Hydrograph and Chloride Pollutograph Analysis of Stony Brook Reservoir Subbasin in Eastern Massachusetts

Hyetographs, hydrographs, pollutographs, and deicing agent applications are analyzed with linear reservoir theory to explain the fate and transport of water and chloride in a small (1 km²) urban watershed subbasin.  The subbasin delivers highway runoff to a public supply reservoir in eastern Massachusetts.  Aquifer, vadose zone, and highway drainage systems have hydrographs with physically plausible exponential decay constants and seasonally varying discharge fractions.  Continuous USGS gage data from April 2006 through December 2012 yield observed water volume and chloride mass fractions delivered by the aquifer to the reservoir, with the latter inferred from specific conductivity logs.  Interflow and runoff calibration of over 160 hydrographs and pollutographs yields water volume and chloride mass fraction estimates for the vadose zone and drainage system.  The chloride mass past the gage is compared with lane mile totals and deicing agent application rate logs over multiple deicing seasons, so that loads may be attributed to state and local sources. The distribution across drainage, vadose zone, and aquifer systems for the Stony Brook Reservoir subbasin is compared with a similar completed analysis of calibrated data from an adjacent (Hobbs Brook Reservoir) subbasin.  The latter subbasin includes detention basins installed as a best management practice for particulate and suspended solid loads in the highway runoff.  The comparison explores the hypothesis that the BMP preferentially partitions deicing agent chloride into the subsurface environment—one expects the Stony Brook peak winter concentrations to be higher than the corresponding Hobbs Brook maxima, but the baseflow and summer concentrations should be lower in the absence of detention basins at Stony Brook.