John E. Tobiason
This research was done to understand the potential fate and transport of a contaminant spill into the Wachusett Reservoir utilizing the model CE-QUAL-W2 V3.6. The Wachusett Reservoir, located in central Massachusetts, is the main water supply for the Boston, MA metropolitan area. The reservoir has a capacity of approximately 65 billion gallons and receives about half of its total inflow from the Quabbin Reservoir, which has a capacity of 412 billion gallons. Water is transferred from the Quabbin Reservoir to the western end of the Wachusett Reservoir intermittently through the Quabbin Aqueduct typically from June through November to meet higher water demands, maintain the water level, and mitigate water quality concerns in Wachusett. The largest outflow from the Wachusett Reservoir is the Cosgrove drinking water intake, located at the most eastern end of the waterbody.
CE-QUAL-W2 is a two-dimensional, longitudinal and vertical, hydrodynamic and water quality model. The model is suitable for simulating water quality and hydrodynamics in Wachusett because the reservoir is relatively long and narrow. Therefore, longitudinal and vertical gradients in velocities, temperatures, and constituents are much larger than lateral gradients. For this study, a model of the reservoir was developed for the year 2009 using inputs for meteorology, bathymetry, initial flow and constituent conditions, inflow quantity and quality, outflow quantity, and outlet descriptions. The 2009 simulation was successfully created and calibrated to match temperature and specific conductivity profile measurements in the reservoir. Models for the years 2003-2008 had been created and calibrated during previous UMass research by Matthews (2007), Sojkowski (2011), and Devonis (2011).
The calibrated CE-QUAL models for the years 2003-2009, verified by the temperature and specific conductivity profiles, were used to simulate potential contaminant spills into the Wachusett Reservoir. Contaminant spills were modeled as a conservative substance to study the effects of seasonal change, various spill densities (temperatures), and turning the Quabbin transfer on and off. Spill dates for each season were chosen based on days with similar meteorological conditions. The approximate contaminant spill arrival time, maximum relative concentrations, and behavior at the Cosgrove Intake were observed and compared for various analyzed scenarios.
During the spring and the fall seasons, the density of a contaminant spill does not typically have an effect on the arrival time or relative concentration of the contaminant at the Cosgrove Intake. Spring spills arrive between 2 and 7 days after the spill occurred, reaching an average maximum relative concentration of 1.0 to 2.9. Fall spills typically arrive between 4 and 11 days with an average maximum concentration of approximately 1.0 to 1.2. During the summer months, when the reservoir is stratified, contaminant spill behavior is more variable, arrival time is usually later, and the average maximum relative concentration is greater. The average arrival times for warm, medium, and cold spills during the summer are 8.4, 11.6, and 12.3 days respectively. The average maximum relative concentration at the Cosgrove for warm spills is 1.9, while for medium and cold spills the average are about 2.6 during the summer months.
Impacts of the Quabbin transfer on spill behavior and relative contaminant concentration were also investigated for the spring, summer, and fall. Turning the Quabbin Aqueduct off after a spill in the summer, when it is normally on, generally does not impact the arrival time of the contaminant at the Cosgrove. However, turning the Quabbin transfer off reduces the variability in the concentration of the contaminant at the intake. Changes in the Quabbin transfer during the spring and the fall have minimal impacts on contaminant arrival time and behavior.
A combined two year model developed from the data for years 2008 and 2009 demonstrates that a conservative contaminant can remain in the reservoir for more than three times the mean hydraulic residence time of 206 days. In contrast, a contaminant with a first order decay rate of 0.02 day-1 results in a 99% decay of contaminant concentration in the outflow after one mean hydraulic residence time. Model results for decaying contaminants show that relatively rapid decay rates (0.10 to 0.66 day-1) are needed to decrease the peak outflow concentration by 99% for a range of peak concentration arrival times of 7 to 46 days for all simulated model years. Additionally, a combined two year model is useful producing more realistic boundary conditions for a 3-D model of the North Basin in the Wachusett Reservoir.