Hydrochemical and isotopic end member mixing analysis to quantify snowmelt contributions to streamflow and lakes during arctic spring: application to hydrologic and mercury mass balance modelling

English Abstract:

Spring snowmelt is the most important hydrologic event of the year in arctic landscapes. During this relatively short period if time, fluxes of water and waterborne contaminants such as mercury to surface waters can far exceed those occurring during the remainder of the water year. Several important studies have reported relatively little mixing of snowmelt runoff with lake water over the melt period. However, on an annual basis, the extent to which catchment runoff mixes with lake water is believed to depend on the relative fractions of rainfall vs. snowfall throughout the year, and the fraction of annual runoff that occurs during the open-water season. This implies that seasonal and inter-annual differences in terrestrial-aquatic interactions may influence the “source-strength” of hydrological and biogeochemical source areas in relation to downstream water bodies.

As part of a broader study focused on these aspects of terrestrial-aquatic connectivity during spring runoff in arctic lake basins, we used hydrochemical and isotopic end member mixing analysis (EMMA) to help quantify the extent of mixing of snowmelt runoff within an 8 ha lake near Iqaluit, NU. Specifically, two and three-component hydrochemical and isotopic EMMA models were developed to calculate volumetric fractions of source waters in inflowing and outflowing streams of the lake over a 2 month period from early June to late July, 2014. Source water composition of inflowing streams ranged from 80% to 0% snowmelt-derived throughout the melt period, with concomitant changes in both mercury and dissolved organic carbon concentrations. Outflowing lake water largely tracked the hydrochemical and isotopic signature of the inflowing streams throughout the entire melt period, and according to EMMA, was dominated by this source water component (~50%). Contributions from melting lake ice and pre-melt lake water also comprised substantial fractions of the remaining 50% of the lake outflow, in approximately equal proportions. Overall, EMMA was found to be a useful tool for inferring interactions between snowmelt runoff, groundwater and lake water in an arctic lake basin, and for studying the mechanisms by which snowpack accumulation and melt serve to couple atmospheric sources of contaminants to arctic freshwaters.