John E. Tobiason
The Wachusett Reservoir, located in central Massachusetts is an unfiltered, ozonated / chloraminated drinking water supply which serves 2.3 million customers in Boston, Massachusetts and surrounding communities. The Massachusetts Department of Conservation and Recreation (DCR) is responsible for managing and protecting the Wachusett Reservoir and surrounding watershed. The Massachusetts Water Resources Authority (MWRA) operates the water supply system and the Carroll Water Treatment Plant (CWTP). In addition, DCR manages and maintains the Quabbin Reservoir and watershed located in western Massachusetts. The Wachusett and Quabbin Reservoirs provide high quality water to consumers due to pristine watersheds and long hydraulic retention time. Modeling the movement of water through the reservoirs can lead to improvement of management, operational practices and procedures to maintain excellent water quality for consumers. CE QUAL W2 is a two dimensional, laterally averaged water quantity and quality modeling tool. In this study it is used to model fecal coliform contamination in the Wachusett Reservoir. Version 3.5, implemented in this research, can model 28 water quality constituents. A generic constituent defined by 1st order decay, an Arrhenius temperature rate multiplier and settling velocity was implemented in this study. The CE QUAL W2 V3.S base code was modified to include light induced decay. This study implemented CE QUAL W2 to sfudy the fate and transport of fecal coliforms originating from a sewage pump station overflow spill into Gates Brook, a tributary which discharges into the Wachusett Reservoir. The period of study was January 2003 to December 2004. The model was calibrated with data from 2003 and 2004. Simulations of sewage spills were conducted with the calibrated 2004 model which represented normal reservoir operation. The 2003 calibrated model was not selected due to the Cosgrove Intake shut down on November 1, 2003. Meteorological, in-reservoir, inflow and outflow quantity, temperature and quality data were required. The data were available from NOAA, MWRA, DCR, USGS and NADP. Values for coliform decay coefficients from prior modeling of Wachusett and Quabbin ReSelYoir and other literature were used as baseline parameters for simulating impacts of a 12 hour wastewater pump station malfunction. The assumed fecal coliform concentration in the untreated wastewater was 108 CFU/100mL. Simulations completed under baseline conditions resulted in the following conclusions: temperature dependant first order decay is the predominant decay mechanism in the Wachusett Reservoir; a spill occurring in June would have negligible fecal coliform impact on water withdrawn through the Cosgrove Intake; fecal coliforms would disperse throughout the reservoir quickly and evenly in the event of a spill when the reservoir is well mixed (early spring, late fall, winter); an early spri,n g spill would be partially diluted by spring runoff and elevated tributary discharges; a fully mixed reservoir, cool water temperatures and low sunlight intensity in the late fall inhibits decay and allows greater amounts of fecal coliform contamination to reach the Cosgrove Intake than at other times of the year; under normal reservoir operation and baseline decay coefficients a spill originating at Gates Brook would not result in water withdrawn at the Cosgrove Intake to exceed the SWTR criteria of 20 CFU/100mL. A set of conservative simulations were completed with decay rate coefficients set to values which minimized decay and maximize fecal coliform transport throughout the reservoir. The following observations were made for the conservative simulations: a spill occurring in June would have negligible impact on fecal coliform concentrations for water withdrawn from the reservoir at the Cosgrove Intake; an inflow spill concentration of 7.0E9 CFU/100 mL, 70 times larger than the baseline value, is required for coliform concentration at Cosgrove Intake to exceed the SWTR requirements; wind direction impacts fecal coliform transport along the reselYoir for spills discharged at the surface and at depth; a mid June wastewater spill occurring on a cloudy day, at the surface of the reservoir, using conservative decay parameters and a strong wind blowing out of the southwest toward the Cosgrove Intake would elevate fecal coliform concentrations at Cosgrove above the SWTR criteria; a similar mid June spill, at the surface of 'the reservoir, using typical decay coefficients and a strong wind originating in the southwest would have a peak fecal coliform concentration 2 days later of 0.6 CFU/100mL, however within 24 hours the fecal coliform concentration throughout the reservoir, including the Cosgrove Intake, would be < 0.2 CFU/100 mL.