2014-08-06_INSPECTION - M1980183

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PERMIT #: M- 2008 -017 INSPECTOR'S INITIALS: TK INSPECTION DATE: July 31, 2014 French drains are installed outside the upgradient sides of some of the gravel pit slurry walls in the South Platte River valley for the purpose of allowing mounded groundwater to drain around the pit to an area downgradient from the pit. Mr. Einspahr's pasture land slopes gently downward several feet to a central low area. Flooding of this pasture was previously reported in 2012, according to the DRMS inspection report dated December 31, 2012. Mr. Einspahr reported the flooding has significantly reduced the use of his pasture for livestock grazing. He also described the current episode of flooding as having developed in the last few weeks, but the pasture has not been completely dry anytime this summer. Mr. Einspahr reported that approximately 2.5 inches of rain had fallen in the area the night before the inspection. During the inspection, the flooded area appeared to be between one and two acres (see attached photos). Maximum water depth appeared to be several feet in the lowest part of the pasture. Although some of the water in the pasture would have come from the rain the night before, the surface drainage area that would report to the flooded part of the pasture appeared too small for the 2.5 -inch rain to have provided much of the water standing in the pasture. The operator monitors alluvial groundwater levels in monitoring well MW -4, located near the northwest corner of the Einspahr pasture (see attached letter from the operator's consultant, Applegate Group, Inc., dated 7/10/14). From December 2010 to March 2014, water levels in MW -4 rose from about 25 feet below the top of the well's casing to 5 feet below the top of the casing. Based on the slope of the land from the well to the flooded pasture, a depth of 6 feet in MW -4 would likely be close to the same elevation of the floodwater surface in the Einspahr pasture. The similarity in elevations between alluvial groundwater in MW -4 and floodwater in the pasture indicate alluvial groundwater is a plausible source of flooding in the Einsphar pasture. The similarity in timing of flooding in the pasture (first reported in 2012), and the rise of the groundwater level in MW -4 (beginning in late 2010), further supports a conclusion that alluvial groundwater is the main source of flooding in the pasture. The completion of a slurry wall around Pit 29 in approximately 2009, and the subsequent rise in water level in MW -4 next to the slurry wall, indicate groundwater mounding occurs next to the slurry wall and is the underlying cause of flooding observed in the Einspahr pasture. The slurry wall that surrounds Pit 29 (operated by Brannan), in conjunction with slurry that surrounds the nearby Stagecoach Pit (operated by Albert Frei and Sons Inc.), can be expected to create a significant constriction in alluvial groundwater flow where the two slurry walls are only about 200 feet apart at well MW -4 (see yellow - colored lines which depict slurry walls on attached Figure 1, dated 10/28/10, from Applegate Group's water monitoring report prepared for Brannan). Of the two pits, Pit 29 is likely the greater contributor of Mr. Einspahr's flooding based on Pit 29's close proximity to the pasture. Based on the observations made during this inspection, DRMS cites the mounding of groundwater on the outside of Pit 29 as a problem, and requires as a corrective action that the operator of the pit submit to DRMS by August 20, 2014 a groundwater mounding mitigation plan. The plan must be submitted as a Technical Revision or Amendment to the permit and must include the following three items: 1. A detailed description of the measures the operator will take to reduce groundwater mounding outside Pit 29 to a level that eliminates flooding on the Einspahr property, 2. A time schedule for implementing the plan, and 3. Enough information for DRMS to estimate the cost of implementing the plan (for establishing the amount of required financial warranty). Page 3 of 5