HEAVY METAL CONTAMINATION OF SEDIMENTS IN AN URBAN ENVIRONMENT: THE LEGACY OF THE NORTH AVENUE DAM IMPOUNDMENT, MILWAUKEE, WISCONSIN, USA

Members of the Lawrence University Geology Department, in conjunction with the Urban Ecology Center in Milwaukee, Wisconsin, are studying sediments from Riverside Park in Milwaukee along the Milwaukee River. These sediments were deposited over a period of more than one hundred fifty years, between 1835 and 1997, behind the North Avenue Dam. During the life of the dam, heavy metals accumulated in the impoundment as a result of urban and industrial runoff. The dam has since been removed, and the river channel has narrowed to a more natural width, leaving a swath of green space where the impoundment once stood. The area that is now Riverside Park once accounted for much of the impoundment of this now removed dam. Our initial sampling took place in the fall of 2004, and will continue with more investigations in the spring of 2005.

Our samples were collected from five different locations in Riverside Park in order to assess the lateral extent of contamination as well as changes throughout the depth profile. The first sample represents a soil that is saturated year round. We then moved up shore and took a sample that is saturated seasonally, between the modern high and low watermarks. We took two samples that are currently dry year round, but would have been inundated while the impoundment was in place. Our final sample comes from above the historic high water mark. We collected all our soil cores at a depth of 12 inches, and separated subsamples based on visible morphology variations in the depth profile. Based on work done by previous investigators, we are analyzing the following contaminants: Zn, Cu, Pb, and Cd. Additionally, we are analyzing for Fe and Mn, as these redox sensitive elements may play an important role in the cycling of the other more hazardous trace elements.

Further investigations of these soils will include particle size analysis and selective sequential extractions. We will perform selective sequential extractions to better understand the mobility of these elements and to give us a better estimate of the threat these elements pose, as well as a first estimate of the bioavailability of the contaminants. Once our analysis has reached a conclusion, thoughts for remediation may be considered.