Implications of Carbon Deposition on the Fate of Semi-Volatile Organic Pollutants in the Great Lakes

Abstract The Laurentian Great Lakes are by far the largest freshwater ecosystem on earth, and their strategic and economical ($4.5 trillion GDP) importance for both the United States and Canada are undeniable. The very large surface area to volume ratio (~100X higher than the ocean) makes them sensitive to airborne pollution, and since the dilution rate the whole-system is small (<1% per year), persistent pollutants tend to accumulate in the system. Consequently, sediment often acts as the final reservoir for hydrophobic contaminants, particularly for the class of compounds called persistent, bio-accumulative toxic compounds (PBTs). I investigated the spatial and temporal impacts of human activities on the Great Lakes during the past 100+ years through the geochemical analysis of >40 sediment cores and >180 grab samples obtained from the five Great Lakes and two seepage lakes in the upper Great Lakes basin. The first phase of the study reconstructed the historical and spatial trends of black carbon (BC) deposition in the Great Lakes as an indicator of past and current industrial and combustion-related emissions to the upper Great Lakes. The BC depositional flux ranged from 0.001-0.164 mg/cm2/yr in Lake Michigan, 0.0003-0.114 mg/cm2/yr in Lake Superior, and 0.004-0.289 mg/cm2/yr in Lake Huron. Geographical information systems (GIS) modeling was used to quantify total BC loading over time in decadal increments during the past 100+ years. The maximum annual BC load to Lake Michigan during the last 100 years was 25.5 Gg (25,500 mt) during the 1940’s. The loading of black carbon deposited to Lake Superior peaked in 1960 at 15.5 Gg/yr (15,500 mg/yr), and then declined steadily. However, BC loading to Lake Superior has started to increase again in the 2000s, and has reached a maximum again in 2010. The loading of BC to Lake Huron, excluding Saginaw Bay reached its maximum annual value of 18.9 Gg (18,900 mg) in 2000, and then has reduced by 25% by 2010. The average BC load to Saginaw Bay during the last 50 years is 8.5 ± 1.5 Gg (8500 ± 1500 mt). The total annual loading of BC to the upper Great Lakes (excluding Saginaw bay) during the last 100 years peaked in 1940 at 55.2 Gg (55,200 mt), and its temporal trend is consistent with the estimates of historic BC emission in the United States. Industrial activity during the build up and operation of World War II had the highest impact in loading of BC to the system, and improvements in the technology of combustion, along with Clean Air Act (CAA), has resulted in a dramatic reduction in BC deposition. The second phase of the study was an investigation of the relationship between the depositional trends of BC and selected semi-volatile persistent bioaccumulative toxic (SV-PBTs) chemicals in the upper Great Lakes. Although direct water-phase discharges of pollutants to the Great Lakes have been greatly diminished over the last few decades, volatile and SV-PBTs primarily reach the lakes through air transport and deposition onto the water surface with subsequent sedimentation to the bottom. BC, total organic carbon (TOC) and organic matter (OM) are well known to strongly sorb hydrophobic compounds like SV-PBTs in aqueous systems. What is comparatively less well studied is the potential for BC to sorb SV-PBTs in the atmosphere acting as a vector for SV-PBT loading to the lakes. In this study, I investigated the spatial and temporal correlations between four categories of SV-PBTs and BC, TOC, and OM. Highly statistically significant correlations between most SV-PBTs and BC were found, with the exception of hexachlorobenzene, alpha-hexachlorocyclohexane, Tetra-PCBs, and Penta-PCBs. In contrast, almost no correlation was observed between either OC and OM and the SV-PBTs, indicating that in-lake OM production does not significantly affect SV-PBT loading to the sediments. The results demonstrate that BC deposition has co-occurred with that of most SV-PBTs, even while BC fluxes have diminished in recent years. The third phase of the study focused on the loading of OM, TOC and total nitrogen (TN) to the sediments as an indicator of eutrophication. The quantity and molecular composition of OM deposited to lake sediments have been used widely to measure anthropogenic impacts on local ecosystems and reconstruct historical environmental conditions. When the rate of oxidation of OM surpasses the ventilation rates of the lake due to the related extended transfer of OM, the dissolved oxygen (DO) can be depleted in the hypolimnion. The results of my analyses show that organic loading to the sediments in the lakes is primarily from in-lake (autochthonous) photosynthesis and not from terrestrial sources. Coincident increases in TOC with concomitant decreases in the TOC/TN ratio provide confirmatory evidence that eutrophication has been found in all five lakes.