Reliability improvement for predicting acid-forming potential of rock samples using static tests Chamteut Oh & Sangwoo Ji & Chul-Min Chon & Giljae Yim & Youngwook Cheong Received: 22 September 2016 /Accepted: 21 March 2017 /Published online: 5 April 2017 # Springer International Publishing Switzerland 2017 Abstract In predicting the acid-forming potential of rock samples, a combination of acid–base accounting (ABA) and net acid generation (NAG) tests has been commonly used. While simple and economical, this method some- times shows low reliability such as categorizing certain samples as uncertain (UC). ABA and NAG tests were modified to selectively recover valid minerals in nature and substituted for the original tests. ABA test overestimated acid-producing capacity (in the case of weathered samples) and acid-neutralizing capacity (in the case of plagioclase-including samples) compared to the modified ABA test. NAG test yielded lower NAG pH compared to modified NAG test for samples with high total C content and low total S content. By comparing the correlation coefficients between acid generation amounts by the two evaluation methods, it was confirmed that modified evaluation method (MEM) has a much higher reliability (R 2 = 0.9582) than existing evaluation method (EEM) (R 2 = 0.5873). It was also concluded that exploiting advantages of both EEM and MEM is recommended where EEM is initially applied for general classification and a supplemented static test of MEM is executed for the purpose of correcting the error of UC categorized samples. Keywords Acid–base account (ABA) . Net acid generation (NAG) . Uncertain (UC) sample . Acid- forming potential . Reliability of evaluation method Introduction Rocks containing acid-forming minerals such as pyrite could generate acid drainage when exposed to water and air (Johnson and Hallberg 2005; Ghorbanzadeh et al. 2015). This acid drainage, due to its acidity, is regarded as a toxic pollutant itself but, moreover, its harmfulness can be exacerbated by dissolving toxic metals from sur- rounding minerals while flowing in a form of stream or groundwater (Ji et al. 2008; Azapagic 2004; Oh et al. 2012; Han et al. 2015). Fe, the most common element in the drainage, can also cause yellow boy which can seri- ously vitiate scenic amenities near the pathway (Brady et al. 1986; Ji et al. 2007; Oh et al. 2015). These environ- mental damages caused by acid drainage are inevitably promoted by anthropogenic activities such as mining and construction works which exposes rocks to an oxidation environment (Bigham and Nordstrom 2000; Park et al. 2015). Depending on the area of occurrence, the drainages are referred to as either acid mine drainage (AMD) or acid rock drainage (ARD). Evaluating the acid-forming potential of rock or soil preexcavation is essential in minimizing the damages by acid drainage (Li et al. 2007; Schumann et al. 2012; Weber et al. 2004). The methods used to evaluate the acid- forming potential are either static or kinetic depending on the way in which the experiment is conducted. Kinetic Environ Monit Assess (2017) 189: 207 DOI 10.1007/s10661-017-5906-6 C. Oh : C.<M. Chon : G. Yim : Y. Cheong Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea S. Ji (*) Climate Change Mitigation and Sustainability Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea e-mail: swji@kigam.re.kr