| Proposed Design | The Property is located within the 100-year floodplain as identified on the Flood Insurance Rate Map. The 100-year floodplain level/base flood elevation is at the level of N Marine Dr., which is north of the Property’s north boundary. All of the Property is within the 100-year floodplain elevation.
The Property is zoned Heavy Industrial and surrounding land uses are primarily industrial. The Property is 7.94 acres in size, with 2.51 acres of the north and central portions of the Property paved with asphalt concrete and occupied by two buildings. The remaining 5.43 acres are unimproved with unpaved gravel surfaces. Steel slag, which covers approximately 4 acres, was used previously as fill in some of the graveled areas.
The Oregon Department of Environmental Quality (DEQ) has required Triggco Real Estate, LLC (Triggco), the owner of the Property, to investigate and characterize the historical total chromium metal contamination that is present in the on-site slag fill and soils located in the western three-quarters of the Property’s graveled area. Based upon the detected total chromium and hexavalent chromium concentrations in the slag/soils, DEQ has required Triggco to remediate the total chromium contamination by capping all the graveled areas with asphalt. DEQ has approved the proposed asphalt capping remedy, which is a component of the planned site repairs and improvements at the Property. The proposed work will also preclude stormwater from coming in contact with the chromium contaminated slag/soil; and, is intended to eliminate/minimize past stormwater permit (NPDES 1200-COLS) benchmark exceedances at the Property. Following implementation of the Proposed Design, stormwater from the site will sheet flow over the asphalt surface and be directed to surface pretreatment ponds and vegetated swales prior to discharge to the adjacent drainage ditch.
Currently, there is no stormwater collection or treatment system for the graveled portion of the Property. Rainwater falls on the graveled parking area surface and infiltrates through the gravels to the underlying soils or the shallow groundwater, or flows downslope to a rock-filled trench along the embankment, where it may pond, until it discharges to the adjacent drainage ditch.
The Proposed Design implements the DEQ-approved remedy and includes minor grading and sloping of the graveled areas to enhance stormwater sheet flow to the new pretreatment ponds. The graveled areas will be paved with 4 to 6 inches of asphalt to provide a low permeability cap over the gravels and to eliminate stormwater contact with the underlying chromium contaminated slag/soils, as well as preventing the co-mingling of groundwater with stormwater. The Proposed Design addresses the following issues:
- Historical stormwater ponding on the Property during heavy rainfall events has resulted in limited use of the Property for truck and trailer parking by JB Hunt, the NPDES 1200-COLS stormwater permittee and current property tenant. The planned sloped asphalt surface and stormwater pretreatment ponds are expected to minimize future ponding in the operational areas of the Property.
- Under the Proposed Design, chromium-contaminated slag/soil associated with the existing on-site steel slag fill material will be isolated from the surface stormwater sheet flow by the placement of a 4 to 6 inch asphalt cap, Stormwater quality compliance issues are associated with the co-mingling of stormwater and the underlying chromium contaminated soils/slag and groundwater. As a direct result of the co-mingling, stormwater discharges from the Property frequently do not meet NPDES 1200-COLS stormwater permit discharge benchmarks for certain parameters. The Proposed Design will isolate the stormwater from the underlying soils/slag and shallow groundwater and treat the collected stormwater by providing stormwater retention and settling of total suspended solids (TSS), biofiltration, and vegetation uptake in the pretreatment ponds and swales. The isolation of the collected stormwater from the slag and underlying groundwater, coupled with on-site stormwater treatment by biofiltration and vegetation uptake is designed to resolve the stormwater discharge permit benchmark exceedances.
As noted above, Triggco is currently working with the DEQ’s Voluntary Cleanup Program to implement these measures to the address the slag/soil contamination and stormwater quality issues at the Property. The proposed repairs and asphalt cap improvements effectively isolate the contaminated slag/soils and preclude the stormwater from coming in contact with the underlying slag/soils and shallow groundwater. The asphalt cap will also prevent site workers from being exposed to the chromium-contaminated slag soils.
The Revised Design complies with the balanced cut/fill requirements of Chapter 24.50.060, F.8. The total fill volume, consisting of the asphalt cap and concrete curb/swale wall, is 3,355 cubic yards (reference Drawings C3 through C6 of the attached Stormwater Management Report). To balance the cut and fill volumes, the existing berms located along the southern and western property lines will be excavated to match the cap surface topography. The proposed pretreatment pond and swale excavation will provide additional cut volume for a calculated total cut of 3,381 cubic yards. This design results in cut volume (3,381 cubic yards), which exceeds the fill volume (3,355 cubic yards) by 26 cubic yards. Thereby, meeting the City’s balanced cut and fill compliance requirements for floodplain areas.
Cut and fill volumes were calculated using three-dimensional modeling in Autodesk 2014 Civil3D software. A three-dimensional surface file was created based on the existing topography (using dimensions and elevations obtained during a topographical survey). The existing surface file was then overlaid with a modeled surface representing the proposed cuts resulting from excavation of the existing berms on the west and southwest sides of the Site, as well as the excavation of the pre-treatment ponds and swales. The cap fill was modeled as a separate surface representing the maximum thickness of the asphalt cap (up to six inches) over the entire slag/gravel area (approximately four acres). The cut and fill volumes were then calculated using Civil 3D’s volume calculation tool.
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| Reason for alternative | Implementing the Proposed Design is necessary to address DEQ findings that “stormwater runoff is carrying the suspended chromium to the drainage from the on-site gravels/slag. Addressing these chromium detections at their source appears to be the best method of resolving this issue. …Triggco has agreed to complete a source control measure on the Property to limit additional chromium from being discharged in stormwater” .
The Proposed Design will improve the overall protection of human health and the environment by eliminating the potential health risk to on-site employees from periodic exposure to the chromium contaminated exposed slag by capping the contaminated slag with asphalt; thereby, preventing employees from coming in contact with the slag material. Similarly, the asphalt cap will preclude stormwater from coming into contact with the slag; thereby, eliminating the potential for entrainment of fine particulate matter containing chromium as TSS in the stormwater. On-site stormwater treatment by biofiltration and vegetation uptake is anticipated to resolve the stormwater discharge permit benchmark exceedances; thereby, eliminating/minimizing adverse contaminant impacts to the discharged stormwater.
Inasmuch as the revised Proposed Design meets the requirements of Chapter 24.50, Section 24.50.060, F.8 – Balanced Cut and Fill Required; Triggco respectfully requests the Administrative Appeal Board to approve the Proposed Design described above and in the attached Stormwater Management Report and issue the Permit for the work.
Thank you for your reconsideration of this appeal.
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