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.

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.
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This historic mine site in the northern Rocky Mountains is comprised of a number of individual mines. Mining began at the site around 1885 with the discovery of silver-bearing vein deposits. Various mines were operated at the site sporadically through the 20th century, with the last operational in the early 1990s. Environmental monitoring at the site began in the early 1980s and partial reclamation efforts were undertaken by the mine operator in the mid-1990s and early 2000s. The operator entered bankruptcy in 2005, reorganized, and entered into a settlement agreement with the State of Montana in 2009. Through the reorganization, a trust was created to address cleanup responsibilities at the mine property and manage the remediation funds.

Trihydro began working at the site in 2011, as a team member supporting the prime consultant. Initial efforts focused on delineation and characterization of metals-impacted waste rock, tailings, sediments, soils, surface water, and groundwater associated with the site. Site characterization activities also included inventorying historical structures and mine workings. Test pits were excavated and monitoring wells installed to evaluate potential waste repository locations. Trihydro’s role included assisting with project coordination, conducting environmental sampling, performing data validation, managing project data in Project Direct® (Trihydro’s custom data and project management application), and evaluating the data.

Subsequent work at the site has included synoptic sampling events along 10 miles of creek, annual groundwater and seep sampling, borrow material investigations, and conducting a formal repository investigation of the preferred, on-site repository location. The repository investigation entailed georeferencing and evaluating mine maps relative to the repository footprint, excavating test pits to evaluate constructability, and installing borings and collecting rock cores to evaluate bedrock conditions. Trihydro also provided assistance during development and public scoping of an Engineering Evaluation / Cost Analysis (EE/CA).

Trihydro is currently serving as the prime consultant for design, bidding, and construction administration of the on-site repository. This engineered repository is designed to contain approximately 400,000 cubic yards of metals-impacted soils, sediments, tailings, and waste rock. Excavation of the repository base is scheduled for the near future, with waste material excavation, repository loading, and capping to occur in subsequent field seasons.

Trihydro continues to assist in routine groundwater and seep sampling, evaluation of surface and groundwater impacts on-site and downstream of the site, and other tasks that arise as part of the ongoing characterization and reclamation of this historic mine complex.
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Trihydro was selected by the City of Cheyenne to prepare design plans and specifications for construction of 850 linear feet of stormwater line in Snyder Avenue in Cheyenne, Wyoming. The purpose of the new stormwater line was to connect an upgraded stormwater collection system in Pershing Boulevard to an existing stormwater line south of the Snyder Avenue and Randall Avenue intersection. The existing stormwater system and outfall line was undersized, resulting in frequent flooding of Pershing Boulevard corridor and the adjoining intersections. Adding the additional stormwater outfall line in Snyder Avenue was a critical component to improving the drainage of the historic neighborhood along the Pershing Boulevard corridor.

A unique design component of the project was dealing with the connection of the new 48-inch pipeline to an undersized existing 36-inch outfall pipeline to Crow Creek. The City had identified the existing 36-inch pipeline in Snyder Avenue to be upsized as part of future drainage project, but that project could not be completed before the start of the Pershing Boulevard project. To prevent the increased flow from the upgraded stormwater system in Pershing Boulevard from flooding the smaller existing downstream pipeline, a series of restrictor plates were designed and installed in the new upstream manholes in Snyder Avenue. The restrictor plates were designed to limit the flow by using the upstream manholes and pipes as small detention cells. After downstream outfall line was upsized the restrictor plates could then be removed to allow full flow in the pipeline.

An additional challenge included routing the 48-inch stormwater pipeline down a narrow two-lane urban corridor, which involved moving existing parallel utilities, as well as threading the pipeline under a 30-inch water main and 12-inch sanitary sewer main. Crossing under the water and sanitary sewer mains required the design of two transition vaults to change from a 48-inch pipe to four 24-inch pipes back to a 48-inch pipe. There were also several areas where the parallel utilities could not be moved to provide the required separation distance between a potable water line and stormwater pipeline. To receive approval from the Cheyenne Board of Public Utilities and the Wyoming Department of Environmental Quality to place the pipes closer, Trihydro negotiated the use of Steel Reinforce Polyethylene (SRPE) pipe with pressure rated joints as an alternative to Reinforce Concrete Pipe. This was the first use of SRPE in the City, and because of the success on this project, the City has opted to use the SRPE pipe product on several subsequent projects.

Construction of the Snyder Avenue stormwater pipeline was completed in October of 2011 and the upstream stormwater system improvements to Pershing Boulevard were completed in June of 2013.
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