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Chapter 6
LABORATORY METHODS FOR ACID-BASE ACCOUNTING: AN UPDATE
Tim Kania
Pennsylvania Department of Environmental Protection
Ebensburg, PA 1593 1
Introduction the percent sulfur in the overburden. The appropriate
calculation factor is somewhat controversial. Sobek et
Laboratory methods for performing acid-base ac-
al. (1978), noting that 3.125 g of CaC03 is theoreti-
counting overburden analysis (ABA) have been thor-
cally capable of neutralizing the acid produced from 1
oughly detailed in previous publications. Sobek et al.
g of S (in the form of FeS2 ), suggested that the amount
(1978) formally presented a step-by-step laboratory
of potential acidity in 1000 tons of overburden could
protocol for performing ABA on mine overburden and
be calculated by multiplying the percent S times 3 1.25.
is frequently cited as the source document. However, hs
T i factor is derived from the stoichiometric relation-
earlier publications described the application of ABA
ships in equation 6.1 and carries the assumption that
principals to mine overburden testing (West Virginia
the C02 exsolves as a gas.
University, 1971;Grube et al., 1973; Smith et al.,
1974; Smith et al., 1976). In 1988 Energy Center,
Inc., under contract to the Department of Environ-
mental Resources, Small Operator Assistance Program Cravotta et al., (1990) suggested that, in backfills
(SOAP), produced a detailed report on overburden where COz cannot readily exsolve, the C 0 2 dissolves
sampling and testing wall et al., 1988). T i latter
hs and reacts with water to form carbonic acid and that
document included a detailed description of considera- the maximum potential acidty in 1000 tons of over-
tions appropriate to planning an overburden analysis burden should then be derived by multiplying the per-
and to collecting the samples; it also added a boil step cent S times 62.50.
to the NP determination methodologies and provided a
detailed description of methods for determining forms The neutralization potential (NP) is determined by
of sulfur. digesting a portion of the prepared sample in hot acid,
ankl then by titrating with a base to determine how
Th~s chapter focuses on aspects ofthe ABA proce- much of the acid the sample consumed. NP represents
dures which have been somewhat dontroversial because carbonates and other acid neutralizers and is commonly
of the effects that they can have on the reported results; expressed m terms of tons CaC03 per 1000 tons of
it will not detail the laboratory protocols required to overburden (ppt). Negative NP values are possible,
perform ABA. and are sometimes derived from samples of weathered
Components of ABA.. rock that contain residual wezithering products which
produce acidity upon dissolution.
ABA is based on the premise that the propensity for
a site to produce acid rnine drainage can be predicted Interpretation of ABA data involves the application
by quantitatively determining the total amount of acid- of numerous assumptions; some of the more significant
ity and alkalinity the strata on a site can potentially assumptions often used are:
produce. all sulfur in a sample will react to form acid;
The maximum potential acidity (expressed as a all material in the sample which consumes acld
negative) and total potential alkalinity (termed neutrali- during digestion in the lab will generate alkalinity in
zation potential) are then summed. If the result is the field;
positive, the site should produce alkaline water, if it is the reaction rate for the sulfur will be the same as
negative, the site should produce acidic water. Sobek the dissolution rate for the neutralizing material.
et al., (1978) defined any strata with a net potential NP and percent sulfur values below certain thresh-
deficiency of 5 tons per 1000 tons (ppt) or greater a s old levels do not influence water quality.
being a potential acid producer. The maximum poten-
tial acidity (MPA) is stoichiometrically calculated from
Chapter 6 - Laboratory Methodsfor Acid-Base Accounting: An Update
As these assumptions imply, interpretation of ABA tential acidity" or MPA that a particular overburden
data is far more complicated than simply summing the sample or column could produce if all the sulfur reacts.
MPA and NP values. Chapter 1 1 titled "Interpretation
Sulfur determinations for ABA are often performed
of Acid-Base Accounting Data" discusses these as-
for total sulfir only, however, determinations for forms
sumptions in more detail.
of sulfur are sometimes included. Sulfur generally oc-
In addition to the percent sulfur and NP detennina- curs in one of three forms in the rock strata associated
tions, two other measured parameters in an ABA over- wth coals in Pennsylvania: sulfide sulik-, organic sul-
burden analysis are paste pH and fizz. Other derived fur, and sulfate sulfur. Sulfide sulfiir is the form
values are calculated from one or more of the measured which reacts with oxygen and water to form acid mine
parameters and from other information such as the drainage. The sulfide minerals most commonly associ-
sample thickness, density and areal extent. The de- ated with coals in Pennsylvania are pyrite and mar-
rived values used may vary somewhat but typically casite, both of whch are FeS2, chemically. Other
include calculations of maximum potential acidity, tons sulfide minerals such as chalcopyrite (CuFeS2)and
of neutralization potential, tons of potential acid@, arsenopyrite (FeAsS) may also be present in small
and tons net neutralization potential for each sample, amounts. Organic sulfur is that sulfur which occurs in
as well as for the entire bore hole. The derived values carbon-based molecules in coal and other rocks with
are also discussed in the chapter of this document significant carbon content; since organic sulfirr is tied
whch deals with the interpretation of ABA overburden up in compounds that are stable under surface condi-
analysis. tions, it is not considered a contributor to acid mine
drainage. Organic sulfur can represent a significant
Paste pH
fraction of the total sulfur found in coal seams. Data
The paste pH test is described in the previously ref- from the Penn State Coal Data Base show that the av-
erenced manuals on ABA protocol (Sobek et al., 1978; erage percent organic sulfur in several frequently
No11 et al., 1988), however, in Pennsylvania, it has mined coals in Pennsylvania ranges from a low of 0.55
fallen into general Qsuse over the past several years. % for the Upper Kittanning Coal to a high of 1.32 %
A portion of the prepared sample is mixed with deion- for the Clarion Coal, with an overall average of 0.74%.
ized water, and then tested with a pH probe after one Sulfate sulfur is often overlooked because in humid
hour. The paste pH test may indicate the number of climates it generally is found in relatively small con-
free hydrogen ions in the prepared sample, but, since centrations due to its high solubility. However, when
pyrite oxidation reactions are time dependent, the paste present in Pennsylvania, sulfate sulfur often occurs in
pH results provide little indication of the propensity of partially weathered samples as the reaction by-
a sample to produce acid mine drainage. In fact, the products of sulfide mineral oxidation. When solubi-
paste pH of a unweathered, high-sulfur sample is likely lized, these weathering by-products are the source of
to be near that of the deionized water, whlle a weath- the contaminants found in acid mine dramage, so when
ered sample with relatively low percent sulfur, but determinations for forms of sulfur are done, sulfate
which includes a small amount of residual weathering sulhr must be considered in the calculation of MPA.
products, may have a significantly depressed paste pH. Alkaline earth sulfate minerals such as gypsum
Because of its limited usefulness in helping predict the (CaSOJ can also contribute to the sulfate sulfiu frac-
potential for acid mine drainage production, the paste tion, but generally are not abundant in coal-bearing
pH test often is no longer performed, and for mine rocks in Pennsylvania. Where they are present, the
permit applications m Pennsylvania, it is not a required W i n e earth sulfate minerals do not contribute to
component of ABA. acidity.
Percent Sulfur Commonly used methods of performing total sulfur
determinations are high temperature combustion meth-
Since acid mine drainage results from accelerated
ods (ASTM D4329), the Eschka Method (ASTM
weathering of sulfide minerals, the amount of sulfur in
D3 177) and the Bomb Washing Method (ASTM
a sample, or in an overburden column, is obviously an
D3 177). Of these methods, the high temperature com-
important component of ABA. As noted above, ABA
bustion methods are the simplest and most frequently
uses the percent sulfur to predict the "maximum po-
used and provide accurate, reproducible results
Chapter 6 - Laboratory Methods for Acid-Base Accounting: An Update
Common methods used for determining forms of sulhr sulfur. The high temperature combustion results
include ASTM D2492 and an EPA method. Noll et compared well on duplicate samples, while the py-
al., (1988) present an A S T W P A Combination ritic results on the same samples did not.
method which the authors of that document felt com- Standards are availablc fiom the National Institute
bined the most desirable features of the other two of Standards for total sulfur but not for pyritic sul-
methods. fur.
Theoretically, the total of the sulfate and sulfide A wide range of methods for determining pyritic
sulfur components should be a better indcator of the sulfur were in use and individual laboratories had
amount of reactive sulfur in a sample than should total their own variations of the methods.
sulfur. However, a laboratory study (Hedin and Er- One of the commonly used methods of pyritic sulfbr
ickson 1988) showed that total sulfur was related more determinations, ASTM D2492, was developed for
strongly to leachate test results than was pyritic sulfur. use on coal and is probably not appropriate for de-
Since pyritic sulfur is the form which contributes most terminations on rock overburden, according to
significantly to acid mine drainage, these results indi- ASTM Committee D-5on Coal and Coke.
cate problems wth pyntic sulfur detemunations. A Figures 6.1 and 6.2 (data taken from Brady and
review of the methods for sulfur determinations de- Smith (1990)) compare total sulhr determinations and
scribed in No11 et al., (1988) reveals that the methods pyritic sulfur determinations of two different laborato-
for total sulfur determinations have a relatively high ries wluch performed analyses on duplicate samples.
degree of precision with few notable interferences and The hgh ? value for the total sulfur determinations
precautions, while the forms of sulfur determination indcate a strong correlation, and the low r2value for
methods described involve lesser degrees of precision the pyritic determinations indicates a weak correlation.
and more numerous potential interferences and precau-
tions. Stanton and Renton ( 1981) examined the nitric
acid dissolution procedure, which is the cornerstone of
the most frequently used methods for determining py-
ritic sulfur, including ASTM D2492; they found the
procedure frequently does not succeed in digesting all
the pyrite in a sample, thus underestimating the pyritic
fraction of the sulfur in the sample. Brady and Smith
(1990) compared total sulfur a d forms of sulfur de-
n
terminations performed by various laboratories. Their
findings include:
While the results generated by each laboratory were
internally consistent in terms of the ratio of pyritic
sulfur to total sulfur, there were sigruficant hffer-
ences between laboratories in the median percent
pyritic sulfurhotal sulfur. Where duplicate samples
were available fiom different laboratories, differ-
ences were noted in the pyritic determinations, but
total sulfur determinations were comparable.
There was no significant difference in the percent.
pyritic sulfur/total sulfur between rock types
(excluding coal). (lks finding contradicts one of
the primary reasons for doing determinations for
forms of sulfur: that some rock types contain sig-
mficant percentages of orgmc sulfur.) .. --.--
With one exception, all laboratories whose data was re 6.2 C w m n & p e r m ~ Iqn%sample analysis
used in the study used a high temperature combus- Tts f ~ @ticaxe; sanlples mxlyzxi by two different
r
tion method for determining weight percent total
fatt?f!". .. ., ... .. . .. . ... . ... .... ... . ... ... .
.
Chapter 6 - Laboratory Methods for Acid-Base Accounting: An Update
The above findings can be summarized as: Total pact on postmining water quality. However, these
sulfur determinations are typically simple to do, are same strata often represent the greatest mass of the
reproducible, and can be calibrated and verified using overburden and can "dilute" the effects of the strata
available standards; pyritic sulfur determinations are with significant NP and percent sulhr if they are in-
done using a variety of methods (sometimes not stan- cluded in the calculations of total NP and MPA for the
dardized, and at least one of which is considered inap- site. In Pennsylvania a threshold value of 30 is often
propriate for rock samples), produce results which are iz
used for NP. A threshold value of a 1 (slight fz)is
often not reproducible between laboratories, and can- also often used. The fizz threshold tends to label a 0 or
not be calibrated and verified using available stan- no fizz as being "bad" and higher fizz ratings as being
dards. Given these considerations, and that pyritic "good." Strata identified as having a 0 fizz will not be
sulfur is the most abundant form in coal overburden counted as contributing potential alkalinity to post-
(but not necessarily in the coal), total sulfur determi- mining water quality which could result in a negative
nations currently provide the best basis for calculating permitting decision. Even with the best intentions of
MPA. the lab personnel performing the test, one cannot ex-
pect objective and reproducible results from a subjec-
Fizz Rating
tive test with a particular outcome pre-labeled as either
The importance of the fizz rating on AE3A results is good or bad. This is not to suggest that the use of
much underestimated and has often not received ap- thresholds is inappropriate, but to point out another
propriate consideration. The fizz test is frequently precaution concerning reported fizz test results.
presented as a minor part of the neutralization potential
Evans and Skousen (1995) suggested a two-tiered
test; however the fizz test can have a large impact on
fizz rating system which would combine the 0 and 1
the reliability and reproducibility of NP data, so it is
fizz ratings into a single category and a 2 or 3 fizz
discussed separately here. The fizz rating can be used
rating into a second category. They reported that dur-
as a check on the NP determination, since there should
ing a round robin sample testing study conducted by
be a qualitative correlation between the two. More
representatives of West Virginia University, Consoli-
importantly, however, the fizz rating determines the
dation Coal Company (Consol), and the Pennsylvania
volume and the strength of the acid which is used to
DER (now DEP) on samples processed at the Penn
digest the prepared sample, which in turn can affect the
State Materials Research Laboratories, the f z ratings
iz
NP determination results (Evans and Skousen, 1995;
varied significantly between laboratories for certain
Skousen et al., 1997). The NP result is then somewhat
samples. The laboratories then used different normali-
dependent on the fizz test results, and the fizz test re-
ties and volumes of acid to perform the NP determina-
sults are a matter of human judgment.
tions on those samples, as dictated by the fizz ratings.
The fizz test is performed by adding one to two The NP values varied considerably, and generally were
drops of 25% HC 1 to a small amount of the prepared higher when a larger volume of acid was used to digest
sample (Sobek et al., 1978). The degree of reaction is the samples. When the Consol lab ran the NP determi-
observed and recorded, according to a four-tiered sys- nations for each sample twice, with a different volume
tem where the reaction is judged to be none or 0, slight of acid each time, the determination that was made
or 1, moderate or 2, strong or 3. (Other systems with with the higher volume of acid produced a higher NP in
more levels have been used for reporting fizz results. each case. The differences were often great enough to
However, given the obvious difficulties inherent to a change the interpretation one would make regarding the
test based on qualitativejudgment, additional levels of alkaline-producing potential of the sample. Table 6.1
judgment can only imply a precision which is not ob- displays fizz and NP data generated by the WW and
tainable.) Consol laboratories during the round robin test and
shows how fizz rating, acid volume and acid normality
There is an additional consideration which further
can affect NP results. Most of the samples included in
complicates the subjective nature of the fizz test.
Table 6.1 were selected for the study because visual
Thresholds for NP and percent sulfur are often used in
observation suggested that they were siderite-rich;
interpreting AE3A. The theory behind using thresholds
therefore, the differences in the fizz results and NP de-
is that strata which produce NP or percent sulfur val-
terminations between laboratories are probably more
ues below the thresholds are thought to have little im-
representative of what one would expect for siderite-
6-4
Chapter 6 - Laboratory Methodr for Acid-Base Accounting: An Update
Table 6.1 Fizz test results and NP determinations for replicate overburden samples.
samples in accordance with both Sobek and Geochem recommendations; autotitrated to pH 70 ..
**@I results based on digestion in 80 ml of 0.1N HCl. NP2 results based on digestion in 20 ml of 0 1 HCI for 0 fizz,
.N
40 ml of 0.1N for 1 fi,and 80 ml of 0 5 for 3 fuz. Manually titrated to pH 70
.N ..
rich samples as opposed to samples with low siderite No11 et a]., (1988). The NP and fizz determinations
content. Skousen et al., (1997) reported that when reported in Skousen et al., (1997) were run on repli-
three different laboratories performed fizz deterrnina- cates of the overburden samples, but the percent in-
tions on replicates of 3 1 samples, all three laboratories soluble residue test was only run by one of the labs.
assigned the same fizz rating to only 13 of the 3 1 sam- As noted by the authors of that study, the method needs
ples. to be hrther tested to validate the proposed rating sys-
Reducing the number of tiers in the f z test should
iz tem and to provide a yardstick for comparing NPs
reduce the amount of judgment required and conse- based on the fizz test to those based on the percent in-
quently the subjectivity of the test. However, running soluble test. One potential problem with the percent
the NP test with a reduced number of fizz test possi- test
insoluble res~due is that, for some samples, the
bilities means that some samples would be digested in results may vary sigruficantly when the percent HCI
dfferent volumes of acid than they would using the used in the digestion is changed (Keith Brady, personal
methods in Sobek et al. (1978) and No11 et al. (1988). communication). The samples stuQed by Skousen et
Users of NP data need to be aware that changing the al. (1997) were subjected to X-ray diffraction and
volume of acid used to dgest a sample can change the characterized as belonging to one of four groups, based
NP results. on their mineral and elemental content. Fe, Ca, S, and
Si. When the percent insoluble residue test was per-
Skousen et al. (1997) described a protocol for a
formed on replicates of some of the samples using dif-
quantitative method of rating overburden samples
fering percents HCl, the results changed significantly
based on the percent insoluble residue. Twenty ml of
for the iron-rich samples (Fe group) whlch included the
10% HC1 is added to 2.0 g of the prepared sample
samples with relatively high siderite content. (See Ta-
which has been dried in an oven. The solution is agi-
ble 6.2.) The results for one of the carbonate-rich
tated until evolution of COz is observed to cease. The
samples (Ca2) also changed significantly. These re-
solution is passed through a weighed filter, the filter
sults raise questions concerning which % HC1 should
plus residue are then dried and weighed, and the per-
be used to achieve results which rate the carbonate in
cent insoluble residue is calculated. The rating is then
the samples in an accurate and reproducible way.
used to determine the volume and strength of acid used
in the NP digestion; for that purpose the carbonate Given the difficulties which the current fizz rating
rating numbers are considered to be equivalent to the system introduces into NP determinations, a reproduci-
fizz rating values described in Sobek et al. (1978) and ble, objective carbonate-rating test could significantly
Chapter 6 - Laboratory Methods for Acid-Base Accounting: An Update
Table 6.2 Comparison of percent insoluble residue to %HC1 used to digest the sample.
(Samples which were digested with all three % HCI were tested at the DEP laboratory in Harrisburg, PA. For the samples di-
gested in 10°/o and 50% HCI, the 10% digestion was done at West Virginia University and the 50% digestion was done at the PA
Geologic Survey. Samples which were digested twice in the same percent HCl were duplicate samples.)
improve the reproducibility of NP data. Until such a calculated from that amount of acid that was neutral-
test is refined, individuals who generate and interpret ized by the sample.
ABA data need to be much more aware of the influence
Carbonate minerals, such as calcite and dolomite,
of the fizz test values on the NP determinations. are known to be the major contributors to groundwater
Where fizz test results and NP values seem to be at
alkalinity in the coal regions of Pennsylvania. The
odds, further testing would be prudent. acid-digestion step of the NP test is suspected of dis-
When a carbonate rating system other than the fa- solving various silicate minerals, whch results in an
miliar four-tiered fizz test is used, data interpretation NP determination that overstates the amount of car-
mil1 have to be adjusted and interpretive rationales will bonate minerals in a sample. Lapakko (1993), worlung
have to be "recalibrated." with rock samples from metals ore in Minnesota, re-
ported that silicate minerals such as plagioclase dis-
Neutralization Potential (NP)
solve and neutralize acid at relatively low pH values
The first step of the NP test is to conduct a qualita- such as those which occur in acid mine drainage or
tive fizz test on a small mount of the prepared sample during a NP titration; however, he also noted that since
as described earlier in this chapter. Based on the f ziz thls dissolution will only take place at low pH values, it
test results, an appropriate volume and normality of is unlikely to help maintain a drainage pH of accept-
HC 1 is selected then added to 2.0 grams of the pre- able quality. His test results, based on leaching stud-
pared sample. (See Table 6.3.) Reagent water is added ies, also indicated that the rate of acid neutralization by
to bring the total volume to 100 m (Noll et al., 1988).
l silicate minerals was not adequate to maintain a drain-
(Note that there are variations between the methods age pH of 6.0 or above.
described in No11 et al (1988) and in Sobek et al. Siderite (FeC03) has long been suspected of inter-
(1978). %s discussion is based on the methods de-
fering with the accuracy of NP determinations and of
scribed by No11 et al.) The solution is boiled for ap-
complicating the interpretation of the data (Meek,
proximately 5 minutes, which is intended to dissolve 1981; Momson et al., 1990; Wirarn, 1992; Leavitt et
potential neutralizers in the sample After the solution
al., 1995). Siderite is common in Pennsylvania coal
is cooled, it is titrated with NaOH to a pH of 7.0; the overburdens. Samples with significant amounts of
end point is to be held for 30 seconds. The NP in
siderite can make it difficult to hold xhe fnl end point
ia
terms of tons per thousand tons of rock (ppt) is then
Table 6.3 Volume and Normality of KC1 used to do NP digestion based on the sample fizz rating. (After
Sobek and others (1978) and No11 and others (1988).)
IZ
F Z RATING HCl VOLUME HCI NORMALITY
None (0)
Slight (1)
Moderate (2)
Strong (3)
Chapter 6 - Laboratory Methodsfor Acid-Base Accounting: An Update
of the titration with NaOH (Noll et al., 1988). If iron parative study, which may be why their results showed
in solution from the siderite is not completely oxidized that the hydrogen peroxide step reduced NP
when the titration is terminated, then the calculated NP
Skousen et al. (1 997) subjected 3 1 overburden
il
value w l be overstated, since complete oxidation of samples of known mineralogy (determined by X-ray
the iron would produce additional acidity An uncer-
diffraction) to four variations of the NP test. The
tain titration end point can obviously affect the repro- variations were defined by the authors ofthat paper as:
ducibility of the NP results. Skousen et al. (1997) also 1) (Sobek), the standard Sobek method (Sobek et al.,
found that laboratories tended to assign different fizz 1978); 2) (Boil), a method that includes boiling of the
ratings to replicates of samples with high siderite con- sample for five minutes during the digestion step (Noll
tent. As noted in the earlier section of this chapter
et al., 1988); 3) (HzOz), the same as the boil method
which dealt with f z ratings, assigning different fizz
iz
except that after digestion the sample is filtered and
ratings to the same sample can change the acid volume
treated with Hz02 before titratlon; 4) (SobPer), the
and strength used in the NP digestion step, which will
same as the Sobek method except that H20zis added to
affect the NP results.
the sample (no filtration) after the first titration.
Meek (1981) and Momson et al., (1990) proposed Among their findings the authors concluded:
adding a hydrogen peroxide step to the NP determina-
The four variations on the NP test produced similar
tion procedures to eliminate the problems with the
method caused by siderite. Momson and Scheetz
results for samples containing l~ttle pynte or
siderite.
(1994) performed ABA tests on four samples using
both the method described in Noll et al. (1988) and The SobPer method gave lower NP values than the
their modified approach. Under the modified method, other methods for samples which included signifi-
after the sample was digested in acid, it was filtered cant amounts of pyrite, due to oxidation of the py-
into a vacuum flask. The filtering was done to ensure rite by HzOzin the unfiltered samples.
that the H202&d not oxidize pyrite or other solids m Compared to the other three methods, the H202
the undigested portion of the sample. Tht: solution was method provided: The lowest NP values for samples
then transferred to a 400 m Pyrex beaker, and the
l with significant siderite content, the best reproduci-
vacuum flask was rinsed with 125 ml of deionized wa- bility between the laboratories which participated in
ter. Five to 7.5 ml of 30 wt % H202was added to the the study; results which were the most consistent
solution whch was then boiled for three to five min- with soxhlet leachate results.
utes. After cooling, the solution was then titrated to Autotitration at a slow setting is preferable to hand
pH 7.0 with NaOH. The NP for each sample was titration, especially for samples with significant
lower when the m d f i e d method was used, and was siderite content.
significantly lower for the three samples known to Skousen et al. (1997) briefly describe a method to
contain a significant amount of siderite. perform NP determinations with the H20zstep. If the
Evans and Skousen (1995) found that NP values hydrogen peroxide step pcrforms according to its in-
were not appreciably different when samples were tent, it should generally decrease the NP's of strata
analyzed both with and without the hydrogen peroxide w1t.h a significant siderite content, but should not ap-
step; however they found that reproducibility between preciably affect the NP values of strata that do not in-
laboratories did improve when the hydrogen peroxide clude significant amounts of siderite. It should also
step was used. They also found that when the hydro- lead to better reproducibility of NP data between labo-
gen peroxide step was performed without filtering the. ratories, especially for samples with significant siderite
solution, the results sometimes did not compare well content.
with other ABA methods, probably due to the oxida- Other Methods of Determining
tion of pyrite in the residue by H202.In fact, oxidation Carbonate Content
of pyrite with Hz02 has been used as a method of pre-
dicting the acid-producing potential of overburden The NP test has been adapted and widely used to
(O'Shay, et al., 1990). Morrison and Scheetz (1994) approximate the carbonate content of mine overbur-
used samples known to include a significant amount of dens largely because it is relatively quick, inexpensive,
siderite (determined by X-ray difiaction) in their com- and easy to perform. However, as noted in this chap-
ter, it may not always provide results which are accu-
6-7
Chapter 6 - Laboratory Methodsfor Acid-Base Accounting: An Update
rate and reproducible. Other methods of determining ing system from 4 to 2 as suggested by Evans and
carbonate content have occasionally been used in Skousen (1995) could result in more consistent NP
Pennsylvania on hlgh risk-sites or on sites where the determinations. However, when the number of fizz
NP test provided questionable results. rating possibilities are reduced, some samples are di-
gested in a larger volume of acid than they would under
Morrison et al. (1990) suggested COz coulometry
the traditional way of performing the tests, resulting in
as an alternative method for determining carbonate
higher NP determinations. The interpretive rationale
content of overburden samples and reported promsing
applied to ABA data will have to be "recalibrated if a
results, however, the method has not been widely carbonate rating system other than the traditionally
adopted for characterizing overburden samples to date. used four-tiered system is ultimately adopted
X-ray dimaction, which can give detailed mfonna-
The quantitative method of rating carbonate content
tion on the overburden mineralogy, has been used on a
of overburden samples by determining the percent in-
few sites in Pennsylvania. In cases where X-ray dif-
soluble residue, as described by Skousen et al. (1997),
fraction has been used and where fizz test ratings and
requires acldtional testing to determine if it could be
NP results seemed in conflict and suggested the pres-
used as a more objective option than the fizz test for
ence of siderite (results which showed significant NP
rating carbonate content. The percent HC1 used to di-
values for samples whch did not fizz), the X-ray dif-
gest the samples may significantly affect the percent
fraction results verified the presence of the siderite.
insoluble residue for siderite-rich samples.
In situations where NP data provlde ambiguous
Siderite, a common mineral in Pennsylvania coal
results and/or where mining presents a risk to sigrufi-
overburdens, can interfere with NP determinations,
cant uses of nearby groundwater or surface water
generally resulting in values that are high relative to the
sources, tools such as X-ray diffraction and COa mu-
amount of calcium carbonate in the sample. Addmg a
lometry are available and should be considered to ver-
hydrogen peroxide step (such as described by Skousen
ify the NP results.
et al., (1997)) to the NP determinations reduces the
Conclusions interference of siderite and does not appreciably affect
NP determinations for samples without significant
Three aspects of ABA overburden analysis labora-
amounts of siderite. NP determinations run with the
tory techques create problems with reproducibility
hydrogen peroxide step provide better reproducibility
and accuracy of data.
between laboratories and produce results whch better
Difficulties in performing forms of sulfur determi- represent the true carbonate content of the rock.
nations can lead to unreliable results if pyritic sulfur
determinations are used to calculate MPA instead of Literature Cited
total sulfur determinations. Since pyritic sulfur is typi- Brady, K.B.C., and M. W. Smith, 1990. Pyritic sulfur
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