Characterisation of Solute Transport in a Seasonal Stream Using Continuous In-situ Water Quality Monitoring

The Supervising Scientist Division conducts water quality monitoring in Magela Creek, a seasonal stream in Kakadu National Park, Northern Territory, to assess the environmental impact of mining activities from the adjacent Energy Resources of Australia Ranger uranium mine. To ensure environmental protection of the Magela Creek system and guide improvements in water management systems on the Ranger mine site, there is a recognised requirement to track and quantify the movement of dissolved salts (solutes) originating from point and diffuse sources within the 600km{2} catchment. The routine surface water quality monitoring program involves collecting and analysing weekly grab samples to ensure that water quality indicators, including uranium, remain within specified guideline values. However, this sampling regime could miss transient system fluctuations caused by natural or mine site inputs. To supplement the grab samples, water quality has been measured on an almost continuous basis over the last two wet seasons using in-situ sensors placed at strategic locations on and off the Ranger mine site. The continuous measurement of key water quality indicators, such as electrical conductivity, gives a more detailed and comprehensive indication of the overall water quality of the system, as well as providing early detection of mining impacts. Annual and event-based loads of dissolved magnesium, a major mine derived solute, have been calculated based on the significant relationship between magnesium concentration and electrical conductivity. These loads may be used for impact assessment and for deriving a solute budget for the system, therefore quantifying input of magnesium from point and diffuse sources on the mine site and surrounding areas. This paper summarises the key aspects of the development of the continuous monitoring method as a robust tool for monitoring water quality in relation to potential mining impact.