Dissolved organic matter in ground water consists of variable and
largely undefined mixtures of humic substances derived from contact
of the underground water with decaying or fully decayed organic matter
(such as lignite, in the case of the Sparta Sand Aquifer). Tannins—a group
of polyphenolic compounds often found in water in contact with organic
matter—are known to have antimicrobial properties. Scalbert (1991)
reviewed the antimicrobial properties of tannins and described different
mechanisms which include: 1) inhibition of extracellular microbial enzymes,
2) deprivation of the substrates required for microbial growth,
or 3) direct action on microbial metabolism through inhibition of oxidative
phosphorylation.