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Background arsenic concentrations in the Reefton goldfield
The Reefton area has abundant gold-bearing rocks in a zone that extends for more than 20 km to the north and south. There has been extensive historic gold mining in the area. The gold-bearing rocks also contain arsenopyrite (FeAsS). Groundwater can dissolve arsenic from these rocks as a natural process of erosion. Gold mining can enhance this arsenic dissolution, and most historic gold mines have abundant dissolved arsenic leaving the sites. This study compares the amounts of naturally elevated arsenic levels in streams to the arsenic being discharged from historic gold mine sites. Prohibition mill siteThe Prohibition mill site at Waiuta is probably the most acutely toxic historic mine site in New Zealand. Residues from the processing plant are lying exposed on the ground surface and in an old tailings dam. Most of these residues contain about 20% arsenic, and near-pure arsenic minerals are present. These arsenic concentrations are more than 1000 times higher than other NZ arsenic-contaminated sites such as orchards and sheep-dips. Some of the arsenic is highly soluble, and surface water typically contains up to 50 mg/litre dissolved arsenic, which is 5000 times higher than the NZ drinking water limit.
The high dissolved arsenic at the site decreases in a dam structure below the site. The dissolved arsenic is removed from the water by adsorption to iron oxyhydroxide that is being generated by oxidation of iron minerals in the dam aggregate. However, water leaving the dam and entering the natural environment downstream still has 2 mg/litre dissolved arsenic, which is 200 times higher than the drinking water limit. The dissolved arsenic concentrations decrease steadily downstream because of dilution. The Blackwater River, which is the receiving stream for the Prohibition mine site discharge, has 0.003 mg/litre dissolved arsenic 14 km downstream from the Prohibition mill site. This water is below the human drinking water limit for arsenic, but is above the ANZECC guidelines for 99% aquatic species survival.
Because the Blackwater River has detectable arsenic concentrations downstream from the Prohibition mine site, it is reasonable to question whether or not the highly contaminated Prohibition site is adversely affecting the water quality in the Blackwater River. It is not possible to answer this question using water chemistry alone. Instead, it is necessary to determine the flux of arsenic in the relevant streams as follows: (a) Prohibition dam discharge water arsenic concentration = 2 mg/litre (d) Blackwater River water arsenic concentration = 0.003 mg/litre Hence, nearly 90% of the dissolved arsenic in the Blackwater River is from sources other than the Prohibition mill site. This arsenic flux constitutes a background value, and is largely derived by groundwater moving through arsenic rich rock in the Blackwater catchment. Globe Hill mining areaThere was extensive underground mining at Globe Hill up to the 1950s. Underground tunnels are now discharging groundwater with strongly elevated arsenic concentrations (up to 50 mg/L). The amounts of water are small (flow rates <1 L/second). These discharge waters enter the Devils Creek catchment which is the main drainage stream for the area.
The mean flow rate of Devils Creek below Progress Junction is 230 L/second. Hence, the total amount of dissolved arsenic leaving the Globe-Progress area is 32 mg/second. This is several orders of magnitude higher than estimated fluxes from most individual adits in the Globe-Progress area, and 15 times higher than the most concentrated adit discharge. These flux estimates suggest that more than 90% of the dissolved arsenic in Devils Creek is derived from background sources.
Both the examples examined in the Reefton area show that about 90% of the dissolved arsenic in the main drainage streams is from natural sources. Even though the historic mine sites can discharge very high levels of arsenic locally, this is diluted downstream. However, the locally high levels of arsenic can have negative effects on stream biota in the immediate vicinity of the mine sites. Further information
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