Noel R. Urban

Response of atmospheric lead to decreased use of lead in gasoline.

Wet-only precipitation collected in urban Minneapolis, MN, and rural, northcentral Minnesota (Marcell) has been analyzed for Pb since the late 1970s. Annual volume-weighted mean concentrations of Pb in precipitation have decreased from 29 to 4.3 ..mu..g/L at the urban site and from 5.7 to 1.5 ..mu..g/L at the rural site in the years 1979-1983. Annual Pb fluxes in precipitation have decreased from 1979 to 1983: 2000 to 370 (ng/cm/sup 2/)/year at the urban site and 430 to 100 (ng/cm/sup 2/)/year at the rural site. Decreases in atmospheric Pb fluxes are closely correlated with decreases in Pb used in gasoline in Minnesota and nationally and with a recently constructed atmospheric Pb source function for the Great Lakes region. 49 references, 3 figures, 3 tables.

Sediment Trap Studies in Lake Superior: Insights into Resuspension, Cross-margin Transport, and Carbon Cycling

As part of the Keweenaw Interdisciplinary Transport Experiment in Lake Superior (KITES) project, sediment traps were deployed at multiple locations along the northern coast of the Keweenaw Peninsula in Lake Superior. Traps were deployed at multiple depths (25 m below water surface to 5 m above sediments) at varying distances from shore (0.5–21 km) during the months May-October over a 3-year period. Material captured in the sediment traps was analyzed for total carbon (C), total nitrogen (N), total phosphorus (P), total copper (Cu), 210Pb, and stable isotope ratios of C and N. Copper in sediment trap material, a tracer for mine residues, indicated transport of material in both directions along the peninsula. Cross-margin transport of material occurred at all trap locations along the coast and in all seasons. The most significant finding was that sediment traps suspended just below the thermocline collected large amounts of resuspended sediments even when the trap was moored in 120–220 m of water (9–21 km from shore). Estimates of the contribution to settling fluxes of organic carbon from resuspended sediments ranged from 10–30% in offshore traps; estimates based on 210Pb agreed well with estimates based on a carbon mixing model. Element ratios suggest that resuspended material in offshore traps originated in near-shore regions. Despite the strong influence of resuspension on the composition of sedimenting material, stable isotope ratios were controlled by processes occurring in the water column. Seasonal variations in isotope ratios may reflect seasonal shifts between predominance by autotrophic and heterotrophic processes in the water column.

Modeling historical trends in Lake Superior total nitrogen concentrations.

ABSTRACT Nitrate concentrations in Lake Superior increased fivefold between 1900 and 1980, and have remained nearly constant since that time. Such rapid changes in concentration in a lake with a long hydraulic residence time (∼190 years) are surprising. We developed a model to better understand the causes of the historical changes and to predict future changes in nitrate concentrations. Historical loadings were reconstructed based on average national NOx emissions estimates, recent (past ∼30 years) atmospheric N deposition data, recent tributary concentration data, and basin-wide runoff estimates. Increases in atmospheric N deposition alone were insufficient to have resulted in the observed trends. However, model runs combining increased atmospheric deposition with increased tributary N loading and/or decreased burial + denitrification mid-century reproduced the observed accumulation of N. Because internal N fluxes are an order of magnitude greater than external fluxes, relatively small changes in the lakes internal N cycle may produce relatively large changes in total N concentrations. Land-use changes in the watershed, particularly increases in logging activity, may have altered riverine N inputs. Regardless of the historical mechanisms leading to the rise in nitrate concentrations, it appears as though the system is currently at or is approaching peak N content.

Community Respiration Rates in Lake Superior

Phytoplankton photosynthesis and community respiration are two key components of the carbon cycle that determine the magnitude of net ecosystem production and the balance between oxygen production and oxygen consumption in lakes. As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, rates of community respiration were measured in 1998 and 1999 in nearand offshore waters along the Keweenaw Peninsula in Lake Superior. Because of the difficulties in measuring low rates of respiration, three methods were employed: bottle incubations, measurements of changes in hypolimnetic oxygen inventories, and rates of CO2 evolution from the lake surface. All three techniques yielded similar rates of CO2 production. Rates of community respiration (bottle incubations) ranged from 2 to 166 μg C/L/d; rates of hypolimnetic oxygen consumption ranged from 3 to 12 μg C/L/d; and rates of CO2 evasion from the lake (positive flux is out of lake) ranged from < 0 to 270 mg C/m2/d corresponding to volumetric rates of < 0 to 11 μg C/L/d. Little change in respiration rate with water depth was noted, but respiration rates near-shore were significantly higher than rates in offshore waters. Higher rates of respiration were measured in the El Nino year of 1998 as compared to 1999, but higher temperatures are not thought to be the direct cause. Rates of respiration were higher than simultaneously measured rates of photosynthesis, and there was a net evolution of CO2 from the lake; the lake appears to be net heterotrophic.

Release of Copper from Mine Tailings on the Keweenaw Peninsula

Over 500 million tons of copper-rich mine tailings were dumped into lakes, rivers, wetlands, and along the shore of Lake Superior between 1850 and 1968. Metals leaching from mine residues have impacted ecosystems throughout the Keweenaw Peninsula as well as Lake Superior. The objective of this study was to elucidate the chemical processes that release Cu from mine tailings into the water. Copper in mining residues from three contrasting environments (lake sediments, wetland stamp stands, and exposed lakeshore tailings piles) was fractionated with a sequential extraction technique (SET) to identify and quantify the labile pools of copper. The SET revealed that the carbonate and oxide fractions were the largest pools of Cu (ca. 50 ∼ 80%) in lakeshore and wetland stamp sands whereas the organic matter fraction was the largest reservoir (ca. 32%) in the lake sediments. X-ray diffraction and SEM confirmed the presence of the copper-bearing minerals cuprite, tenorite, malachite, and elemental Cu. Size fractionation studies suggested that weathering of native (elemental) Cu results in enrichment of particle surfaces with Cu oxides and carbonates; fine particles also are enriched in these phases. Both laboratory titrations and computer modeling suggested that aqueous Cu concentrations are limited by mineral (malachite and copper(II) oxides) dissolution and precipitation reactions. Concentrations of DOC and pH depressions caused by microbial activity strongly affect the dissolved Cu concentrations. At some sites, aqueous concentrations of copper approach equilibrium with a Cu oxyhydroxide that has a solubility intermediate between that of cupric hydroxide (Cu(OH)2) and tenorite (CuO).

The Benthic Nepheloid Layer (BNL) North of the Keweenaw Peninsula in Lake Superior: Composition, Dynamics, and Role in Sediment Transport

As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, measurements were made of the extent and the composition of the BNL in Lake Superior off the northwest coast of the Keweenaw Peninsula. Between 1998 and 2000, transmissometer profiles were obtained frequently along three transects oriented perpendicularly to the coast. Large-volume water samples were filtered, and the BNL particles were analyzed for major elements (C, N, P) and 210Pb. In 1998 and 1999, an ROV was deployed to facilitate sampling of this layer. These measurements together with ancillary data from the KITES project show that the BNL develops each summer when the lake is stably stratified and disappears in early fall as the thermocline erodes. Particles within the BNL are enriched in biogenic elements and 210Pb relative to material in surface sediments and relative to material collected in sediment traps, but are enriched in phosphorus relative to epilimnetic seston. There is a large inventory of solids suspended within the BNL (approximately 100,000 metric tons in a 20-km band along the Keweenaw Peninsula); this inventory appears quickly (1–2 weeks) in summer and disappears equally quickly via settling in fall. The large inventory of organic carbon indicates that particles are concentrated from a large area of surface waters into a considerably smaller region where the BNL exists.

210Po and 210Pb distributions and residence times in the nearshore region of Lake Superior

[1] The naturally occurring radionuclide, 210 Pb, and its decay daughter, 210 Po, were measured in the Keweenaw Peninsula region of Lake Superior. Water, suspended particles, sediment trap material (settling particles), and sediment cores and grab samples were collected along three transects that stretched from 1 to 20 km from shore. Departures from secular equilibrium (activity ratio of 210 Po: 210 Pb = 1) were observed for most samples. 210 Po-deficiency was observed in both suspended particles (TSP) with a ratio of 0.43 ± 0.05 (±95% confidence interval (CI)) and settling particles with a ratio of 0.57 ± 0.04; higher ratios in the settling particles resulted from an admixture of resuspended sediments. Ratios in the dissolved phase were 0.45 ± 0.12. Approximately 83% and 85% of total 210 Po and 210 Pb in the water column was in the particulate phase. No evidence of biological uptake of Po was found. Seasonal and spatial variability in activities and ratios was small. Using steady state solutions to the mass balance equations for both isotopes, similar residence times in the water column were calculated for 210 Po and 210 Pb (55 � 75 days in a 150-m-deep water column). It was possible to calibrate a one-box model for the paired isotopes so that the model output closely matched rates of sediment and isotope resuspension estimated from sediment traps. However, this calibration required a fractionation of the isotopes during resuspension. The particle settling velocity was estimated to be 2.3 m d � 1 , which also is in agreement with the estimate (2.4 ± 2.2 m d � 1 ) from sediment traps. These results indicate rapid fluxes of radioisotopes and sediments through the water column largely driven by resuspension of sediments in nearshore areas. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 4860 Oceanography: Biological and Chemical: Radioactivity and radioisotopes; 4239 Oceanography: General: Limnology; 9345 Information Related to Geographic Region: Large bodies of water (e.g., lakes and inland seas); KEYWORDS: polonium-210, lead-210, Lake Superior Citation: Chai, Y., and N. R. Urban (2004), 210 Po and 210 Pb distributions and residence times in the nearshore region of Lake

Measurement and Modeling of Atmosphere-Surface Exchangeable Pollutants (ASEPs) To Better Understand their Environmental Cycling and Planetary Boundaries

Pollutants (ASEPs) To Better Understand their Environmental Cycling and Planetary Boundaries Judith A. Perlinger,*,† Hugh S. Gorman, ‡ Emma S. Norman, Daniel Obrist, Noelle E. Selin, Noel R. Urban,† and Shiliang Wu#,† †Department of Civil & Environmental Engineering, Michigan Technological University, Houghton, Michigan 49931, United States ‡Department of Social Sciences, Michigan Technological University, Houghton, Michigan 49931, United States Department of Native Environmental Science, Northwest Indian College, Bellingham, Washington 98226, United States Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada 89512, United States Institute for Data, Systems, and Society and Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Department of Geological & Mining Engineering & Sciences, Michigan Technological University, Houghton, Michigan 49931, United States

Historical sediment mercury deposition for select South Dakota, USA, lakes: implications for watershed transport and flooding

PurposeSelect South Dakota, USA water bodies, including both natural lakes and man-made impoundments, were sampled and analyzed to assess mercury (Hg) dynamics and historical patterns of total Hg deposition.Materials and methodsSediment cores were collected from seven South Dakota lakes. Mercury concentrations and flux profiles were determined using lead (210Pb) dating and sedimentation rates.Results and discussionMost upper lake sediments contained variable heavy metal concentrations, but became more consistent with depth and age. Five of the seven lakes exhibited Hg accumulation fluxes that peaked between 1920 and 1960, while the remaining two lakes exhibited recent (1995–2009) Hg flux spikes. Historical sediment accumulation rates and Hg flux profiles demonstrate similar peak and stabilized values. Mercury in the sampled South Dakota lakes appears to emanate from watershed transport due to erosion from agricultural land use common to the Northern Great Plains.ConclusionsFor sampled South Dakota lakes, watershed inputs are more significant sources of Hg than atmospheric deposition.