NAG Test Protocol & Interpretation Guide (ASTM D7786-22)

The NAG Test Protocol is designed to simulate accelerated oxidative weathering of sulfide-bearing mine wastes—such as tailings, waste rock, and overburden—to evaluate their potential to generate acid over decades to centuries. Unlike static geochemical tests that measure instantaneous acid-base accounting (e.g., Acid Base Accounting), ASTM D7786-22 employs a two-stage procedure: first, samples are reacted with 30% H₂O₂ to oxidize reactive sulfides (e.g., pyrite, pyrrhotite) to sulfuric acid; second, the resulting solution is titrated to quantify total acidity and neutralizing capacity (primarily from carbonates and hydroxides), yielding net acid generation values. The protocol specifies strict controls on particle size (<75 µm), reaction time (24 h ± 1 h), temperature (23 ± 2 °C), and reagent purity to ensure reproducibility and inter-laboratory comparability. Results are reported as NAG pH (pH of the final suspension after titration to endpoint), NAG acidity (total acid produced, corrected for neutralization), and NAG Neutralization Potential (NP), enabling classification into ARD risk categories (e.g., potentially acid generating, non-acid generating, or transitional). This kinetic–semi-quantitative approach bridges the gap between simple screening tests and complex long-term column leaching studies, supporting robust environmental management decisions in mine planning, closure design, and regulatory compliance.