This report summarizes the basic chemistry associated with mineral dissolution, weathering, biotic processes and mixing, all of which may contribute to changes in seepage chemistry during structural transit. This report also provides references, overviews of planning, sampling, quality assurance, and interpretation of seepage chemistry investigations, and examples from successful seepage investigations performed over the past 20 years by Bureau of Reclamation Dam Safety Program professionals. Summary references are also recommended for the reader who wishes to investigate specific topics more thoroughly.