2012-12-10_REVISION - M2004009 (9)

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2.0 GROUNDWATER MOUNDING / SHADOWING 2.1 BACKGROUND Due to the low permeability of the backfilled shale material, it is possible that up- gradient groundwater levels may rise to levels that could impact adjoining property owners to the east. The Schell house occurs approximately 250ft up- gradient of the backfill boundary and the Timmreck house occurs approximately 183ft up- gradient of the backfill boundary. The Andreason domestic well occurs west of the Cell 2C backfill area, however is unlikely to be affected by a groundwater shadow since the groundwater flow vector is northwest (See Figure 1). Currently, pumping is occurring in Cell 2A and therefore establishment of baseline water levels cannot be achieved by measuring the monitoring well presently or in the near future at this location. Instead, this plan will piece together some historic and published data to approximate baseline water levels. 2.2 HISTORIC /BASELINE GROUNDWATER LEVELS / MOUNDING ANALYSIS Previously, a portion of Cell 2 A had been allowed to fill with groundwater due to shutdown of the pumps. On July 14, 2012 the water level of the pond in Cell 2 A was surveyed at 4938.32ft. July would normally be considered a time when the seasonal groundwater levels would be expected to be near their maximum. This is due to the fact that the local groundwater levels are strongly influenced by regional irrigation and stages of the local ditches. Irrigation ditch diversions in the area typically peak in July and tail off in August -Oct. It should be noted however, that the summer of 2012 was considered a drought year. Additional baseline groundwater levels were measured during a subsurface investigation that was conducted by Rocky Mountain Consultants (RMC) in October 1999. Elevations are given in Table 1. For the purpose of estimating historic seasonal high groundwater levels, the July 2012 pond water level in Cell 2 A ( "July Pond ") is used to back calculate up- gradient water levels based on the published regional water table gradient given in the USGS Open File Report 02 -338. The eastern edge of the July Pond in Cell 2 A occurred 675 ft down - gradient from the observation well on Schell's property (108A). Multiplying that distance by a gradient of 0.0031 ft/ft (USGS OFR 02 -338) and adding to the July Pond elevation, yields a water level of 4940.41ft. This elevation compares reasonably well with the observed October 2011 water level of 4938.51 ft. at well 108A considering that October groundwater levels would be expected to be a few feet lower than July. The October 1999 RMC test hole TH -39 occurs 202.55 ft up- gradient of the July Pond. Multiplying that distance by a gradient of 0.0031 ft/ft (USGS OFR 02 -338) and adding to the July Pond elevation yields a water level of 4938.95 ft. This elevation compares reasonably well with the observed October 1999 water level of 4937.5 ft ( +1.44 ft difference) of TH -39 considering that October groundwater levels would be expected to be a few feet lower than July. Page 4 of 6