CHEMICALLY AND PHYSICALLY STRATIFIED FLOW IN A SUBURBAN STREAM

A suburban Boston, Massachusetts stream, Bogle Brook, was monitored at the exit of a small pond for conductivity and streamflow during a study of road salt contamination during the period from November 2015 to May 2016. Two Aquatroll 200 probes were tube mounted from February 2016, one near to and open to the streambed and another at approximately mid-depth approximately 15 cm above, with the bottom sealed and only the sides open to sample a higher part of the water column. Data were recorded every fifteen minutes, and calibrated against chemical analyses by ion chromatography. Measured stream discharge was from two to eight cubic feet per second, and the sodium chloride load was 100 to 400 kilograms per hour during the period of measurement.

Compared to a very small nearby watershed also adjacent to an Interstate highway, discharge and salt content increase and decrease more slowly and peak at lower salinity because most adjacent roads drain through wetlands rather than directly into the main stream.

Physically and chemically stratified flow occurs in the stream, as differences between the monitoring probes in temperature, conductivity, calculated density, and calculated and measured salt content occur. Some differences are apparently due to ice formation and melting on the pond surface, while others may be related to baseflow into the stream. Density inversions occur occasionally, with less dense water entering the stream from below, correlated with dry periods in the spring but not directly with either salt content or temperature. There appear to be two parallel linear correlation trends for conductivity between the two probes, suggesting two different flow regimes.

Estimated total salt flux from the catchment area above this site must take into account the inhomogeneity of the water in the stream.