A comparison of test methods in evaluating a rock's potential to produce acid rock drainage

Abstract

Acid base accounting (ABA) is the measurement of a material's ability to produce acid. It measures acid potential (AP) and neutralization potential (NP) to obtain the neutralization potential ratio (NPR) of a material. NPR is equal to NP/AP. Static tests are a widely accepted and used method of determining a material's NPR. These tests measure the sulphur content of a sample to determine AP. They also measure a sample's NP by various means, depending on the method. Limitations of static tests are that not all of the sulphur within a sample may be readily available to produce acid, hence there is an overestimation of AP. Also, the vigorous testing conditions employed to determine NP do not mimic field conditions, hence there is an overestimation of NP. For this reason these tests are used primarily as preliminary indicators as to which rocks have the potential to produce acid. Seven rock samples were collected from a road-cut on Highway 7 in New Brunswick. The samples were collected to ensure a representative sample of the road-cut material. Three static tests were performed on the samples; the Sobek Method, the Modified Sobek Method, and the B.C. Research Initial Method. Thin sections of each sample were also fabricated. The mineralogy of each sample was studied in order to determine the specific minerals present. Results of NP, AP, and NPR from each test are compared to determine which test is most accurate in determining the rock's potential to produce acid rock drainage. In all samples, the NPR was lowest when measured by the B.C. Research Method.