Deposition of bottom sediment in Lake Ellyn, Glen Ellyn, Illinois
Cowan, Ellen A.
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Detention of urban stormwater allows suspended sediment and sorbed chemical constituents to accumulate as bottom sediment within detention ponds and lakes. Resulting reduction of storage capacity and accumulation of heavy metals were studied in 10.2-acre (4.1 x 10⁴m²) Lake Ellyn, a detention pond located in a 534-acre (2.2 km²) watershed in the Chicago Metropolitan area- Dominant sediments are sandy-muds and muds; the most frequent mean grain size is 7.5 phi. Direct measurement of accumulated sediment at 380 points in the lake indicates an average sedimentation rate of .8 in/year (2.0 cm/yr), which represents a cumulative loss of 13.1 percent of lake volume over the past ten years. Maximum thickness is 3.3 feet (1.0 m) near the principal inflow, with zero thicknesses occurring near the shoreline. Sediments are derived by two depositional processes, pelagic lake suspension and gravity flow. Gravity flows deposit coarse particles in near-shore areas and near inflows; offshore sediments are deposited predominantly from pelagic suspension. Relative to other Illinois lakes, Lake Ellyn bottom sediments have highly elevated concentrations of cadmium, copper, and lead, elevated concentrations of zinc, and normal concentrations of iron. Enrichment is attributable to automobile exhaust, atmospheric deposition, and domestic chemical use within the watershed. Concentrations are relatively higher in silt and clay (<62 micron) fractions than in sand (>62 micron) fractions.