OR/17/056 Impact of mining

Lapworth, D J, Stuart, M E, Pedley, S, Nkhuwa, D C W, and Tijani, M N. 2017. A review of urban groundwater use and water quality challenges in Sub-Saharan Africa. British Geological Survey Internal Report, OR/17/056.

Introduction

This is a specific problem to particular parts of SSA which have a legacy of mining and problems of poor waste management, limited urban planning and rapid urban and peri-urban expansion. This can result in many informal settlements on or close to mine waste tips with potential for soil and water quality degradation and significant associated health risks for local residents.

Copper belt

The sulphide Zn–Pb–Cu ore deposits of the Central African Copperbelt in the Democratic Republic of Congo and Zambia are mostly found in deformed shallow marine platform carbonates and associated sedimentary rocks of the Neoproterozoic Katanga Supergroup (Kampunzu et al., 2009). Economic ore bodies, that also contain variable amounts of minor Cd, Co, Ge, Ag, Re, As, Mo, Ga, and V, occur mainly as irregular pipe-like bodies associated with collapse breccias and faults as well as lenticular bodies subparallel to bedding.

Kipushi, in the Democratic Republic of the Congo, and Kabwe in Zambia are the major examples of carbonate-hosted Zn–Pb–Cu mined deposits with important by-products of Ge, Cd, Ag and V in the Lufilian Arc, a major metallogenic province famous for its world-class sediment-hosted stratiform Cu–Co deposits (Kampunzu et al., 2009).

Gold belt

In the Migori Gold Belt, Kenya, gold occurs in quartz veins within mafic volcanics (Ogola et al., 2002). Mining was done on a large-scale up to the time of independence and thereafter reverted to artisan miners. This tends to be unregulated and managed with lack of awareness of metal poisoning issues. Panning is carried out along the river profiles and there is visible evidence of pollution from colouration and siltation. There are elevated concentrations of lead, mercury and arsenic in rivers and stream sediments.

Impacts

Mining and opencast workings can impact the environment and human health via a variety of chemical and physical routes (Morris et al., 2003) summarised in Figure 5.1. Human exposure can occur by accidental ingestion of mine wastes or waste contaminated soils by hand-to-mouth transmission, inhalation of dusts blown from tailings or waste piles, inhalation of gases or atmospheric particulates generated by smelting or roasting, and consumption of vegetables that take up metals from waste-contaminated soils or that accumulate mine waste dusts on their leafy parts (Plumlee and Morman, 2011). Nwankwo and Elinder (1979) measured cadmium, lead and zinc concentrations in soils and leafy vegetables close to a lead-zinc smelter at Broken Hill, near Kabwe, Zambia. The highest Cd concentrations were found in spinach, cabbage and rape.

Groundwater contamination

Sources of groundwater contaminants can be mine drainage, acid mine drainage (AMD), tailings lagoons and waste rock dumps (Morris et al., 2003). The routes to groundwater are shown in Figure 5.1. Mine waste piles can contain clay-sized particles to boulder sized blocks of unmineralised or partly mineralised rocks (Plumlee and Morman, 2011). Tailings contain ore, host rocks, and alteration minerals that were ground to <500 μm particles, and are often enriched in non-economic iron sulphides, crystalline silica, and alumina-silicates. AMD forms when oxygenated rain or groundwater oxidizes iron sulphides exposed in mine workings, mine wastes, or tailings impoundments. AMD can contain high levels of potentially toxic metals (e.g. lead, cadmium, nickel) and metalloids (e.g. arsenic, antimony) leached from the ores.

File:OR17056fig5.1.jpg

Figure 5.1    Main pathways of mine contamination to human receptors including the groundwater pathway (after Klinck and Stuart, 1999).

Post mining, secondary minerals form by the weathering of mine wastes, and these can include soluble salts formed by the evaporation of AMD. These salts commonly contain high levels of iron, other metals, and acid, and easily dissolve when they come into contact with water (Plumlee, 1999). Calcines and particulates produced by roasting or smelting of sulphide ores include water-soluble chlorides, oxides and sulphates of the ore metals; weathering can transport the metals into iron or manganese oxides. Smelter slag contains metal-enriched glass, residual sulphides, and metal chlorides, sulphates, and oxides.

Contamination of drinking-water supplies by mine wastes, AMD, and mineral processing solutions can introduce low pH, sulphate, calcium and magnesium derived from AMD, and its interaction with the aquifer matrix can introduce iron, manganese and a range of other heavy metals. Water can also be contaminated with asbestos, and other fibrous silicate minerals and by processing chemicals such as cyanide (Plumlee and Morman, 2011).

Mining of nickel-copper-cobalt ore at Selebi-Phikwe, Botswana, started in 1974. The area is underlain by high-grade metamorphic rocks with a 30 m weathered zone. Seepage from a tailings dam was identified as a major source of groundwater pollution (Schwartz and Kgomanyane, 2008). Seepage water had a pH in the range 1.7 to 2.8 and contained high concentrations of sulphate (5680 mg/L), nickel (6230 µg/L), copper (1860 µg/L) and cobalt (410) µg/L. Groundwater has a relatively low pH (generally <6) and contains relatively high concentrations of calcium and magnesium. Most of the heavy metal content is therefore scavenged within 500 m by iron and manganese oxides, but the sulphate is not attenuated.

Investigation of the impact of acid mine drainage in Selebi-Phikwe, showed that shallow soils had reacted with the leachate at the surface (Shemang et al., 2003). This had led to relatively conductive shallow layers containing sulphate, from pyrite oxidation, and Cu and Zn enhanced above background levels.

Zambian copperbelt

The Zambian Copperbelt has a high proportion of tailings impoundments, residue heaps, and extensive ore deposits close to high-density informal settlements (von der Heyden and New, 2004). Seepages from active and decommissioned tailings deposits pose a substantial threat to groundwater that may be used as a source of domestic water by the local residents.

At a site in the Chambishi catchment von der Heyden and New (2004) identified a high solute plume, possibly derived from a tailings impoundment either by dry deposition of tailings dust or, more likely, by leaching from the tailings material. The plume was 500–700 m down-gradient of the tailings dam and characterised by high concentrations of sulphate, calcium, magnesium, cobalt, nickel and zinc in groundwater (Table 5.1). Within the main plume heavy metals were low, thought to be due to the precipitation of hydroxides and sulphides and sorption to organics and clays. While pH remained buffered in the impoundment and groundwater the plume was assessed as not posing a major risk. In an assessment of current and post-closure pollution potential from mining (Limpitlaw and Smithen, 2003) identify a wide area that has been impacted by copper mining in the upper Kafue area.

Branan (2008) assessed Kabwe to be one of the ten most poisonous places on earth due to contamination of soils by heavy metals (e.g. Ikenaka et al., 2010; Nakayama et al., 2011; Tembo et al., 2006). Branan (2008) reports studies carried out in Kabwe that indicate tens of thousands of residents are suffering from severe lead poisoning.

Table 5.1    Summary of concentrations of metals in solid tailings and groundwater in Chambisi, Zambia (after von der Heyden and New, 2004).

		Average concentration (range) (mg/L)		Average concentration (range) (µg/L)
Type		Al	Fe	Co	Cu	Ni	Zn
Solid tailings		35 000	63 000	17 000	21 000	1700	2500
Groundwater	Tailing	0.032 (0.02–0.05)	2.05 (1.68–2.42)	24.7 (23.8–25.7)	10.1 (1.7–18.5)	7.5 (5.4–9.6)	65.4 (39.7–91.1)
	Plume western	0.03 (0.02–0.05)	5.09 (2.19–7.83)	124 (<50–206)	12.6 (10.0–24.0)	8.1 (4.7–16.0)	79.3 (33.0–133)
	Plume eastern	0.03 (0.02–0.04)	4.16 (0.22–3.68)	9.7 (7.0–10.0	7.8 (3.0–11.3)	4.7 (2.7–5.9)	46.2 (16.5–83.6)
	Background	0.107 (0.09–0.12)	0.352 (0.00–0.52)	6.6 (3.3–9.9)	15.1 (13.0–17.3)	3.0 (1.5–4.5)	23.9 (12.8–35.0)