Groundwater composition in a discrete, water-bearing fracture in granitic rock at the Underground Research Laboratory, Manitoba, was monitored during a period of underground excavation of adjacent rock to determine the impact of conventional blasting techniques and rock fracturing on the concentration of dissolved constituents. This work has relevance to the study of hydrogeochemical anomalies associated with seismic activity. Short-lived anomalies such as decreases in dissolved anion (Cl,F,Br,SO4) and gas (He,Rn) concentrations and concurrent increases in NO3 and O2 concentrations were seen shortly after two blasts located opposite the groundwater sampling site. A third blast downstream of the site resulted in variability in dissolved gases concentrations 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 hydrochemical 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.¿ 1997 American Geophysical Union