Substance-specific water quality criteria for the protection of South African freshwater ecosystems: methods for derivation and initial results for some inorganic toxic substances

Abstract

Freshwater ecosystems form the resource base on which water users, such as the agricultural, recreational, domestic and industrial sectors depend. These essential resources therefore need to be protected and maintained in a healthy state. The Department of Water Affairs and Forestry is currently developing water quality criteria for the protection of South African freshwater ecosystems, to complement the existing National Water Quality Guidelines for domestic, industrial, agricultural and recreational use. This paper describes the methodology for the derivation of in-stream water quality criteria for inorganic toxic substances. Criteria are calculated from the results of acute and chronic toxicity tests on a number of representative species, using local data where available, and relying on international databases to supplement local information. Conservative numerical criteria are provided for aluminium, ammonia, arsenic, boron, cadmium, chlorine, chromium, copper, cyanide, fluoride, lead, manganese, mercury, molybdenum, selenium, vanadium and zinc. For each toxic substance, threshold levels at which chronic and acute toxicity effects on aquatic biota can be expected are indicated. The criteria can be applied in water quality evaluation, impact assessment, and in the setting of discharge permit conditions.