2012-06-01_REVISION - C1981012 (4)

Home Browse Search

Interoffice Memorandum The lowest reach of ditch D49 (directly above the permit boundary) includes two very steep slopes. The proposed method for controlling erosion on these slopes is shotcrete. The Division is not confident that this is the best method. NECC should provide assurance that this will work or consider other options: grouted rip rap, a slope drain with full pipe, and a slope drain with a half pipe. Some of the ditches and culverts on Map 14 for TR -65 may be drawn incorrectly. Another culvert may be needed for ditch D54 to provide access to the Soil Storage Stockpile. It is unclear if culvert C51 provides a function or if it is in the correct location on the map. The ditches near the proposed Sediment Drying Area may be drawn incorrectly: ditch D34b should not be connected to ditch D52 and ditch D34a should likely be extended west to the road that accesses Containment #2. These particular errors should be corrected and other drainage features checked by NECC and consultants for additional errors. Rule 4.05.13 With MR -116, NECC has submitted a plan to add additional surface water and groundwater monitoring points. The Division will assess this issue with our review of MR -116, and no additional information is needed for TR -65. Rule 4.10.3 It would be worthwhile to add a sediment sump below the Soil Storage Stockpile to prevent this resource from being transported down the valley. The data for the drill logs of the four West DWDA geotechnical boreholes did not indicate the presence of groundwater, and the May 1 inspection did not reveal sign of seeps or springs in the area. Furthermore, an assessment of alluvial well data in the AHR also indicates that it is unlikely that groundwater will rise to the surface (or near the surface) in the area of the proposed DWDA. This is based on the following comparison with two nearby wells: • Wells PAW -1 and PAW -2 were used for comparison. They are approximately 6,000 feet down the valley from the proposed DWDA. • These two existing wells are closer to the river than the proposed toe of the DWDA per Map 8. • An assessment of topo maps revealed that the surface elevations of these wells are significantly closer to the elevation of the river (at the nearest points to the river from each location): o The surface elevation at PAW -1 was estimated to be 37 feet above the river. o The surface elevation at PAW -2 was estimated to be 21 feet above the river. o The surface elevation at the toe of the DWDA was estimated to be 46 feet above the river. • Over a 28 -year time period (1984 — 2011) of monitoring water levels in these two wells, water levels never came closer to the surface than 5.7 feet in PAW -1 or 6 feet in PAW -2. Given the fact that alluvial groundwater did not come within five feet of the surface at these lower locations, it is highly unlikely that the groundwater will rise to an area that is higher and likely drier. If you have any questions or would like me to look at any additional items regarding TR -65, please let me know. Thanks, Rob