In 1994, EPA became aware of a subsurface oil plume in the SCA vicinity, which was later shown by chromatographic analysis to be caused by co-mingled petroleum products released between 1978 and 1991. In addition to a light non-aqueous phase liquid (LNAPL) plume floating on ground water in the Kingshill Aquifer and on perched ground water, dissolved-phase petroleum hydrocarbon constituents were present in ground water. Although ground water is not utilized at the SCA facility, the Kingshill Aquifer is used upgradient of the facility and elsewhere on the island with less saline conditions. Consequently, the Kingshill Aquifer is an important source of drinking water in some parts of St. Croix.
Wind resource data from the nearby St. Croix airport showed that the site was well suited for wind energy applications. Oil recovery began in 2002 using four WTCs to drive pneumatic total-fluid pumps in six recovery wells. Recovered oil and co-mingled ground water are pumped to a separation tank where oil and water are separated via gravity separation. The oil then is reclaimed and used as feedstock at an adjacent petroleum refinery, as permitted under RCRA, and the water is transferred to a permitted wastewater treatment system prior to discharge to the Caribbean Sea.
Each WTC is powered by a windmill with 4.3-ft blades that begin rotating at a wind speed of 4 mph. When wind speed exceeds 30 mph, the blades furl and turn out of the wind. The air compressor is located directly behind the windmill, on a hinged tower. For maintenance or occasional hurricane protection, the combined blade/compressor unit can be lowered to the ground. Each WTC is designed to generate approximately 45 psi of operating pressure.
A 165-W solar panel array was installed in 2003 to provide electricity for an enhanced fluid-gathering system. In 2006, two WEGs were installed to supplement power for electric submersible total-fluid pumps that were installed in several wells to increase oil recovery by the four WTCs. Two additional WEGs (for a total of four WEGs) and an additional PV array (220 W) were installed the following spring to power four new recovery wells for enhanced hydrocarbon recovery.
In November of last year, a compressed air “makeup” line was added to supplement the air from the WTCs and to maintain a continuous 55-psi operating pressure in the pneumatic recovery pumps when the WTCs are not providing sufficient air pressure. To date, electricity from the utility grid still is not required for the recovery pumps or to collect and transport the recovered total fluids to the separation tanks.
The volume of free-product oil recovered during the second half of 2007, reflecting the recent system upgrades, averaged 91.5 gal/day. During that period, the recovery system employed five pneumatic totalfluid submersible pumps and five electrically powered total-fluid submersible pumps. The submersible pumps recover both free product and ground water, and enhance the groundwater gradient to facilitate capture of free product away from the well bore. Oil “skimmer” pumps capture the oil layer in the well bore but do not create an enhanced gradient for oil to move to the well bore. Semi-annual fluid gauging and analysis of ground-water samples from nine monitoring wells around the perimeter of the plume indicate that both the LNAPL and dissolved constituent plumes are not migrating and remain on the SCA site.
This treatment design was selected due to the absence of onsite electricity upon project startup. SCA estimates that the system’s capital costs (including installation of WTCs, WEGs, and PV panels) total nearly $40,000, approximately half the cost to connect to the power grid. The WTCs are no longer manufactured; as a result, future upgrades may include replacement of the WTCs due to unavailability of replacement parts, or custom fabrication of replacement parts.