Remediation System Design & Implementation
Natural Gas Compressor Station
Adams County, Colorado
Regulatory Agencies Involved
Colorado Oil & Gas Conservation Commission
Trihydro was contracted to design and implement a remediation strategy at a natural gas compressor station site in Colorado. The station was built in the 1970s, and continues to operate today. Historical records show that a natural gas condensate spill of about 40 barrels occurred in the 1990s and was addressed by soil excavation to a depth of about six feet at the time. However, hydrocarbon-impacted soil and groundwater were later discovered by the client during an on-site investigation conducted as part of due diligence in connection with their purchase of the station from its previous owner in 2011. Five to seven feet of free product (condensate) was identified on the water table at depth of 100 feet in two monitoring wells onsite. In addition, hydrocarbon impacts were observed in the vadose zone and an aqueous-phase benzene plume extended off-site from the station. Trihydro was tasked with developing a remedial strategy to address the hydrocarbon impacts in soil and groundwater and satisfy cleanup goals in compliance with Colorado Oil and Gas Conservation Commission (COGCC) requirements.
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Trihydro recommended and implemented a Laser Induced Florescence (LIF) and Membrane Interface Probe (MIP) investigation to cost-effectively and rapidly delineate, in detail, the lateral and vertical extent of the hydrocarbon impacts in the vadose zone. The overriding goal of the LIF and MIP investigation was to accurately locate the source material in order to facilitate some removal and expedite cleanup of groundwater. The LIF survey was successful in identifying source material in a relatively discrete area split between a shallow zone lying above a clay lense and a somewhat different distribution of hydrocarbons, at depth, below the clay. A small number of soil boring and geotechnical analyses provided the data to quantify soil properties. A compositional analyses of the free product provided information on the relative volatilization potential of the hydrocarbons. These data were used to evaluate remedial technologies for hydrocarbon recovery and/or treatment. The compositional analyses of the free product and the soil conditions were favorable for the use of soil vapor extraction (SVE), both to achieve source reduction in soil and also effect reduction of free product in the site monitoring wells.
A pilot test was performed a few months after the LIF and MIP work to determine the hydrocarbon concentrations in the extracted soil vapor and identify flow rates that could be achieved. This data led to the development of a two-phase SVE system design-implementation plan. Phase I consists of the use a high mass-removal internal combustion engine to provide both the vacuum for soil vapor recovery and the combustion for off gas control. The advantages of this unit are that it can provide a high mass recovery rate, has the potential for high vacuum, and it does not require electric power, which is significant because there is no electrical service at the compressor station. As concentrations follow a natural decline curve over time, other technologies become more cost-effective. For the lower concentration Phase II period, a generator-powered blower and catalytic oxidizer are planned, which is considerably more efficient at lower soil vapor concentrations. The need for Phase II will be evaluated following a 6 to 24 month period of evaluation and monitoring of groundwater quality. The phased approach for the project has the potential of shortening the remediation time and lowering the project cost.