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~ ~ ~ri1C Mr. Scott Gudahl January 28, 1997 Page 2 constructed parallel to St. Vrain Creek and the South Branch. Most of the proposed gravel lakes will be lined. Plans are to construct two lined lakes west of the cement plant and two lined lakes at the southeast edge of the project near 61st Street. Four gravel lakes are currently present north of the South Branch. Three of these existing lakes are unlined. Potential impacts to wetlands and nearby wells can be mitigated utilizing engineered means such as unlined ditches and/or infiltration galleries to maintain the water table in these areas. EXISTING MONITORING WELL NETWORK Western Mobile maintains a network of 37 monitoring wells located within the proposed mine (in areas of current and future mining) and on neighboring properties. Additionally, staff gauges are present in surface water features located near the wells. The locations of these monitoring wells and staff gauges are shown on Plate 1. The purpose of these wells is to document groundwater levels, surface/groundwater interaction, and general groundwater quality in the alluvial aquifer at and near the mine site. Wells PZ-1 through PZ-25 were installed in early 199A. Wells PZ-26 and PZ-27 were installed in December, 1995, and wells PZ-28 through PZ-37 were constructed in October and December, 1996. Groundwater level and staff gauge measurements are performed monthly in all 37 wells. Groundwater samples from wells PZ-7, PZ-11, PZ-12, PZ-16, PZ-23, PZ- 26, and PZ-27 are analyzed for general groundwater quality parameters semi-annually. In addition, two domestic wells (the Zapf and Darby wells) located approximately 2,500 to 3,000 feet east and downgradient of the project are also sampled and analyzed for general groundwater quality semi- annually. Results of the laboratory analyses indicate total coliforms are commonly detected in groundwater above the human health standard. In addition, secondary drinking water standards for dissolved manganese and dissolved iron are commonly exceeded, and secondary standards for total sulfate are locally exceeded. These conditions are typical for shallow alluvial groundwater. A contour map of groundwater elevations measured on December 12, 1996 is included on Plate 1. NEIGHBORING WELLS State records indicate 23 wells (excluding those permitted to Western Mobile) are permitted on or within 500 feet of the project (Western Mobile, 1996). Five of these permits appear to be monitoring wells permitted to a previous aggregate mining company (Frontier/Gregory) in the east half of Section 27, Township 3 North, Range 70 West. Three of the wells are also permitted to Frontier/Gregory and may be permits for unlined ponds in Section 27. The remaining 15 permitted wells appear to be water sources for neighbors of the project. The location of many of these wells is known only to the quarter-quarter section. The depths and construction details of the wells are unknown. However, based on the approximate locations of the wells and the lack of a major bedrock aquifer in the area, these wells are most likely constructed in the valley alluvium. It is our understanding that Western Mobile is in the process of performing an inventory to determine well locations, depths, construction details, and yields in order to identify wells potentially impacted by the project. Based on our experience in the area, these alluvial wells are typically shallow, hand- excavated, and of low efficiency (slow recharge rate). Inefficient wells are particularly susceptible to minor water level fluctuations that can be caused by numerous factors such as changes in precipitation, irrigation patterns, or mining.