Trace elements dispersion from a tailings dam and speciation in surrounding

agricultural soils

Kombat Mine, Otavi Mountainland, Namibia

Marta Mileusnić, Stanko Ružičić University of Zagreb, Croatia

Benjamin Siyowi Mapani, Akalemwa Fred KamonaIsaac Mapaure, Percy Maruwa Chimwamurombe

University of Namibia, Namibia

STUDY AREA - Kombat

Grootfontein districtlatitude 1943´Slongitude 1743´Ealtitude of 1.590 metres

2 000 inhabitants

Agriculture:(1) livestock production(2) maize, potatoes, vegetables and subtropical fruits

Kombat Cu mine mining: 1962 – 2008 (46 years) 24,550,280 t of copper ore

tailing dam (about 300 million T)

Area: 20 ha Height: 3-4 m

winter

summer

Sub tropical climate

Summers-very hot-heavy downpours-northeastery winds

Winters (May –June)-mild-dry winters - easterly winds

Main agents of material transfer from tailings into surrounding areas

WINTER - WIND SUMMER - RAIN WATER

(as run-off in episodes of heavy downpours

RESEARCH GOALS

(1) to determine lateral distribution and seasonal variability of trace metals, as well as dominant metal binding mechanisms in the agricultural soil

(2) to identify dominant type of metal dispersion from the tailings impoundment (mechanical or chemical)

(3) to carry out preliminary assessment of environmental risk associated with tailings dam

SAMPLING

- after results of Kribek et al.

(2006)

2) polluted soil

3) control soil

1) tailings

-2 sampling campaigns:-Wet season – march-Dry season – august

Analytical work

(1) bulk mineralogy of soils and tailings (XRD)

(2) Pseudototal metal content of soils and tailings (aqua regia digestion, AAS)

(3) Binding sites of metals (sequential leaching, AAS)

Fraction Label Reagent

Exchangeable ADS/EXC Ammonium acetate

Acid (carbonates)

CARB Acetic acid

Reducible FEMN Hydroxylamine hidrochloride

Oxidisable OR/SUL Hydrogen peroxide

Modified after: TESSIER et al. (1979) and SULKOWSKI et al. (2006)

RESULTS

Mineral content tailings: calcite and dolomite, quartz, micaceous material, +/-gypsum

soil: quartz, calcite, dolomite, K-feldspar, plagioclase, clay minerals.

pHtailings: circum-neutral (7.0-7.7)soil: alkaline (8.1-8.6)

Pseudototal metal concentrations (median values in mg/kg)

- no seasonal variation in concentrations

Canadian soil quality guidelines (CECME, 1999) for agricultural soils

Element

Tailings Polluted farmland soil

Control soils

Cu 1328 112 18

Pb 946 117 17

Zn 107 49 27

Cr 16 20 27

Ni 3 9 12

Co 6 5 5

Cd 3 BDL BDL

Metals binding forms in polluted soils

0

50

100

150

200

250

300

350

400

450

500

ADS-EXC CARB FEMN OR/SUL RES

soil fraction

mg/

kg

Cu

0

100

200

300

400

500

600

ADS-EXC CARB FEMN OR/SULF RES

soil fraction

mg/

kg

Pb

CONCLUSIONS

(1) Cu and Pb showed significantly high concentration in tailings.

(2) Cu and Pb in all analysed samples of farmland soils west of the tailings dam exceed maximal allowed values for agricultural soils.

(3) Seasonal variations - not observed.

(4) The main binding site for Cu and Pb is oxide fraction

(5) mechanical dispersion of elements from tailings dam prevails

(6) carbonate minerals dominate in tailings - drainage is neutralized - lead and copper immobile

(7) Pb and Cu relatively strongly bound to the soils + high pH of soil- small risk for their release in soil water and to groundwater

(8) study by Casteel et al. (2006) - lead associated with iron oxide shows intermediate bioaccesibility while ingested

(9) Rehabilitation of tailings dam necessary

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