Clear Creek Associates completed a hydrogeological assessment of an aggregate mining operation located in Fairfax County, Virginia. The plant consists of two mine pits with operating levels approximately 350 feet below land surface. Aggregate consisting of diabase and hornfels (contact metamorphosed siltstone) are mined and processed at the facility. The purpose of the study was to evaluate geologic and hydrogeologic conditions in the vicinity of the mine, focusing specifically on the effect of mine operations on the local aquifer. Clear Creek Associates completed an analysis that relied on the following tasks:

Field Investigation – Clear Creek geologists completed a field investigation that relied on geologic mapping, water level monitoring, and water quality sampling. The geologic mapping provided key information pertaining to the placement of key geologic units and the relationship between the occurrence and flow of groundwater within the local (near pit) system. Measurements collected during water level and quality monitoring were compiled and used to augment the records review.

Well Inventory/Records Review – Clear Creek developed an inventory of all wells located within 10,000 feet of the mine.  The inventory relied on driller logs provided by the County. The inventory included the following information: well location, date drilled, well depth, water level (at time of completion), lithology, and pump setting. A well inventory map was generated using the well location information, and map series overlays were developed using GIS to present key information, including total depth and water levels. Hydrogeologic cross sections were prepared using the lithologic data from the drill logs and the results of the in-pit geologic mapping.

Analysis – Clear Creek Associates completed an analysis using the information collected during the field investigation and the well inventory. The analysis addressed the study objectives by presenting an interpretation on the hydraulic effects of mining on the local aquifer system. The mine owner is using the results of the analysis to develop future mine plans.