Lake-based magnetic mapping of contaminated sediment distribution, Hamilton Harbour, Lake Ontario, Canada

The remediation of toxic sediment in harbours and urban waterways requires detailed mapping of contaminated sediment distribution and thickness. Conventional methods rely on interpolation of pollutant concentrations from widely spaced core samples but can lead to significant errors in estimating sediment distribution. An improved approach, as demonstrated by recent work in Hamilton Harbour in Lake Ontario, is to estimate pollutant levels from proxy measurements of sediment magnetic properties. Measurements from 40 core samples collected within the harbour show that the magnetic susceptibility of a contaminated upper layer of sediment is one to two orders of magnitude greater than in the underlying uncontaminated ‘pre-colonial’ sediments. The susceptibility contrast results from elevated levels of urban-source magnetic oxides and is sufficient to generate a total field anomaly (ca. 5–40 nT) that can be measured with a towed magnetometer. Systematic lake-based magnetic surveying (>500 line km) of the harbour using an Overhauser marine magnetometer identifies well-defined positive magnetic anomalies that coincide with mapped accumulations of contaminated sediments on the harbour bottom. Forward modelling of the anomalies shows that the magnetic response is consistent with a contaminated upper layer thickness of up to 5 m. Apparent susceptibility maps calculated from magnetic survey data show a close spatial correspondence with core-derived magnetic susceptibilities and provide a rapid means for classifying contaminated sediments. Detection of shallow magnetic anomalies is dependent upon a closely spaced survey grid (<75 m line spacing) and careful post-cruise processing to remove diurnal, regional and water-depth related variations in the magnetic field intensity.