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An assessment of waters of the Chicot aquifer

Study Description

The Sub-Unit Survey is designed to provide a broad assessment of the water-quality conditions of the most important present and future ground-water resources of the study unit, as described in the Ground-Water Study Design section of the "Design of the National Water-Quality Assessment Program: Occurrence and Distribution of Water-Quality Conditions, Circular 1112".

This study focuses on a regional-scale survey of ground-water quality of the Chicot Aquifer System, a sole-source aquifer in southwestern Louisiana that is susceptible to surficial contamination. Analyses from the Sub-Unit Survey will provide extensive water-quality data on the aquifer. Data from these studies will be useful to assess the current effects of land-use practices on groundwater quality in the study area, and will aid water quality managers in making land-use decisions to protect this natural resource for future generations. In addition, results of analyses for the Chicot and Chicot Equivalent Aquifers will be compared against each other, and serve as a basis for analysis of data from the Rice Land-Use Study and the Urban Land-Use Study.

Sites

Potential sampling locations are distributed in a random grid within the aquifer boundaries according to well-selection methods published in "Ground-Water Data-Collection Protocols and Procedures for the National Water-Quality Assessment Program: Selection, Installation, and Documentation of Wells, and Collection of Related Data, Open-File Report 95-398". Thirty of the closest available domestic wells were then selected from this set for sampling. Sites are located on private property, please contact Rob Fendick [rfendick@usgs.gov] for information on wells sampled.

Methods and Analysis

In the summer of 2000, thirty domestic wells were sampled for water chemistry. Water-sampling methods are the same as those used for the Rice Land-Use Study. These methods include the use of all-Teflon tubing, sampling and preservation chambers, mobile canopy, as well as the "clean-hands/dirty-hands" procedure, all methods employed to reduce sample contamination. These sampling protocols are described in "Ground-Water Data-Collection Protocols and Procedures for the National Water-Quality Assessment Program: Collection and Documentation of Water-Quality Samples and Related Data, Open-File Report 95-399". With the exception of volatile organic compounds, the Sub-Unit Survey sample analytes are the same as for the Rice and Urban Land-Use Studies. Water levels are obtained when possible; however, many well owners use jet pumps which do not allow access for determining water levels. For those wells with jet pumps, water-level data are determined from original driller's logs.

• Volatile Organic Compounds
• Dissolved Major Ions
• Dissolved Nutrients
• Dissolved Organic Carbon
• Dissolved Pesticides
• Dissolved Trace Elements
• Radon

Standard NAWQA Land-Use/Land-Cover Forms are used to give spatially detailed information about land use in the vicinity of each well. Land within a 500-meter radius of each well is carefully surveyed for vegetative cover, crop types, livestock, hydrologic regimes, human and industrial use, and other kinds of land cover. These data are used in analysis to assign each site a relative percentage of land devoted to rice agriculture and determine point sources that may influence data. These methods are described in "Ground-Water Data Collection Protocols and Procedures for the National Water-Quality Assessment Program: Collection, Documentation, and Compilation of Required Site, Well, Subsurface, and Landscape Data for Wells, Water Resources Investigation Report 98-4107".

For questions about our aquifer sub-unit water-quality surveys, please contact Robert Fendick [rfendick@usgs.gov], 225-298-5481.