Impact of urban development on the chemical composition of ground water ina fen-wetland complex

A 15-month-long hydrogeologic investigation of a fen-wetland complex in nor theastern Illinois, USA indicated the encroachment of ground-water-borne an thropogenic contaminants into two of three high quality fens. Ground-water flow directions and chemical evidence indicated that plumes of ground water with anomalously large concentrations of Na+ and Cl- originated from a pri vate septic system and from rock salt spread on an adjacent road. The conta mination, in turn, had an adverse effect on fen vegetation; within the plum es, diverse vegetation was replaced by the more salt-tolerant narrow-leaf c attail (Typha angustifolia). Ground water of the third fen contained large concentrations of SO42- as high as 516 mg/L. The SO42- anomaly was observed on a transient and/or seasonal basis in the fen ground water and in an adj acent marsh and pond. Isotopically light delta(34)S values in these waters indicated that the addition of SO42- resulted from the oxidation of pyrite within underlying pear and/or pyritic gravel. However, the large SO42- conc entrations had no discernible effect on fen vegetation. The results of this investigation indicate how easily construction of houses with private sept ic systems and deicing agents from roadway maintenance can contaminate fen ground water with relatively large concentrations of Na+ and Cl-, resulting in a significant loss of biodiversity in fens.