A draft copy of this report is posted for comment. Here is the summary:
The watershed tributary Northwest Creek (NWC) is estimated to have added 1439 kilograms of Nitrogen (analyzed and reported as TN) and 88 kilograms of Phosphorus (analyzed and reported as TP) nutrients to Sylvan Lake in the April-October period of 2014. Total Nitrogen (TN) and Total Phosphorus (TP) compositions of water quality samples and the estimated cumulative flow in the NWC stream were used to calculate lake loading by those nutrients. Incident precipitation on the 12.8 square kilometre catchment area between April 1 and October 31 was 4.46 million cubic metres (Mm3). As flow in NWC was not measured continuously, the runoff coefficient method was applied and an assumed value of 0.30 yielded a discharge volume of 1.34 Mm3 of surface flow to Sylvan Lake. Mixing of that NWC flow with the total lake volume of 420 million cubic metres would have diluted the nutrient loads from NWC and might have increased the TP concentration in the lake by an estimated 1%.
The analysis was based on water quality data collected between mid-April and mid-May, 2014. Assumptions were made about nutrient concentrations in the Northwest Creek tributary, including in up-slope branches that collect surface flow from a winter feedlot area, summer-month pasture land, and other agricultural and forested areas. We suspect that the TN and TP values quoted above underestimate the true discharges into Sylvan Lake.
The extent to which tributary flows mix with the inventory of water in the lake is unknown. No attempt has been made to quantify the dispersion. Composite water quality sampling over the lake area is not designed to provide evidence of areal or vertical concentration gradients.
This project was co-funded by the Sylvan Lake Watershed Stewardship Society and the Sylvan Lake Management Committee. Alberta Environment and Sustainable Resource Development provided expert assistance for water quality sample collection and some stream flow measurements.