ASSESSING THE NATURAL RECOVERY OF A HIGHLY DISTURBED LAKE CONTAMINATED WITH HG USING SEDIMENT CHRONOLOGIES AND A MULTI-ELEMENT APPROACH

Deer Lake is an impoundment located near Ishpeming, MI, USA. Iron mining assay laboratories located in Ishpeming disposed of Hg salts to the city sewer whose outfall was located along an inlet to Deer Lake. An effort to remediate the system in the mid 1980s which consisted of a drawdown of the impoundment in order to volatize Hg from the sediments did not result in recovery of the system. Since the mid 1990s the system has been managed to allow natural recovery. To assess the effectiveness of this remediation strategy, this study investigated the recovery state of the system and estimated the time frame for recovery. Sediment cores were collected to determine historical trends in accumulation rates and concentrations of Hg and other metals. Sedimentation rates and sediment ages were estimated using ²¹⁰Pb, ¹³⁷Cs and stable Pb values. Because of high ²¹⁰Pb activities at the base of the core, event based dating and a modification to the application of the constant rate of supply model was used to supplement the ²¹⁰Pb data. Selected results are that 1) drawdown significantly influenced sedimentation patterns causing negative slopes for the ²¹⁰Pb profiles apparently related to the influx of older sediment, 2) periods of iron production correlate to Hg loading indicating the point source for contamination, a relationship not previously identified, 3) Hg:Al ratios indicate a recent change to a watershed pathway for Hg loading and 4) Hg concentrations and fluxes had decreased from their peak, remain elevated, and recently are unchanging. Estimating complete recovery time is challenging in this system because natural recovery was arrested and deeper, uncontaminated sediments were not recovered, meaning reference conditions could not be determined. By analogy to the effects of the lake drawdown; however, the current lack of natural recovery might be related to recent drought and based on Hg loading history should start again. Exploring various estimates for reference (e.g., recent rates of wet Hg deposition, Hg conditions in reference lakes, elemental ratios), reveals time for recovery of less than 10 years. This study demonstrates that estimates of environmental states of highly disturbed lake systems can be made, but that traditional assessment techniques must be supplemented with other estimates for age and reference states.