According to the Groundwater Branch of the Kentucky Division of Water, "Groundwater is a vital, renewable natural resource that is widely used throughout Kentucky. Wells and springs provide approximately one-third of public domestic water supplies in the state. Surface streams, the major source of Kentucky's water supply, are primarily sustained during base flow by groundwater discharge from adjacent aquifers. This resource is susceptible to contamination from a variety of activities at the land surface. Once contaminated, groundwater can be difficult or impossible to remediate."
Quality of Groundwater in the County
The quality of groundwater in the Mississippian Plateaus Region varies considerably from place to place and is determined by the water's geologic source and the length of time it has been in contact with the rocks. Generally, deeper wells produce more mineralized water; however, deeper wells are less likely to become polluted by man's activities. In Trigg County water obtained from most drilled wells in limestone aquifers is considered hard. Common salt and hydrogen sulfide are the two naturally occurring constituents most often encountered in objectionable amounts in groundwater. Water obtained from wells and springs in many limestone aquifers is subject to pollution and, at a minimum, should be treated to eliminate bacterial contamination. It is a good practice to have any residential water source tested at least once a year.
At a time when surprisingly little information is available on groundwater quality, groundwater contamination has become a major environmental issue. Reliable information about water quality is necessary in order to develop plans for protecting groundwater. The absence of accurate and broad perspectives on groundwater quality may lead to inappropriate and ineffective regulatory policies. Because groundwater supplies a large percentage of rural drinking water and water for agricultural use, rural landowners have become increasingly concerned about the quality of groundwater. The Kentucky Farm Bureau, Kentucky Division of Conservation, University of Kentucky Cooperative Extension Service, and the Kentucky Geological Survey conducted a water-quality survey of nearly 5,000 rural domestic wells. The results are discussed in "Quality of Private Ground-Water Supplies in Kentucky." Additional references are contained in the Water Research Library on the Kentucky Geological Survey's Web site.
Salt water (saline water) is found below fresh groundwater at variable depths throughout the entire state of Kentucky. Depths to the saline groundwater range from 50 feet or less down to 2,000 feet below land surface in Kentucky. "Salinity" is defined as a measure of the quantity of dissolved mineral matter or total dissolved solids (TDS) in water, reported in parts per million (ppm) or milligrams per liter (mg/L); the two forms of measurement are usually equivalent. The term "salt" or "table salt" as used by most people is pure sodium chloride. Sodium and chloride are generally the major component of saline waters in Kentucky, but are not the only constituents. Water having a TDS concentration of less than 1,000 ppm is classified as fresh and water having a TDS concentration of 1,000 ppm or more is classified as saline. Recommendations by the U.S. Public Health Service for drinking water suggest that total dissolved solids should not exceed 500 ppm, but less than 1,000 ppm may be used. In agriculture, the recommended TDS levels vary with uses, as shown in the following table, which was taken in part from "Fresh-Saline Water Interface Map of Kentucky" (Hopkins, 1966).
Being aware of the depth to saline groundwater is valuable when planning a water-supply well. Drilling a well too deep through the freshwater interval may cause a good well to be unsuitable for various uses. Care must be taken to prevent contamination of the freshwater zones by the deeper saline waters. Properly constructed water wells will screen the production zone in the targeted aquifer and isolate all other zones by casing and properly grouting and cementing of the space outside the casings in the boreholes.
In Trigg County the fresh-saline interface slopes from elevations of -100 feet mean sea level on the western side of the county, up to 400 feet to the eastern end the county. This corresponds to depths of less than 100 feet below the level of the principal valley bottoms in the eastern part of the county, down to depths as great as 400 feet in the western side of the county.
Sensitivity of Groundwater to Pollution
According to the Groundwater Branch of the Kentucky Division of Water, Trigg County has areas of moderate-high to high sensitivity to groundwater pollution (see "Groundwater Sensitivity Regions of Kentucky"). The hydrogeologic sensitivity of an area is defined as the ease and speed with which a contaminant can move into and within a groundwater system. The sensitivity assessment addressed only the naturally occurring hydrogeologic characteristics of an area. Possible impacts of huamn activity upon groundwater, such as mining, logging, industry, and the use of pesticides, injection wells, and landfills, were not considered in the production of this map. Because of its small scale and generalized nature, this map is not intended for site-specific use, such as detailed land-use planning for city, county, or State agencies. The map should prove useful as a broad-scale management, educational, and planning tool, however.