Modeling of Eutrophication in the Tualatin River to Facilitate a TMDL Update

A phosphorus TMDL has been in place for The Tualatin River in Oregon since 1988 that established strict effluent limits for phosphorus. To achieve the limit requires the use of alum to remove phosphorus from wastewater effluent. Since 1988, the river has changed dramatically in terms of operations, flows, and water quality. The EPA has recently proposed a new aluminum standard in Oregon that may necessitate curtailing the use of tertiary alum. A combined effort of modeling eutrophication in the river, data collection, and an experimental elimination of alum addition at the treatment plants was conducted in the summer of 2019. Results suggested that algal growth in the river is no longer phosphorus-limited, but is more sensitive to algal ‘seeding’ from upstream sources, including a treatment wetland. The treatment plants may be able to cease alum addition without harmfully affecting water quality in the river.