2013-07-19_REVISION - M2010049 - Laserfiche WebLink

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.AWES, LLC Fort Collins, Colorado, USA June 27, 2013 Varra Companies 8120 Gage Street Frederick, Colorado 80516 Attn: Mr. Garrett Varra RE: Shadow /Mounding Analysis AWES No. 1 -3 -1320 Dear Mr. Varra: You have requested that I provide an opinion on possible effects that lining excavated gravel mines can have on local groundwater hydrology - specifically shadow and mounding effects created by the presence of a low permeability barrier. My opinion is that lining active or inactive gravel mines will deflect groundwater around the barrier, but hydraulic gradients changes at any distance away from the barrier will be insignificant. My rational for this opinion is provided below. Varra Companies, Inc., (Varra) has two lined pits used for water storage that were lined in 2003 (Pit 112) and 2010 (Pit 110). The method of lining was compacting several feet of clay and shale bedrock on the side walls of the pits, thus creating a low permeability liner. Varra installed 13 monitoring wells at each of these locations and obtained water levels on a periodic basis. Pit 110 consists of two lined pits and Pit 112 is a single pit. The locations of these pits are depicted on Figure 1. The monitoring well locations are depicted on Plates 1 through 4, which are included in Attachments A and B. In order to evaluate the effects of low permeability liners on the local groundwater hydrology, I used the geo- statistical model Surfer® to evaluate groundwater flow direction and gradients in the vicinity of the pits. I input water level data obtained from the monitoring wells for the months of January and July 2012, at each location. I chose these months to evaluate possible changes that might occur from irrigation returns in the vicinity of the pits. After the model runs were competed, I used a vector analysis to depict variation in gradient and flow direction. Model runs for Pits 112 and 110 are presented in Attachments A and B, respectively. A review of both Attachments show that groundwater is being deflected around the pits, but the changes in gradients are so minor the model also predicts that groundwater will flow through the pits as well - this indicates the liners are competent and that there is little to no hydraulic connection between the aquifer and pond. For graphical purposes, I superimposed red lines that represent the primary groundwater flow patterns in the vicinity of the pits. The vector analysis shows very minor changes in hydraulic gradients up and down stream of the pits indicating shadow and mounding are not occurring. AwU-s 4809 Four Star Court, Fort Collins, CO 80524 970 -590 -3807