Development of a Chloride Standard Exceedance Map for Louisiana

Within Louisiana, two regional aquifers are designated as “sole source aquifers” by the U.S. Environmental Protection Agency (USEPA), as follows: the Chicot Aquifer System and Southern Hills Aquifer System. In addition, another aquifer is designated as an “area of groundwater concern” by the State of Louisiana: the Sparta Aquifer. These aquifers, among others, are part of a larger coastal aquifer, where saltwater intrusion is an ongoing process, both from the movement of connate groundwater and seawater. Salinity has been used in the past to identify areas of concern in these aquifers, but it does not provide a quantitative means to determine where the extracted groundwater meets drinking water standards. However, this information is easily derivable from geophysical well logs. One of the groundwater components of interest, and a relatively good environmental tracer, is chloride. Groundwater containing more than 250 mg/L chloride (a non-enforceable aesthetic standard established by the USEPA) is generally deemed not suitable for drinking without prior treatment. The authors have prepared maps of Louisiana to assist regional water managers, developers, property owners and drillers. These maps show the elevation and the depth below grade of the greater than 250 mg/L chloride concentration surface. These maps are geologic unit non-specific. They are based on a cumulative database of groundwater chemistry, and rely only on geophysical logs in areas where data distribution is poor. These maps generally show changes (e.g. saltwater intrusion) compared to the 1968 and 1988 saline groundwater maps in northern and southwestern Louisiana.