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

A 15-month-long hydrogeologic investigation of a fen-wetland complex in northeastern Illinois, USA indicated the encroachment of ground-water-borne anthropogenic 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 private septic system and from rock salt spread on an adjacent road. The contamination, in turn, had an adverse effect on fen vegetation; within the plumes, diverse vegetation was replaced by the more salt-tolerant narrow-leaf cattail (Typha angustifolia). Ground water of the third fen contained large concentrations of SO₄^2- as high as 516 mg/L. The SO₄^2- anomaly was observed on a transient and/or seasonal basis in the fen ground water and in an adjacent marsh and pond. Isotopically light δ³⁴S values in these waters indicated that the addition of SO₄^2- resulted from the oxidation of pyrite within underlying peat and/or pyritic gravel. However, the large SO₄^2- concentrations had no discernible effect on fen vegetation. The results of this investigation indicate how easily construction of houses with private septic 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.