Long-term trends in sodium (Na) and lead (Pb) in Great Lakes sediments over the last 170 years. Na, which comes from road salt, indicates a persistent increasing trend. Pb, which is released from the burning of fossil fuels, increased during the 20th century. But, after Pb was banned from use in gasoline there has been a recovery in sediment concentrations in more recent years.
The Laurentian Great Lakes - Lakes Superior, Michigan, Huron, Erie and Ontario - are the most studied system in lake geochemistry and have well-preserved chronological profiles.
Metals play numerous critical roles in natural and human-influenced characteristics of lake ecosystems, so patterns in the historical records of metals from sedimentary cores provide important information about environmental baselines and human impact.
In two studies recently published in the journal PeerJ, researchers from the Natural Resources Research Institute (NRRI) at the University of Minnesota Duluth summarized what was previously known of metals paleogeochemistry in the Laurentian Great Lakes, then performed a geochemical analysis of eleven sediment cores throughout the Great Lakes basin.
Specifically, relevant studies of Great Lakes geochemistry are presented as a detailed literature review, followed by encyclopedic descriptions of metals, their likely natural and human-influenced sources of elements, their known history from previous paleoecological studies, and their status as potential contaminants of concern.
The researchers then analyzed the history of metals over the last two centuries using sediment cores from throughout the Great Lakes. Cluster analysis indicated that metals could be grouped based on their source or contaminant status. Two contamination indices were applied to determine the extent and threat of human-influenced contamination. Metals records were related to known histories of human activities around the Great Lakes to make linkages between human-influenced stress and Great Lakes sediment contamination.
The researchers found that natural conditions and human-influenced impacts have been measured over time through the sourcing of metals from catchments, industry, agriculture, mining, and other human activities. With a few exceptions, it is difficult to pinpoint sources of contaminant metals due to their ubiquity in minerals comprising the bedrock and soils in the Great Lakes catchment. But, the study did identify several key trends:
Lead pollution in sediments is largely related to combustion of leaded gasoline, which caused a rise in lead and related metals during the 20th century. Since regulatory removal of lead from gasoline in the 1970s it has notably declined in recent sediments across all of the Great Lakes.
Fluctuations in calcium compounds are often related to large "whiting events", calcite precipitation during summer algal blooms. Observation of very high calcium was common in the 1970s and 1980s, especially in Lake Ontario, coinciding with well-known blooms and visible whiting events.
Salt pollution is a prevailing phenomenon in the Great Lakes. In particular, rock salt used for road deicing has led to increased salinity in lakes. The rise in sodium is clearly apparent in all of the Great Lakes sedimentary profiles.
Sediment contamination guidelines help managers assess risk to biota and other water uses. In several cases (e.g. nickel, copper, arsenic) these guidelines were exceeded in the very oldest (natural, pre-impact) sediments suggesting that the appropriateness of these guidelines may be questionable.
"The Great Lakes are sensitive to environmental changes such as pollution by metals, and it is clear that, while there has been remedial success, these paleolimnological results indicate ongoing enrichment of contaminants," said Meagan Aliff, an NRRI research staff scientist and aquatic scientist who co-led the studies. "The impact of certain management practices, such as the banning of leaded gasoline and improvement of municipal sewage treatment, is visible in the cores and demonstrates the efficacy of these practices. Certain analytes, especially sodium, are still increasing in sedimentary records indicating they may require further management."
For future studies, the researchers recommend that mineralogical contents of the cores should be considered so that elements can be more accurately characterized to natural sources. Further, extension of coring efforts to nearshore areas would allow for more specific identification of the sources of metal contaminants of concern.
The two studies - Metallic elements and oxides and their relevance to Laurentian Great Lakes geochemistry and Anthropocene geochemistry of metals in sediment cores from the Laurentian Great Lakes - were coauthored by NRRI staff Euan D. Reavie, assistant water initiative director; Sara P. Post, senior staff scientist; and Lawrence M. Zanko, senior research program manager for the minerals and metallurgy research group.
Funding was provided by the U.S. Environmental Protection Agency.
