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Terracon Helps Georgia Town Find a Solution to Long-term Water Problems

Thunderbolt Well

 High chloride concentrations in the drinking water wells of Thunderbolt, Ga., a coastal community, have been a documented problem since at least the early 1900s. In 2009, Thunderbolt had to abandon a second well in town because of increasing chloride concentrations, leaving the town with only one working water well. Thunderbolt had previously abandoned two other wells because of increasing chloride concentrations potentially related to saltwater intrusion in the vicinity of the well field. In 2011, Thunderbolt officials worried high chloride levels in Well No. 2, the town’s only remaining well, would force them to drill a new well or rely only on water piped in from nearby Savannah, Ga.
In the past two years, chloride concentrations in Thunderbolt’s remaining drinking water well rose from 100 parts per million (ppm) to 216 ppm, which was alarming as the Environmental Protection Agency’s (EPA) secondary maximum contaminant level MCL for chloride is 250 ppm. The town of Thunderbolt explored the option of drilling a new production well to offset the loss of the previously abandoned wells. However, the high cost of the project, over one million dollars, and life span uncertainty of the well due to localized hydrogeologic and geochemical conditions of the aquifer, caused the town to put off construction of the new well and look for alternative options.
At this point, Thunderbolt town council members hired Terracon’s Savannah, Ga., office to identify the source of the high chloride water entering the well and to develop alternatives for repairing the well. Terracon was selected due to expertise in regards to well assessment and rehabilitation in the area and knowledge of local conditions. Terracon met with several of the council members, the city manager, and the mayor to discuss their water quality problems and determined the latest borehole inspection methods could be used to identify the problem.

After the source of the high chloride water was identified, Terracon evaluated the data to determine if reconstruction of the well would reduce observed chloride concentrations and test the reconstruction plan prior to implementation. If reconstruction was possible, this would save the town a substantial amount of funds.

The initial phase of the hydrogeologic assessment of Well No. 2 was completed by performing a series of tests using video and geophysical logs to determine current well construction (casing depth, size, and total borehole depth), production profile, and water quality profile of the existing borehole. The city did not have any well construction information, so the video was key to visually inspect the well casing, the grout seal between the casing and the formation, and to generate a well construction log.

The difference in water quality was visible. Two distinct water quality zones were identified based on the results collected. A chloride (“salinity”) zone was identified between 315 to 350 feet below the top of existing casing, with chloride concentrations measured to be 400 to 488 ppm. Fresh water was identified below the high chloride zone with chloride concentrations of 7.8 to 5.8 ppm, which is much more common for the area around Savannah.

A layer of lower permeability rock was also identified between 360 to 390 feet below the top of existing casing, which was tested and determined to contribute little flow to the borehole. This proved beneficial for the reconstruction scenario as the high chloride water would be impeded from seeping into the water with lower chloride concentrations.

Terracon selected a depth of 380 feet to set an inflatable packer for the packer test, used to simulate production rate, drawdown, and water quality under the proposed well reconstruction scenario. The packer test results indicated that the lower fresh water zone was capable of meeting the town’s water demand needs and water quality goals.

Terracon also created a 2-D model of the well using data collected to simulate groundwater flow and chloride concentrations around the well to predict long-term effectiveness of well modification on chloride concentrations. The well and the adjacent aquifer system were simulated with AnalyzeHOLE, a U.S. Geological Survey wellbore analysis tool for simulating flow in wells and aquifer systems, using the borehole geophysical and water quality data collected during the investigation.

Terracon presented the assessment results and well reconstruction design recommendations for a new casing diameter and final casing depth for the well reconstruction phase of the project to the Thunderbolt town council during a special council meeting.

Upon completion of reconstruction of the well, water quality samples were collected during an 8-hour test, which showed that Terracon’s design recommendations had successfully sealed off the high chloride water. Final water quality data showed that chloride concentrations were in the range of 7 to 8 ppm.

Since October 2011, the town of Thunderbolt has collected water quality samples monthly with results of low chloride concentrations. Terracon has continued to work with the town to monitor chloride levels at the well and provide recommendations in order to optimize production while maintaining decreased levels of chloride.

According to Linton Smith, Thunderbolt Town administrator, the council had been bracing for a worst-case scenario of installing a new well, which would have cost about $1.2 million. The town had anticipated using Special Purpose Local Option Sales Tax (SPLOST) funds, but with the evaluation and well repair totaling only $140,000, Smith said, the SPLOST money could be available for other infrastructure needs.