Isotopic, geochemical and biological tracing of the source of an impacted karst spring, Weldon Spring, Missouri

Weldon Spring is consistently enriched in O-18 relative to other karst spri ngs in east-central Missouri and western Illinois, suggesting an evaporated source component. Regional potentiometric head maps of the shallow aquifer suggest that Prairie Lake, an artificial lake built between 1954 and 1982, could represent this component. Isotopic, biological and chemical tracing of the spring conclusively verify the hypothesis that this lake has impacte d Weldon Spring. Mixing calculations indicate that Weldon Spring is now com prised of approximately 80% lake water and 20% groundwater. Recent measurem ents indicate that the discharge rate of the spring is now approximately 10 times the rate prior to the construction of the lake, confirming the augme ntation of flow by a new source. Analysis of the isotopic trends indicates that the subsurface travel time is short, and suggests that the conduits co nnecting the lake and the spring may be progressively enlarging. (C) 2001 A EHS.