2025-03-11-minutes-solid-waste-and-maintenance

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Ityegraiss LPL and Ryegrass Balehll Landfill Upgradeent Groundwater Investigation Maim County Solid Wow feet below the Facility occurring above the Vantage Interbed. The Vantage interbed near the Facility has a slight westerly dip related to historical folding and faulting of the basin. 3.5 Groundwater Gradient The gradient of the Shallow Aquifer was observed to be southwesterly at a rate of 0.032 to 0.067 feet per foot, or 167 to 354 feet per mile. Both the First and Third Quarters of 2023 show a southwesterly gradient across the Facility. However, there is a strong westerly gradient between the LPL and Clerf #4. The Clerf #4 elevation is approximately 2,350 feet above mean sea level (ANISL) and is a surficial expression of the Shallow Aquifer. Poison Spring, Upper Clerf, Lower Clerf, and Clerf #3 are similarly surficial expressions of the Shallow Aquifer and elevations are representative of the water table elevation. Potentlometric maps for the First and Third Quarter 2023 events are included In Appendix A. 3.6 Groundwater Flow Velocity HWA (2004) identified the horizontal hydraulic conductivity at well 137 to be 1.3 x 10-4 cm/sec, or 0.37 feet per day based upon drawdown data. HWA estimated the average effective porosity to be 25%; however, calculations in this report used a more conservative value of 15%. Based an these measurements and the gradient ranging from 0,032 to 0.067 in 2023, the estimated horizontal flow velocity ranges from 0.08 to 0.16 feet per day, or 29 to 60 feet per year. From the groundwater flow velocities, the minimum horizontal travel time from the LPL to Clerf #4 is 60 years. 3.6.1 Evidence of Upgradient Sources As discussed in the 2023 Annual Report (Parametrix 2024) the LPL Was first developed in 1996. In 2004, 87 was installed just southwest and downgradient of the LPL. By 2005 and 2006, nitrate concentrations at B7 were already 20 to 40 mg/L. The minimum horizontal travel time from the LPL to B7 is 6 years. This would require vertical travel times through 70 to 185 feet of basalt to be within 4.5 years for the LPL to be the source of the nitrates. These travel time requirements are further evidence of a potential upgradient source of nitrates in B7 other Than the LPL. 3.7 Cross Sections Two hydrogeologic cross sections, A -A' (West to East) and B-B' (North to South), were developed to assist with evaluatingthe target drilling depth for the new monitoring well with respect to the subsurface geology below the Facility (Figure 2). These are attached as Figures 4 and 5. Cross Section A -A' (Figure 4) is very similar to the cross section developed by HWA (HWA 2004) when the Ryegrass CDL was updated to an LPL under WAC 173-340-400.The cross section trends west - east from Clerf #4 Spring through B7, the LPL, and the proposed new upgradient well B8. Cross Section B-B' (Figure 5) trends from north to south from the proposed new well BS through the Balefrll Landfill, wells B2 and 64, to just beyond Upper Clerf Springs. As shown on the cross sections, the proposed new upgradient monitoring well B8 would be drilled from an elevation of approximately 2650 AIVISL to the bottom of the Shallow Aquifer near elevation 2460 AMSL, or approximately 190 feet bgs. The well would be screened in the pillow basa Its, palagonite, and vesicular basalt of the flow bottom zone of the Wanapum Basalt (Gingko Flow): April 2024 1553-1763-01-0 Revised May 2024