Application of WASP for Modelling and Management of Naphthenic Acids along Athabasca River, Alberta, Canada

Abstract

This study investigates the potential impact of OSPW (oil sands processed water) in terms of naphthenic acids (NAs) in the event that OSPW which contain NAs were discharged to a stretch of the Athabasca River of Alberta, Canada. A one-dimensional model WASP7 (water quality analysis simulation programme) was hydro-dynamically calibrated and validated for the data 1999–2008. The model represented the field data quite well except frozen seasons. The sensitivity analysis showed that the concentrations of NAs in the Athabasca River due to OSPW discharged to the river will be most sensitive to changes in the discharge rate of OSPW and concentrations of NAs in the OSPW. The WASP7 was applied to investigate how to achieve acceptable concentrations of NAs (≤0.15 mg/L) along the river, assuming NAs are degraded by natural dilution, biodegradation, sorption, photodegradation or combinations of these processes. If only the dilution effect is considered for an OSPW discharged at 1.5, 3.0 and 4.5 m3/s (initial NAs concentration of 120 mg/L) to the river, respectively, all the NAs concentrations simulated would exceed an allowable limit of ≤0.15 mg/L, which indicates that dilution effect alone is not sufficient to decrease the concentration of NAs in the river. Similarly, by only considering a photodegradation rate of 0.005/day, the concentrations of NAs would decrease by approximately 0.1–3 %. However, by only considering a maximum biodegradation rate of 0.4/day, NAs discharged to the river can be decreased by 5–90 %. If a more moderate biodegradation rate of 0.2/day is assumed but photodegradation is also considered at 0.0025/day, then for the same three discharge rates of OSPW to the river, the allowable OSPW NAs have to be limited to 8.02, 4.09 and 2.77 mg/L to limit NAs at 0.15 mg/L. This implies that high biodegradation itself is more effective than a combination of moderate biodegradation and photodegradation in degrading NAs. Through multiple numerical experiments with WASP7, it seems that to limit the concentrations of the final NAs in the Athabasca River within 0.15 mg/L, it will be crucial to limit the OSPW NAs and the OSPW discharge rate to the river.