Acidic and metal-laden drainage from the portals of abandoned mines is a serious environmental problem in wet, mountainous regions where steep hydraulic gradients ensure that fresh recharge waters are continuously flushed through the workings. Hydrogeological characterization of these sites is hampered by their geological complexity and their frequent inaccessibility. This paper presents a material balance model for the groundwater capture zone associated with influx to a mountainside adit. The model can be used to estimate bulk hydraulic properties of the rock surrounding an adit from hydrograph data commonly recorded at its portal. This application is illustrated using data from the Myra Falls mining camp, Vancouver Island, British Columbia. By simultaneously fitting base flow recession slopes for hydrographs of discharge, water table elevation and head in an underground borehole, the model yields plausible and well-constrained estimates of hydraulic conductivity, porosity, skin factor, and hydraulic gradient.