OVERBURDEN SAMPLING AND ANALYTICAL TECHNIQUES

Acid Base Accounting and Leaching Tests

Characterization of the acid or toxic potential of overburden material is generally accomplished by either a technique known as acid/base accounting (Smith and others, 1982; Sobek and others, 1978) or leaching tests (Caruccio and others, 1977, 1981, 1982; Caruccio, 1984; Geidel, 1979).

Acid/base accounting is utilized nationwide to assess acid or toxic potential and consists of the following three fundamental measurements:

1. Ph of a pulverized sample mixed with distilled water to the consistency of a thin paste.
   
2. Measurement of total disulfide content (mainly pyrite).
   
3. Measurement of total neutralization potential.
   

Paste Ph is a measure of the sample's immediate acidity or alkalinity and reflects current geochemical conditions. Samples with a paste Ph of 4.0 or less are classed as acid/toxic regardless of the pyrite and neutralizer balance (Smith and Sencendiver, 1982; Sobek and others, 1978).

Solubility and mobility of many trace elements and metals in water are strongly dependent on Ph. For example, at a Ph 4.0 or lower, elements such as aluminum, copper, manganese, zinc, lead, and chromium can be released into ground or surface waters.

Maximum potential acidity is calculated from the pyrite content of the sample and the four-step chemical reaction of pyrite to acid formation (Sobek and others, 1978). This calculated value represents the maximum or worst case condition of acid production. The actual acid production rate and completeness of reaction cannot be estimated using the acid/base technique.

Neutralization potential measures the sum total of carbonates, alkaline earths, and bases available to neutralize acidity and represents the most favorable condition. Calculations of maximum potential acidity and neutralization potential are structured to equate the two measurements to a common basis for comparison. The resulting values, expressed as calcium carbonate equivalent, are compared to compute a net acid-producing or neutralizing potential. Materials exhibiting a net acid production potential of 5 tons/1,000 tons of overburden material or more as calcium carbonate equivalent are classed as toxic or potentially toxic (Smith and Sencindiver, 1982; Sobek and others, 1978; Sturm and others, 1984).

The primary advantages of the acid/base accounting method are:

1. Short turn-around time for sample processing.
   
2. Low cost.
   
3. Relatively simple analytical procedures.
   
4. Relatively simple interpretation of results.
   

The principal disadvantages of acid/base accounting are:

1. The method predicts maximum potential acidity and maximum neutralization capability and implies a 1:1 acid to base reaction. Actual acid production and neutralization release rates cannot be predicted with this technique nor can the completeness of the reaction be assessed.
   
2. Acid/base accounting assumes all acid production is attributable to iron disulfide minerals (chiefly pyrite) and that no acid is produced by sulfate or organic sulfur forms. Sulfur fractionation studies of some Western overburden material have shown that about half of the total sulfur is present in organic forms and that acidity is being produced that cannot be accounted for by pyrite alone.
   
3. Measurement of neutralization potential utilizes a hot acid extract to measure carbonates and bases. Recent work in Texas suggests that this extraction procedure may overestimate neutralization potential and that a modified method may be needed.
   

Application of this acid/base accounting method to overburden handling and placement plans throughout the country has generally been moderately effective in eliminating or reducing adverse water quality impacts. Acid/base accounting is typically considered state-of-the-art for overburden analysis.

Leaching or simulated weathering tests have been advocated by Caruccio and others (1977, 1981, 1982), Caruccio (1984), and Geidel (1979) as an alternative overburden analysis method. The procedure is designed to mimic the conditions expected to occur in regraded spoil. Samples are subjected to alternating water leaching and exposure to moist air. Leachate is collected periodically and analyzed for pH, acidity, sulfate, and any other constituents of interest. Supplemental information may also be obtained by petrographic study of pyrite morphology. Caruccio has indicated that fine-grained pyrite with a large surface area is much more reactive and likely to produce acidity than coarse-grained pyritic material (Caruccio, 1969, 1984; Caruccio and others, 1982).

Results of leachate analyses are plotted as a function of time resulting in data on rate of reaction and the quantity of acidity or alkalinity produced. Leaching simulations have been applied to eastern and midwestern minesites and are reasonably good predictors of short-term (2 or 3 years) drainage quality.

Caruccio (1984) suggested that acid/base accounting be used as an initial screening test for overburden samples. Materials containing a large net neutralization potential do not require further analyses. If total pyrite exceeds about 1 percent or if the neutralization potential is small, Caruccio recommends leaching tests to more accurately predict drainage quality.

The main advantages of leaching tests are summarized as follows:

1. Test methods are designed to simulate field conditions.
   
2. Reaction rates (kinetics) can be evaluated.
   
3. Leaching of overburden constituents other than acidity and alkalinity can be evaluated.
   

The primary disadvantages associated with leaching tests are:

1. Test time is lengthy; about two months are required to conduct one analysis.
   
2. Analysis is expensive.
   
3. Long-term predictive capability of leaching tests are uncertain.
   
4. Data interpretation requires more sophisticated review than the acid/base accounting method.