Acid Drainage Acid Drainage

Part I. Reconnaissance survey of acid drainage in Tasmania

There are over 4000 recorded mineral activity sites in Tasmania and of these about 681 sites are classified as metal-related abandoned mines of various categories. About 215 of the abandoned mines have a history of base metal mining and contain sulphidic rock materials that are either acid-forming or have the potential to form acid when exposed to oxidising conditions. Acid drainage (AD) resulting from oxidation of sulphidic materials at several abandoned mines in Tasmania is identified as one of the major sources of metal pollution affecting water quality in catchments impacted by past mining activities.

Results of the survey show that surface waters draining many of the abandoned mines with base metal mining history contain elevated levels of sulphate and dissolved metals (Al, As, Cd, Cu, Pb & Zn) at widely varying pH range. This variation in impacted waters is largely attributed to the geochemical distribution of acid forming rocks, physical setting of the mines and local environmental conditions. Insignificant variation in the dissolved and total metal analysis in AD indicate that downstream metal mobilisation may be mainly in dissolved form.

AD impacted surface waters show a broad linear pattern of metal distribution from high acid/extreme metal to low acid/low metal. A similar trend was shown by sulphate distribution. An elevated level of sulphate downstream from acid generation sites is indicative of active sulphide oxidation taking place at point source. A strong correlation between dissolved metals and sulphate distribution suggests that sulphate analysis in waters may be used as cost effective AD indicator mapping tool.

High sulphate/high metal waters are found to be closely associated with abandoned mines containing sulphidic rock materials with high net acid producing potential (NAPP). Many of the volcanic-hosted mineralised metasediments in the West Coast region generally have high positive NAPP and very low to negligible acid neutralising capacity (ANC). Carbonate-hosted base-metal abandoned mines were found to be generally high acid producers mainly because of the presence of high NAPP ore rocks and restricted availability of ANC of the host rock. Waste rocks and tailings materials at many of the abandoned mines commonly contain anomalously high concentrations of metals. A similarly high level of metals reflected in surface waters and stream sediments suggest that there is a significant release of these metals into the receiving environment during natural weathering processes under Tasmanian climatic conditions.

This report presents new data and the compilation of existing data on water chemistry and geochemistry associated with abandoned mines in Tasmania. The report focuses mainly on AD from abandoned mines although recently closed and currently operating sites are also reviewed in light of overall spatial distribution of AD in Tasmania. An important outcome of the survey is generation of AD impact maps along with an inventory of acid-producing abandoned mines.

Many of the abandoned mines investigated in this survey show serious AD and metal pollution problems. The extent and seriousness of the impact on the receiving environment is difficult to assess from the currently available information. Detailed hydro-geochemical characterisation along with baseline data on mass loadings and environmental parameters need to be assessed prior to recommending effective remediation of problem sites identified in this survey.

The report produced as part of this project includes an inventory of acid-producing abandoned mines in Tasmania and water chemistry and geochemical data. Three 1:500 000 scale maps were also produced as part of this project; printed copies may be obtained from Mineral Resources Tasmania.

Acid drainage report (Tasmanian Geological Survey Record 2001/05) (pdf file, 1895 kb)

Acid drainage report (appendices) (pdf file, 303 kb)

Map 1: Distribution of low pH-high sulphate surface waters in catchments impacted by abandoned mines (jpg image, 3870 kb)

Map 2: Distribution of low pH-high metal surface waters in catchments impacted by abandoned mines (jpg image, 3875 kb)

Map 3: Geochemical distribution of potential acid forming rocks hosting abandoned mines in Tasmania (jpg image, 3113 kb)

 

Part II: Reconnaissance survey of acid sulphate soils in Tasmania

Acid sulphate soils (ASS) are pyritic sediments with potential to generate acid when exposed to oxidising conditions. These sediments were formed in environments where key ingredients: sulphate, organic matter and iron oxides were present under anaerobic conditions. Low-lying estuarine and coastal sediments, remnant seawater lakes and backswamps formed during the Holocene period (< 10, 000 yrs) are considered ideal environments for the development of ASS. Tasmanian State Policy on Water Quality Management (1997) identifies acid drainage from ASS as one of the many diffuse sources of water pollution.

This report presents results of a reconnaissance survey of acid sulphate soils (ASS) in Tasmania. Desktop evaluation of background information and data from this survey were used to generate an ASS distribution map of Tasmania. The results of the survey indicated that about 200 km of Tasmanian coastline is likely to be affected by the occurrence of Holocene sediments with potential to host ASS. Evidence of acid drainage from disturbance of ASS in backswamps and in coastal sediments under intensive agriculture suggest that metal-rich acidic waters pose a serious pollution problem in receiving waters proximal to heavily drained areas.

This investigation indicated that ASS may be extensively developed around the northern Tasmanian coastline and in inland backswamps at elevations exceeding 20m AHD. Field evidence of ASS was encountered in a number of locations in Tasmania. Evidence of potential acid sulphate soils (PASS) and actual acid sulphate soils (AASS) occurrence in some sections of coastal sediments and in drained backswamps suggest that there is a need for further investigation into identifying priority impacted areas for management of ASS in Tasmania.

Report and appendices (Tasmanian Geological Survey Record 2001/06) (749 kb)

Map 4: Distribution of acid sulphate soils in Tasmania (jpg image, 3843 kb)