Eutrophication of Rivers by Nutrients in Treated Kraft Pulp Mill Effluent

Abstract

Abstract In 1972, the start up of a 1200 ADT/day bleached kraft pulp mill in Kamloops, British Columbia, resulted in a massive increase in algal standing crop in the Thompson River below Kamloops Lake. While phosphorus loading from the pulp mill was believed to be responsible for this change, the actual elevation in dissolved phosphorus concentration downstream of the discharge was below the detection limit (3 µg PL-1) of the analytical procedure used in the joint Federal-Provincial Task Force Study of the Thompson River in 1973-75. Research at Environment Canada’s Experimental Troughs Apparatus (EXTRA) located at Chase, B.C., has proven that the ambient concentration of phosphorus required to saturate the specific growth rate of attached diatom communities with a concomitant increase in algal standing crop is very low (ca. 1 ug PL1). Hence, although the concentration of dissolved phosphorus in kraft mill effluent (KME) is typically below 0.5 mg PL1 (ca. 1 µg PL1 total phosphorus), even at in-river dilutions of 100-fold, the steady-state elevation of soluble phosphorus is high enough to stimulate algal production in rivers which are phosphorus-limited. In rivers that are nitrogen-limited, nitrogen in fully treated KME can also stimulate algal production. The atomic ratio of available N and P in the McKenzie River, Oregon, is ca. 2:1 clearly indicating a nitrogen-limited system. The discharge of secondarily treated KME to this river near Springfield, Oregon, has also increased algal production. Stream-side flume experiments measuring the effect of treated KME additions on algal growth have shown that the concentration of dissolved inorganic nitrogen (DIN) available for algal uptake (ammonium-N; ~200 µg NL-1 and nitrate-N; ~ 50 ug N L-1) in KME was high enough to increase specific growth rates from 0.30 to 0.56 divisions d-1 during the summer, even with the effluent completely diluted in the river (0.5% v/v). At other seasons of the year, KME did not stimulate algal growth at concentrations occurring in the river. In an experiment with phosphorus added in pulses to river algal communities, the attached diatoms were able to rapidly uptake the phosphorus spikes and continue growing rapidly. Algae exposed to a P concentration of 60 µg PL-1 for only one min each hour grew as fast as those algae exposed to a continuous enrichment concentration of 1 µg PL-1. This showed the importance of stable nutrient loading characteristics in controlling eutrophication of rivers.