Residence Time of Freshwater in Boston's Inner Harbor

Abstract

Two field tracer studies and some numerical model experiments were used to analyze the residence time of freshwater in Boston's inner harbor, and thereby help evaluate water quality impacts of combined sewer overflows. An exponential filter was used to modify the freshwater fraction approach for variable freshwater inflow and to reanalyze data from Bumpus et al. (1953). Results showed an inverse relationship between residence time and inflow rate, with times ranging from 2 to 10 d. An instantaneous dye study gave a residence time of 3.75 d, consistent with the freshwater measurements for conditions of summertime low flow. A three-dimensional (3D) numerical model applied to a schematized domain was able to reproduce trends observed in both the freshwater and dye studies.