Application of a simple binary mixing model to the reconstruction of lead pollution sources in two Mississippi River floodplain lakes
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A simple binary mixing model is used to determine the isotopic ratios of lead (Pb) pollution sources to a lake located near a smelter closed because of excessive Pb aerosols (Horseshoe Lake Madison County, Illinois, USA). As a control, we also examine a relatively unpolluted lake in a rural area of Southern Illinois (Horseshoe Lake Alexander County). Sediment cores were taken from both lakes and analyzed for Pb and Pb isotopes by ICP-MS. The mixing model shows that Madison County Horseshoe Lake had 3 different sources of Pb in its history. The first source is sediment from the Mississippi River with an intermediate 206Pb/207Pb ratio (1.223 ± 0.009) which dominates inputs in pre-settlement times. From 1750 to 1933, the source of pollution Pb has the high 206Pb/207Pb ratio (1.256 ± 0.005) characteristic of ore from the southeast Missouri Pb mines. The most recently deposited pollution Pb comes from a source with a low 206Pb/207Pb ratio (1.202 ± 0.005). This source is similar in isotopic composition to pollution Pb found by several other investigators in the Eastern US and probably represents the mixture of ores used in modern industrial processes. It is unclear from the isotopic composition whether this source at Horseshoe Lake is the local Pb smelter or vehicle exhaust. The sediment core from Horseshoe Lake, Alexander County, shows a less variable isotopic composition. The binary mixing model showed a source composition of 1.225 ± 0.003 before 1850 and 1.231 ± 0.003 after this date. The change does not indicate a pollution source, but may be a shift in the sources of natural sediment with slightly different isotopic ratios to the lake. Our results show the value of simple binary mixing models to reconstruct the isotopic composition of Pb sources to lakes.