TECHBriefs

By Jeffrey L. Binder, PG

The occurrence, containment and treatment of hydrocarbon seeps along creeks and surface water bodies are problematic for both compliance and aesthetics reasons. Numerous controls and remedial alternatives can be evaluated and implemented, but the actual long-term effectiveness and viability can be mixed. Many remediation techniques using reagents where oxygen is introduced can be problematic due to rapid consumption of the oxygen by anaerobic metabolisms and the impact on iron in the water and soil. Problems with aerobic treatment have led to the consideration and use of anaerobic treatment processes. Anaerobic treatment has found some success in the treatment of chlorinated compounds and, more recently, petroleum contamination in groundwater aquifers. In land treatment of hydrocarbon wastes, manure and other organic material, such as straw and wood chips, have been found to be beneficial in the biodegradation process. These two materials add air space and microbes resulting in a more porous material to enhance aerobic degradation process.

This project sought to innovatively combine hydrocarbon biodegradation methods by creating a treatment trench that used the existing creek bed and used a bio-augmentation mixture that included, in part, limestone chips, cow manure, wood chips and gypsum to combine aerobic and anaerobic treatment methods for organics and metals. As a result, groundwater interception along with aerobic treatment for free-phase hydrocarbons and anaerobic treatment for dissolved-phase organics and metals were incorporated into the design of the interim remedial measure (IRM).

The project consisted of numerous investigations to determine the extent of groundwater impacts and identify contaminants of concern prior to the construction of the IRM. The IRM construction activities involved modifying an oxbow segment of the creek where oil seeps had historically been observed. A biologically active interceptor trench was then constructed in the former oxbow. The intent of this treatment trench was to collect any free-phase hydrocarbon and/or treat the impacted waters by using in situ bioremediation prior to discharge to the creek channel.

This particular project dealt with surface water impacts from impacted groundwater migrating into the receiving creek. The primary focus for this IRM was to use passive treatment technologies and to modify the natural conditions for the design and implementation of this remedial alternative. Based upon analytical results of samples of impacted groundwater, the main constituents of concern were benzene and lead. This project was designed to treat impacted water to meet stream-quality discharge parameters.

Project Overview The construction plan for the IRM included: (1) obtaining a 404 permit from the Army Corps of Engineers; (2) bypassing the oxbow segment of Skull Creek where oil seeps had historically been observed; (3) constructing a biologically active interceptor trench in the former oxbow; (4) regrading the site to control stormwater flow; and (5) restoration/mitigation plantings to enhance the creek’s ecosystem.

Anaerobic Treatment and Hydrocarbon Seepage Control Using Stream MorphologyA Natural Geomorphic Feature Can Serve as an IRM Structure

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IRM Construction Activities Prior to construction activities a 404 permit was obtained from the U.S. Army Corps of Engineers. To facilitate channel relocation activities, treatment trench installation and site regrading activities, brush and trees were removed from the work areas. Most of the vegetation was removed from the areas adjacent to the oxbow. Much of the woody debris was chipped on-site and used as a component of the treatment trench backfill material.

Topsoil was stripped from this area and stockpiled for reuse elsewhere on site. Clay borrow material excavated from the relocated channel was stockpiled within the oxbow area for use as fill material during regrading activities.

The relocated channel construction focused on minimizing the need for bypass dewatering. This was done by completing most of the relocated channel while maintaining coffer dams at both the upstream and downstream ends of the channel. As segments of the relocated channel were excavated, a geotextile fabric was installed as a material separation layer. Riprap was placed over the geotextile, per the design. Once the relocated channel was complete, the downstream coffer dam was removed. This allowed for completion of the transitional area that would be part of the newly

Site Description The Skull Creek Oil Seeps IRM is located on the eastern edge of a former refinery remediation site, near Cushing, Okla., in Payne County. The IRM is bounded on the west and south by county roads (across from which is the remainder of the remediation site), the north by vacant land that was previously occupied by a topping plant and on the east by vacant land. Skull Creek flows across the IRM area from southwest to northeast.

Physical Setting The IRM is located adjacent to Skull Creek in an oxbow meander of the former creek channel. This geomorphic feature provided the physical basis for the eventual location and design of the IRM.

Site History Historically, a number of refineries operated to the west and north of the IRM area. Several refineries previously operated on the remediation site from the early 1900s until the early 1970s.

During a 2002 inspection, oil was observed seeping from the north bank of Skull Creek in the vicinity of the future IRM. The oil seepage was coming from a 6- to 12-inch-thick basal gravel conglomerate unit approximately 2 feet above the base flow level of the creek.

Photo 1: Oil seepage was observed in the oxbow segment of Skull Creek.

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relocated channel. The upstream coffer dam was then removed, allowing flow through the newly constructed channel.

Oxbow Subgrade Preparation Following successful diversion of Skull Creek through the relocated channel, the former oxbow subgrade was prepared for backfill placement. Impacted soils (when encountered) were removed and temporarily stockpiled on polyethylene liner material prior to being transported to the designated investigation derived waste storage area.

Subgrade preparation included determining the depth to mudstone and the overlying basal gravel conglomerate layer (the layer where oil seeps had historically been observed), as the performance of the treatment trench depends on effective hydraulic communication between the basal gravel layer and the treatment trench.

Oxbow Backfilling and Treatment Trench Installation The oxbow subgrade was prepared concurrent with the excavation. As excavation of areas of the oxbow were completed, the areas were backfilled with clay borrow material from the relocated channel excavation. Clay borrow material was placed in lifts and compacted with a vibrating padded drum roller.

The treatment trench excavation was backfilled with a bio-augmented granular material (referred to as lower zone material) and covered with limestone material (referred to as upper zone material). Prior to backfilling, a trial mixing operation of these materials was performed. The bio-augmented granular material was originally specified to consist of approximately 60 percent gravel, 15 percent limestone cobbles, 15 percent limestone sand, 6 percent wood chips, 2 percent cow manure and 2 percent gypsum (by volume). After initial field trial operations and problems with achieving a consistent mixture ratio, the volume of cow manure and gypsum was approximately doubled to assure availability of these two components for treatment.

Each component was important to the successful operation of the system. The limestone helps the pH of the treated water remain slightly alkaline, the preferred

environment for the microbes. The sawdust/wood chips provide a host site to which dissolved hydrocarbons will tend to attach (giving the microbes more time to metabolize), along with nutrients and additional food for the microbes. The manure provides nutrients, a source of microbes (especially sulfate-reducing microbes), and still more food for the microbes. The gypsum provides a source of sulfate from which the sulfate-reducing bacteria will be able to strip oxygen to “breathe.”

The upper zone limestone material consisted of 90 percent to 95 percent limestone gravel and 5 percent to 10 percent wood chips. This zone was designed to capture free-phase hydrocarbons and to aid in providing residence time for aerobic microbes to metabolize the hydrocarbons. However, complete degradation of the free-phase hydrocarbon is not anticipated.

After installation of the treatment trench, and prior to completion of a weir structure installation, a temporary coffer dam was installed just upstream of the weir structure. The coffer dam served several functions: (1) allowed installation of the weir structure in dry conditions; (2) minimized the potential for release of impacted water to Skull Creek prior to the effective treatment of the water; and (3) provided an opportunity to biologically condition the treatment trench by recirculating waters collected behind the dam.

A sump pump was installed just upstream from the coffer dam to allow recirculation of trench waters until the temporary clay dam was removed.

Weir Structure Installation As part of the treatment trench construction, a weir structure was installed as not only an overflow but also as an underflow structure. This allows the structure to act as a skimming device (oil-water separator) to prevent free-phase hydrocarbon from discharging to the creek outfall. A stainless steel weir plate was mounted to the downgradient face of the precast concrete structure to measure flow.

Prior to backfilling the interior of the weir structure with limestone gravel, a sump and a weir riser pipe were installed. For more information, please email

jbinder@burnsmcd.com.

Jeffrey Binder, PG, is an associate geologist and hydrogeology and geology section manager at Burns & McDonnell. He earned a Bachelor of Geology from the University of Missouri at Kansas City.

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A riprap-filled discharge trench was constructed from the weir structure to the relocated channel. The discharge trench was excavated, a geotextile filter fabric was installed, and riprap was placed to complete the discharge trench.

Operations and maintenance (O&M) for the IRM weir involves visual observation and measurements of product occurrence with removal using a passive skimmer system to collect the free-phase hydrocarbon for proper disposal.

Site Regrading and Restoration Coincident with the treatment trench and weir structure installation, the area within the former oxbow was regraded and restored. Clay borrow material excavated during construction of the relocated channel was used to raise the grade within this area and to backfill the landform within the former oxbow. Regrading was performed to promote drainage of stormwater runoff away from the treatment trench and toward the newly constructed channel and to keep stormwater from ponding within the former oxbow area.

Following completion of the earthwork and final grading, the area was reseeded with native grass species.

To prevent flooding from beaver dams, it was necessary to extend the proposed mitigation area along the creek. Trees and shrubs that could not be planted on the northwestern bank were planted farther upstream on the southeast side, as an extension of the originally proposed mitigation area. All new plantings were fenced to protect against beavers.

IRM Monitoring Points Monitoring wells and piezometers were installed following completion of the treatment installation to monitor water quality and water levels.

In addition to the groundwater monitoring, outfall samples are collected on a regular basis at the location where effluent from the IRM enters Skull Creek.

Summary of Conclusions This project provided effective use of a natural geomorphic feature as a structure for the IRM. The IRM met the goal by using a passive bioremediation technique that is capable of controlling free-phase hydrocarbon migration and treatment of impacted groundwater prior to discharge to a surface water outfall. Mitigation of the riparian habitat was implemented to promote a viable environment alongside the IRM.

No reported exceedences of water quality parameters or flow have been identified. The anaerobic process exhausts the sulfate portion of the bio-augmented treatment so a regular (every three years) amendment of gypsum is surface applied by broadcasting gypsum pellets using a fertilizer spreader.

Whereas all free-phase hydrocarbons will not be treated, the treatment trench underflow weir structure will provide a means to skim the product, thereby helping its discharged water meet the water quality standards at the outfall on the creek. This system will serve as a passive, low-maintenance and long-term remediation solution.

Photo 2: Ongoing discharge monitoring has helped provide data to measure the effectiveness of the IRM.