Engineered wetland removes subsurface hydrocarbons while providing beneficial reuse

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British Petroleum (BP) is operating an engineered wetland to treat petroleum-contaminated groundwater at a 300-acre site in Casper, WY. Release of petroleum products during refinery operations from 1908 until 1991 resulted in significant petroleum hydrocarbon contamination. Over 44,000 yd3 of LNAPL were removed by dual-phase recovery wells and oil/water separators constructed in 1981, but additional cleanup measures were needed to address dissolved-phase contaminants. Augmented by a cascading aeration system, the engineered wetland is achieving non-detect concentrations of target compounds such as benzene, toluene, ethylbenzene, and xylenes (BTEX) while allowing concurrent reuse of the property for commercial and recreational purposes.

In 1998, BP and the City of Casper agreed to a cleanup strategy accommodating redevelopment of the site, including a golf course, office park, riverfront trails, and a whitewater kayak course. The plan involved construction of an engineered wetland capable of treating up to 11,000 m3/day of gasoline-contaminated ground water for up to 100 years while blending into the golf course. Designs were initiated in 2000, pilotscale wetland tests were begun in 2001, and construction of the full-scale wetlands project was completed in 2003. Maximum concentrations of dissolvedphase hydrocarbons included 330 mg/L benzene and 530 mg/L total BTEX. The site is located in alluvial sand and silt deposits along the floodplain of the North Platte River. Fluctuations in the river’s water level cause the water table to fluctuate within the surficial aquifer, creating a “smear zone” due to remaining LNAPL.

The resulting large mass of petroleum hydrocarbons sorbed to the aquifer matrix gradually desorbs and dissolves into the water, causing elevated hydrocarbon concentrations. Refinery demolition involved excavating and recycling more than 200 miles of underground pipes and recovery and onsite crushing of more than 300,000 tons of foundation concrete. The concrete was reused as aggregate for the wetland treatment system. Construction involved grading of more than 1 million yd3 of soil for the golf course and installation of more than 60 dualphase recovery wells feeding the groundwater treatment system. A five-month pilot test was conducted to refine the final design and establish sitespecific parameters for contaminant degradation. Test results indicated that potential iron fouling of the wetland media could be addressed by including a cascade aeration system to oxidize iron and a surfaceflow wetland to precipitate it. Testing under both aerated and non-aerated conditions demonstrated that ground-water aeration could degrade recalcitrant compounds 45% more effectively, based on the increase in firstorder rate constants. Scaling up from the pilot system required a 1,200-fold increase in reactor volume.

Two center-feed, radial-flow treatment beds were constructed using crushed foundation concrete as the subsurface flow-engineered wetland medium. This configuration maximized flow distribution at a rate of up to 6,000 m3/day. One wetland is 360 feet in diameter, and the second is 65% smaller due to space constraints. To withstand winter temperatures as low as -35°F, the cells were insulated with a 6-inch mulch layer. Emergent facultative wetland plants such as bulrushes, switchgrass, and cordgrass were planted in each of the four treatment cells of both wetlands. After first passing through an oil-water separator to remove any remaining free product, ground water is pumped from the recovery wells to an aboveground “forced bed” cascade aerator that transfers atmospheric oxygen to the ground water, thereby enhancing contaminant volatilization and oxidizing ferrous iron (Figure 3). Off-gas from the cascade aerator is routed to a soil-matrix biofilter to control potential air emissions of BTEX. The aerated fluid travels through 60 feet of pipe 3 feet below ground surface to one of two free-water surface wetlands operating in parallel.

Residence time in the free-water surface wetlands is approximately 0.5 days. From the surface wetlands, water finally passes through additional subsurface pipes to the center of each subsurface radial wetland, where it radiates under natural hydraulic conditions toward the perimeter of four wedge-shaped treatment zones for additional biodegradation and phytoremediation (including phytovolatilization). Aeration pipes are flushed periodically with citric acid to remove mineral deposits that potentially interfere with air delivery and encourage algae growth after routine fertilization of adjacent golf course turf. Since full-scale operation began in May 2003, hydraulic loading of the system has averaged 2,600 m3/day (700,000 gal/day). Concentrations of target contaminants in ground-water samples collected from the aerator effluent show benzene and total BTEX concentrations approximately 50% lower than in aerator influent. Concentrations in wetland effluent are below detectable limits prior to discharge into Soda Lake, a basin created by former refinery effluent discharge.

The engineered wetland was selected over other in-situ biological strategies, such as bioremediation or phytoremediation, due to the need to control hydraulic gradient. Construction costs totaling $3.4 million saved an estimated $12.5 million compared to pumping and treatment using stripping towers and activated carbon. O&M consists primarily of sampling, monitoring, and maintenance of the recovery well system. The Wyoming Oil and Gas Conservation Commission became the anchor tenant of the new office park in March 2004, ten months after the full-scale wetland system began operating. The golf course and other recreational facilities were completed in 2005 as surface features of the wetlands continued to blend with the community

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