CONSTRUCTED TREATMENT WETLANDS AND WATER EFFECT RATIO STUDY TO ACHIEVE STORM WATER COMPLIANCE AT SAVANNAH RIVER SITE

George M. Huddleston III and Eric A. Nelson

______________________________________________________________________________________ AUTHORS: George M. Huddleston III, Project Scientist, ENTRIX, Inc., 102 East Main St., Pendleton, SC 29670; Eric A. Nelson, Fellow Scientist, Savannah River National Laboratory, Bldg. 773-42A, SRNL-SRNS, Aiken, SC 29808. REFERENCE: Proceedings of the 2008 South Carolina Water Resources Conference, held October 13-14, 2010, Columbia Metropolitan Convention Center, Columbia, SC. ___________________________________________________________________________________________________________________

Abstract. This project focused on compliance with stringent metals limits for storm water runoff and non-contact cooling water from the 33-acre drainage basin of the H-02 Tritium Extraction Area at Savannah River Site near Aiken, South Carolina. Proposed monthly average NPDES Limits for total metals were: copper, 7 µg/L; lead, 1 µg/L; and zinc, 100 µg/L. Compliance was achieved by constructing a treatment wetland system, conducting water effect ratio (WER) studies on wetland effluent for metals, and negotiating site-specific NPDES permit limits based on WER results. The constructed wetland treatment system consisted of a 3.3-million gallon retention/equalization basin and two (0.5 acre) wetland cells planted with giant bulrush. The constructed wetland system was designed to sequester metals in the hydrosoil substrate. The system operates entirely by gravity and requires no power or chemical addition. The permit limit negotiation resulted in daily average (total) limits of: copper, 32 µg/L; lead, monitor and report; and zinc, 110 µg/L. The completed system came online in January of 2008. The site-specific NPDES permit limits went into effect in October of 2008. The constructed wetland system consistently achieves NPDES compliance at a low operating cost.

INTRODUCTION Constructed wetlands can be effectively designed and utilized for removal of metals from stormwater and industrial effluents (Nelson et al., 2002; Eggert et al., 2008; Huddleston and Rodgers, 2008; Huddleston et al., 2009). The H-02 outfall is a permitted surface water compliance

point (NPDES permit number SC0000175) at the Savannah River Site (SRS) near Aiken, South Carolina. The discharge from the H-02 outfall flows approximately 660 feet before entering the waters of the State, Crouch Branch. In 2006, Washington Savannah River Company, which operates SRS for the United States Department of Energy, began design a surface flow constructed wetland system for treatment of the H-02 outfall and compliance with NPDES requirements. Once the treatment system was constructed and operational, a water effect ratio (WER) study was conducted on the constructed wetland effluent. The results of that study were submitted to South Carolina Department of Health and Environmental Control (SCDHEC) for consideration in setting final NPDES limits for the H-02 outfall. This paper describes the design and performance of the constructed wetland system presents findings of the WER study, and reconciliation of the H-02 NPDES permit requirements.

METHODS H-02 Outfall Characterization. The H-02 outfall drains a mostly impervious 33-acre watershed and contains cooling water, laboratory drain waste, steam condensate, and storm water. Parameters targeted for reduction in the H-02 outfall were copper, lead, and zinc. Each parameter exceeded average and/or maximum NPDES permit limits prior to wetland construction (Table 1).

Table 1. H-02 outfall flow and concentrations of target constituents.

Parameter

Draft NPDES Limit

2005-2006 (n=17) Units

Flow Average - 0.111 MGD Copper Average 7 24.0 µg/L Maximum 9 35.5 µg/L Lead Average 1 2.2 µg/L Maximum 20 4.2 µg/L Zinc Average 100 67.1 µg/L Maximum - 122.5 µg/L

Constructed wetland system design. The constructed wetland system was designed to store and treat stormwater runoff and cooling wastewaters from the H Area at SRS. The system consists of a 3.3 million gallon earthen detention basin with appurtenances, a flow splitter box with an isolation valve, two (2) 0.53-acre surface flow wetlands, and associated piping (Fig. 1). The detention basin is sized to hold up to 3.3 million gallons before water overflows to a principal spillway. The detention basin is equipped with an emergency spillway sized to handle the 100-year, 24-hour peak runoff rate to protect the structural integrity of the earthen basin. The wastewater discharges from the basin into the gravity pipeline to the splitter box and flows through a normally open plug valve. When the wastewater reaches the splitter box the waste stream is equally divided by a set of rectangular weirs into two even streams, one for each wetland cell.

Each wetland cell is 0.53 acres with a design water depth of 30 cm (12 in.) providing a 48-h residence time. The cells are lined with a geosynthetic pond liner and have a freeboard of 24 in. Wastewater is applied to each cell evenly by discharging from a PVC pipe with exit holes drilled evenly across the full length of the pipe (and the width of each cell). The wetland cells consist of a pond liner covered with 1 ft of uncompacted soil to protect the liner, 2 ft of uncompacted hydrosoil (growth medium for the

plants), 12 in. of wastewater, and 2 ft of freeboard. The hydrosoil is sandy upland soils from the site amended with mulch for a carbon source until the plants can provide carbon from their decaying mass, slow release fertilizer, and gypsum as a sulfur source to ensure an excess of sulfides in the wetland cells. The hydrosoil is planted with giant bulrush (Schoenoplectus californicus) spaced on 1.5-ft centers to promote a high density of plants upon maturity (Murray-Gulde et al., 2005). Wastewater exits the wetland cells through one of four evenly spaced outlet control boxes. These boxes are pre-manufactured of PVC and allow the water depth to be adjusted up to three feet in depth by installing or removing 6-in. stop logs. Wastewater exiting the wetland outlet control boxes discharged to the H-02 outfall compliance monitoring station. Water effect ratio study. A water effect ratio study was conduced on wetland effluent samples over three sampling events from October 2007 to January 2008 in accordance with U.S. EPA (1994) methods.

RESULTS AND DISCUSSION Construction of the H-02 wetland system was completed in July 2007, and the WER study was completed in March 2008. The WER study encompassed three sampling events of wetland effluent, and resulted in new total copper limits of 32 µg/L and 42 µg/L for the daily average and daily maximum, respectively. Lead was reclassified as “monitor and report” (from the original limits of 1 µg/L and 20 µg/L for the daily average and daily maximum, respectively). The daily maximum total zinc level was set at 110 µg/L. In this case, the WER study resulted in higher metals limits due to the toxicity mitigating effects of dissolved organic matter, mineral particles, and metal chelators such as sulfides provided by the treatment wetlands.

Following the WER study and issuance of the final NPDES permit in October 2008, total copper, lead, and zinc have averaged 11.0 (±4.2), <2.0, and 11.6 (±6.4), respectively, in the wetland effluent from November 2008 to April 2010. The H-02 outfall has remained in compliance since construction of the wetland treatment system. Other attributes of the wetland system include low operation and maintenance needs, the ability to treat multiple constituents, reduction of toxicity, and pH buffering of the effluent.

ACKNOWLEDGMENTS The authors appreciate the contributions of John Gladden and Rege Strauss (Savannah River National Laboratory), Gary Mills (Savannah River Ecology Lab), and S. Michele Harmon (University of South Carolina, Aiken.

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Fig. 1. H-02 constructed wetland treatment system, Savannah River Site.

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