Arsenic is a well known element that is very poisonous to human and other living organisms once it is mobilised into the environment due to either natural or anthropogenic causes. Soil and water contamination by arsenic has been reported all over the world.
The Mole River Arsenic Mine is located in the northern New South Wales, Australia. It was one of the largest arsenic mines in the New England Fold Belt and its geological formation was associated with the intrusion of plutons and with volcanic activity in the Late Permian to Early Triassic. Arsenic trioxide (As2O3) and arsenic pentoxide (As2O5) were produced during the period 1924-1935 by Roberts Chemicals Limited from arsenopyrite mined from the sporadic lenses, bungs or pipes which are hosted in the metasedimentary Texas beds. A total of 19000 tonnes of arsenopyrite ore was mined and processed at the site. The mine was closed in 1940 but it was not until 2000 that it was partially rehabilitated.
Some environmental research studies have been undertaken to define the arsenic contamination at the site and in the adjacent areas, however the geochemical and physical factors relating to arsenic mobility at the site have not been thoroughly investigated. This study was therefore proposed to fill the gap in this knowledge. The site was surveyed in detail, so that different areas could be accurately defined and characterised in terms of their soil types, vegetation, hydrology and extent of human impact. The leachability of arsenic, the hydraulic conductivity of the soils, the water release characteristics of the soils, site erosion, and the geochemical modelling of arsenopyrite oxidation were studied in detail for each area. The total concentrations of arsenic and other elements (Mg, Al, Fe, Mn, P, Cu, Zn and Pb), the soil mineralogy, the contents of organic matter and carbonate, the water chemistry and some other physicochemical parameters were also determined. The data was used to quantify the potential for arsenic release from the site, its mobility and the overall stability of the site. Finally, some recommendations for the rehabilitation of the site have been put forward.
Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES) and Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) were used to determine the concentrations of elements in total digests, in leachates obtained from leaching test using step 1 of the BCR sequential extraction scheme, and in surface water. The contents of organic matter and carbonate were determined from Loss on Ignition. Mineralogy was determined by X-Ray Diffraction. The Geochemist’s Workbench® software and its database were used to model the oxidation of arsenopyrite. To determine soil physical factors, the intact core method was used for the measurements of soil bulk denisty, porosity, and initial soil water content. The constant potential supply method was used for the determination of hydraulic conductivity. Water contents at six different water potentials were determined by using sand bath, a vacuum pressure assisted ceramic suction plate, and a pressure plate apparatus. Finally, the RETC software was used to interprete the water contents in a wide range of water potentials.
The concentrations of arsenic on the site soils ranged from 22 to ~ 300000 mg/kg, with an average of 44700 mg/kg. These values exceeded by about 4000 times the recommended arsenic levels issued by the National Environment Protection Council for soil quality (ANZECC/NHMRC, 1992) and were four times higher than the arsenic level found in one of the previous studies. The concentrations of arsenic varied in different soil types and greatly depended on the degree of the disturbance of the soil by the previous mining activities. Although the mine site was rehabilitated in 2000, the soils at the site have remained extremely contaminated by arsenic.
Two important sources of the arsenic are the waste rock heap and the residual processed materials in the furnace and the flue. The results of soil mineralogy derived from this study (and from the existing knowledge) showed that arsenopyrite (FeAsS), chalcopyrite (CuFeS2), arsenolite (As2O3), pharmacolite [Ca(HAsO4).2(H2O)], clinoclase [Cu3AsO4(OH)3], claudetite (As2O3) and krautite (Mn(HAsO4).2H2O) dominate in the waste rock and in the residual processed materials. Arsenic had been transported from these sources in AMD water to the soils. Much of the arsenic was held by ferrihydrite (5Fe2O3.9H2O), jarosite [KFe3(SO4)(OH)6] and goethite [α-Fe3+O(OH)] and clays, and arsenic was also found to coprecipitate with iron to form scorodite (FeAsO4.2H2O). These associations were proved through significant correlation of arsenic with other elements such as Al, Fe, Mn, Cu, Pb and finely grained soils (FGS).
Leaching tests showed that the order of the leachablity of the elements in soils was Mn>Zn>Cu>P>Mg>As>Al=Fe>Pb and an average of 2.2% of the arsenic was leached from the soils. The leachability of arsenic was strongly dependent on the availability of the easily mobile fraction of arsenic bound to soils, and the forms of the solid phase which bind arsenic in the soils. Leached arsenic was not only derived from the easily mobile fraction bound to the soil surface, but also from dissolution of arsenic-bearing carbonates because the leachablility of arsenic was associated with the leachability of Mg and Mn and the content of carbonates in the soils. The absence of any associations of arsenic with Al, Fe, Cu, Zn, and Pb showed that arsenic was still held to the mineral forms of these metals in the soils. Although the leachablility of arsenic was low, the actual concentrations of mobile arsenic was elevated (200 mg/kg in average). Under natural conditions, arsenic in the soils at the mine is potentially mobilised with seepage or runoff water and eventually discharge into the local aquatic system. This migration however is strongly controlled by the degree of erosion of the soils at the site and by the textures, hydraulic conductivities and the water release characteristics of the soils.
Measurements of the density, porosity, hydraulic conductivity and water content of the soils showed that the movement of water in the soils is not favoured for the retention of arsenic. The soils, particularly where they have been strongly disturbed by mining activities, have low densities (1.3 g/cm3), high porosities (50%) and hydraulic conductivities in the range of 400 – 17000 cm/day. The values of water content in a range of water potentials of 1 – 15000 hPa show that the soils are unlikely to be able to retain water. Approximately 50% of total water is lost at a water potential of 50 hPa (corresponding to the field capacity), indicating that when the mine site is subjected to rain, the soil water is immediately transported within the soil and arsenic is likely to be carried with the water.
Soil erosion can be also a significant factor in the migration of arsenic. The erosion rate for the site has been estimated at 29 tonnes/ha/year which shows that the site has been subject to heavy erosion. An interpretation of rainfall records and soil profile shows that the heavy erosion has occurred at least twice since the site was rehabilitated in 2000. When the soils were saturated by high intensity rainfall, any rainwater falling on the mine site catchment area would have formed erosive runoff which would have physically carried arsenic-bearing materials to Sam’s Creek, located immediately south east of the site.
The results of sediment and water chemistry confirmed the migration of arsenic to Sam’s Creek. The concentrations of arsenic in sediments upstream of the site were relatively low (240 mg/kg), but very high (2880 mg/kg) in the sediments collected next to the mine site, but then they decreased downstream of the mine site. This decrease in arsenic concentration in the downstream sediments plus the associations of arsenic with Mg, Al, Fe, Mn, Cu, Zn and Pb and the presence of ferrihydrite, scorodite and carbonate shows that arsenic is co-precipitated and adsorbed immediately downstream. Water chemistry in Sam’s Creek showed that arsenic in the water ranged from 12 to 2461 μg/L and increased downstream from the mine site. This increase in arsenic in the water indicates the remobilisation of arsenic bound to the sediments (derived from dissolution of scorodite and carbonates). Although the Sam’s Creek water had a high level of arsenic, which exceeded the ANZECC guideline values for irrigation, livestock and human drinking water, its level had fallen to below these recommended values after the water from Sam’s Creek is discharged into the Mole River. The mechanisms leading to the decay of arsenic in the Mole River are dilution and co-precipitation. The reduction in Fe concentration in the Mole River water and arsenic in Sam’s Creek water, and the presence of scorodite in the sediments at the confluence suggest co-precipitation of arsenic with iron. However, dilution apparently plays a vital role in reduction of arsenic in the Mole River water. Although the arsenic level was below the recommended values for water use, the Mole River water still suffered from elevated concentrations of Fe and Ni and thus it is not recommended for irrigation, or for livestock and human consumption.
It is concluded that geochemical and physical factors have helped to define the sources, the level of contamination, and the pathway of arsenic at the Mole River Arsenic Mine site. The results also showed that the mine site has not been chemically and physically stabilised as it is evident that the waste rock heap and the residual processed materials in the collapsed furnace and the flue are undergoing ongoing oxidation and dissolution; soil erosion is occurring on the barren, non-vegetated catchment areas of the site and arsenic is being mobilised and transported to the aquatic system. These environmental concerns require the long-term stabilisation of the mine site, and the implementation of an effective rehabilitation program.
Geochemical modelling of arsenopyrite oxidation showed that the oxidation of arsenopyrite in the presence of oxygen and water at pH 5.5, but without the adsorption and buffering capacity of non-sulphide minerals, led to a very acidic solution (pH 2.3) with a high arsenic content. When the non-sulphide mineral (calcite) took part in the reaction, the acidity remained unchanged, but the solution has a higher concentration of arsenic since adsorption was not taken into account in this model. However, when adsorption was considered, only 20% of the arsenic was removed from the solution. Any rehabilitation program should therefore provide for not only the revegetation of the catchment, the reconstruction of the sedimentation ponds, the building of a new repository into which all the strongly contaminated materials should be placed, but should also include the application of lime or limestone to the waste rock heap to reduce the acidity. The lime or limestone should be applied during the reshaping, reconstruction and capping of the waste rock heap, and the amount of lime needed to maintain the pH at around 5 should be determined. It is also recommended that the capping of the waste rock heap should be finished in a concave shape to avoid water infiltrating into it. In addition, Fe salts should be applied in the furnace and to the flue to increase the coprecipitation of any dissolved arsenic in runoff. The banks of Sam’s Creek next to the mine site should be stabilised by suitable riprap materials. The implementation of these measures should ensure the long-term stability of the Mole River Arsenic Mine.