Speciation of arsenic in water and sediments, in some mines within the Pine Creek Geosyncline, Northern Territory Australia

Abstract

This study of arsenic speciation in water and sediments from five mine sites within the Pine Creek Geosyncline (PCG), Northern Territory, examined transportation and fixation of arsenic forms due to one of the following discrete processes:

• alkaline (cyanide) seepage from tailings dam;
• acid drainage to retention pond prior to overflow;
• acid drainage run-off across bushland;
• erosion of mineral particulates from small scale mining; and
• transport of organic-rich alluvial gold tailings.

The study of arsenic species was based on operationally-defined forms in soils, sediment and evaporates and enabled interpretation of the key processes associated with arsenic retention. Modified sediment sequential extraction (Chunguo and Zihui, 1988) and HCl/Chloroform extraction (Chappel et al., 1995) procedures were used to measure the arsenic species as follows : soluble and loosely bound As, Al-As, Fe-As, Ca-As, Fe(OH)3 occluded As, organic bound As and residual As in sediment phases using the sequential As extraction procedure; and As (III), As (V), inorganic As, organic As and As(t) soluble in strong acid using the HC1/chloroform extraction. The arsenic speciation data for 13 species extracted from 117 spatial and 20 depth profile sediment samples and 26 water samples, was used to determine the mechanism of arsenic dispersion occurring at the various mine sites.

The study found that the following retention processes were involved in immobilising arsenic dispersion:


(i) In wetland systems, as at Tom's Gully gold mine anabranch, through precipitation with iron oxyhydroxides, calcium from alkaline groundwater and organic matter from detrital material;
(ii) Utilisation of retention ponds to act as sediment trap to retain arsenic and metal contaminants from alkaline and acidic source seepages as demonstrated at Goodall gold mine. Arsenic levels up to 1 ,700µg/g were held in the form of ferric arsenate, calcium arsenate and residual arsenic;
(iii) Seepage should be directed into retention ponds or borrow pits;
(iv) High levels of iron and organic matter helped in retention of residual arsenic and other mineral particulates during erosion or controlled process water discharges at Millars Battery Union Reefs mine, McKinlay River Union Reefs mine and MLN 95 Copperfield Creek at Pine Creek; and
(v) Natural geomobilisation of arsenic was observed in upstream sediments at Copperfield Creek (5-8µg/g), Mt. Bundey Creek (10-12µg/g), upstream Ryan's Creek (10-12µg/g) and downstream East branch Ryan's Creek (7µg/g). Erosion of mineralisation containing arsenic was observed in the McKinlay River upstream and downstream (23-26µg/g) and upstream Ryan's Creek boundary of MLN 1049 (24-40µg/g).

These features, as identified, can be utilised for future mine water management design.

Date of Award	2003
Original language	English
Supervisor	David Parry (Supervisor)