Derelict Mines - Macleay Catchment
Arsenic and Antimony Assessment Stages 2 and 2a

GHD report for NSW Department of Industry
October 2016


Cover of Derelict Mines - Macleay Catchment - Arsenic and Antimony Assessment Stages 2 and 2a

The Macleay River Catchment hosts many areas that contain relatively elevated concentrations of certain metals and metalloids, including gold, arsenic and antimony. The presence of these mineralised areas within the Macleay Catchment has led to a mining history of over 140 years. Historically, mineralised mining waste, including tailings and waste rock, were consciously disposed of in-stream, and/or poorly stored on many mine sites, as was the practice of the day. Subsequent erosion has seen mineralised waste deposited into tributaries within the Macleay Catchment. This, in turn, has resulted in elevated concentrations of arsenic and antimony in stream sediments for in excess of 300 kilometres within the Macleay Catchment.

A study was commissioned to address Strategy 30.1 of the Macleay River Estuary: Coastal Zone Management Plan; an initiative borne to improve water quality in the Macleay River by Kempsey Shire Council with financial assistance from the NSW Government’s Estuary Program as administered by the NSW Office of Environment and Heritage (OEH). Specifically, Strategy 30.1 aims to identify and manage existing sources of contamination from mines in the upper Macleay catchment, thereby seeking to prevent further contamination of waterways by addressing those upstream contaminant sources.

Stage 1 of the study involved a desktop review of existing information, which was reported by GHD to the NSW Derelict Mines Program in 2015. Stage 1 used existing sediment and water quality data to identify the top five sub-catchments within the Macleay that contributed the largest arsenic and antimony flux. These were Bakers Creek, Chandler, Commissioners Waters, Hickeys and Mungay Creeks, and Apsley sub-catchments. Stages 2 and 2a of the study included undertaking additional mineral waste, sediment and surface water sampling and analysis within those nominated five sub-catchments to attempt to identify key point or diffuse sources of arsenic and antimony for priority remedial action.

This document reports on Stages 2 and 2a of the study that included an assessment of key arsenic and antimony sources from derelict mine sites within the Macleay Catchment, such that priority rehabilitation works may be implemented. Based on the data assessed during Stage 1, the key contributor of antimony to the Macleay Catchment was found to be the Bakers Creek sub-catchment (around 77 percent of total antimony flux). This, presumably, was mostly sourced from historic mine workings and in stream sediment within the Hillgrove Mineral Field. Ashley and Graham (2001) reported that Bakers Creek and some of its tributaries had been historic repositories for up to seven million tonnes of mineralised waste rock and tailings. Ashley and Graham (2001) subsequently concluded that the in-stream mineral waste was likely to be the major source of arsenic and antinomy contamination in Bakers Creek; the remediation of which it is unlikely to be cost-effective based on current technology.

The second largest antimony contributor by flux was the Chandler sub-catchment (around 4.5 percent), which hosts the Halls Peak and Rockvale derelict mines, amongst others, followed by the Hickey and Mungay Creek sub-catchment that contributes some 3.2 percent of the antimony flux—the latter possibly sourced from the Mungay Creek Antimony Mine. Then came Commissioners Waters (around 1 percent) and Apsley (0 4 percent). It was considered that Apsley may have been identified based on the method used, being a function of catchment size. i.e. a potentially small point source contribution and relatively large catchment area; thereby yielding a relatively significant antimony flux. The balance was comprised largely of naturally mineralised though unmined catchments (around 8 percent), the transitional Trunk Macleay around 4 percent) and the depositional Macleay Floodplain (around 1 percent).

The top five arsenic contributors were found to be the Chandler sub-catchment with around 35 percent, Bakers Creek with around 25 percent, Commissioners Waters (7.2 percent), Apsley (2.6 percent) and Hickey and Mungay Creek sub-catchment with 1.2 percent. The balance was comprised largely of naturally mineralised though unmined catchments (around 10.6 percent), the transitional Trunk Macleay (around 10 percent) and the depositional Macleay Floodplain (around 3 percent).

It is important to recognise that an estimated 13 percent of the antimony flux and around 30 percent of the arsenic flux appears to be sourced from non-mining catchments that are naturally elevated in antimony and arsenic, the Trunk Macleay and the Macleay Floodplains themselves.

To further investigate which individual sites within the top five antimony and arsenic generating sub-catchment listed above were contributing the most contamination, GHD collected the following samples during Stages 2 and 2a:

  • 27 mineral waste samples from 10 individual mine sites/mining areas in 5 subcatchments
  • 74 sediment samples from 15 sites in 6 sub catchments (Warbro Brook was the addition)
  • 52 surface water samples from 14 sites in 6 sub catchments (Khans Creek being the exception; Warbro Brook being the addition).