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Groundwater in the area occurs in both the sedimentary rocks and in the alluvial aquifer of the St. Vrain Creek. Groundwater levels in the sedimentary strata are well below the bottom of C-Pit. Ponded water currently in C-Pit is from surface runoff, collected precipitation, Boulder Feeder Canal seepage, and runoff from the sprinkler system. Southdown drilled two water wells immediately east of the permit area and had to advance the holes 800 feet or more to encounter a useable water supply. The exposed highwalls of C-Pit are dry except for seepage in response to rainfall or snowmelt, or from ditch seepage. The direction of groundwater flow in the sedimentary strata is to the east, primarily along bedding planes. The western highwall of C-Pit was dry this year, before the water began to flow in the Boulder Feeder Canal. About 3 days after the Canal started flowing, seepage appeared in the C-Pit highwall. This fact indicates that roundwat r is o serve seeps. figures A, 36 and 3C show a sequence of photographs of the western highwall of C-Pit before and after the Canal started flowing. There is no apparent hydraulic connection between C-Pit and the alluvial aquifer. The alluvial aquifer of St. Vrain Creek is in contact with the sedimentary strata east of C-Pit but not in contact with C-Pit itself. Groundwater in the alluvial aquifer is typically 5-8 feet below ground surface and flows east-southeast, generally towards St. Vrain Creek. Further evidence of the lack of potential for leachate generation from the disposed CKD is provided by an analysis of the pH of the dust control water and 1 precipitation runoff that collects along the C-Pit highwall downgradient of the disposed CKD. The water that collects along the highwall is either in direct contract with CKD or is dust control water or precipitation that fell on the CKD and drained to this low area. If the CKD were not hydrated in its disposed state, the runoff water could be expected to have a much higher pH than natural waters that may occur in the area. In order to determine the potential impact of the disposed CKD on area water quality, field pH measurements were made on April 7, 1999 at the edge of the water that had collected along the C-Pit hghwall. The pH measurements were taken in the water at approximate 100-foot intervals from the northeast to the southwest end of C-Pit. The water depth varied between about 6 inches to one ' foot at the measuring stations. The pH measurements ranged between 8.34 and 8.50 standard pH units. These pH values are similar to the 8.04 field pH measured at A-Pit. The results of the C-Pit pH survey suggest that the quality of water that contacts and runs off the disposed CKD is similar to the natural water quality in the area and that the disposed CKD does not pose a significant threat to groundwater quality. r ,3