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- November 25, 2025
Background
Beginning in 2024, McAlister GeoScience was brought in to evaluate an approximately 106.61 acre property along Denny Creek Road in North Bend, Washington and was found to contain elevated arsenic levels in soil due to historic mining activity in the area. Previous investigations confirmed several hotspots exceeding Washington State Department of Ecology Model Toxics Control Act (MTCA) cleanup levels, and an onsite stockpile of asphalt grindings also required proper management. McAlister GeoScience was brought in to define the extent of these impacts and complete the necessary cleanup to restore the area to safe, compliant conditions.
Environmental Assessment
As part of the work completed between 2024 and 2025, McAlister GeoScience conducted both a Phase I and Phase II Environmental Site Assessment to define the extent of arsenic impacts on the property. During the Phase II investigation, soil samples were collected across the affected area and analyzed by a Washington State certified laboratory using EPA approved methods. The results confirmed several locations where arsenic exceeded MTCA cleanup levels and identified approximately 10,000 cubic feet of arsenic impacted soil and 10,000 cubic feet of asphalt grindings requiring removal.
In 2025, our team returned to complete the remediation. All impacted soil and asphalt materials were excavated and hauled to an approved disposal facility. After excavation, McAlister GeoScience used an X-ray fluorescence (XRF) analyzer to field screen the exposed areas, followed by collecting laboratory confirmation samples to verify that all remaining soils met Washington State MTCA Method A cleanup standards.
Conclusion
Following the excavation and off-site disposal of approximately 10,000 cubic feet of arsenic impacted soil and 10,000 cubic feet of asphalt grindings, all confirmation samples collected in 2025 were reported below Washington State MTCA Method A cleanup levels. With the impacted materials removed and the remaining soils meeting regulatory standards, McAlister GeoScience concluded that no further action was necessary for the property with respect to arsenic impacts.
Outcome
The remediation was completed efficiently and without complications, giving the client a clean site and clear documentation for their records. With the environmental work closed out and all standards met, the client was pleased to move forward with confidence.
Key Findings
- Arsenic:Several soil samples collected during the Phase II investigation showed arsenic levels above MTCA Cleanup Levels.
- Extent of Impacts:Elevated arsenic was limited to a defined area on the property and did not extend beyond the identified boundaries.
- Material Volume: Approximately 10,000 cubic feet of arsenic impacted soil and 10,000 cubic feet of asphalt grindings were identified for removal.
- Post-Excavation Results:After excavation, all confirmation samples showed arsenic levels below state cleanup standards, confirming that the impacted soil had been successfully removed.