IMPACT OF BLASTING ON GROUNDWATER COMPOSITION IN A FRACTURE IN CANADAUNDERGROUND RESEARCH LABORATORY

Groundwater composition in a discrete, water-bearing fracture in grani tic lock at the Underground Research Laboratory, Manitoba, was monitor ed during a period of underground excavation of adjacent rock to. dete rmine the impact of conventional blasting techniques and rock fracturi ng on the concentration of dissolved constituents. This work has relev ance to the study of hydrogeochemical anomalies associated with seismi c activity. Short-lived anomalies such as decreases in dissolved anion (Cl, F, Br, SO4) and gas (He, Rn) concentrations and concurrent incre ases in NO3 and O-2 concentrations were seen shortly after two blasts located opposite the groundwater sampling site. A third blast downstre am of the site resulted in variability in dissolved gases concentratio ns but showed no impact on dissolved anion concentrations. The results are compared with various models used to account for hydrogeochemical fluctuations associated with earthquakes. The data best fit a general form of the aquifer breaching/fluid mixing model in which hydrochemic al responses are caused by localized changes in hydraulic conductivity along the plane of a fracture which, in turn, cause changes in mixing ratio of groundwater at the monitoring site.