*Bruce W. Downing, M.Sc., P.Geo. is Senior Geologist, Gamah International Ltd., Vancouver.

Editor's Note: This page is based on a paper by Bruce Downing that was originally developed to explain the importance of natural ARD in an environmental context in the Province of British Columbia, Canada. Some references, therefore, are to specific Regulations and Regulatory Guidelines applicable in this Province. (updated in July 2000)


The dynamics of acid rock drainage (ARD) have been described in numerous papers and are essentially reproduced on various pages of this site. Surface water creeks, rivers, lakes, etc. and sub-surface groundwater contain many chemical compounds, usually in very small concentrations. Many of these chemicals occur naturally while some are attributable to human activities. Often they are present in both surface and groundwater as a combination of anthropogenic and natural sources. Groundwater ARD effects on terrestrial animals and plants, and to a lessor degree aquatic organisms, resulting from groundwater combining with surface water are detectable distant from the actual source due to groundwater flow regime/path and type of substrate it passes through and/or comes in contact with.

While there are many examples of combined effects (e.g. the disposition offertilizerss and pesticides used in agriculture), the subject of this page is Acid Rock Generation from natural sources and events. The exposure of sulphide bearing material occurs naturally, but can be increased (often by millions of tons) through human activities (anthropogenic) such as highway construction, mining, quarrying, civil engineering works (for example the construction of the Halifax, Nova Scotia, airport) and by logging activities.

The term Acid Rock Drainage (ARD) is a misnomer as it implies drainage from an acid generating source, whereas the acid generating source may not be connected with drainage. Acid Rock Drainage is often referred to as Acid Mine Drainage, which occurs only in association with mining activity. The term Acid Mine Drainage, as commonly used in published literature, implies that acid generation only occurs at mine sites, and does notrecognizee that the process is a naturally occurring one that has been taking place for millions of years. Therefore the term Acid Rock Generation is a more appropriate term for the description of natural events, even though Acid Rock Drainage (ARD) is the accepted generic descriptive term in western Canada..

ARD is produced by atmospheric oxidation of the relatively common iron-sulphur minerals pyrite (FeS2) and pyrrhotite (FeS), and any other products generated as a consequence of these oxidation reactions in the presence of Thiobacillus ferrooxidans. Significant amounts of heavy metals may be solubilised by this process. Acid Rock Drainage is often referred to as Acid Mine Drainage (AMD) which occurs only in association with mining activity. When the process occurs naturally, geologists measuring the levels of metals or sulphate in surface or ground waters usually infer the presence of a mineral deposit. Surface to subsurface alteration of rocks resulting from ARD, either in-situ or at some distance from the source of generation, are usually oxidized, and referred to as gossans, Photographs 1-3.

Photograph 1: Gossan, Stewart Area, BC (courtesy Placer Dome Canada, S. Gardiner)

Photograph 2: Gossan; Recently Uncovered Beneath Glacier, Stewart Area, BC

Photograph 3: Gossan, Northern BC


Generally, all iron-sulphide bearing deposits have a rust or yellow surface expression (oxidized cap) due to oxidized sulphide minerals, Photographs 1-3, and result from three processes, namely, sulphide oxidation and dissolution, precipitation of secondary minerals and pervasive rock leaching. Statements such as

"The recently staked, ¼ . in north-central British Columbia centres on a 200 metre wide vegetation "kill zone" caused by a high concentration of sulphide mineralization over an area of gossanous rock" (Northern Miner, 28 August 1995)

are somewhat typical of natural ARD related to mineralization. Gossans have been a target of mineral prospectors throughout human history and there are numerous examples of gossan discovery leading to major economic mining projects. Explorationists use natural ARD products as an aid to mineral exploration. Photograph 4 shows large massive sulphide boulders on a glacier that are indicative of ARD, and are used by prospectors to follow up and locate the source of the mineralization.

Photograph 4: Highly Oxidized (ARD) Boulders on Frobisher Glacier, Windy Craggy Area, Northwestern BC

Mechanisms of sulphide oxidation and gossan formation are well described by Taylor and Thornber (1995). These authors demonstrate that an understanding of the processes and conditions that can change a particular sulphide assemblage into a gossan, or subsequently into an ironstone, is based on assemblage origin, with mineralogy and geochemistry being described in terms of climate, topographic relief, type of mineralization, nature of wall rocks, nature of groundwaters, weathering history and intrinsic properties. The types of ironstones and gossans (as natural ARD products) are shown in Figure 1.

Figure 1: Mechanisms of sulphide oxidation and gossan formation from Taylor and Thornber (1995)

Ironstone (ferricrete) is unconsolidated surficial material cemented together by iron oxyhydroxides forming in water drainages (creeks) or slopes downstream/downslope of weathering iron sulphides. This is generally indicative of natural ARD. Ironstones may also form in fault zones, or have their iron oxide origin several metres (or hundreds of metres) below the surface, and iron oxide solution is transported to the surface by groundwater percolating along dilation zones such as faults, (Photographs 5 and 6, Gataga and Atlin Lake).

Photograph 5: Iron Oxides Developed in Soil (Permafrost) over Gossan, Windy Craggy Area, Northwestern BC

Photograph 6: Iron and Manganese Oxides Developed in Soil (Permafrost) over Gossan, Windy Craggy Area, Northwestern BC

Surface (and near surface) waters may also affect the immediate environment as shown in Photograph 7. Gossan development over the same deposit may vary in texture and composition as a function of parental material (Thornber et al, 1981). Soil development will also be affected by the underlying gossan as shown in Photograph 8. After gossans develop they may be eroded, become buried by volcanic activity or be modified by other erosional and depositional regimes. Gossan formation is a feature noted on acid generating waste dumps and tailings.

Photograph 7: Ferricrete in Creek.

Photograph 8: Ferricrete in Soil Horizon (Gravels), Atlin, BC

Gossan discolouration is sometimes a desired feature such as decorative stone on buildings (Photograph 9) and as part of landscaping (Photograph 10 and 11). Unfortunately, ARD material is also used as fill (from a quarry) by uniformed contractors, such as observed at the marina on Harrison Lake, B.C. (Photograph 12).

Photograph 9: Decorative gossan discolouration on buildings

Photographs 10 and 11: ARD material used as part of landscaping

Photograph 12: ARD material as fill

Many metal-tolerant plants occur naturally in areas of acid rock drainage, Photograph 13. Though high metal toxicity will leave areas of "kill zones" , plants do survive on the fringes and with time gradually overgrow the "kill zones". Many paleogossans are covered with vegetation. A biogeochemistry exploration technique that analyses plant and tree material is used as a guide to buried metal deposits (Brooks, 1972).

Photograph 13: Birch Tree Growing in Massive Sulphide Lens, Trail, BC

It is interesting to note on topographic maps that some creeks have ARD related descriptive names such as Red Creek or Sulphide Creek or Bitter Creek, Iron Creek and Alum Creek in the case of the Summitville Mine area in Colorado. Many waterways such as creeks and lakes may be devoid of life due to natural ARD effects which can dramatically affect their pH and metal(s) content and toxicity to life forms. When acidic streams merge with non-acidic streams, iron oxides are often precipitated causing the stream sediments and gravels (boulders) to become stained (Photograph 14).

Photograph 14: Iron Oxides Precipitation at Confluence of Creeks, Berg Copper Deposit, Northern BC (©Fletcher & Baylis)

Gossans can form today and have formed over geologic time (sometimes up to several tens of thousands of years) and may form mineral enriched mantos or supergene zones (Figure 2). Supergene oxidation and leaching of copper bearing intrusions (porphyry) give rise to chalcocite enrichment that forms some of the richest and largest copper deposits in the world. Age dates of supergene alunite from porphyry copper deposits in Chile yield K/Ar ages ranging from about 34 to 14 million years (Sillitoe & McKee, 1996). A paper by Downing (2000) discusses the acid rock generation/drainage in mineral deposits throughout time. Thus, acid rock generation has taken place for millions of years and is not just a recent phenomena. A similar process occurs at many mines where dump leaching is used, or at waste and tailings dumps where natural leaching is prominent (Chermak and Runnells).

Figure 2: Mature Gossan Profile

Gossan and related alteration can be recognized and mapped using satellite image data..This method has been used extensively and successfully by explorationists in locating prospective mineral sites generated from natural acid rock generation and related alteration of rocks - particularly in non-vegetated (arid) terrain. The method (or Crosta technique) is a multivariate statistical technique (or principal component transformation) which uses multi-spectral image channels. These detect anomalous concentrations of hydroxyl, hydroxyl plus iron-oxide and iron-oxide, and display the pixels (or area) in red-green-blue space colour. The technique and a case study is reviewed in a paper by Loughlin (1991). Again, this method maps natural acid rock generation products - see Photograph 11.


Photograph 15: Crosta Technique, Peru; Orange - Areas of Anomalous Iron Oxide, Blue - Areas of Anomalous Clay Alteration, Green - Areas of Anomalous Iron Oxide and Clay Alteration (Photograph courtesy of G.Tomlins, Pacific Geomatics, Vancouver, 1998).


A literature review of current ARD research, indicates a frequent lack of understanding of trace element geochemistry with respect to ARD. Naturally occurring trace element toxicity resulting from ARD may have an impact on the environment. A report by the Geological Survey of Canada (Reichenbach, 1993) indicates how black shales can be considered as environmentally hazardous in Canada. Black shales commonly host complex assemblage of elements that have a significant influence on the environment. A major black shale area covering several hundred square kilometres in north-eastern BC and the Yukon is acid generating. The acid produced by these shales is natural, not a product of mining. It is also a natural process to which the environment has adjusted. A report by Kwong and Whiteley (1992a) describes natural acid rock drainage at McMillan Pass, Yukon. Based on measurements of 12 parameters, the criteria for drinking water and aquatic life failed to meet acceptable standards. Another study carried out by the BC Geological Survey (Koyanagi and Panteleyev, 1994) on northern Vancouver Island demonstrated the source and extent of natural acidity in waters draining areas of altered and mineralized rock. Results of sampling and analysing streams throughout B.C. have identified over 100 occurrences of natural acid generation (Price & Errington, 1995). Other studies on natural ARD and natural metal leaching have been conducted by Posey et al. (2000), Mast et al. (2000), and Yager et al. (2000). Natural ARD/ metal leaching is becoming more recognized and documented.

Ficklin et al. (1992) have attempted a geochemical classification of mine drainages and natural drainages in mineralized areas. Their conclusions indicate that natural drainages in mineralized areas produce waters that are chemically similar to mine drainage waters. Groundwater can carry natural ARD effects a long way and contaminate soil.

Determining the "natural" levels of metals and other solutes in surface and ground waters is a pre-requisite to planning a mine development. Remediation standards, which are based on rather rigid guidelines (e.g. drinking water standards), may be unrealistic because they ignore the natural geochemical character of mineralized areas before and after mining. Determination of natural background concentrations of dissolved components in water at mining, milling and smelting sites have been investigated by Runnells et al. (1998), Alpers et al. (2000), and Davis et al. (2000). ARD prediction and treatment are the subject of substantial research effort by governments, the mining industry, universities and independent research establishments.


Metal concentrations in the environment are dependant upon variations of metal content in naturally occurring media (i.e. bedrock, soil). Knowledge of local and regional geochemical processes can lead to a better understanding of the impact of metals on the environment. It also provides technical constraints on environmental regulations which are incorporated into the regulatory process. This page deals only with natural (background) ARD and does not consider anthropogenic sources.

Variations in the composition of biota and water are influenced by soils which, in turn, are dependant on the geochemical nature of bedrock. The biogeochemistry of metals in natural aquatic systems is complex. Principles and concepts of metal biogeochemistry in surface water systems are discussed by Elder (1988), and the natural geochemistry of the environment is discussed in detail in many publications, including Environmental Geochemistry by Fortescue (1980) and The Natural Geochemistry of Our Environment by Speidel and Agnew (1982). Thorton et al. (1983) discuss the interaction between geochemical and pollutant metal sources in the environment and the implications for the community.

There are different definitions of the term 'background'. In the mineral/mining industry, background is defined as " the abundance of an element or any chemical property of a naturally occurring material in areas where the chemical pattern has not been affected by the presence of a mineral deposit" (Thrush et al., 1990). In geochemical exploration terms, background is defined as "the abundance of an element, or any chemical property of a naturally occurring material, in an area in which the concentration is not anomalous" (Bates and Jackson, 1987). For the purpose of identifying contaminated sites, elevated metal concentrations associated with mineral deposits should be considered part of the background variability.

Natural background concentrations of elements in the environment are dependant upon interactions between rock, soil, water and vegetation. If the dominant source of the parent material is non-mineralized and unaltered, metal concentrations tend to be low. If the source is mineralized and/or altered, then higher levels of elemental concentrations can be expected. Mineralized areas include both metallic and non-metallic minerals. These levels can be measured from baseline studies around specific sites determined by regional sampling programs such as the stream sediment surveys sponsored by both the federal and provincial governments. Concentrations of substances in soil, stream and lake sediments, and water are dependant upon both the geological and biological environments within specific drainage basins and cannot be used as overall standards because they may be different for each drainage basin, compared with or measured against the values for the total landscape.

Tables of elemental concentrations are available for a variety of environmental matrices but these are generally from non-mineralized sites. Crustal average abundances, known as Clarke values (Clarke, 1924), for various elements can be used as a measure of background. Caution should be exercised, however, in using these values since secondary iron and manganese oxides, which are 'strong scavengers' for many heavy metals released during the weathering process, have an impact on the natural background concentrations of some metals (Jenne, 1968).

Anomalous concentrations of elements are of importance to exploration geologists because they generally indicate mineralization that is the source of anomalous values. There is no general standard for anomalous values or concentrations because they are dependant upon the area/region and medium that is sampled and upon statistical determinations of the background and threshold concentration levels. Anomalous values, which may ultimately lead to the development of a mine, may be several orders of magnitude higher than those presently accepted by (in British Columbia) the Ministry of Environment, Lands and Parks (MOELP) as maximum acceptable concentrations. A study of several mineral deposits in BC by Bradshaw et al. (1975) outlines the use of natural background and anomalous element concentrations, in various media, in locating the deposits with respect to the region in which they occur. Sutherland-Brown (1973) describes the variation of natural metal concentrations of five tectonic belts in British Columbia in terms of potential mineral exploration.

Metal concentrations in soil and water exert a strong influence on vegetation. A study of plant uptake of heavy elements as function of element speciation shows the necessity of understanding metal species availability and method of analysis (Dunn,1989). The distinction between "total" metal and "available" metal is critical to the understanding of potential pollution/contamination problems. Plant stress may occur resulting in non-vegetated or "kill" zones if there is natural contamination. Recognizing stressed plant communities is a technique used by explorationists to identify potential mineralized areas

The maximum acceptable concentrations of substances in found in many government regulations are based on land use classifications and do not take into account the natural background variations based on bedrock and Quaternary media. There is a geological background variation which influences these concentrations.

Natural background metal concentrations vary with lithological boundaries. Differences also exist within lithologically similar bedrock. Since lithological boundaries do not coincide with political/administrative boundaries, the standards for maximum acceptable concentrations should be consistent for all provinces/states and/or should be adjusted according to lithological boundaries.



As ARD is essentially controlled by bedrock geology, then an understanding of the geological environment is extremely important. Conceptual models for ore formation and mineralization provide the ARD practitioner and regulator with some ideas as to the potential size of metal leaching and mobility both from a natural and anthropogenic context. An understanding of ore deposit models is basic in determining the natural background values which reflect the primary character of the mineralization and alteration. The geo-environmental concept is discussed in published papers by Alpers & Nordstrom (2000) and Kwong (2000).



Because metal concentration varies with pH, methods for sampling, sample preparation and analysis must be standardized . The most important factor in these analyses is the Total versus Available concentration. Total is a measure of all the element content within a sample, whereas Available is a measure of a portion of the total element that can directly participate in biological reactions. Availability is related to mobility in the environment. It is considered to be the Dissolved portion of a water sample (practically taken by analysis after filtration at 0.45 µm). In most mining studies elements are reported as Dissolved or both Dissolved and Total, depending upon regulatory requirements. Where metals are present as complexes, however, they may be both Dissolved and unavailable. Total concentration may have little or no effect on the environment if it is unavailable, whereas available concentration is usually an indicator of direct biological effects (bio-availability). Both the bio-accumulation (the accumulation of elements by organisms) and bio-availability of zinc and copper are discussed in papers by Van der Zee and de Haan (1992) and Luo and Rimmer (1994). The common parameter discussed in both papers is the soil solution concentration of metals and its effect on plant growth.



Surface and ground water are essentially two separate entities because they are influenced by different media (i.e. bedrock vs. gravel) and/or by the same parameter but in different ways (i.e. temperature). Consequently there should be different guidelines and standards for surface water and ground water. Other parameters such as sediment loading and time of year are factors which affect metal concentration in water.

An examination of published water quality data from the Tatshenshini area of BC (Claridge & Downing, 1993) indicate that 9 of 19 sites have dissolved Cu and Zn values above the maximum allowable concentrations as per Schedule 7 of the BC Contaminated Sites Regulation for freshwater/aquatic life. These data are derived from undisturbed natural sites.

Stream Sediments

Regional stream sediment surveys cover approximately 70% of the province of British Columbia. Stream sediments reflect the composition of bedrock occurring in drainage basins and are used to locate areas of potential mineralization within these drainage basins.

Many of the toxic substances found in rivers are attached to, and transported with, sediment. Any 'maximum acceptable concentration of substances' should recognize natural background variations in concentrations.

In British Columbia a large regional stream sediment database has been assembled and is available from the BC Geological Survey. Using maximum acceptable concentrations of Schedule 6 of the BC Contaminated Sites Regulation, examination of a subset of this database (NTS 104B - Iskut area) indicates that 10 elements are above these levels using the different soil classification categories. The data are tabulated in the accompanying Table 1. Results indicate that the source material, barren vs. mineralized, is the dominant influence of metal concentration. Ten sample sites returned pH values <5.5 which indicates acidity. For some of these sites high element concentrations correlate with pH (pH<5.5).

Table 1: Number of Contaminated Sites for Various Total Metal Concentrations,

Map Sheet 104B



RANGE, ppm





































































































Government geochemical data (stream and lake sediment) should be used by federal and provincial ministries to develop a set of area specific background and acceptable metal levels. Painter et al. (1994) have published several maps, covering various areas of Canada, in which they show natural background levels for eight elements. They discuss the importance of reconnaissance geochemistry and its environmental relevance. They note that the concentrations of metals in the natural environment vary widely, and in some areas unaffected by human activity may reach levels that elsewhere have been considered to have an effect on the ecosystem. A preliminary directory of trace element databases has been compiled and is available for the Vancouver, BC area, (Delaney & Turner, 1994). These data sets characterize natural background geochemical baseline concentrations and monitoring studies over time. Thorton et al (1983). have suggested using stream sediments as an estimate of trace element concentrations for water and could be useful in the selection of critical sites for routine water sampling.


In a paper on metals in agricultural soils by Logan and Traina (Allen et al., 1993), the authors observe that the interaction of soil with surface and groundwater is a fundamental process controlling the solubility of trace metals in water and their impact on water and soil quality.

There is a wide range of trace element concentrations in soils and related materials. Soil surveys are often conducted by mineral explorationists over areas of poor outcrop exposure in order to delineate high concentrations of elements which might indicate changes in geology or indicate mineralization. Levels in Schedule 6 of the BC Contaminated Sites Regulation do not differentiate between the A, B or C soil horizons or the mesh size of the sample fraction used to obtain these concentrations. Certain soil horizons concentrate some elements more than others, depending upon the organic matter content. The recognition of residual versus transported soils is equally important for metal concentrations. After a two year study on the biogeochemistry of copper and cadmium in two different ecosystems, Keller et al. (1992) concluded that soil properties are more important for determining the movement of metals in soil than the properties of metals themselves.

The geochemistry of soils is also affected by airborne particulate material from natural sources such as volcanoes.

Variations in metal content are also associated with different levels of soil classification. A geochemical survey of some soils in Missouri (Tidball, 1976) shows that the variation of element concentrations between Soil Series is much larger than variation within a given Series.

Because background concentrations vary, statistical analyses should be undertaken to calculate the background, threshold/anomalous values, (Fletcher et al., 1986).


Metal Complexes

Metals can be present in various media as inorganic and organic complexes, colloids, adsorbed on colloids, adsorbed on inorganic and organic material, precipitates or incorporated within biological materials. Speciation is complex and dependent on temperature, pH, dissolved oxygen, concentration and other factors. Examination of Eh - pH diagrams indicates the complexity of metal speciation (Garrels and Christ, 1965). Both total and dissolved metals must be taken into account because chemical species of a particular metal can have different impacts on aquatic organisms (i.e.bio-availability). Both the uptake and toxicity of metals which have undergone speciation in plants and animals and "free" metal species are the most bioactive and toxic (Logan & Traina in Allen et al, 1993). Natural waters are multicomponent electrolyte solutions. A study by Williamson and Parnell (1994), demonstrates the use of selective, sequential extraction of metals from lake sediments and its implications for selective metal remobilization under variable environmental conditions, both natural and anthropogenic. This has a direct bearing on natural background metal concentrations. The closure plan for the Bell Mine in British Columbia recognizes copper complexing and the importance of the complexing capacity of Babine Lake and Hagen Arm.

"As the complexing capacity is in great excess of existing background concentrations of copper, it suggests that the current Cu concentrations in both Hagen Arm and Main Lake are not imposing environmental stress on the resident biota" (Noranda Minerals, 1992; Singleton, 1987).

Reports by the National Research Council of Canada on various metals in the Canadian environment discuss in detail metal complexing and toxicity. A recent paper by Gulson et al. (1994) discusses the importance of lead complexes and their bioavailability to humans at the Broken Hill mining community in Australia. The importance of metal complexes should be recognized in any regulation(s) dealing with metal concentrations and their impact upon the environment.



High metal concentrations are a widespread naturally occurring phenomena.

There are in excess of 11,375 documented naturally occurring mineralized sites throughout British Columbia (MINFILE), of which approximately 400 are coal, 600 industrial minerals and 10,000 base and precious metals. Most of these sites would be classified as "contaminated sites" under the BC Contaminated Sites Regulation. The majority of these sites are shown on the mineral deposits and principal mineral occurrences map produced by and available from the BC & Yukon Chamber of Mines (1994).

The present Assessment Report Database contains over 23,000 Assessment Reports containing data resulting from mineral exploration programs in British Columbia. The data cover geology, geochemistry, stream & lake sediments, soil, hydrogeochemistry, geophysics, prospecting, physical work (i.e. trenching) and drilling. Reports, recorded since 1947, generally cover data which originated from surveys over natural undisturbed land and do not include 'urban' lands. These reports document extensive areas of mineralization.

A database search indicates the numbers of reports describing the following are: 170 on hydrogeochemistry, 1117 on stream sediments and 4033 on soil. Assessment reports generally list all analytical values with maps indicating the major elements of interest. Most soil reports generally indicate some anomalous elemental value(s) and in most cases these will exceed the maximum acceptable concentrations shown in Schedule 6 of the Contaminated Sites Regulation. These 4033 reports cover soils at sites which are scattered throughout the province ( Figure 15). They would now be considered as contaminated sites.

In general, the MINFILE and Assessment Report databases represent the largest known listing of potential 'naturally' contaminated sites in British Columbia.



The natural phenomena called Acid Rock Drainage has had a major impact upon the earth’s environment for millions of years. Researchers today have only just begun to understand the process, but at the same time are investing time and money into controlling a process that has occurred and will occur naturally, in perpetuity, over geologic time. Our environment (vegetation, wildlife and fish) has adapted and will adapt to natural ARD. If the basic ingredients are present, then ARD will occur. If one of the ingredients is removed, then the process will either stop, or slow down and eventually stop.

In order to create suitable limits for a potentially developed site, a thorough review of background characteristics must be carried out using existing government databases before applying the designation "contaminated" to a site.



Bruce Downing: This page is a collection of notes and talks that the I have used in teaching the public about Acid Rock Drainage (ARD). I would like to thank the late Chris Mills for many discussions on this topic, and for permission to put this page on ARD at Enviromine in the hope that it will spur others to consider the natural ARD events in geological history, and possibly send any of their studies and photographs to this web site for possible future inclusion.


©The contents of this web page are protected by copyright law. Please contact the authors for permission to re-use the contained information.




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