SARGENT, B. PIERRE, and C. PAUL FREDERICK, U.S. Geological Survey, Baton Rouge, LA
Abstract: Using a Geographic Information System to Determine the Chicot Aquifer System Surficial Confining Unit's Thickness and Location of Sand Lenses, Southwestern Louisiana
The Chicot aquifer system in southwestern Louisiana has a surficial confining unit that is undifferentiated from overburden material near the surface. This confining unit is composed of fine sand, silt, and clay, through which recharge flows to the underlying aquifer units. The thickness of the confining unit was mapped using water-well and petroleum-well data from federal and state agency databases, and supplemental information from previous publications. Thicknesses were estimated from 128 petroleum-well geophysical logs, 260 water-well geophysical logs, and 1,705 water-well driller's logs. Although an abundance of petroleumwell geophysical logs are available from southwestern Louisiana, logging commonly did not start at or near the ground surface, which rendered the logs unusable. Water-well driller's logs vary in quality, and those used were compiled from paper-files of more than 10,000 well records. Thickness values from driller's logs were not used if the values did not match thickness values from other nearby geophysical and driller's logs.
The thickness of the confining unit was estimated using kriging, an interpolation method. Kriging can handle spatial variability, smooth out random "noise," and mitigate the impact of "hot spots" where there is a great quantity of data. Universal kriging was used instead of ordinary kriging because the data were highly correlated in a particular direction.
The distribution of shallow domestic wells completed in sand lenses within the Chicot surficial confining unit was compared to the distribution of sand lenses obtained from the log data. This provided independent corroboration of areas where sufficient water is available for rural domestic use.
AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana