Freshwater storage impoundment mitigates increased chlorides in Abercorn Creek

Public Affairs Specialist
Published Oct. 31, 2014
A similar structure of a water storage impoundment located in North Carolina.

A similar structure of a water storage impoundment located in North Carolina.

Illustration shows functionality and location of proposed freshwater storage impoundment.

Illustration shows functionality and location of proposed freshwater storage impoundment.

The maroon box shows the location of the proposed site.

The maroon box shows the location of the proposed site.

Editors Note: This is the sixth in a series of articles to explain environmental monitoring efforts associated with the Savannah Harbor Expansion Project (SHEP). This series will focus on the various monitoring activities that must take place as construction begins.

SAVANNAH, Ga. – Understanding the reverberating environmental impacts of the Savannah Harbor Expansion Project has been a sizable undertaking for the Corps’ and its partners. Preserving high water quality standards is at the forefront of the Corps’ efforts to minimize adverse environmental effects that may result from the expansive project.

By deepening the Savannah River to the Garden City terminal, slightly elevated levels of salinity may extend further upstream, increasing chloride concentrations in Abercorn Creek which is a vital water source for Savannah residents and local industries.

Studies indicate that extreme spring high tides combined with drought conditions – such as when the Savannah River flow at Clyo, Georgia, falls below 4,000 cubic feet per second – could temporarily increase chloride concentrations in Abercorn Creek at high tide. If left unmitigated, the higher chloride levels could increase maintenance costs for industries and water quality for municipal users of Savannah River water.

The planned construction of a freshwater storage impoundment to store raw water solves this problem. The city’s treatment plant can use water from the impoundment on occasions when high tides and low stream flow result in higher chloride levels.

“During normal operations, all of the water withdrawn from Abercorn Creek will pass through the storage impoundment,” said Joseph Hoke, a Savannah District civil engineer. “This will provide an operational benefit by mixing the intake water and minimizing the twice daily fluctuations of pH and turbidity that are presently seen at the water treatment plant due to the tidal influence at the Abercorn Creek intake.”

If chloride levels climb, the city’s water treatment plant operator will turn off Abercorn Creek intake pumps and take raw water from the storage impoundment. The impoundment will serve as an intermediate holding pond for raw water from Abercorn Creek before reaching the city’s water treatment plant.

“Dependency on surface water sources is expected to increase due to requirements to reduce groundwater pumping from the aquifer combined with population growth,” said Bryan Robinson, also a Savannah District civil engineer. “The raw water is pumped about 7 miles from the Abercorn Creek intake in Effingham County to the treatment plant in Port Wentworth, Georgia.”

The U.S. Geological Survey (USGS) Savannah field office has pre-monitored fluctuations in chloride levels at the intake since July of this year. From July 25 to Oct. 20, the average chloride level at the intake was 10.2 milligrams per liter (mg/l). The maximum chloride level was 15 mg/l recorded July 27 and Aug. 23-24, according to the USGS National Water Information System website.

According to model projections by the Corps, the USGS and the Georgia Ports Authority (GPA), the maximum daily average chloride levels in Abercorn Creek are expected to be 62 mg/l as a result of the harbor deepening. City regulations prohibit chloride concentrations exceeding 250 mg/l according to drinking water standards established by the Environmental Protection Agency. The long-term average chloride level is predicted to increase from 11 to 13 mg/l.

“Even considering both the maximum and long term average increases, levels remain well below the drinking water threshold of 250 mg/l,” said Jason O’Kane, SHEP project manager.

There is no potential for salinity to increase to a level where it could be detected by taste, said Hoke.

“The concern is for potential chemical byproducts resulting from prolonged contact of elevated chlorides with copper pipe and lead-based solder in older water lines,” he said. “The availability of the water in the storage reservoir will alleviate those concerns.”

Higher chloride also raised municipal concerns for the pipelines and industrial processes that are sensitive to heightened concentrations.

According to city estimates, corrosion damage to the city’s 750 miles of water distribution pipeline, comprised of 60 percent steel, could amount to $22 million in replacement costs.

In response, a 2011 Corps investigation report concluded that corrosion to copper and lead is unlikely, and impacts to steel can be controlled by raising the pH of the treated water supplied to the distribution system.

The Corps will monitor water quality in Abercorn Creek for five years post-construction to assist the city in refining its operational controls and determine the frequency of needing to use the impoundment storage. The City of Savannah will assume maintenance and operations responsibility for the impoundment once the facility is accepted from the contractor, said Hoke.

The 97 million-gallon impoundment will occupy land on the northeast quadrant of I-95 and GA Highway 21, which is approximately the midpoint of the 7.5 mile raw water pipeline system.

The approximately two-year construction project is scheduled to begin during the summer of 2015 and be operational before the completion of inner harbor dredging.

Additional information can be accessed at the 2013 Environmental Assessment and the SHEP monitoring website.