Mitigation of Soil Vapor Intrusion Through Application of Zero-Valent Iron & Emulsified Vegetable Oil

Transcription

1 Mitigation of Soil Vapor Intrusion Through Application of Zero-Valent Iron & Emulsified Vegetable Oil Keith B. Rapp (, Inc., Minneapolis, MN), Clint Bickmore (OnMaterials, LLC, Escondido, CA) William Newman (Remediation & Natural Attenuation Services, Inc., Brooklyn Center, MN) Kristin Colberg (H.B. Fuller Company, St. Paul, MN) William Anthony (, Inc., Joplin, MO), Robert Wojciak (APECS Environmental Consultants, Piney Flats, TN) ABSTRACT: The goal of this investigation and remediation program were to reduce, and eliminate the threat of volatile organic vapors emanating from the soil and groundwater environment and posing a risk to human health and the environment at a chemical manufacturing plant in upstate New York. The chemicals below the manufacturing plant represent legacy contamination, unrelated to current operations. Soil vapor intrusion (SVI) can occur in buildings and structures when volatile organic vapors are present in the subsurface directly below or next the foundation of a building. Several years of subsurface investigations established the hydrogeologic and contaminant transport conditions at this Site, indicating shallow groundwater conditions and one (1), primary source area contributing volatile organic compounds (VOCs) to soil vapor beneath the building. Exposure to volatile chemicals due to vapor intrusion does not necessarily mean that adverse health effects will occur, but regulatory guidelines have established guidance criteria for chemical exposures. A baseline SVI sampling program in sub-slab and indoor air exhibited elevated VOCs in both media. A corrective action program was designed to reduce and eliminate the VOCs in soil and groundwater environment, utilizing emulsified vegetable oil (EVO) and nano-scale zero valent iron (nzvi) structured particles injected into the subsurface sediments, thus reducing the potential for SVI into the manufacturing plant. INTRODUCTION Attention to potential SVI sites is an emerging issue across the US, however, it wasn t until work by the Massachusetts Department of Environmental Protection (MDEP) in the 1990s allowed regulators to begin to understand the significance of SVI (ITRC, January 2006). Annually, SVI has progressively become a more significant environmental issue for regulators, industry leaders, and concerned residents, as information, data, and exposure assessments become more widespread and understood. Vapor intrusion requires three components: 1) a source; 2) a building or enclosed structure; and, 3) a pathway from the source to the inhabitants. Occasionally, vapors may accumulate in structures at levels that pose immediate safety hazards (explosion), or present acute health effects or aesthetic problems (odors), however, most SVI cases reveal chemical concentrations are at low or non-detectable levels (EPA, November 2002). In residences with low concentrations one of the primary considerations is whether the chemical exposures pose an unacceptable risk for chronic health effects due to long-term VOC exposures at low levels. Depending on site conditions, constituent levels and monitoring, mitigation steps are often taken to reduce or eliminate exposures to soil vapors, including sealing cracks in the building foundation, adjusting the buildings heating, ventilation and air-conditioning (HVAC) system to maintain a positive pressure to prevent infiltration of subsurface vapors, installing a sub-slab depressurization system beneath a building, or reducing the level of VOCs in the soil and groundwater environment. 1

2 The discovery in May 1996 of VOCs in soil and groundwater at a chemical manufacturer of food-grade cleaning and dairy industry sanitizers in upstate New York (Figure 1) led to the investigation and implementation of Interim Remedial Measures (IRM) in October 2008 for an in-situ, enhanced reductive dechlorination (ERD) bioremediation program. This IRM was designed to reduce and eliminate VOCs in the soil and groundwater at the Site, and thus to address SVI concerns inside the chemical plant. A series of subsurface investigations revealed soil, soil vapor, groundwater, and surface water contamination emanating from shallow soil source areas impacted with tetrachloroethene (PCE), trichloroethene (TCE), 1,1,1-trichloroethane (111TCA), and the degradation products 1,1-dichloroethene (11DCE), 1,1- dichloroethane (11DCA), cis-1,2-dichloroethene (cdce), trans-1,2-dichloroethene (tdce), vinyl chloride (VC), chloroform, and non-chlorinated VOCs including methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), methyl blue active substances (MBAS), benzene, toluene, ethyl benzene, and xylene (BTEX) compounds. Due to the age of the apparent releases, significant breakdown products including 1,1-dichloroethene, 1,1-dichloroethane, cis-1,2-dichloroethene, trans-1,2-dichloroethene, vinyl chloride, and chloroform have been detected. This corrective action program describes the technologies and methods used to detect and monitor SVI in the manufacturing plant, and the means to reduce and eliminate VOCs in the subsurface through the injection of ERD amendments to effectively reduce the risk of chemical exposure from VOCs for workers in the plant. FIGURE 1. USGS Topographic map showing project location. Site Description. A complex operation and manufacturing history accompanies this property, which was developed in 1910 with the construction of part of the existing 33,700 ft 2 (3,130 m 2 ) manufacturing plant, with further additions and improvements completed in the 1930s and 1950s. Past manufacturing history includes a boiler factory and circuit board manufacturing operations until 1979, when the Site was sold to a manufacturer of food grade cleaners and sanitizers used in dairy and related industries. The Site is a 2.7- acre (1.1-hectare) strip of land containing the chemical plant, which was built in stages, slab on-grade. Due to the age of the plant, sumps, underground utilities, foundation cracks, pressure changes, and other means have allowed vapors to migrate into the building. This property also has an accompanying truck loading and unloading areas, parking lots, and an above ground storage tank (AST) containment area. The Site is approximately 0.85-miles (1.36-kilometers) northwest of Seneca Lake in Ontario County, New York. Beneath the plant, the Site is underlain by several unconsolidated units of varying composition and thickness, typical of the gently undulating glacial topography in the Finger Lakes Region of upstate New York. The near surface sediments range between 0 and 4 feet bgs, consisting of a layer of mixed fill (gravel to silt), underlain by native silt and clays ranging in thickness from 1 to 8 feet thick. Sand and sandy loam underlie this unit, ranging from 2 to 10 feet thick, variable in composition, and interbedded laterally with discontinuous layers common of a deltaic or fluvio-lacustrine depositional environment, followed by a confining clay and silty clay between 9 and 14 feet bgs. No soil or groundwater receptors provide a completed exposure pathway, and the surficial glacial sediments do not comprise an aquifer. Limestone and dolostone bedrock underlies the glacial sediments. 2

3 FIGURE 2. West-East cross section of Site illustrating geologic profile. METHODS AND MATERIALS In 2007, a baseline sampling program was initiated to establish the SVI conditions at four (4) sub-slab points through the plant floor, co-located with four (4) points inside the plant. The air and vapor samples were collected in clean, laboratory certified, six (6) liter Summa canisters, and analyzed for TO-15 volatiles. Helium tracer gas was used in a clean, closed container to establish no airflow between indoor air and the sub-slab sample probes. The samples were collected with clean, laboratory supplied flow controllers over a period of approximately eight (8) hours, at an estimated flow rate of approximately 10 ml/minute. Sub-Slab sampling locations consist of stainless steel probes extending below the plant floor and sealed-off from the plant interior to ensure no passage of air\vapor between the building and soil. The indoor samples are collected generally 3 to 4-feet above the sub-slab sampling locations. The sampling locations were based on known VOC source areas in the subsurface soil and groundwater environment. The initial IRM strategy for the injection program was to inject fluids into the subsurface from the bottom of the boring and inject as the rods were extracted upwards. One-hundred six (106) injection points were established, whereas the injection strategy was modified from location to location due to subsurface pressures, notice of ERD amendments achieving the required radius of influence (ROI), permeability changes, ejection of ERD amendments at the surface, and other factors. After the requisite ERD amendments were injected into the more permeable zones the rods were either advanced or retracted and additional fluids were injected into the formation above or below the initial injection intervals. High pressure piston pumps injected the ERD amendments below the fracture pressure of the glacial sediments. This provided good vertical and lateral coverage of the ERD amendments, and potentiometric data shows that laterally the radius of influence (ROI) was greater than the 7.5-foot design ROI. 3

4 The injection tool was a Geoprobe Model 6610DT and a GP300 piston pump, utilizing municipal potable water supply and distributed to multiple injection points via a proportional metering system. Flooding techniques implemented during the injection program ensured treatment in both the saturated and unsaturated zone. The primary source for VOCs in soil and groundwater is the MW-118 area, north and adjacent to the manufacturing plant, as depicted on Figure 3. FIGURE 3. Trichloroet hene concentrati ons in groundwate r in August 2008, prior to injection program. The EVO used in the IRM was Newman Zone, consisting of a non-anionic solution of stable, sub-micron droplets that are small and uniform in size containing fast- and slow-release electron donors. The EVO contains a proportion of sodium lactate in solution which is used to immediately stimulate microbial growth within hours of injection and rapidly produce anaerobic conditions at the injection site. As the sodium lactate is depleted by microbes, the slow fermentation of the vegetable oil provides a continuous supply of hydrogen and volatile fatty acids (VFAs) in the ERD zone. The hydrogen and VFAs support anaerobic microbial activity for an extended period of time, often years after injection. The soy oil is held in suspension as sub-micron droplets, which remains stable during injection (even under pressure), and does not degrade until the stabilizing additives (primarily surfactants) have been consumed by microbes. The EVO serves as a concentrated source of electron donors, with an electron equivalent dose for the lactate, soybean oil, and food grade additives over 175 electron equivalents per kilogram. The nzvi structured particles in the injection was Z-Loy, from OnMaterials LLC, which is packaged as a concentrated non-aqueous suspension. The Z-Loy nzvi contains discrete, sub-micrometer and nanocrystalline particles as multi-particle aggregates of smaller nano-particles. The nzvi is comprised of discrete, irregular sub-micrometer particles with a median particle size of about 200 nm and a typical surface area of 15 m 2 /g. The multi-particle material limits aggregation and allows for greater subsurface mobility and subsurface product placement. The multiple sized particles reduce the potential for aggradation and precipitation of the iron in the subsurface. Pre-injection microbial analysis indicated a moderate population of dechlorinating bacteria, including Dehalococcoides, Desulfuromonas, Dehalobacter, and Desulfitobacterium. RESULTS AND DISCUSSION The IRM injection program provided for an infusion of total organic carbon (TOC) into the groundwater flow system, which coupled with the nzvi reduced the subsurface environment anaerobically, provided a food-source for the indigenous biological community. The injection techniques employed distributed the ERD amendments to both the saturated zone, and because of the injection probe extraction techniques, the vadose zone was also saturated (flooded) to the ground surface, providing ERD 4

5 amendments also to the source zone soils as well. The injection program was conducted outside-in so that contaminated groundwater was not driven off-site by the IRM program. Five (5) rounds of groundwater sampling have established the destruction of chlorinated VOCs in the source area soils and groundwater at monitoring well MW-118, as noted on Table 1. TABLE 1. Groundwater Concentrations for Primary Chlorinated VOCs at MW-118 Date PCE (ug/l) TCE (ug/l) 111TCA (ug/l) MW-118 1/12/06 4,900 2,700 3,900 MW-118 9/26/07 3,000 1,100 7,700 MW-118 8/4/08 3,200 1,500 3,000 MW /18/ MW-118 4/29/09 < ,700 MW-118 8/4/09 < 10 < 10 2,300 MW-118 4/15/10 < 25 < 25 1,100 MW-118 8/10/10 < 10 < MW /18/ injection The IRM groundwater monitoring program has established continued anaerobic conditions throughout the area of groundwater impacted with VOCs, more than 2-years post injection. Initial rebound was noted in 111TCA concentrations, followed by further reductions in groundwater as the result of soil flushing and mobilization of VOCs into groundwater. Following the IRM injection program, SVI sampling was conducted to determine the effectiveness of the ERD program on soil vapor in sub-slab (SS) probes and indoor air (IA) in the manufacturing plant. The sampling location with the highest VOCs below the manufacturing plant was SS-3, and the sampling location with the highest VOCs in indoor air was IA-4, as noted in Table 2. TABLE 2. Summary Table of SVI Locations with Highest VOC Levels 111TCA Carbon Tetrachloride PCE TCE SS-3 3/27/ , /31/08 < /20/10 60 < IA-4 3/27/ /31/ < /20/ all results in ug/m 3 5

6 Post injection, the PCE, TCE, and 111TCA concentrations dropped more than 50%, and carbon tetrachloride (CT) concentrations decreased below the laboratory method detection limit (MDL). Correspondingly, the indoor air results decreased for CT, and remained relatively constant for the other chlorinated VOCs, to soil vapor concentrations below guidance levels, indicating further vapor mitigation actions are unnecessary to be protective of human health for workers in the manufacturing plant. CONCLUSIONS The injection program efficiently and effectively distributed the EVO and nzvi fluids to both the saturated and unsaturated zone, resulting in reductions in VOCs in groundwater within 3- months, with some VOCs decreasing by as much as an order of magnitude under pre-injection levels. FIGURE 4. Tetrachloroethene concentrations in groundwater (November 2010) and Sub-Slab probes (December 2010), following the IRM injection program. The high pressure flooding techniques produced ROI greater than 10 feet from each injection point, resulting in overlapping injection amendments. The injection techniques brought ERD amendments vertically to the surface, introducing oxidizing compounds to VOCs in soil. Continued groundwater monitoring in the ERD zone shows that reductive dechlorination daughter products are the only remaining VOCs detected in groundwater at monitoring well MW-118. Follow-up SVI sampling in December 2010 confirmed the continued destruction of VOCs in soil and soil vapor to below guidance levels, indicating vapor mitigation is unnecessary to be protective of human health. 6

7 REFERENCES Interstate Technology & Regulatory Council, January Vapor Intrusion Pathway: A Practical Guideline. January Prepared by The Interstate Technology & Regulatory Council - Vapor Intrusion Team, 172 pages. U.S. Environmental Protection Agency, November OSWER Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils (Subsurface Vapor Intrusion Guidance). November EPA530-D pages th Avenue North Minneapolis, MN Tel: Fax: