In-Situ Containment and Treatment of a Free Phase Hydrocarbon Plume beneath Plant Infrastructure

Transcription

1 In-Situ Containment and Treatment of a Free Phase Hydrocarbon Plume beneath Plant Infrastructure M. Brewster 1, S. Penny 1, B. Odell 1 and T. Jorgensen-Nelson 2 Abstract A very shallow (1 2 m deep) hydrocarbon free product plume was identified on the groundwater surface at an Alberta Gas Plant. The plume extended beneath Plant infrastructure, which precluded excavation of the contaminated material. Therefore, an in-situ remediation approach was required. The primary remedial goal was to recover hydrocarbon mass and prevent further expansion of free phase and dissolved phase plumes, until such time that complete clean up could be undertaken. Following the review of various options, a trench-and-gate remediation system was selected and implemented. The concept of was to use the natural hydraulic gradients to passively deliver contaminated groundwater to a treatment system. Groundwater flow would be directed through permeable trenches to the treatment gate(s). For this project a two-stage treatment process was implemented. In the first stage hydrocarbon product is separated and retained. In the second stage, an aeration system is used to sparge volatile dissolved hydrocarbons and oxygenate the groundwater. This trench-and-gate system was constructed in 2000 and began operation in Since beginning operation, a hydrocarbon film has collected in the free product separation gate. Analytical results for water samples collected from the treatment gate and re-infiltration gallery show dissolved hydrocarbon concentrations were below laboratory detection limits, indicating successful treatment of the dissolved phase.

2 Introduction During a soil and groundwater investigation program in 1998, the presence of hydrocarbon free product was detected in the subsurface of the Liquefied Petroleum Gases (LPG) Recovery Area at an Alberta Gas Plant. A follow-up investigation delineated an area of approximately 3,100 m 2 with hydrocarbon free product up to 0.03 m in apparent thickness on the uppermost groundwater surface (water table). A phased approach was undertaken towards developing a free product recovery and dissolved phase treatment system. In 1999, Phase I involved the installation of a hydrocarbon recovery trench across the core of the plume along a former utility conduit route. This paper describes the results of Phase II of the program: the installation of a Trench and Gate system at the downgradient edge of the plume to provide containment and treatment of hydrocarbon contamination. Site Description The free product contamination originated from the LPG (Liquefied Petroleum Gases) Recovery Area of the Plant, where lean oil is used to strip (condense) propanes and butanes from the gas stream. The free product plume straddles the two upper hydrogeological zones (Figure 1). The plume source area exists beneath the process facilities and lies within the 1.5 to 2 m of coarse angular gravel fill. The gravel fill has a hydraulic conductivity on the order of 10-5 m/s. The fill was used to raise and level the area prior to construction of process facilities. Relatively low permeability silt till underlies the gravel fill. The till is in turn underlain by siltstone bedrock at a depth of 6 to 8 m below ground surface (bgs). Approximately 10 m downgradient (south) of the Recovery area, the gravel fill thins out in a lateral transition to the native silt till. The ground surface elevation drops approximately 1 m to the original grade. The till is fractured and contains laterally discontinuous sand lenses. Hydraulic conductivity measurements are on the order of 10-7 m/s in shallow piezometers. In deeper piezometers (> 6 m), hydraulic conductivity measurements of the till are in the 10-8 to 10-9 m/s range. The groundwater surface occurs at approximately 1.5 m bgs in the fill and as shallow as 0.5 m bgs in the till. Groundwater flow across the area is south-southwest (Plant grid). A conservative estimate of the lateral groundwater flow velocity is on the order of 0.3 m/year. Remedial Goals Based on a review of remedial options for the site, it was recognized that excavation and ex-situ land surface treatment ( land-farming ) would be the fastest and most costeffective approach to achieve the ultimate goal of complete free product removal and compliance with remedial objectives. However, given the proximity and partial encroachment of the free product plume beneath the Plant process area and the LPG

3 storage bullets (87,000 Gallons each), complete excavation of hydrocarbon impacted soils was not feasible. These obstacles presented serious safety and logistical concerns that weighed against the potential gains from complete hydrocarbon removal at that time. Furthermore, the proximity of this area to existing operations leaves it vulnerable to recontamination. Figure 1 : Hydrogeological Setting PHASE II: TRENCH AND GATE TREATMENT SYSTEM HYDROCARBON RECOVERY TRENCH (TRENCH #1) PIEZOMETER LOCATION APPROXIMATE AREA OF FREE PRODUCT APPROXIMATE PLUME OF DISSOLVED PHASE HYDROCARBON CONTAMINATION CROSS-SECTION LOCATION MONITORING WELL LOCATION A more pragmatic goal was therefore selected: to recover free product in a cost-effective manner and to prevent further degradation from plume expansion. Specifically, the objectives of the remediation program were as follows: 1. Containment of the free product and dissolved phase plumes to prevent further expansion of the area of impact; 2. Recovery of free product to eliminate surface seepage of hydrocarbons; and, 3. Treatment (removal) of dissolved phase hydrocarbons from the downgradient edge of the plume. Overview of Trench and Gate Technology Traditionally, groundwater remediation has consisted of pump and treat systems. However, Starr and Cherry (1994) introduced an in-situ groundwater treatment system: the Funnel and Gate system. This system uses an impermeable barrier to funnel

4 groundwater into an in-situ reactor to treat the contaminated plume. Bowles (1997) outlines the three disadvantages with the Funnel and Gate system: 1. the funnel width has to exceed the plume width, in order to capture the streamlines which veer around the end of the walls (Fitts, 1997); 2. unless the walls are set into a low hydraulic conductivity unit, some streamlines will also short circuit the remediation system by travelling below the walls (Shikaze and Austrins, 1995); and, 3. groundwater may tend to mound behind the funnel walls due to the damming effects of the walls (Bowles, 1997). Bowles (1997) modified the Funnel and Gate system into a Trench and Gate system. This system consists of an impermeable funnel with the addition of a high hydraulic conductivity drainage trench along the inside edge of the funnel, and a high permeability downgradient re-infiltration gallery. Hoyne (2000) numerically modelled and visualized the two systems using three-dimensional groundwater modelling software. Hoyne (2000) showed that the Trench and Gate system had a much larger capture zone than the Funnel and Gate system. Furthermore, the modelling work also illustrated that the impermeable funnel barrier is often not required for the Trench and Gate to function effectively. The concept of the Trench and Gate approach is to use the natural groundwater flow to deliver contaminated groundwater to a treatment system, rather than impose artificial gradients through pumping. Figure 2 shows a conceptual Trench and Gate design. It is particularly well-suited to low permeability settings such as the till beneath the LPG area where active pumping is unlikely to be effective. Trench and Gate Application For this application the Trench and Gate system consisted of the following components: a permeable V shaped trench directing groundwater flow to the gate ; a free product separation gate; a dissolved phase treatment gate; and, a post-treatment re-infiltration gallery. Groundwater flow is directed through the permeable trenches to the product separation gate. This chamber is used to trap floating hydrocarbons as outflow occurs only through the bottom of the gate (Figure 2). Groundwater travels from the product separation gate to the dissolved phase treatment gate via a connector pipe, which runs beneath the two southern LPG bullets. In the dissolved phase treatment gate, an aeration system is used to sparge volatile hydrocarbons and to oxygenate the water. This provides a dual stage treatment by first

5 stripping volatile contaminants from the water and secondly promoting aerobic biodegradation of any remaining dissolved hydrocarbons. Outflow from the dissolved phase treatment gate is returned to the groundwater flow system through the re-infiltration gallery. Figure 2 : Conceptual Trench and Gate Remediation System

6 Construction The Trench and Gate system was constructed in the available open space between two sets of LPG storage bullets at the downgradient end of the free product plume. Figure 1 shows a plan view of the constructed Trench and Gate system. The bulk of the excavation was completed using a Hitachi EX 200 excavator equipped with a wrist bucket. However, space restrictions below the LPG bullets required a mini excavator be used to install the connector pipe from the collection gate to the treatment gate. The collection trench was a 58 m long, 0.9 m wide and 2 m deep. The collection trench and the re-infiltration trenches were filled with 40 mm drainage rock. Three stacked rows of 150 mm diameter weeping tile were placed along the collection trench to facilitate lateral fluid movement. Approximately 0.6 m of clay soils compacted to 95% standard proctor density (SPD) were placed over the gravel to minimize infiltration. Woven geotextile was placed over the drainage rock to separate the drainage rock from the overlain clay soils. A 30 mil very low density polyethylene synthetic liner was draped along the downgradient edge of the collection trench to provide an impermeable barrier. The collection and treatment gates were constructed of 0.9 m and 1.8 m diameter metal culverts, respectively. The aeration system installed in the treatment gate consisted of a network of 5 cm (2 inch) diameter slotted PVC pipe connected to the Plant s compressed air system. The air supply pressure is approximately 25 kpa. The re-infiltration gallery consisted of approximately 30 m of 2 m wide and 2 m deep gravel filled trench. A geotextile was placed over the surface of the gravel and a 0.6 m thick layer of compacted clay was placed over top. The Trench and Gate system was constructed in Fall The aeration system was installed in Spring Because all the components are located sub-grade, the Trench and Gate system was designed to operate continuously year round. Performance Review The performance of the Trench and Gate system was evaluated via the following two metrics: 1. product recovery volumes in the product separation gate; and, 2. groundwater quality before and after treatment in the dissolved phase treatment gate. Table 1 shows the analytical results for water samples collected from the product separation gate and the treatment gate (after treatment) on two occasions in 2001 (September and November) and in June 2002.

7 Table 1: Trench and Gate Treatment System Water Quality (Dissolved Hydrocarbon Concentrations) Monitoring Station Date Benzene Toluene Ethylbenzene Xylenes total TPH (C 3 -C 10 ) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) CDWQG* Product 24-Sep < < Separation Gate 13-Nov <0.002 < Jun < Treatment 24-Sep-01 < < < < <0.1 Gate 13-Nov-01 < < < < < Jun-02 < < < < <0.1 * Canadian Drinking Water Quality Guidelines (Health Canada, 1996). --- denotes no criteria superscript 1 denotes values exceeding CDWQG. Results indicate that the treatment gate has been effective in stripping dissolved volatile hydrocarbons from the groundwater. This treated groundwater is automatically released through the re-infiltration gallery. The Trench and Gate system was installed at the downgradient end of the plume to ensure containment of free product. During the construction program, it was observed that the plume extended almost to the location of the product separation gate. To date, no free product has been removed from the separation gate; however, a hydrocarbon sheen has been observed in the gate, suggesting the edge of the plume is advancing towards the system. Additionally, no free product has been observed in monitoring wells downgradient of the Trench and Gate system indicating the plume remains contained. Summary and Conclusion Since it began operation in 2001, the Trench and Gate system has achieved the remedial goals of preventing further expansion of free product and dissolved phase plumes and in removing dissolved hydrocarbon at the downgradient edge. Significant product has yet to enter the Trench and Gate system, which is consistent with the very slow groundwater flow rate (approximately 0.3 m/yr). However, the system is well positioned to capture future downgradient product migration.

8 References Bowles, M The Trench and Gate Groundwater Remediation System. Master of Science Thesis. Department of Geology and Geophysics University of Calgary. November, Fitts, C.R., Analytical Modelling of Impermeable and Resistant Barriers. Ground Water, Vol. 35, No. 2., March-April, pp Health Canada, Guidelines for Drinking Water Quality (6 th Edition). Cat. No. H48-10/1996E, ISBN Hoyne, W.E Three dimensional Flow and Transport Modelling of a Trench and Gate System in a Low Permeability Till. Master of Science Thesis. Department of Geology and Geophysics University of Calgary. March, Shikaze, S.B. and C.D. Austrins, A 3-D Numerical Investigation of Groundwater Flow in the Vicinity of a Funnel-and-Gate System. Proceedings of the 5 th Annual Symposium on Groundwater and Soil Remediation. Toronto, Ontario, Canada, October 2-5, Starr, R.C. and J.C. Cherry, In-situ Remediation of Contaminated Ground Water: The Funnel-and-Gate System. Ground Water, Vol. 32, No. 3, pp Komex International Ltd., Suite 100, th Ave SW, Calgary, AB, T3B 0M6 2 Confidential Employer I:\CIJS\cij148\reports\paper.doc