Authors
Year of Publication: 1993
Abstract:
Computer modelling of the oxygen balance of the Athabasca River is being used to assess the potential water quality impacts of expanding mill development and to set appropriate effluent standards for the mills. Sediment oxygen demand (SOD) is an important variable in the oxygen balance of rivers, in particular during the winter under low flow conditions and extensive ice cover.
In 1989, an in-situ chamber method of SOD measurement was used to determine winter SOD rates in the Athabasca River. In 1990, a number of improvements were incorporated and the method used to determine temporal and spatial trends of SOD on the river. The objectives of the 1992 study were similar to those in 1990 but incorporated a number of study locations extending downstream to Fort McKay.
During low flow conditions in mid February, sampling sites suitable for the in-situ method were found in the upper reaches of the study area (e.g. Windfall bridge, Whitecourt and Smith). The method was, however, found to be somewhat limited in the lower reaches (Athabasca, Calling River and Fort McMurray) during the same low flow period. Physical constraints including an abundance of deep and soft, fine sediments, excessive ice thickness and unsuitable water depth and velocity, necessitated the development of a comparable method best suited to SOD measurement in areas of low gradient and velocity that featured an abundance of fine sediment.
The sediment core method, as it was named, was tested in March in conjunction with the chamber method at Hinton, Whitecourt, Athabasca, Alpac. Given observed river break-up in late March and its anticipated influence upon study results, measurements of SOD were not conducted at points downstream of the Alpac study location.
The principle findings of the 1992 study were as follows:
• the lowest rates of SOD was measured at Windfall bridge (mean = -0.003 g02/m2/day) located 170 km downstream of Hinton and 30 km upstream of Whitecourt;
• the highest rates of SOD were measured at sites located immediately downstream of the Weldwood pulp mill at Hinton (mean = 0.33 g02/m2/day) and the Millar Western Ltd. pulp mill at Whitecourt (mean = 0.53 g02/m2/day);
• At Smith, approximately 220 km downstream of the Millar Western Ltd. pulp mill effluent, SOD rates declined (mean =0.10 g02/m2/day) but were higher than those measured at Windfall bridge;
• Immediately downstream of the town of Athabasca's sewage effluent, SOD rates (mean = 0.21 g02/m2/day) approximated those measured at Hinton, Blue Ridge and Fort Assiniboine in previous Athabasca River studies;
• The Athabasca River experienced an increase in SOD over the winter months. In conjunction with low flows and stable river conditions, the build-up of SOD was likely due to the continuous addition of organic nutrients and material which result in an increase in benthic biomass and respiration. Under the influence of increasing streamflows in late March, which scour accumulations of organic material and associated organisms, SOD rates were observed to decline;
• The chamber and sediment core methods of SOD measurement were somewhat similar in design and operation and during simultaneous deployment yielded similar SOD rates. Given that the sediment core method can be used in habitats not suitable for the deployment of open or closed chambers (and vice versa), the 1992 results indicated that estimates of SOD can be obtained in most situations, regardless of site specific characteristics.
Future development of the sediment core method could focus upon the interactive effects of substrate quality and disturbance, particle resuspension, test volumes, flow rates and top-water velocity. Continuous monitoring of dissolved oxygen concentration with a meter would provide valuable information concerning maximum SOD rates and the time period required for incubation. Meter measurements would limit the potential error that may be associated with decreasing sediment core water volumes during the incubation period.
Continued measurement of SOD, particularly in the lower river reaches, will provide information valuable for river oxygen balance modelling and the development of appropriate effluent standards.
