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A study of urban land-use effects on shallow groundwater

Study Description

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 Urban Land-use Study is to determine whether land-use practices in an urban setting (Lafayette, Louisiana) are affecting the water-quality of underlying shallow aquifers. National study objectives specific to issues of urban water-quality can be found in the "Urban Land-Use Study Plan for the National Water-quality Assessment Program, Open-File Report 96-217".

A variety of chemicals are purposefully used and accidentally released in urban settings. Fertilizers and pesticides are applied to private and commercial properties for landscape upkeep, as well as for golf course and playing-field maintenance. Automobiles release a variety of chemicals and compounds, such as combustion products, residues from wear and tear, and those involved in upkeep. Various other chemicals leach from pavement and building materials.

The primary national question is what chemicals are potentially reaching drinking water sources beneath urban areas? Urban areas in southern Louisiana sit high above their respective drinking water sources. This does affect how our local study is applied in context of analyzing nation-wide data. In siting our study, we elected to study how contaminants move into shallow groundwater, irrespective of whether that water is used for a drinking-water source.

Sites

For this study, potential sites for observation wells must be in a residential or commercial area built between the years 1960 and 1990. Additionally, the majority of a 500-meter radius around each potential well must be covered by this land use. Description of the study rationale and site selection in urban areas is described in "Urban Land-Use Study Plan for the National Water-quality Assessment Program, Open-File Report 96-217".

Once these target areas are identified with GIS, potential well locations are selected using a random sampling approach. Wells were then drilled and developed in the closest locations to the potential well locations where USGS personnel were given permission to do so. Methods used for randomized selection and well development are described 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". Since sites are located on private property, please contact Rob Fendick [rfendick@usgs.gov] for information on wells sampled and their locations.

Methods and Analysis

Starting in 2001, twenty-eight 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". 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.

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".

• 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 urban groundwater-quality surveys, please contact Rob Fendick [rfendick@usgs.gov], 225-298-5481.