3800 RIVERFRONT BLVD 2025-07-23 - Laserfiche WebLink

1 Consent Decree Exhibit C 2. Method PQL, OR 3. Accepted background concentrations if higher than the lowest (most stringent) cleanup level determined via #1, above. Since the highest beneficial use of Site groundwater is proposed to be the protection of surface water quality, the proposed point of compliance for the deep aquifer is located: 1. Downgradient of the landfill, between the landfill and the point of discharge into the Snohomish River, 2. Outside the boundary of landfilled materials, 3. No further than 100' east of the most easterly existing railroad tracks, and 4. Within property able to be restricted by institutional controls under the Consent Decree (property controlled by the City or BNSF). Appropriate institutional controls prohibiting the withdrawal of groundwater for domestic water supply are included to ensure that if existing regulations change, the restriction will remain with the Site in perpetuity. 3.2.3 Nature and Extent of Groundwater Contamination Groundwater samples collected from the Everett Landfill/Tire Fire Site indicate that water quality in the deep aquifer is generally in compliance with cleanup standards. This condition is evident in data from both the earlier and recent site investigations. Evaluation Monitoring as described in the Compliance Monitoring and Contingency Plan (CMCP, attachment CAP-2) will utilize consistent sampling procedures and an improved monitoring well network in order to confirm conditions in the deep aquifer. Shallow aquifer data do not indicate that widespread contamination exists in the leachate. See Tables 3-2 and 3-3 for contaminants detected in the deep and shallow aquifers. Figure 3-2 shows the existing groundwater monitoring well network. All groundwater data collected to date support the conclusion that there is a low risk of shallow groundwater impacting the deep aquifer above the cleanup levels even at a significant distance upgradient of the compliance point. Reasons for this conclusion are as follows: • High natural organic content of shallow aquifer material, including significant quantities of peat, enhances degradation of contaminants. • Horizontal flow in the shallow aquifer is two to four orders of magnitude greater than potential vertical flow through the aquitard. • Low groundwater flow gradients and low permeability sediments present in both aquifers allow time for chemical breakdown reactions to occur. • The leachate collection trench, installed as an interim action under an Ecology Enforcement Order, acts as a hydraulic barrier for the shallow aquifer preventing flow of leachate to the east. • The western one-third of the landfill overlies natural glacial silt and sand soil that are not considered aquitard soils (i.e., the clayey silt or organic soil). Refuse has been placed in this western one-third of the landfill and has generated leachate (based on observed conditions during recent drilling). Monitoring Wells MW -2, -4, -10, and -15 were completed in this part of the landfill where the aquitard is absent. Water quality FINAL - March 2001 3-4 Everett Landfill Tire Fire Site Cleanup Action Plan r� 1 1 1 1 1 1 1 1 1 1 1 L 1 n J' 1