Any discussion of ABA methods seems to be an invitation to controversy. This is a field with strongly held opinions that are frequently in conflict. Rather than make suggestions and recommendations, I have tried to report the most recent findings of North American workers in both the hard rock metalliferous and coal mining spheres. I leave readers to draw their own conclusions from the reported data and the cited references.

I am indebted to my colleagues Keith Brady of Pennsylvania DEP, Charles Bucknam of Newmont,  Tom Durkin of South Dakota DENR, Kim Lapakko of Minnesota DNR, Rick Lawrence of UBC, Shannon Shaw of Robertson Geoconsultants, Jeff Skousen of WVU, Brian Soregaroli, formerly of Klohn Crippen and Rik Vos of BCRI for assisting me in various stages of this compilation. I would also like to thank all of the subscribers to the listserver Enviromine_Technical who took part in the lively discussion of ABA methods during May of 1997.


Acid-Base accounting (ABA) is a screening procedure whereby the acid-neutralizing potential (assets) and acid-generating potential (liabilities) of rock samples are determined, and the difference, net neutralizing potential (equity), is calculated. The net neutralizing potential, and/or the ratio of neutralizing potential to acid-generation potential, is compared with a predetermined value, or set of values, to divide samples into categories that either require, or do not require, further determinative acid potential generation testwork. Just as different methods of accounting (e.g. cash or accrual basis) will present different sets of books to an auditor, so different methods of conducting ABA testwork will generate different sets of sample data for evaluation. Rules and guidelines have been developed by mine regulatory and permitting agencies (e.g. Price and Errington, 1995, Steffen Robertson and Kirsten, 1992, Miller, 1995, Brady et al., 1994) for ABA procedures that may be likened to the rules and guidelines of financial accounting.

In its most basic form ABA is simply a screening process. It provides no information on the speed (or kinetic rate) with which acid generation or neutralization will proceed, and because of this limitation the testwork procedures used in ABA are referred to as Static Procedures. However, the evaluation of ABA data in conjunction with mineralogical and petrological data can, for certain ABA procedures, give a degree of neutralization rate prediction to ABA.

The potential for a given rock to generate and neutralize acid is determined by its mineralogical composition. This includes not only the quantitative mineralogical composition, but also individual mineral grain size, shape, texture and spatial relationship with other mineral grains. The term "potential" is used because even the most detailed mineralogical analysis, when combined with ABA, can give only a "worst case" value for potential acid production and, depending upon the NP procedure used, a "worst case", "most likely case" or "best case" value for potential neutralization capability. The field generation and neutralization of ARD represents the degree to which these potential values are realized in practice.

Neutralization Potential Procedures

Because of the complexity and difficulty of translating detailed mineralogical data into Neutralizing Potential values (see Paktunc, 1998a, 1998b, 1998c and Lawrence and Sheske, 1997) for the large number of samples evaluated in a typical ABA screening program, chemical procedures have been developed as a substitute for mineralogical procedures. However, to maximize the information obtained from chemical procedures in ARD prediction, they should always be used in combination with determinative mineralogical methods.

These chemical procedures are generically termed Neutralization Potential determination methods or procedures. The most common of such procedures are described on the ABA Procedures Page of this site and are:

Despite individual procedural differences, these methods all involve:

Note that the last step is an accounting convenience – a sample with no calcium carbonate content, nor even any carbonate content, may nevertheless produce a Neutralizing Potential value that is recorded as g/kg or kg/tonne CaCO3. The methods determine the quantity of the sample that is dissolved by the prescribed quantity of acid for the sample and procedure used.

The procedures listed above differ primarily in the degree of aggressiveness of acid reaction with the sample – i.e. the variety of minerals attacked by acid and the degree of mineral dissolution. The following Table 1 summarizes the described procedure conditions and suggests, qualitatively, which minerals are dissolved by each procedure. 

Table 1: Procedure Conditions for Neutralization Potential (NP) Determination










To reach pH 6.0


Up to 1 week


Ca + Mg carbonates
BCRI Initial


To reach pH 3.5


16-24 h2


Ca + Mg carbonates

Possibly chlorite3, limonite4

Modified Sobek


Determined by Fizz Test


24 h


Ca + Mg carbonates

Some Fe carbonate, Biotite, chlorite, amphibole5

olivine (forsterite-fayallite)



Determined by Fizz Test


Until gas evolution ceases (test including titration up to 3 h)

Elevated (c. 90 deg C)

Mineral carbonates

Ca-feldspar, pyroxene, olivine (forsterite-fayallite)7,8

Some feldspars (anorthoclase>orthoclase>albite); ferromagnesians – pyroxene, hornblende, augite, biotite2

Sobek – Siderite Correction

Procedure as for Sobek, but with peroxide correction for siderite

Ca + Mg carbonates, excludes Fe + Mn carbonates. Otherwise as per Sobek.
Net Carbonate Value (NCV)

Method uses combustion-infrared analysis, not acid digestion

Calcite, dolomite, ankerite, siderite
Inorganic Carbon – Carbonate6

Method uses Leco furnace or equivalent, not acid digestion

Mineral carbonates


  1. 1Lawrence and Wang, 1997 – actual pH extremes were pH 0.35 to 5.2
  2. 2Downing and Madeisky, 1997
  3. 3Warren, 1996
  4. 4Mills, 1997
  5. 5Kwong and Ferguson, 1997
  6. 6It is common practice to determine inorganic (mineral) carbonate for samples subject to NP determination for comparison with measured NP.
  7. 7Lapakko, 1994
  8. 8BHP Diamonds Inc., 1995a, 1995b, 1996.
  9. The chemical formula for the chlorite group of minerals is A5-6Z4O10(OH)8 where A = Al, Fe2+, Fe3+, Li, Mg, Mn2+, Ni and Z = Al, B, Fe3+, Si, while that for biotite is K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH,F)2 (Fleischer, 1983). Note that both contain the (OH) group. The general chemical formula for the feldspar group of minerals is XZ4O8 where X = Ba, Ca, K, Na, NH4, Sr and Z = Al, B, Si (Fleischer, 1983). Note the absence of the (OH) group.
  10. Although NP is reported as kg CaCO3/tonne or tonne CaCO3/1000 tonne, the carbonate minerals that contribute to NP include calcite, dolomite, (Ca,Mg)CO3, magnesite, MgCO3, ankerite, Ca(Fe2+, Mg,Mn)(CO3)2, siderite, FeCO3 and rhodochrosite, MnCO3. However, the Fe and Mn components of carbonates hydrolyze to generate hydrogen ions, and therefore have no net neutralizing power (Skousen et al., 1997). Carbonate NP values must be reduced to account for Fe and Mn carbonates, if such carbonates are present in the sample.
  11. The low pH end point for the Sobek method is significant, since Morin et al. (1995) in their International Kinetic Database of 396 kinetic tests from 60 mines show that pH fell below a value of pH 2 for only one test - see also The Fizz Test (Sobek Method), below.

There is very little data published on mineral reactivity during NP determination by any of the above procedures.

Lawrence and Wang (1996) analyzed the leachate from samples tested by the Sobek and Modified Sobek procedure for aluminium and other cations contained in aluminosilicates. From the higher cation concentrations in the leachate from the Sobek method, the authors concluded that the Sobek method dissolved more silicate minerals than the Modified Sobek.

Downing (1996) compared the quantities of various elements dissolved (mg/kg) for waste rock samples tested by the BCRI Initial and Sobek methods. For the five samples tested the Sobek method dissolved more aluminium than the BCRI Initial method by factors of 1.5, 8.2, 11.2, 12.4 and 3.9 to 1. Again, this clearly demonstrates increased dissolution of aluminosilicate minerals by the Sobek method over the BCRI Initial method.

Mills (1997) analyzed leachate from eight waste rock samples tested with the BCRI Initial method for a number of cations including aluminium and potassium. Enhanced levels of aluminium and potassium (for all samples) in the leachate were taken as evidence of the dissolution of potassium aluminosilicates.

Kwong and Ferguson (1997) studied 26 samples treated by the Modified Sobek method and variations, and utilized X-Ray Diffraction (XRD) to determine mineralogical changes. They concluded that biotite, chlorite and amphibole contributed to the determined NP, whereas quartz, muscovite, plagioclase and K-feldspar did not.

In general NP determined by the various methods increases with the degree of aggressiveness of the method in the order NP-Sobek > NP-Modified Sobek > NP-BCRI Initial > NP-Lapakko. In addition, for the Sobek and Modified Sobek methods, an overestimation of the fizz test rating will generally yield a higher NP value because of the action of the additional acid. The fizz test will be examined later in this discussion.

While the role of carbonate minerals in ARD neutralization has been well established, that of aluminosilicate minerals has not. Sherlock et al. (1995) and Downing and Madeisky (1997) have discussed the behaviour of aluminosilicate minerals in the neutralization process from a theoretical viewpoint and commented on the practical implications. Downing and Madeisky (1997) have proposed the use of a lithogeochemistry-derived pair of Net Buffering Capacity indices – a short term index based on carbonate content, and a long term index based on calcium plus potassium content.

Lawrence and Wang (1997) have suggested the following relationship between mineral reactivity and method of NP determination (Table 2): 

Table 2: Relationship Between Mineral Reactivity and NP Determination Method




More Reactive

Less Reactive


Ca-feldspar, Olivine

Pyroxenes, Amphiboles

Sorosilicates, Phyllosilicates

Plagioclase feldspar




Modified Sobek


These relationships are not inconsistent with the comparisons made earlier. Another table of reactivity (Table 3) of neutralizing minerals (at pH 5) that has been widely quoted (e.g. Kwong, 1993a, 1993b) is that of Sverdrup (1990)

Table 3: Relative Mineral Reactivity (after Sverdrup, 1990 and Kwong, 1993a, 1993b)





Calcite, aragonite, dolomite, magnesite, brucite


Fast weathering

Anorthite, nepheline, olivine1, garnet, jadeite, leucite, spodumene, diopside, wollastonite


Intermediate weathering

epidote, zoisite, enstatite2, hypersthene, augite, hedenbergite, hornblende, glaucophane, tremolite, actinolite, anthophyllite, serpentine, chrysotile, talc, chlorite, biotite


Slow weathering

albite, oligoclase, labradorite, montmorillonite, vermiculite, gibbsite, kaolinite


Very slow weathering

K-feldspars, muscovite



Quartz, rutile, zircon


1See Schott and Berner (1983, 1985), Grandstaff (1980), Hutchison and Ellison (1992), Banfield et al. (1991), Loughnan (1969) and Luce et al. (1972);

2See Grandstaff (1977)

While these values for relative reactivity are given for pH 5, rather than the lower pH values used for most NP determination procedures, they are generally consistent with previous discussion (although the categorization of garnet as fast weathering is seems inappropriate). Kwong (1993a) has suggested that all of the minerals in the Dissolving, Fast weathering and Intermediate groups (Relative reactivity 1.0, 0.6 and 0.4 respectively) be considered as having practical neutralizing capability in the field.

That aluminosilicate minerals are dissolved to some extent by ARD, and therefore act as neutralizing agents, is clearly shown by aluminium analyses of ARD from established sites. The following Figure 1 shows some of the 1995 monitoring data for the 2200 Level adit discharge at the former Britannia mine in British Columbia. 

Brit2200.gif (6550 bytes)

Figure 1: Metal Concentration Monitoring Data, 2200 Level Adit, Britannia Mine, BC, 1995 (Data source: Gordon Ford, BC Ministry of Environment)

It can be seen that aluminium analyses range from about 35 to 65 mg/l, with an ARD pH of about 3, indicating significant dissolution of aluminosilicate minerals. In this case the use of an NP determination method that considers only carbonates as a source of neutralizing capacity would probably underestimate the true, or field, neutralizing capacity.

Blowes et al. (1992) have reported elevated concentrations of both silica and aluminium in tailings water, as have Alpers and Nordstrom (1990) for waste rock drainage.Alpers and Nordstrom (1990) also concluded, based on computer speciation modeling, that at Iron Mountain, California the minerals albite, chlorite, sericite, epidote and calcite contributed to ARD neutralization. Morin et al. (1988) reported that tailings water migrating from the uranium tailings impoundments studied was saturated with respect to aluminium and silica.

Downing and Madeisky (1997) measured the NP of four low carbonate samples as a function of time using the BCRI Initial Method (the standard BCRI Initial Method typically takes 16-24 hour). For comparison, they also conducted similar tests with the Canadian Reference Material for Standard Acid Base Accounting, NBM-1 which is carbonate bearing. Their results for the four low calcite samples, and for the Standard, are shown below in Figures 2 and 3.

largeNP1.gif (10523 bytes)

Figure 2: NP v Time (BCRI Initial Method) for Four Low Carbonate Samples (From Downing and Madeisky, 1997)

largeNBM1.gif (7322 bytes)

Figure 3: NP v Time (BCRI Initial Method) for Canadian Reference Material for Standard Acid Base Accounting NBM-1 (from Downing and Madeisky, 1997)

This data tends to indicate that where carbonate is the predominant neutralizing species (i.e. the NBM-1 Standard), the BCRI Initial Method gives an NP value which does not increase with time after the normal (16-24 hour) test duration, whereas the slower reaction rates of non-carbonate neutralizing minerals give NP values that are time dependent. The degree of time dependency appears to be a function of mineralogical composition - as would be expected from previous discussion. Further, it seems likely that the NP for the four low carbonate samples is contributed by mica, chlorite, pyroxene and amphibole.

It should be clear from the above discussion that no single NP determination method will give the true, or field, NP for any given sample. As a very broad generalization, it may be said that, for a sample containing carbonates and reactive silicates:

  1. The Lapakko and Inorganic Carbon-Carbonate methods will tend to give a 'worst case' neutralization potential, since the carbonates are credited, but other minerals are not.
  2. The Sobek method will tend to give a 'best case' neutralizing potential since all carbonates and other minerals soluble at the lowest pH of the test will be credited.
  3. The BCRI Initial and Modified Sobek methods will tend to give a 'most likely case' neutralization potential, since the carbonates, and only the most reactive silicates are credited.

The above statements notwithstanding, it should be understood that even the Lapakko and Inorganic Carbon-Carbonate methods may overestimate the real, or field, neutralizing potential of materials. This is possible because some of the neutralizing minerals present may be inaccessible to ARD because of physical placement or entrainment, or because of 'armouring' by metal precipitates (Hyman et al., 1996)


The Fizz Test (Sobek Method)

The Fizz Test used in the Sobek and Modified Sobek methods to determine the quantity of acid used in the digestion is subjective in that it requires a judgement by the test operator.

Lawrence and Wang (1997), Soregaroli and Lawrence (1997) and Skousen et al. (1997) have compared different laboratory assessments of Fizz Rating and examined the effects of such differences on measured Neutralization Potential (NP).

Lawrence and Wang (1997) evaluated 112 samples of waste rock and tailings and 8 certified reference standards (concentrates, ores and metallurgical) to determine the effect of Fizz Rating on NP by the Sobek method. The following Table 4 shows the typical end pH and range of end pH obtained in testwork for the four Fizz Ratings None, Slight, Moderate and Strong.

Table 4: pH End Points for Various Fizz Ratings using the Sobek NP Procedure (from Lawrence and Wang, 1997)




No Fizz

Slight Fizz

Moderate Fizz

Strong Fizz









NP values (Sobek method) reported for three samples using a Strong Fizz rating exceeded those obtained with a Slight Fizz rating by 527%, 400% and 337%. The authors have reported this work in detail in Lawrence and Wang (1996). In their Experimental Objectives and Methods, the authors state:

"To compare NP value variations in the Sobek procedure when different fizz ratings were used for each waste sample tested. This was done since the assigning of a fizz rating can be subjective. In addition, some test laboratories, for convenience, do not carry out a fizz test in strict accordance with the Sobek procedure. Instead, acid additions are made according to a strong fizz rating."

Soregaroli and Lawrence (1997) considered four Dublin Gulch waste rock samples with Fizz Ratings of Moderate, Weak, Moderate and Weak by Chemex Laboratories and Strong, Strong, Strong and Strong by University of British Columbia laboratories. The NP values determined (Sobek method) by UBC laboratories for their (Strong) Fizz Ratings exceeded those by Chemex for their (Moderate and Weak) ratings by 586%, 417%, 686% and 223%.

Skousen et al. (1997) examined 31 samples of overburden from coal mines in Pennsylvania and West Virginia, using three independent laboratories (PADEP, WVU and CONSOL). All three laboratories assigned the same fizz rating to16 of the samples, of which 11 were from the 'Si Group' of 17 samples containing mostly quartz and clays and little or no carbonates (calcite and siderite). There was no agreement for the 'Fe Group' (6 samples containing 18-65% siderite and little or no calcite), complete agreement for the 'Ca Group' (3 samples containing  23-90% calcite and no siderite) and some agreement, 2 of 5, for the 'S Group' (5 samples containing 1-63% pyrite).

Where there were differences in fizz ratings, they were generally by one category - 2 samples gave a two-category range and one a three-category range. The authors compared NP values determined using the reported fizz ratings with NP values obtained using the next higher fizz rating. They concluded:

"The NP values were higher for all Fe group samples and variable for the other groups when using more acid. It is evident that NP results for samples containing siderite are more sensitive to the assigned fizz rating than samples without siderite. For example, sample Si14, a gray shale shale devoid of siderite, showed little variation in NP (7-8 mg/1000 mg) when the fizz rating was increased from 1 to 2, while the NP values for sample Fe5, containing 18% siderite, increased from 64 to 234 mg/1000 mg when the fizz rating was increased from 2 to 3. When greater amounts of acid were used during sample digestion, most samples yielded a higher NP value."

The authors suggest an alternative procedure to the Fizz Test to determine the carbonate content of rocks, and report experimental testing of the alternative procedure on 26 of the 31 samples previously examined.

It is clear from the above discussion that the subjectivity of the Fizz Test used in the Sobek method may, depending upon the sample, produce widely differing NP values for a single sample. It is also likely that the consistent use of a 'Strong' rating, rather than a determined rating, may lead to unusually high NP values that may be misleading.

Finally, it would be useful to see the effects of fizz rating on NP values determined by the Modified Sobek method.


Acid Generation Potential Procedures

In comparison with Neutralization Potential, the determination of acid generation potential is less fraught with difficulties. When no organic material containing sulphur is present in samples, potential for acid generation is attributed to the potential oxidation of sulphide minerals to sulphate (sulphuric acid). Sulphide minerals include the common iron minerals pyrite, FeS2 and pyrrhotite, Fe1-XS, and metallic sulphides such as chalcopyrite, CuFeS2, sphalerite, ZnS, galena, PbS, etc. The sulphide sulphur content is taken to react stoichiometrically with oxygen and water to form sulphuric acid which has an equivalence in calcium carbonate, and hence acid generation potential in kg CaCO3/tonne or tonne CaCO3/1000 tonne is calculated.

Samples may also contain sulphate minerals such as gypsum, CaSO4.2H2O, anhydrite, CaSO4 and barite, BaSO4 (previously oxidized samples may also contain a wide range of sulphate-containing oxidation products such as melanterite, FeSO4.7H2O, brochantite, Cu4(SO4)(OH)6, jarosite, KFe3(SO4)2(OH)6 and alunite, KAl3(SO4)2(OH)6 ). Sulphate minerals have no potential to oxidize to sulphuric, acid although some oxidation products, such as melanterite, may dissolve, hydrolyze and generate acidity (this is generally of more concern in kinetic tests, such as humidity cells, than in static tests).

Where sulphate mineral content is very much less than sulphide mineral content (and in the absence of organic sulphur), sulphide sulphur may be approximated to total sulphur. Whether acid generation potential may be satisfactorily calculated from total sulphur instead of sulphide sulphur will, therefore, depend upon sample sulphate mineral content, and will be site-specific. Unless total sulphur analysis is used, it is necessary to conduct additional analytical procedures in order to determine sulphide sulphur.

Sobek et al. (1978) describe analytical procedures to determine total sulphur, sulphide sulphur, acid-leachable sulphate sulphur, acid-insoluble sulphate sulphur and organic sulphur. Acid-leachable sulphate sulphur includes gypsum and anhydrite, and acid-insoluble sulphate sulphur includes barite. The necessity of determining all of these sulphur components for any give suite of samples is rare. For example, for western Canadian metalliferous mines it is common practice to determine sulphide sulphur as the difference between total sulphur by Leco furnace and acid-soluble sulphate sulphur. The occasional necessity to correct for barite can be determined from barium assays obtained from whole-rock analyses. Acid generation potential is calculated from sulphide sulphur content. It is infrequently acknowledged that each analytical stage added to the sulphide sulphur determination introduces an additional analytical error, and that these errors are cumulative.

All mines in British Columbia are currently subject to ARD Guidelines for Mine Sites in British Columbia (Price and Errington, 1995). These Guidelines, which encourage a site-specific philosophy for potential ARD prediction,   are under review at the present time, and were the subject of two papers at the 4th International Conference on Acid Rock Drainage (Price et al., 1997a; Price et al., 1997b). Price et al. (1997b) state:

'The recommended procedure for determining the maximum potential for future mineral acid generation is to calculate it using the sulphide-S content measured using an expanded sulphur speciation analysis. Complete sulphur speciation may not be required if there are no organic-S or sulphate-S components present, however, these assumption should be verified. As many base metal deposits contain significant sulphate-S; the use of total-S as a sulphide-S measure may result in a large over-estimation of AP.'

According to Brady (1997) and Hyman et al. (1996) current Appalachian practice for coal mine overburden samples favours acid generation potential (termed MPA, Maximum Potential Acidity) from total sulphur determined by Leco furnace. The regulatory process does not prohibit the use of expanded sulphur forms analysis, but the use of total sulphur is considered more reliable (Brady, 1990).

For further discussion of this subject, see Day (1991).


Screening Assessment Criteria

After the Neutralizing Potential (NP) and Acid Generation Potential (AP or MPA) have been determined for a sample it is necessary to combined these two values in a manner that allows comparison with set criteria based on experience or regulation. The two methods of combination commonly used are:

The former is the preference for Appalachian coal mines, and the latter for western Canadian metalliferous mines.

From the previous discussion of NP and AP determination methods it should be clear that an NNP or NPR value for a sample will reflect the method used to determine NP and AP, but will be particularly affected by the method used for NP determination. Since potentially large ranges of values for NP can, depending upon the procedures used, be obtained for a single sample, it follows that both NNP and NPR are also sensitive to procedures. This has, in the past, resulted in the reporting of a myriad of sub-classes of NP, AP, NNP and NPR that are all procedure-specific, and has resulted in considerable confusion among practitioners of ARD (or AMD) prediction. These reporting problems have occurred less in Appalachian coal mining, where the Sobek NP and total sulphur MPA have been used fairly consistently, than in North American metalliferous mining. It is partly to ensure consistency of reporting that the British Columbia Guidelines recommend Sobek NP and sulphide sulphur AP (although see the previous section on the The Fizz Test (Sobek Method)).

For 39 Pennsylvania coal mine sites, where historical data has been maintained, it has been reported that material with an NNP (Sobek NP - MPA) greater than 15 tonne CaCO3/1000 tonne had alkaline drainage. For 78% of the same sites, material with an NNP less than or equal to zero had acidic drainage (Brady et al., 1994). For these sites the "grey zone", or range of NNP for which prediction is difficult, is represented by material with an NNP of greater than zero, but less than 15 tonne CaCO3/1000 tonne. Brady has commented (Brady, 1997) that the siderite modification to the Sobek method (Skousen et al., 1997) may reduce the range of the "grey zone". Other NNP criteria have been proposed and discussed by Smith et al. (1974, 1976), Surface Mine Drainage Task Force (1979), Skousen et al. (1987), British Columbia Acid Mine Drainage Task Force (1989), Ferguson and Morin (1991), Patterson and Ferguson (1994), Perry and Brady (1995) and di Pretoro and Rauch (1988).

Paste pH has been considered as an assessment criterion by Smith et al. (1974, 1976), Surface Mine Drainage Task Force (1979) and Miller and Murray (1988) and % Total Sulphur an an assessment criterion by Brady and Hornberger (1990) and Miller and Murray (1988).

NP has been considered as an assessment criterion by Brady and Hornberger (1990), Brady et al. (1994), Perry and Brady (1995) and di Pretoro and Rauch (1988).

In British Columbia, the existing Guidelines (Price and Errington, 1995) state:

"The screening criteria used to evaluate a need for further testwork based on acid base accounting tests are:

Basically, these Guidelines defined a "grey zone" for NPR greater than 1, less than 4. However, based on Price et al. (1997b), it is expected that the new Guidelines will define two NPR "grey zones" (Sobek NP/sulphide sulphur AP) as shown in Table 5 below (from Price et al., 1997b):

Table 5: Neutralization Potential Ratio (NPR) Screening Criteria (from Price et al., 1997b)

Likely < 1:1 Likely ARD generating
Possibly 1:1 - 2:1 Possible ARD generating if NP is insufficiently reactive or is depleted at a faster rate than sulphides
Low 2:1 - 4:1 Not potentially ARD generating unless significant preferential exposure of sulphides along fracture planes, or extremely reactive sulphides in combination with insufficiently reactive NP
None > 4:1 No further ARD testing required unless materials are to be used as a source of alkalinity

Traditionally in British Columbia, where sample NPR values have fallen in the "grey zone", kinetic testwork (humidity cells or columns) have been conducted to clarify sample status with regard to acid generation potential. However it should be possible to characterize material in the 4<NPR>2 zone adequately by mineralogical and petrographic testwork. Since the Guidelines have not yet been published, there have been no mines permitted under them and the manner in which samples within the 4<NPR>2 will be required to be tested is unknown.

A most useful method of presentation for NPR values is their plot against sulphide sulphur on logarithmic axes, as is shown below in Figure 4 for ABA data for the proposed Dublin Gulch mine in the Yukon Territory (Soregaroli and Lawrence, 1998) and in Figure 5 on the Quality Assurance/Quality Control page for the Colomac mine in the Northwest Territories. For further discussion of such plots, please refer to the QA/QC page.

Figure 4: NPR versus Sulphide Sulphur for Ore and Waste at the Proposed Dublin Gulch Mine, Yukon Territory (Soregaroli and Lawrence, 1998)

NPR as an assessment criterion has been discussed by Patterson and Ferguson (1994), Ferguson and Morin (1991), Ferguson and Robertson (1994), Cravotta et al. (1990), di Pretoro and Rauch (1988) and Filipek et al. (1991).


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Banfield, J.F., Jones, B.F. & Veblen, D.R. (1991), An AEM-TEM Study of Weathering and Diagenesis, Albert Lake, Oregon: I, Weathering Reactions in the Volcanics, Geochim. Cosmochim. Acta, v55, p2781-2793.

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