GROUNDWATER AND WETLAND CONTRIBUTIONS TO STREAM ACIDIFICATION - AN ISOTOPIC ANALYSIS

Stream water pH may be influenced by (1) the flow paths and (2) the residence time of water that contributes to streamflow, when these hydrologic factors interact with the biogeochemical processes that neutralize H+ ions in the catchment. This paper presents measures of the volumes of groundwater contributing to streamflow, the groundwater residence times, and the sources of stream water acidity found during spring runoff in three basins on the Canadian Shield. Isotopic hydrograph separations were used to estimate the relative contributions of groundwater to spring runoff. The contributions of old (premelt) groundwater to spring runoff were greater (60%) in a well‐buffered, third‐order basin than in a more acidic first‐order basin (49%). Using a simple mixing model, a larger groundwater reservoir (420 mm unit depth) and longer residence time (162 days) were estimated in the third‐order basin. The lowest stream pH (4.8) was observed in a second‐order basin with a wetland that collects drainage from about 79% of the basin. In this basin the principal source of H+ ions was the conifer‐sphagnum wetland. We conclude that the hypotheses that the pH of these streams was proportional to (1) a fraction of streamflow contributed by groundwater or (2) the residence time of water in a basin are rejected. More attention must be focused upon the source of acidity generated in wetlands, since these are ubiquitous in small basins. Copyright 1990 by the American Geophysical Union.