Decision-tree construction and analysis indicate the quantitative occu
rrence of sulfate-reducing bacteria in a groundwater basis. The decisi
on-tree analysis allowed determination of the most important measured
variables (discriminators) associated with the occurrence of sulfate-r
educing bacteria. Those variables may be used to improve reliability a
nd validity of the tree. Statistical validation of endpoint data (leav
es) is described, and decision trees are presented that increase the u
nderstanding of individual wells and hypothesize causes for sulfate-re
ducing bacteria.