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Groundwater Studies in Rice-Growing Areas
A study of water-quality of shallow aquifers in an area of rice agriculture
Land-use studies assess the concentration and distribution of water-quality constituents in recently recharged ground water, and examine the natural and human factors affecting the quality of shallow ground water underlying various land uses. The general objectives and methods of these nationwide studies are described in the Land-Use Studies section of "Design of the National Water-Quality Assessment Program: Occurrence and Distribution of Water-Quality Conditions, Circular 1112".
The objective of the Rice Land Use Study is to determine whether chemicals (such as fertilizers or pesticides) used in rice agriculture are leaching into underlying shallow aquifers. Specifically, this study samples waters from the Chicot Aquifer of southwestern Louisiana, where rice is grown over much of the area. Propanil, molinate, thiobencarb, and more recently fipronil, are the major herbicides and insecticides used in the rice-growing areas and are potential contaminants of shallow ground water.
The rice land-use ground-water study is co-located with Bayous Lacassine and Des Cannes, and the Mermentau River, sites sampled as part of the Fixed Surface-Water Study, and the Mermentau Basin Pesticide Surface-Water Study. Attempts will be made to compare concentrations, locations, and times between surface-water and ground-water samples to trace sources and transport of pesticides. Comparisons will provide evidence needed to assess the potential of leaching as a contamination source for ground-water, as well as rice-field runoff as a potential source of contamination for surface-water.
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". Wells were drilled in the closest locations to the grid points where USGS personnel were given permission to do so. Since sites are located on private property, please contact Rob Fendick [firstname.lastname@example.org] for information on wells sampled and their locations.
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Methods and Analysis
Starting in 1999, thirty observation wells were drilled and developed at the selected sites by study unit personnel according to published guidelines and procedures for well installation, described in "Guidelines and Standard Procedures for Studies of Ground-Water Quality: Selection and Installation of Wells, and Supporting Documentation, Water Resources Investigation Report 96-4233". Twenty-eight of the thirty sites are located within the rice growing area. The remaining two sites are in forested areas and will serve as reference wells.
Well drilling terminates at first water where maximum leaching potential exists. Well depths are between 10 to 100 feet below land surface; screened intervals are 5, 10, or 15 feet, depending on the grain size of the individual sand unit. Grain size ranges from well-sorted sands to silty, fine sands. Hydraulic conductivity was estimated using the Hvorslev method for unconfined aquifer systems. Hydraulic conductivity ranges from 1.52E-02 cm/s to 6.91E-05 cm/s.
Wells have been sampled for major ions, trace elements, nutrients, pesticides, and radon. Groundwater samples are collected according to methods published 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". 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.Constituents analyzed:
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 Ground water-quality surveys, please contact Rob Fendick [email@example.com], 225-298-5481.
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