Abstracts

Evaluations of Groundwater Resources of Southern Mississippi

Author(s): Hoffman, J.

Virtually all water used for public and domestic drinking water supplies in southern Mississippi is derived from underground sources. Much of the area is underlain by a thick section of fresh water-bearing sediments and multiple aquifers are available at most locations. Although some wells in Wayne and Jones Counties are screened in other aquifers, the most widely used aquifers in southern Mississippi are developed in beds of sand that occur within sediments above the Vicksburg Group. Southwest of a line from Warren County through Wayne County, these sediments dip in a general gulfward direction at rates ranging from 30 to 100 feet per mile and form a wedge thickening southward to more than 5,000 feet in southern Hancock County and southwestern Wilkinson County. The aquifers within this interval have been termed the “Miocene aquifer system”. Because these deposits range from late Oligocene to Pliocene age, it is proposed that the name Grand Gulf aquifer system is more appropriate. Fresh water is available from these aquifers nearly everywhere within the 17,000 square mile area of their occurrence, even extending out from the coast beneath the barrier islands along Mississippi Sound and beyond, possibly as deep as 2,500 feet at Ship and Cat Islands. For years, there was little effort to systematically subdivide the many sand intervals that function as separate aquifers within the Grand Gulf system, an interval often consisting of 2,000 feet of fresh water-bearing sediments. Because of the growing need to make informed decisions concerning water use, staff initiated a study to delineate the individual aquifer units within the Grand Gulf system from interpretations of borehole geophysical logs. Office of Land and Water staff members have measured water levels and collected water samples in wells throughout southern Mississippi for many years. In early 1992, investigation of the potential for intrusion of saltwater into the aquifers that are sources of drinking water along the coast started. Results of this investigation found no evidence of saltwater intrusion in the confined aquifers that are sources of water supplies along the coast. In 2011, after reports of increased mineral leasing activity in southwestern Mississippi associated with fracking and its attendant requirements for water, the staff began a study focusing on potential groundwater availability in Amite and Wilkinson Counties that identified the specific aquifer intervals already being used for public and domestic drinking water supplies and those which might supply water for fracking without resulting in adverse impacts upon others.

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