Автор: D. Banks

Источник: http://www.image-train.net/products/IT_ASC2_proceedings.pdf

GEOCHEMICAL PROCESSES CONTROLLING MINEWATER POLLUTION

Minewater is a subset of groundwater, subject to broadly similar hydrochemical processes. In “normal” groundwaters, access to oxidising species is poor and acid-base reactions tend to dominate over oxidation reactions. Acid-base reactions such as carbonate dissolution and silicate hydrolysis consume protons and carbon dioxide, and release alkalinity and base cations. In mines, the atmospheric environment is rapidly introduced to the deep reducing geosphere (or vice versa in the case of mine waste deposits). This carries the possibility of intense and rapid oxidation of sulphide minerals such as pyrite, to such an extent that these acid-generating redox reactions may dominate over acid-base “neutralisation” reactions and result in the phenomenon of “acid rock drainage” (ARD). In ARD, a negative correlation is typically observed between pH and concentrations of many metals and metalloids, base cations and sulphate. This correlation is due to (i) genetic co-variation – generation of pro- tons, sulphate and metals in sulphide weathering reactions, (ii) pH-dependent solubility of many ARD-related metals and (iii) low pH intensifying carbonate dissolution and silicate hy- drolysis to release aluminium, silica and base metals. This paper examines the reactions in- volved in ARD generation and neutralisation, and attempts to clarify key concepts such as pH, Eh, alkalinity, acidity and equilibrium constants.

Introduction

Groundwater is often defined as water occurring within the subsurface geological environ- ment. Mine water is thus merely a type of groundwater, subject to the same geochemical processes as “normal” groundwater. Mine water often appears very different to the pure spring water beloved of poets and bottled water manufacturers, however: it may be highly acidic, brightly coloured and packed full of salts and potentially toxic metals. So what is it that creates the difference between our flask of Evian and our sampling bottle of mine water from San Jose silver/tin mine? Why are some mine waters alkaline (Banks et al., 2002b), while others are acidic and rich in aluminium, iron and other metals (Banks, 1994)? This pa- per attempts to provide an introduction to some of the answers. We should start, however, by examining, in outline, some of the processes which give all groundwaters (including mine waters) their characteristic chemical signatures.

The Hydrochemistry of Groundwater

Every groundwater has its own unique hydrochemical fingerprint. This is derived from the in- terplay of various processes at various stages along the groundwater’s flow path:

Recharge Chemistry

Groundwater may retain some of the characteristics of the water (rainfall, snowmelt or infil- trating river water) that was the source of its recharge. For example, newly recharged groundwater will often contain:

• An isotopic signature (2H, 3H, 18O) characteristic of the rainfall at the geographic loca- tion, time and altitude of recharge
• An “atmospheric” chloride content, which may increase with decreasing distance from the coast (due to marine aerosols in the atmosphere, Banks et al. 1998).
• A content of atmospheric “pollutants” from industry or motors: nitrate, sulphate, chloride
• A high content of dissolved oxygen

Water-Rock Interaction

This innocent term covers a huge range of geochemical processes that describe various ways in which groundwater reacts with minerals in the subsurface. The most important of these will be discussed in the next section. In general, water-rock interaction processes in “normal” groundwater tend to result in:

• Consumption of dissolved O 2 and CO 2
• Elevation of pH and production of alkalinity
• Release of base cations

Mixing Along its flow path, groundwater may mix with other water “facies”, e.g.

• Mixing with deep saline “formation” water within the aquifer
• Mixing with intruding saline water from a surficial source (e.g. sea water)

Water-Rock Interaction

The main reactions taking place between water and mineral phases in the subsurface fall into four main categories:

Dissolution Reactions E.g. halite: NaCl ? Na + + Cl - or fluorite: CaF 2 ? Ca ++ + 2F -

What is so Special About Mine Water ?

The earth’s immediate subsurface is a reaction front between the atmosphere (generally oxidising and acidic) and the geosphere (generally reducing and basic). Groundwater is the circulating medium which carries atmospheric reactants (oxygen, carbon dioxide) into the geosphere. The zone of groundwater circulation is typically the zone where redox and acid- base reactions occur.

In normal groundwater environments, the contents of oxidisable minerals (e.g. pyrite) are so low or the access to oxidising species (e.g. oxygen) is so poor, that acid-base reactions (which consume protons) dominate over redox oxidation reactions (which may generate protons). Thus, “normal” groundwaters have typically neutral to slightly alkaline pH (Freng- stad & Banks, 2000), dominated by base cations (Ca ++ , Mg ++ , Na + ) and bicarbonate. Fig. 1:

The zone of groundwater circulation is a reaction front between the oxidising, acidic atmos- phere and the reducing, basic geosphere. The zone of groundwater circulation is character- ised by acid-base and redox reactions. The rate of reaction will increase where flaws in the geosphere (e.g. mines) allow rapid circulation of water and oxygen, or where geosphere ma- terial is transported directly to the surface environment (mine waste tips).

When we dig mines, we introduce the rapid circulation of oxygen and water into the deep geosphere, in zones where there are high concentrations of oxidisable minerals (sulphides).

Similarly, when we create mine waste tips, we are bringing deep sulphide-rich geosphere up into the atmosphere, with often excellent access to circulating water and oxygen. Thus, in mine or spoil tip environments, oxidation reactions may dominate over acid-base (neutrali- zation) reactions, resulting in the phenomenon of acid rock drainage (ARD). mine waters may be dominated by:

• Low pH
• Elevated sulphate concentrations
• Elevated concentrations of metals However, if pyrite / marcasite is also present, the acidic environment generated will assist in mobilising other metals such as Zn and Pb, whose solubility is pH-dependent.

Characteristics of Mine Waters

From the Tables, the following points should be noted:

There is considerable variation in mine water chemistry, even amongst mines of the same type. This can be ascribed to factors such as: access to and rate of circulation of water and oxygen, neutralisation potential of host rocks and ambient groundwater, mor- phology and mineral content of sulphide minerals, age of mine discharge. Particularly aggressive (i.e. metal-rich, acidic) mine waters would thus be expected where:

• Mines or spoils have a high pyrite content, especially if fine-grained
• Mines or spoils have good access to oxygen (i.e. unflooded mines)
• Water throughput is low. Lack of dilution may produce very aggressive mine waters (e.g. San Jose)
• Recent flooding of a mine has resulted in a “first flush” of accumulated pyrite weather- ing products from a mine
  • Metals mines generally have the potential to generate more aggressive mine waters than coal mines.
  • The most aggressive discharges emanate from recently flooded (or, sometimes, pump- ing) mines.
  • As pH and alkalinity increase, contents of metals, sulphate and base cations generally decrease.
  This is due to several factors:
• Metals, protons and sulphate are all released upon sulphide weathering. A co-variation in these parameters would thus be expected Low pH promotes hydrolysis of carbonates and silicates, thus releasing base cations to the water
• Low pH promotes solubility and mobilisation of most heavy metals (including Fe, Zn, Cu, Al, Pb etc.)
• Chloride concentrations are independent of pH. The final water in Table 3 is from natural landslipped, broken strata (pyritiferous Millstone Grit shales) at Mam Tor mountain, Derbyshire, UK. This demonstrates that “acid rock drain- age” can have a purely natural origin (Vear & Curtis, 1981, Banks, 1997, Banks et al., 1997a).

Orgreave, near Sheffield, UK. Coal mine spoil from a deep Carboniferous Coal

Measures mine. The formation water is saline and alkaline and has not yet been fully flushed from the mine waste (Banks et al., 1997a).

The “First Flush” Phenomenon

While a mine is worked and dewatered by pumping, sulphide oxidation proceeds in dewa- tered strata. Intermediate oxidation products (sulphate and hydroxysulphate minerals) ac- cumulate in these unsaturated strata.

These secondary products may form stalactites, stalagmites, efflorescences and growths in abandoned mines, as was observed in the San Jose mine in Oruro, Bolivia (Banks et al., 2002a).

When the mine closes and floods, rising mine water levels dissolve and mobilise all these accumulated secondary products and acidic pore waters, resulting in a highly potent “first flush” of concentrated mine water. +

With time, the high initial acidity and metal concentrations of the first flush decay. The con- centrations often seem to follow a quasi-exponential decay curve, indicative of flushing out of accumulated oxidation products (“vestigial” acidity / contaminant loading) from the mine system with fresh recharge water. The concentrations tend, in the long term, towards a steady state contaminant loading that reflects new, ongoing (“juvenile”) acidity production from pyrite weathering.

Figure 2 shows the decay of “first flush” concentrations following the overflow of Wheal Jane tin mine in Cornwall. In fact, Younger (2000) and Younger et al. (2002) have studied empirical data and found that the time taken for exponential decay flushing (t f ) can be re- lated to the time taken for the mine to fill with mine water following cessation of pumping (t r ). In other words, the rate of decay is related to the floodable mine volume, as one would ex- pect in a flushing model. Younger (2000) and Younger et al. (2002) found that t f is approxi- mately equal to four times t r

Evolution of contaminant loading from Wheal Jane tin mine, following its overflow in 1992. Note the exponential decay of “first flush” concentrations towards a steady state (after Younger et al., 2002). Fig. 3: Time taken for decay of “first flush” concentrations to a steady state condition (after Younger et al., 2002). Stratification of chemistry during flooding within Wheal Jane tin mine, Cornwall (after Younger et al., 2002). Note that such stratification may be destroyed by turbulence once the mine overflows or is pumped (Nuttall et al., 2002). It should be remembered, however, that the volume being actively flushed may not be the total volume of the mine. Some mine systems develop a stratification (see Figure 4), where only the upper portion of the mine is being actively flushed, and the lower part contains “stagnant”, highly contaminated mine water.

Can We Predict the Quality of Mine Drainage Water

Well, no, not really. There are usually too many unknown variables. We can, however, make some general observations.

1. Acidic, contaminant-loaded mine waters are characteristic of mine systems which are unsaturated, with rapid throughflow of water and good access for oxygen, e.g. workings from surface outcrops, which may be under-drained by a sough or adit (Figure 5a).
2. Flooded workings (especially coal mines), with poor access for oxygen and slow wa- ter throughflow, are often characterised by more circum-neutral mine waters (Figure 5b).
3. Some researchers have found tentative correlations between coal mine water iron concentrations and (a) stratigraphical proximity to marine beds in the UK Carbonifer- ous Coal Measures (characterised by high sulphur contents) and (b) distance to out- crop of most closely associated coal seam (MCACS) – Figure 6.

Younger (2000), having studied some 81 UK coal mine discharges concluded that:

• If the worked seam was within 25 m (stratigraphically) of a marine bed, peak first-flush concentrations in excess of 100 mg/l Fe could be expected, Otherwise, concentrations not exceeding some 10s of mg/l are likely
• For coal mines >0.5 km from the outcrop of the shallowest worked seam, long term Fe concentrations of around 7 ± 1.6 mg/l were typical. If <0.5 km, an Fe concentration of 19 ± 2.9 mg/l was regarded as characteristic.
• Fe concentrations could be related to the sulphur content of the worked seam, if known

Precipitation of Ochre and Other Minerals in Recipient Watercourses

When minewater emerges from a mine, increased access to oxygen and possible increase in pH on mixing with recipient waters, may cause oxidation, hydrolysis and precipitation of metal oxyhydroxides or other salts. For example, ferrous iron may oxidise and precipitate as an orange ferric oxyhydroxide (“ochre”), which may be written as Fe(OH)

The overall ochre precipitation reaction is thus proton-generating. It is thus potentially self- limiting: generation of protons may lower the pH to a point where ferric ions no longer pre- cipitate as a hydroxide, unless adequate neutralisation capacity (i.e. alkalinity) is present in the water. Similarly, dissolved aluminium may precipitate as a white hydroxide

However, aluminium is only soluble in the most acidic mine waters. Thus, precipitates from acidic mine waters tend to contain aluminium hydroxide and may be whiter or yellower in colour than those precipitating from more circum-neutral mine waters, which are typically more reddish-orange in coloration.

Many other minerals may be found in stream bed precipitates. In very aggressive mine wa- ters, (for example, San Jose, see Table 1) both calcium and sulphate concentrations may be high, leading to precipitation of gypsum:

In the early life of a mine waste tip, for example, any calcite present would be used up rapidly by the oxidation of pyrite. Thus, initially, the presence of minor amounts of calcite would be expected to maintain a relatively high pH in the leachate from the mine waste.

Due to fast reaction kinetics, the calcite would eventually be consumed, if present in less amounts than pyrite. pH would then drop, as silicate weathering would be too slow to effec- tively neutralise acid generated by pyrite oxidation. With time, however, the pyrite content would diminish and silicate neutralisation might become significant. Thus the leachate drain- ing from a mine waste pile comprising:

• a dominant fraction of silicate waste rocks
• some residual pyrite
• a minor content of calcite,
• residual deep saline and alkaline pore water

• Phase 1: Flushing of saline pore water from waste rock. Alkaline saline leachate
• Phase 2: Consumption of calcite by acid generated by pyrite weathering. Circum-neutral pH.
• Phase 3: Calcite fully consumed. pH drops dramatically as pyrite oxidation proceeds unbuffered
• Phase 4: Pyrite begins to be used up. Silicate buffering becomes significant. pH rises. Of course, in real, heterogeneous, mine wastes, the picture will not be so simple. Such flushing and geochemical processes may take places in zones, related to the progressive inward / downward migration of recharge water and oxygen, or within “hot spots” or prefer- ential pathways with good access to water and oxygen throughflow.

Other Types of Mine Water Pollution

Of course, although sulphide oxidation, leading to the phenomenon of acid rock drainage, is the most familiar type of mine drainage pollution, other issues may also arise (Banks et al., 1997b):

• Salinity: saline mine waters may be derived from salt mines, or from deep coal (e.g. Tilmanstone, Kent: Buchan 1962) or metals mines (e.g. San Jose, Bolivia: Banks et al., 2002a), where deep saline formation waters may be encountered. Alternatively, saline waters may be derived from intrusion of sea water in near-coastal mines. At Tilmanstone, disposal (infiltration) of saline mine waters led to significant contamina- tion of the regionally important Chalk aquifer.
• Ammonium: deep coal mines, especially in shaley, mudstone environments (e.g. in the East Midlands of the UK: Banks et al., 1997a,b) are known to produce pumped mine waters with a high content of ammonium (and a high salinity, Downing & Howitt, 1969). Discharge of ammonium to recipient watercourses is regarded as a significant water quality issue in this context.
• Nitrate: use of nitrogen-based explosives in mining or quarrying can lead to elevated nitrate concentrations in groundwaters. This phenomenon is documented to have led to the contamination of a borehole near a quarry at Koppera, Norway (T. Moseid, pers. comm.) and is suspected at some Siberian iron ore mines (Banks et al., 2002b).
• Oil / drilling fluids: Contamination by oils or drilling fluids used in the mining or quar- rying process is also a recognised threat (Arnesen & Iversen, 1995).
• Radium and barium: in deep coal mine waters, which are low in sulphate (reducing environment), the solubility product of the relevant sulphate mineral (barite) may not be a limiting factor for barium solubility. Thus, elevated concentrations of barium, and its chemical analogue, radium, may occur. This phenomenon is documented from the Tyneside coalfields of the UK, and the Silesian coalfields of Poland (Banks et al., 1997b, Lebecka et al. 1994).
• Organics: the presence of organic micro-contaminants in mine waters from coal or lignite mines is poorly researched. However, a tentative link between refractory fluo- rescent substances in waters related to lignite exposures with Balkan endemic neph- ropathy has been suggested (Goldberg et al., 1994). Minewater: an Environmental Resource While minewater pollution tends to experience an unremittingly negative press coverage, it should be remembered that mine waters can also be regarded as an environmental re- source, as documented by Banks et al. (1996):
• Mine water discharges may provide low-N, bacteriologically pure baseflow to rivers: for example, the Rivers Drone and Dove in Derbyshire and Yorkshire, UK, which are otherwise very highly loaded with sewage effluent.
• Mine water discharges from limestone-hosted sulphide mines can be of high enough quality to be used as sources of drinking water (e.g. Meerbrook Sough, Derbyshire, UK)
• Mine waters can be sources of minerals (such as alkali salts, barium: Tyneside, UK) or can be employed as mineral water spas (Matlock, Derbyshire, UK [Albu et al., 1997] and Joachimstal)
• Ferruginous mine waters have been used in practice as flocculating agents at sew- age treatment works (Buxton, Derbyshire, UK: Roberts & Leach, 1985). Ferric sul- phate is a recognised flocculent salt.
• Mine water is an ideal source upon which to base heat pump solutions for space- heating and cooling of housing complexes, large commercial developments or public buildings. Examples can be found from Scotland, Norway, Canada and the USA (Banks et al., 2002c, in prep.)

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Contact
David Banks
Holymoor Consultancy
86 Holymoor Rd, Holymoorside
UK - S42 7DXChesterfield, Derbyshire
Tel.: + 44 1246 23 00 68 Email: david@holymoor.co.uk