Neutralization of acid runoff by groundwater discharge to streams in Canadian Precambrian Shield watersheds

Abstract

Application of environmental isotope techniques during storm and snowmelt runoff from the Harp Lake and Turkey Lakes watersheds on the Precambrian Shield of central Ontario, Canada, indicates that subsurface pre-event water is a major component of the runoff hydrograph. Storm runoff from two sub-basins in the Harp Lake watershed is comprised of 40–90% groundwater, depending on rainfall amount and intensity, and antecedent soil-moisture conditions and water-table depth. Because the alkalinity of the groundwater is large compared to the acid precipitation (pH 3.8–4.5) characteristic of this region, groundwater discharge results in significant neutralization of runoff acidity. Snowmelt-runoff studies in the Turkey Lakes watershed indicate that much of the first meltwater from the acidic (pH ≈ 4.5) snowpack infiltrates the subsurface, displacing higher-pH groundwater to the stream. Since the first meltwater from an ablating snowpack is more acidic than the snow itself, meltwater infiltration limits the extent of stream acidification during snowmelt runoff. Hence, the physical displacement of near-neutral-pH pre-event subsurface water during storm- and snowmelt-runoff periods partially buffers the alkalinity of baseflow. As long as the acid neutralization capacity of the geologic materials storing the subsurface water is not exhausted, groundwater contributions to runoff will continue to neutralize acid runoff.