The production of chemical heterogeneity in Upper Mystic Lake

Chemical heterogeneity in a lake may arise from spatially and temporally nonuniform inputs or from spatially nonuniform transformation rates within the water body. The potential for heterogeneity to persist at a given spatial scale then depends on the relative magnitudes of the hydraulic residence time, the timescale for mixing, and the timescale for transformations to and from the chemical species of interest. Through simple scaling arguments, the potential for heterogeneity in total arsenic (As) and arsenic speciation were predicted for Upper Mystic Lake, a small urban lake near Boston, Massachusetts. Specifically, heterogeneity in total arsenic was expected during periods of high inflow-which are typical of spring conditions-given a sufficient magnitude of inflow variation. Heterogeneity in As(III) was expected throughout the year, whenever spatial variations in the net As(V)-to-As(III) transformation rate exceeded a few percent per day. Observed heterogeneity was consistent with these predictions. Although modest, heterogeneity in total As was highest in early spring (7%, expressed as the spatial standard deviation divided by the lakewide mean). By contrast, heterogeneity of As(III) was as high as 36%, indicating that spatial variation in oxidation and reduction rates is sufficient to contribute to heterogeneity. Heterogeneity in As(III) varied significantly from month to month.