For use in all LRRs.

Positive identification of dark-gray or black iron monosulfide concentrations with value of 4 or less and chroma of 2 or less, starting at a depth of 25 cm (10 inches) or less from the soil surface.

User Notes: Positive identification of this indicator requires a minimum of 2 separate observations of iron monosulfide (FeS) concentrations in the soil occurring as stains, coatings, soft masses, or pore linings. Care should be taken to observe the occurrence of FeS immediately following excavation as these compounds can oxidize rapidly with exposure to the atmosphere. The presence of FeS concentrations is confirmed by documenting dark-gray or black colored areas within the soil matrix and its subsequent degradation using either: (A) oxidation following exposure to the atmosphere or with application of an oxidizing agent such as dilute hydrogen peroxide, both of which result in an increase in Munsell value of 1 or more; or (B), the evolution of hydrogen sulfide gas following application of dilute hydrochloric acid. See the “Glossary” for a description of methods to identify FeS (fig. 21).