This report updates ground-water information pertaining to the lower unit of the Chicot aquifer in the Orange County area, Texas and Louisiana. The period of data collection was from 1980 to the spring of 1985. Some data collected prior to 1980 are presented to establish long-term trends and relations. The lower unit of the Chicot aquifer, which consists of sediments of Pleistocene age, is confined and underlies all of the study area. The base of the aquifer ranges from about 400 feet below National Geodetic Vertical Datum of 1929 in the northwestern part of the county to about 1,000 feet below National Geodetic Vertical Datum of 1929 in the southeastern part.

The lower unit of the Chicot aquifer is the main source of freshwater for several cities, communities, industries, housing subdivisions, and individual homeowners in Orange County. The total pumpage from the lower unit of the Chicot aquifer in Orange County decreased from a historical maximum of 23.1 million gallons per day during 1972 to an estimated 15.2 million gallons per day during 1984. The average industrial pumpage during 1980-84, 10.5 million gallons per day, decreased substantially when compared to 1963-79, when an average of 15.6 million gallons per day was withdrawn. This is in contrast to municipal pumpage that increased from an average withdrawal of 5.3 million gallons per day during 1963-79 to 7.3 million gallons per day during 1980-84. The use of surface water decreased from a peak withdrawal of 58.1 million gallons per day during 1981 to 41.4 million gallons per day during 1984.

From the spring of 1980 to the spring of 1985, water levels in the lower unit of the Chicot aquifer in the Orange County area ranged from rises of as much as 14 feet to declines of as much as about 3 feet. Water levels rose throughout most of the area. The greatest rise in water levels occurred in and near the city of Orange, whereas the greatest decline occurred northwest of Vidor.

Most of the water in the lower unit of the Chicot aquifer is fresh, but the water quality can vary greatly within short distances. Chloride concentrations determined during 1980-84 ranged from 10 to 1,700 milligrams per liter. The larger chloride concentrations were measured where salinewater coning and updip migration are occurring. In general, chloride concentrations remained constant during 1980-84.

A relation exists between chloride concentrations and specific conductance. It was determined that chloride concentrations (milligrams per liter) generally can be estimated by multiplying specific-conductance values (microsiemens per centimeter at 25 °Celsius) by 0.29 when the specific conductance is between 500 and 5,600 microsiemens per centimeter at 25 °Celsius.