A Monte-Carlo chemical budget approach to assess ambient groundwater flow in bedrock open boreholes

In low-permeability rocks, ambient groundwater flow in open boreholes may go undetected using conventional borehole-flowmeter tools and alternative approaches may be needed to identify flow. Understanding ambient flow in open boreholes is important for tracking of cross contamination in groundwater. Chlorinated volatile organic compound (CVOC) concentrations from three open boreholes set in a crystalline-rock aquifer (two of three open boreholes) and a siltstone aquifer (one of three open boreholes) were examined using a new approach and associated software program called the AFCE (Aqueous-Flow-Concentration-Estimator). The program allows comparison of coupled chemical datasets through a Monte-Carlo simulation and a chemical-budget approach to assess ambient groundwater flow in open boreholes. The coupled datasets required for the comparison include aqueous CVOC concentrations from groundwater samples from (1) discrete fractures, such as those measured from temporary deployment of straddle-borehole packer assemblies; and (2) the concentration of the open borehole (wellbore) water, as measured by a vertical profile of passive samplers from within the same open borehole. Because results from the passive samplers represent a composite mixture of the results from the discrete samples under ambient groundwater-flow conditions, potentially at unknown proportions, the comparison between coupled datasets affords the ability to discern likely water contributions of CVOC from discrete fractures (or fracture zones), and which fractures may be dominating the water chemistry of the open borehole.