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We made observations from vertical moorings of thermistors and current meters in Lake Ontario to study the spring thermal bar circulation, its effect in inhibiting the horizontal mixing, and spatial and temporal variability of vertical mixing during this period. Thermal bar progression rates were compared with two analytical models; both produced overestimates during the early part of the thermal bar and underestimates during the late spring. In a zone between the shore and the thermal bar, currents were influenced by the local wind stress and flowed in a counterclockwise direction. The mean cross-shore flows in the nearshore zone were reduced during the thermal bar period. The alongshore horizontal exchange coefficients were higher than cross-shore exchange coefficients. During the thermal bar period, the magnitude of both alongshore and cross-shore exchange coefficients decreased when the bar was still within middepth (<40 m); when it was further offshore, the thermal bar did not have any significant effect on alongshore exchange coefficients, but cross-shore exchanges decreased marginally. The vertical exchange coefficients were high in the water column because of convective mixing during the thermal bar period. During the evolution of spring thermal bar, the vertical mixing decreased considerably in the nearshore regions under stable stratification, whereas the high vertical mixing levels continued in the offshore region for a longer period. [ABSTRACT FROM AUTHOR]

The alternate strategy of open-lake discharge may alleviate the need for unusually stringent treatment needed to meet water quality goals of the Hamilton Harbour Remedial Action Plan (RAP). The latest update of the RAP recommended a study of the possibility of offshore discharges. A study conducted for the City of Burlington has proposed a location for outfall in Lake Ontario. This paper utilizes a combination of physical limnological data and mathematical models to predict the waste plume characteristics for the proposed outfall in the lake. Near-field dilutions obtained from a mixing zone model show that, for treated effluents with a discharge condition of 2 m³/s at the proposed outfall site at Burlington, the dilution ratios are in the range of 13:1 to 28:1 for weak to moderate currents during summer stratification. Winter dilution ratios increased to 21:1 to 96:1 for moderate currents. The recommended site for openlake outfall provides acceptable near-field dilutions for treated effluents under typical lake currents and density structure. The extension of outfall to a location farther offshore is only marginally beneficial. With the proposed Burlington outfall location and discharge conditions, no far-field contamination is observed near the beaches or nearby water intakes for typical summer and winter conditions. Thus, this study indicates that by discharging the treated sewage from an outfall in Lake Ontario it is possible to achieve the Hamilton Harbour RAP goals. [ABSTRACT FROM AUTHOR]