Modified Low Flow Frequency Analysis Applied to a Wastewater Allocation Study.
The South Nation River was studied for its ability to assimilate wastewater effluents from municipal and industrial point sources for both existing and anticipated future conditions. The watershed, located in Eastern Ontario, is rural with community populations ranging up to 3500 residents. Currently there are ten communities, all with lagoon wastewater treatment, and two industries with custom designed wastewater treatment processes that discharge into streams within the watershed. In the future, an additional five communities and one industry are expected to install sewerage and wastewater treatment facilities.
Streamflows drop to very low values during the late summer and early fall, and as a result all facilities are restricted to spring and/or late fall discharges. Provincial Water Quality Objectives (PWQOs) are often exceeded for phosphorus and ammonia. In this case, Ontario Ministry of Environment (MOE) policy stipulates that the receiver cannot be further degraded (MOE, 1984). In assessing assimilative capacity of the watershed, the traditional approach is to compute n-day low flows with a 20-year return period (usually 7 or 30 days) over the entire discharge season. This computation, in the case of the South Nation River, yielded predictions of 0.0 m3/s for 7Q20s and 30Q20s at several locations in the watershed, particularly during the fall period. This would have resulted in severe restrictions in municipal growth. A wasteload allocation scheme which allowed greater utilization of the available assimilative capacity was therefore required. An alternative approach was developed for the low flow frequency analysis component of the study. Complete study details are presented by Gore & Storrie (1992).
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