Petroleum-impacted soil was encountered during a routine upgrade of petroleum storage and distribution infrastructure in 2006. During that upgrade, previously unknown underground storage tanks (USTs), and associated piping, were uncovered and subsequently removed from two locations. The excavation that followed included removal of approximately 4,500 tons of petroleum-impacted soil to a depth of approximately 13 feet. However, impacted soil extending beneath the convenience store, dispensing island/canopy supports and, around municipal utilities located within the DOT right-of-way would have been cost-prohibitive for removal.
Site investigation activities conducted subsequent to backfilling the excavation and completing the petroleum storage and distribution infrastructure upgrade indicated a subsurface consisting of glaciofluvial silty-sand and gravel as the primary water bearing unit overlain by a lower permeability mixture ranging from clay to sand. Analytical results of groundwater, encountered at approximately 5.0-to-7.0 feet below grade, indicated an area centered around the dispensing island/canopy that was characterized by total volatile organic compound concentrations in excess of 5,000 parts per billion. The total volume of petroleum-impacted groundwater within this area was estimated at 100,000 gallons.
Based on the distribution of petroleum impacts documented within soil and groundwater and, the proximity of those impacted media to site features and/or infrastructure, Aztech recommended a low-temperature form of ISCO that was able to achieve site cleanup goals and was cost-effective. This approach was based primarily on the application of sodium percarbonate, ferrous sulfate and calcium peroxide (and other reagents) to a series of strategically placed application wells.
Sodium percarbonate, when mixed with water, is highly soluble and rapidly releases hydrogen peroxide as a by-product. In the presence of a suitable catalyst (such as dissolved iron), the hydrogen peroxide can react to produce a modified Fenton’s chemical reaction. The Fenton’s chemical reaction is a highly effective method to degrade synthetic organic compounds, such as petroleum hydrocarbons and organic solvents, into carbon dioxide and water. Delivering hydrogen peroxide to the subsurface by way of dissolving sodium percarbonate in water is highly stable and safe when compared to the issues involved with handling the higher concentration liquid forms of hydrogen peroxide. As such, the reagents applied included alternating additions of ferrous sulfate followed by sodium percarbonate. This application method provided both the hydrogen peroxide and iron catalyst required to initiate controlled, low temperature Fenton’s chemistry within the subsurface. A biodegradable, citrus-based solvent was also applied in order to de-sorb and emulsify the hydrocarbons from the soil, and make them available for consumption by the ongoing Fenton’s reactions.
The final step in the ISCO application was to prolong the Fenton’s reactions by the addition of calcium peroxide to the subsurface. When applied in an environment characterized by a pH below its natural pH of 12, calcium peroxide slowly dissolves and releases hydrogen peroxide. Peroxide enrichment of the subsurface via slowly dissolving calcium peroxide (and earlier application of ferrous sulfate) provides an ongoing mechanism for Fenton’s chemistry to continue over a relatively long duration (i.e. several months).
Infrastructure to facilitate the ISCO application included using a track-mounted drill rig to collect soil cores and advance hollow stem augers at 14 strategic locations within the three areas of impacted soil at the site. This includes six permitted locations within the shoulder of a state roadway; three locations beneath the dispenser island/canopy; three locations adjacent to the convenience store and two locations within the backfilled excavation area. Petroleum hydrocarbons at each drilled location were most concentrated within a specific layer of higher permeability. As such, each PVC application well was designed to target that specific preferential flow pathway. The drilling program was conducted during normal business hours with little disruption to retail or gasoline sales.
Prior to initiating the ISCO application, baseline data was collected. This included laboratory analysis of groundwater samples, and measurement of water quality field parameters (temperature, dissolved oxygen, specific conductance, pH and, oxidation reduction potential). Additionally, composite soil and groundwater samples, representing the soil and groundwater conditions to be addressed by the ISCO approach, were collected for the purpose of bench-scale testing. This bench scale testing would help to develop a site-specific “recipe” and outline the appropriate steps for implementing the ISCO application. The ISCO application was conducted over eight business days during normal daytime hours.
Post-application observations include an increase in dissolved oxygen, specific conductance and oxidation/reduction potential in site monitoring wells. The odor of the citrus based solvent can also be detected in selected site monitoring wells. Each of these observations verify that the various reagents introduced during the ISCO application have been distributed to the targeted areas in the subsurface. Monitoring the effectiveness of the application is ongoing.