Groundwater and Wetland Contributions to Stream Acidification: An Isotopic Analysis

Abstract

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.Weis, C., Streamflow generation in acidified headwater baSih s during spring runoff: An isotopic and geochemical approach, M.S. thesis, 264 pp., Trent Univ., Peterborough, Ont., Canada, 1989. Weis, C., and K. Devito, Tracing of flow paths in a conifer swamp, paper presented at

Geochemical and hydrologic controls on the composition of surface water in a high‐elevation basin. Sierra Nevada, California

“…Episodic acidification occurs at present deposition levels. Furthermore, the susceptibility of catchments to acidification increases as the hydrologic residence time decreases during snowpack runoff (Bottomley et al 1986;Wels et al 1990). Water held in storage before snowpack runoff Provides onlv a small fraction of stream flow during runoff, in contrast to montane areas of eastern North America.…”

“…Pre-event water at the ELW contributes only a small fraction of stream flow during the period of snowpack runoff. The use of a single Cp for hydrograph separation of stream flow during runoff generally underestimates the contribution of groundwater to stream flow in basins with small groundwater reservoirs (Wels et al 1990). Our results suggest that much snowpack runoff in the ELW infiltrates soils and unconsolidated materials, undergoes reactions with soil water and soil exchangers, and then is discharged to stream flow.…”

“…We made the assumption that the Si content of the soil solution and groundwater were the same (e.g. Wels et al 1990). The concentrations of Si used for Cp were calculated by averaging all Si measurements in the soil solution (Fig.…”

“…Basic assumptions of this mixing model are that the chemical content of event water is significantly different from that of pre-event water, that event water is characterized by a single value or variations in the value are accounted for, that contributions of soil water are insignificant, and that surface storage contributions are negligible (Sklash and Farvolden 1979;Wels et al 1990). Pre-event water was the component of surface flow from water stored in soil (vadose) and groundwater reservoirs before snowpack runoff began.…”

Source waters and flow paths in an alpine catchment, Colorado Front Range, United States

“…Soil and groundwater chemical and isotopic analyses have been used to provide quantitative, multicomponent mass balance models of streamflow during snowmelt at watersheds in eastern North America [Wels et al, 1990]; but generally such models have not been well developed for watersheds of the Colorado Rocky Mountains, in part, due to the difficulty of access during snowmelt. Further advances in understanding the sources and sinks of N during snowmelt will require the development of multicomponent runoff models that utilize solute and isotopic measurements of source waters [Burns, 2002].…”