Weld County Waste Disposal Site (EPA 7003 RCRA): Closure of an exploration and production waste disposal facility
Patrick Flynn, H S Resources, Inc.
David Boyce, BP Amoco
Thomas Murphy, LT Environmental, Inc.
Site Background
The Weld County Waste Disposal, Inc. (WCWD) site was a 40-acre commercial waste disposal facility originally permitted to receive oil field brines and other petroleum related liquid waste derived from oil and gas exploration and production (E&P) activities. The facility was built in the late 1970's and was permanently closed in May 1995. Prior to closure, liquids received were off loaded into a concrete receiving sump and transported via aboveground pipe to a clay-lined impoundment (Pond C) where liquid hydrocarbons were separated from the produced water (brine) and recycled. The water was transferred to a second clay-lined impoundment (Pond D) where enhanced evaporation occurred as the primary water disposal method.
The work was conducted in fulfillment of the requirements of an U. S. Environmental Protection Agency (EPA) RCRA 7003 Administrative Order (Orders) received by the owner and the two largest local producers who had previously used the facility. EPA developed the Orders in response to environmental concerns as a result of the operational practices.
The structures containing the waste products at the time of closure consisted of the concrete receiving sump, eight ASTs, two impoundments, and a sludge drying bed. Since August 1995, remedial measures were initiated to eliminate the waste present. These measures included:
- Removal of over 647,000 bbls (>27,000,000 gallons) of water through the operation of an enhanced evaporation system. A residual 15,000 bbls of heavy brine was injected into a permitted Class I well;
- Removal of 4,300 bbls (180,000 gals) of nonhazardous hydrocarbon liquids and 20 yd3 of BS&W solids from the ASTs and receiving sump. The waste was disposed of at a commercial biobed operation; and
- A sludge landfarming program of an estimated 20,000 yd3 of sludge with elevated hydrocarbon and salts.
Sludge Landfarm Program
The objectives of the program were to:
- Eliminate the potential for wildlife to be exposed to the sludge;
- Reduce the accumulation of free water in the pond from precipitation;
- Provide easy access to the pond for subsurface characterization activities;
- Reduce the potential for vertical migration of organic and inorganic compounds;
- Provide oxygen, nutrients, and proper substrate (wood chips and manure) to initiate in-place biodegradation of the hydrocarbons; and
- Reduce the oily matrix to allow for more precise chemical characterization of the sludge.
Establishment of Cleanup Objectives
To accomplish the on site landfarming project, chemical specific cleanup objectives were established through the risk assessment process that protect potential human receptors. The primary contaminants of concern of benzene, polynuclear aromatic hydrocarbons (PAHs), and arsenic were evaluated. The objective was to establish what the maximum concentration of these compounds could be after the sludge treatment so as not to pose a threat based on potential future land use scenarios. This process was concurrent with the implementation of the landfarming program. The cleanup goals established were derived from risk-based concentrations (RBCs), background values, and soil to groundwater transfer concentrations using the SESOIL/AT123D fate and transport models. Cleanup goals based primarily on the more conservative RBCs were established for benzene (0.65 mg/kg) and the PAHs (chemical specific). Background values were used to establish an arsenic cleanup goal of 22 mg/kg.
Implementation of Landfarming Program
The initial mixing of bulking material and nutrients was conducted in May 1997 in Pond C and in March 1998 in Pond D. An estimated 4,125 yd3 of wood chips and 1,000 yd3 of cow manure were added increasing the sludge volume by 25%. After conducting baseline analyses, the initial nutrient addition consisted on six tons of urea and one ton of monoammonium phosphate. Tilling of the amended sludge occurred on average three times per month from June 1997 through September 1998. A second nutrient application occurred in June 1998 with 3.8 tons of urea and 0.65 tons of monoammonium phosphate added.
Performance Results
Periodic performance monitoring of the amended sludge occurred. By September 1998, the benzene concentration in the sludge was non detect at reporting limits below the cleanup goal of 0.65 mg/kg. The observed benzene reduction since the inception of the aggressive tilling program is 99%. This is accompanied by an overall reduction in total hydrocarbons (TRPH) concentration of 84%. Although reporting limits of two PAHs exceeded the established RBC cleanup goals, mitigation was achieved through the use of the SESOIL/AT123D models and closure design criteria (i.e. soil cover). The models demonstrated that as a result of biodegradation in the groundwater, the potential pathway was not completed. Arsenic concentrations did not exceed the established background values for the area.
Site Closure
Permanent closure was achieved through the elimination of the waste products and removal of the remaining structures. The ASTs were dismantled and removed from the site. The concrete sump was rubbleized and buried on site. Pond C containing the remediated sludge from both ponds was backfilled with berm material, a soil cover with drainage controls constructed, and the site revegetated. Final EPA approval is eminent and post-closure monitoring occurring to document the success of the closure activities.