One of the methods used to assess the effects of runoff water from the Ranger uranium minesite (NT) on the adjacent receiving-waters uses a form of ‘early warning’, biological monitoring – so-termed ‘toxicity monitoring’ – where reproductive output (egg production) of the freshwater snail, Amerianna cumingi, is measured. This snail species demonstrates relatively high toxicological sensitivity to the main mine wastewater contaminants (Mg and U). Refinements to the monitoring technique have been made periodically since 1991 when the method was first introduced. A BACIP design is employed in which paired (P) control (upstream, C) – impact (downstream, I) response ‘differences’ are compared before (B) and after (A) potential mine-related disturbances.

Statistical tests for impact detection have invariably resulted in ‘no change’ in responses between the several tests conducted in the wet season just completed (A period) and all previous wet seasons (B). However, after the 2009–10 wet season and relative to previous years, significantly greater egg production was observed downstream of Ranger compared with upstream. As a consequence of this finding, recent efforts have been directed at better understanding the environmental conditions (viz water quality, hydrology, snail husbandry) affecting the production of snail eggs. This work helps distinguish mine-induced from natural effects on snail egg numbers for impact assessment purposes. Factors found to affect egg production include flow rates of creek waters through test containers, culturing method of snails, and the water quality variables, water temperature and electrical conductivity (EC) where a significant interacting effect has been observed.