the Ghyben-Herzberg principle. The base of the fresh-water sands ranges in depth from about 100 feet over the salt dome near Hockley to more than 3,000 feet in the northeastern part of the county.
Before large-scale ground-water withdrawals were begun, the hydraulic gradient sloped gently toward the coast. Then, as large quantities of water were withdrawn a large cone of depression was established, the hydraulic gradient was reversed, and salt water began to move slowly toward the centers of pumping. The rate of movement of the salt water is very slow and the closest salt water is probably 5 miles from centers of pumping in the deeper sands. However, the threat of salt-water intrusion is present and the rate of advance of the salt water should be watched by means of strategically placed observation wells.
Other less probable potential sources of salt-water contamination which are discussed include upward movement of salt water from below, vertical movement around salt domes or along faults, downward seepage from surface sources, and contamination through leaking wells.
