Withdrawals, water levels, and specific conductance in the Chicot aquifer system in southwestern Louisiana, 2000-03
The Chicot aquifer system is the principal source of fresh ground-water supplies in southwestern Louisiana. Much of the area is rural and rice cultivation is the primary agricultural activity. About 540 million gallons per day were withdrawn from the aquifer system in southwestern Louisiana in 2000. Potentiometric-surface maps of the aquifer system were created for June 2002 and January 2003 to determine where water-level declines occur due to seasonal ground-water withdrawals. During June 2002, water levels in the aquifer system were more than 40 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29) in parts of Acadia, Calcasieu, Evangeline, and Jefferson Davis Parishes, in an area that generally coincides with rice-farming areas. During January 2003, water levels were more than 30 feet below NGVD 29 in these areas.
From June 2002 to January 2003, water levels generally recovered between 5 and 20 feet in the Chicot aquifer system in most of Acadia and Jefferson Davis Parishes, southeastern Calcasieu Parish, and southern Evangeline Parish, in an area that generally coincides with rice-farming areas. These water-level changes are representative of the areal extent and magnitude of typical seasonal water-level fluctuations that occur in the aquifer system in response to seasonal ground-water withdrawals for rice irrigation.
The presence of saltwater has been documented in the Chicot aquifer system beneath coastal parishes and in some areas where the aquifer system merges with the stratigraphically adjacent Atchafalaya aquifer. Data collected during the period 1943 to 2003 from 1,355 wells screened in the massive, upper, and "200-foot" sands of the Chicot aquifer system and the Atchafalaya aquifer were used to delineate areas having similar specific conductance values and determine areas where wells are affected by saltwater. Near the outcrop area, specific conductance values in the Chicot aquifer system generally are less than 150 µS/cm (microsiemens per centimeter at 25 degrees Celsius). Specific conductance values increase south and east of the outcrop area. Specific conductance values generally range from 151 to 500 µS/cm in rice-farming areas of northwestern Acadia Parish, southeastern Allen Parish, western Evangeline Parish, and northern and central Jefferson Davis Parish. Specific conductance values generally range from 501 to 1,000 µS/cm in most of the remaining rice-farming areas. Specific conductance values often exceed 1,000 µS/cm in an area along the border between Calcasieu and Jefferson Davis Parishes near Iowa, Louisiana, parts of northeastern Cameron Parish, an area of northwestern and central St. Landry Parish; parts of Vermilion Parish, and several areas along the eastern boundary of the study area where the Chicot aquifer system merges with the Atchafalaya aquifer. The maximum specific conductance value, 12,100 µS/cm, is from a well in Cameron Parish.
During 2000-03, specific conductance was measured in 521 water samples from 166 wells screened in the Chicot aquifer system or the Atchafalaya aquifer. Specific conductance values exceeded 1,000 µS/cm in water samples from wells in Calcasieu, Cameron, Jefferson Davis, St. Landry, St. Martin, St. Mary, and Vermilion Parishes. Specific conductance values exceeded 2,000 µS/cm in only two wells—an irrigation well located about 2 miles south of Iowa and a USGS observation well used to monitor saltwater encroachment in east-central Vermilion Parish. Specific conductance values increased steadily at one well, from 1,090 µS/cm in April 2000 to 2,860 µS/cm in April 2003. Nearby wells did not show similar increases.
Specific conductance was measured hourly during pumping at two irrigation wells between 2000 and 2003. Specific conductance values were greater than 1,000 µS/cm in both wells, indicating the presence of saltwater near the wells. Specific conductance values generally fluctuated about 150 µS/cm at both wells, but no long-term trends in the specific conductance were evident in either well.
|USGS Numbered Series
|Withdrawals, water levels, and specific conductance in the Chicot aquifer system in southwestern Louisiana, 2000-03
|Scientific Investigations Report
|U.S. Geological Survey
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