Soil Management Plans

CATEGORY OF SERVICE, LOCATION:

Site Assessment, Risk Assessment and Site-Wide Soil Management Plan Preparation, Felton, California.

TYPE OF CLIENT:

Commercial Property Leasing and Management – 33 Acre Former Wood Treatment Facility.

BACKGROUND:

The Subject Property was formerly a wood treatment facility and lumber yard. Wood treatment activities occurred from approximately 1945 to 1983 under various operators and consisted of pressure treating a variety of wood products with the chemical Wood-Last, a water-based chromated copper arsenate (CCA) solution. Remedial investigations identified CCA-impacted soil at the Site, which led to the excavation and removal of impacted soil to 1986 cleanup levels. Case closure was approved by several local oversight agencies in 1988, however, no land use covenant (LUC) was established at the time of closure.

The Client engaged Trinity in early 2015 to evaluate shallow soils for arsenic prior to conducting proposed construction plans. The Santa Cruz County Department of Environmental Health (SCCEH) reopened the case, identifying the contaminants of potential concern (COPCs) as arsenic, total chromium, hexavalent chromium, copper, and formaldehyde. In 2016, Trinity developed an Interim Soil Management Plan (ISMP) for managing potentially impacted shallow soil at the site during future construction activities.

In 2019, the California Environmental Protection Agency Department of Toxic Substances (DTSC) assumed the role of regulatory oversight agency for the site via entering a Standard Voluntary Agreement (SVA) with the property manager. In coordination with the DTSC, Trinity developed a work plan to further characterize COPCs related to wood treatment, including an unauthorized release from a former diesel AST. The subject property is currently operating as a commercial/industrial space of approximately 33-acres, consisting of multiple businesses (tenants).

CLIENT GOALS FOR PROJECT:

Primary goal for this project was to delineate the presence and concentration of COPCs at the site and develop a Soil Management Plan (SMP) for potential subsurface tenant improvement projects, and ultimately to obtain a Certificate of Completion from the DTSC.

PROJECT APPROACH:

• Trinity recommended the client switch regulatory agencies from the SCCEHS to the DTSC, a regulatory agency with more resources, risk assessors and guidance for metal impacted sites.

• Trinity acted as a liaison between the Client and the DTSC to help facilitate development of an SVA agreement and develop a work plan for site-wide assessment.

• Trinity conducted risk assessment to identify arsenic as the primary COPC, showing that concentrations of copper, chromium, petroleum hydrocarbons and formaldehyde do not pose human health risks at the site.

• Trinity developed a conceptual site model (CSM) to represent the nature, extent, and fate of contaminants at the site and evaluate potential impact to human and ecological receptors.

• Trinity cost-effectively delineated arsenic impacts in shallow soil by conducting a site‑wide soil sampling event consisting of 125 shallow soil borings on a grid-like pattern, sampling in 0.5-foot depth intervals to effectively develop high resolution vertical arsenic profiles across the site.

• Trinity facilitated calculating a site-specific arsenic background soil concentration of 17 mg/kg based on historical and current site data, following DTSC guidance.

• Trinity compared arsenic (naturally occurring) concentrations in groundwater from nearby wells to concentrations reported from onsite wells, to determine if arsenic is naturally occurring or from CCA leaching from soils.

• Following DTSC guidance, Trinity facilitated a site-specific relative bioavailability of arsenic study (RBA) via in vitro bioavailability tests (IVBAs) to determine the percentage of bioavailable arsenic in site soil.

• Trinity addressed petroleum hydrocarbon data gaps to evaluate impacts related to the diesel AST release, showing that data meets the site-specific and general criteria listed in the California Regional Water Quality Control Board (CRWQCB) Low-Threat Underground Storage Tank Closure Policy (LTCP).

RESULTS, RESOLUTION, NEXT STEPS:

• Trinity’s investigations between 2015 to 2020 evaluated human health risks to receptors and identified arsenic as the primary COPC while effectively delineating vertical and horizontal arsenic impacts in soil across the 33-acre property.

• Trinity addressed petroleum hydrocarbon data gaps related to the diesel AST release and evaluated human and environmental risks based on the LTCP guidance, concluding that remaining impacts pose a low threat to human health, safety and the environment.

• IVBA results determined a bioavailability of arsenic in soil of approximately 33%, which, compared to the default of 66% used in DTSC risk assessment, resulted in RBA-modified arsenic soil screening level.

• Trinity demonstrated that former wood treatment activities at the site have not impacted local groundwater, showing that arsenic concentrations in local groundwater are similar to regional groundwater, citing specific stratigraphic formations underlying the site that are known to contribute to naturally occurring arsenic levels in regional aquifers.

• Develop a cost-effective remedial action or mitigation plan with the Client to protect receptors from potential exposure, likely an asphalt cap. 

• Amend the ISMP and develop a new SMP under DTSC oversight to aid the client in future construction planning and land use.

• Submit all necessary documents per the DTCS SVA, including a Response Plan for public review, long-term Operation and Management (O&M) plan of engineering controls, Financial Assurance Cost Estimate, LUC and O&M Agreement, and Response Action Completion Report.

• Obtain a Certificate of Completion from DTSC.