ABSTRACTThe “ring average” method of generating trend surfaces has been applied to water chemistry data from shallow water wells in southern Illinois. The method is a slight modification of a well‐known geophysical technique. Basically the method acts as a filter which removes anomalies with dimensions smaller than the diameter of the ring. The trend maps generated by the technique are presented and discussed in this paper. Between the Salem‐Woodward structural block and the Wabash Valley Fault Zone water chemistry shows marked changes. These changes suggest that this is a zone of upward moving waters from deep stratigraphic units. The process of reverse osmosis filtration appears to explain some observed phenomena, but by no means all of them. The reasons why the bounding structural features are so effective is not clear