Impacts on downstream water quality

This research area is closely aligned with research on sediment/contaminant delivery from mine sites to stream systems. Once the quantity of sediment eroded from a mine site, transported through a catchment and delivered to the stream system has been determined, the impact of this sediment on stream water quality needs to be assessed.

Location of the gauging stations upstream and  downstream of Ranger (Left) and Jabiluka (Right). The direction of stream flow  along the creeks is also shown.

Location of the gauging stations upstream and downstream of Ranger (Left) and Jabiluka (Right). The direction of stream flow along the creeks is also shown.

The most obvious impact of suspended sediment is to reduce light penetration through the water column and therefore reduce the level of photosynthetic activity. Turbidity, which is related to suspended sediment concentration, can cause adverse affects at levels as low as 5 NTU. Elevated suspended sediment loads can affect fish and benthic organism respiration, feeding, reproduction and change in community structure. Local studies at Jim Jim Creek in Kakadu National Park found that macroinvertebrate community structure was impacted at suspended sediment increases of 17 mg/L. Impacts on fish populations were detected at concentrations of 100 mg/L.

A gauging station network has been implemented within the Magela Creek catchment to determine baseline sediment transport characteristics in the catchment against which the impact of future mine rehabilitation can be assessed. Flow and fine suspended sediment (mud) are monitored at gauging stations located upstream and downstream of Ranger (along Magela and Gulungul Creeks) and Jabiluka (along Ngarradj). Turbidimeters have been installed at all stations to indirectly monitor mud concentration in the streams.

Along Magela Creek, the equipment was installed on floating pontoons, which means that turbidity data were collected approximately 0.2 m below the surface for all flow conditions. Along Gulungul Creek and Ngarradj, turbidity sensors were contained within a plastic tube which extended from the stream bank to the water channel, and positioned approximately 0.2 m above the bed level at each station.

Water samples are collected at each station by an automatic pump sampler over a range of flow conditions. Sample collection is triggered by changes in stream turbidity. The mud concentration (silt and clay fraction) ( 0.45 mm) in each sample is determined by sieving, filtering and oven drying techniques (Erskine et al., 2001). These data, along with concurrent in situ turbidity measurements, are used to derive statistically significant relationships between mud concentration and turbidity for each station.

Monitoring station along Magela Creek

Monitoring station along Magela Creek

The flow and fine suspended sediment (mud) concentration data collected at these stations have been used to derive both mud concentration and event-based mud load trigger levels which can be used for impact assessment. The impact assessment technique we have used is a Before-After-Control-Impact Paired Difference design (BACIP), which is the methodology recommended in the Australian Water Quality Guidelines (AWQG). Mud concentration trigger values can be applied on a day-to-day basis to identify/flag a need for immediate action. Event-based mud load trigger values will be used to assess the behaviour of the rehabilitated landform on an annual basis.

Research projects

  • Monitoring stream sediment movement in Ngarradj
  • The impact of Cyclone Monica (April 2006) on riparian vegetation, in-channel large wood loadings, channel erosion and tree fall in the Ngarradj catchment
  • Monitoring sediment movement in Gulungul Creek
  • The impact of Cyclone Monica (April 2006) on stream sediment loading and tree fall in the Gulungul catchment
  • Assessment of continuous Magela Creek turbidity data upstream and downstream of Ranger mine
Monitoring station along GulungulCreek

Monitoring station along Gulungul Creek

Relevant Supervising Scientist reports

SSR172
Moliere DR, Boggs GS, Evans KG, Saynor MJ & Erskine WD 2002. Baseline hydrology characteristics of the Ngarradj catchment, Northern Territory. Supervising Scientist Report 172, Supervising Scientist, Darwin NT.

SSR179
Evans KG, Moliere DR, Saynor MJ, Erskine WD & Bellio MG 2004. Baseline suspended-sediment, solute, EC and turbidity characteristics for the Ngarradj catchment, Northern Territory, and the impact of mine construction. Supervising Scientist Report 179, Supervising Scientist, Darwin NT.

SSR183
Moliere D 2005. Analysis of historical streamflow data to assist sampling design in Gulungul Creek, Kakadu National Park, Australia. Supervising Scientist Report 183, Supervising Scientist, Darwin NT.

Journal Papers

Evans KG, Martin P, Moliere DR, Saynor MJ, Prendergast JB & Erskine WD 2004. Erosion risk assessment of the Jabiluka mine site, Northern Territory, Australia. J. Hydrologic Engrg  9(6) 512-522.

Saynor MJ, Erskine WD, Evans KG & Eliot I 2004. Gully initiation and implications for management of scour holes in the vicinity of the Jabiluka Mine, Northern Territory, Australia. Geografiska Annaler 86 (2), 191–203.

Moliere DR, Saynor MJ & Evans KG 2005. Suspended sediment concentration-turbidity relationships for Ngarradj – a seasonal stream in the wet-dry tropics. Australian Journal of Water Resources 9(1), 37-48.