This chapter is concerned with some of the pollutants and ancillary chemicals found in treatment marshes that are not principal targets for design. Some have the potential to modify wetland processes but are not themselves design targets. Modifiers include pH, dissolved oxygen, and sulfur. Others are secondary targets for treatment, meaning their removal is of considerable interest, but not a direct factor in design. BOD and pathogens fall in this category. Subsequent chapters will deal with the larger categories of trace metals (Chapter 13) and trace organics (Chapter 14).