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Mercury Accumulation in Alpine Lakes, Colorado

This study investigates the accumulation of mercury in alpine lakes in Colorado and its potential effects on human health. The research compares wet and freeze-dried methods, evaluates trends in mercury accumulation, and examines factors that contribute to worsening mercury accumulation. The study also explores the impact of mercury deposition, snow melt, increased carbon production, and wildfires on alpine lakes.

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Mercury Accumulation in Alpine Lakes, Colorado

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  1. Mercury Accumulation in Alpine Lakes, Colorado David Manthorne, USGS Mark Williams, CU-Boulder

  2. Problem Statement • Concern for mercury arose from human health effects caused by mercury ingestion from freshwater and marine fish, in which mercury bioaccumulates • Documented bioaccumulation of mercury in fish has occurred in many high elevation lakes in Sweden (Johansson et al., 1995) • In 1998, five water bodies in Colorado were put on the EPA section 303(d) list for impaired water bodies – Hg content in fish exceeded advisory levels of 0.5mg kg-1

  3. MERCURY and BRAIN FOOD • Mercury is toxic to the developing fetal brain • Exposure to mercury in the womb can cause learning deficiencies and delay mental development in children

  4. Mercury in the Environment • In a recent EPA report to Congress (1994), they pointed to coal fired utilities as the major anthropogenic source of mercury • It is suggested that deposition of mercury and organochlorines may increase with elevation because of cold condensation and orographic precipitation (grasshopper effect) • This may be cause for concern in warm mid-latitude climates where water resources fall mainly in the form of snow (70%)

  5. NOT A PROBLEM IN THE WEST?

  6. Mercury Loading in Wetfall

  7. MOUNTAINS ENHANCE MERCURY PROBLEMS • Snowfall increases with elevation • Mercury deposition may increase with altitude • Mercury transport associated with carbon • Carbon transport increases during snow melt • Mercury transport from soils to lakes may be greater than in catchments without snow melt • Increasing nitrogen deposition • May increase lake productivity • May increase mercury sequestration in lakes

  8. Study Objectives • To compare wet vs. freeze-dried methods • To evaluate trends in mercury accumulation in alpine lakes of Colorado • Place these results in context by comparing to other sites • Evaluate other factors that may worsen Hg accumulation in Colorado lakes: • Cold-condensation • Increasing Hg deposition with elevation • Snow melt • Increased carbon production from N deposition • Fires

  9. Pristine Lake Hayden, Craig Black Lake Green 4,5 Denver Navajo Lake Navajo, San Juan PP Sulfate/Mercury emissions (1,000/yr SO2)

  10. Lake Sediment Cores - Sample Collection • Lake sediment cores were collected with gravity corer from each lakes deepest point • Samples were extruded in the field in .5 to 1cm intervals • Samples were kept cold until they could be frozen • Wet and freeze-dried samples were digested and analyzed with CVAFS

  11. Methods • Lake sediments were dated with 210Pb activity • Sediment mass accumulation was calculated as the dry weight per section and years each section represented • Hg mass flux per section calculated as Hg concentration times sediment accumulation • Hg flux ratios were calculated as surface Hg flux divided by background Hg flux

  12. Navajo Lake Concentrations Black Lake Concentrations Digestion Comparison - Concentrations

  13. Hg Concentrations in Sediment

  14. Sediment Accumulation Rates

  15. 1825 Hg Flux Ratio = 4.4

  16. 1821 Hg Flux Ratio = 4.0

  17. Hg Flux Ratio = 3.8 1841

  18. 1984 Hg Flux Ratio = 3.2 1856

  19. 1988 1968 1856 Hg Flux Ratio = 3.0

  20. Front Range Lakes – Historical trends

  21. MERCURY DEPOSITION INCREASING IN FRONT RANGE • Highest rates of mercury accumulation in history are now • Mercury accumulation will get worse before it gets better • Front Range lakes and reservoirs at risk • Brown cloud?

  22. West Side Lakes – Recent Decline?

  23. POWER PLANTS ??? • Unclear why max rates have declined since mid-60’s • Difficult to obtain information on emissions • Stack heights increased? • Future trends unclear

  24. SOUTHWEST CO: COMPROMISED DATA • Mining activity comprised data • USGS snow survey: highest mercury content in SW CO • Need additional samples

  25. NUTRIENT ENRICHMENT PRISTINE LAKE

  26. NUTRIENT ENRICHMENT ENHANCES Hg IN LAKES

  27. POWER PLANT EMISSIONS • Mercury sources • Nitrogen sources • Stimulate algal production in lakes • Enhanced lake productivity increases mercury sequestration • More mercury enters the food chain

  28. Mast (unpublished), 2001 Mercury and DOC in Streamwaters

  29. CARBON and MERCURY • TOC/DOC increases mercury transport to lakes and reservoirs • DOC increases production of MMHg • DOC mobility increases during snow melt • Alpine areas at risk

  30. MERCURY AND WILDFIRES: A SMOKING GUN? • 95% of mercury stored in biomass volatilized • 90% as elemental Hg • 10% with aerosols • Biomass burning may account for 25% of global emissions • Large increase after wildfires?

  31. CABALLO RESERVOIR: NM • 2,930 ha burned in ’95 • THg increased 650% • MMHg up 3,000 % • TOC up 600% • Ratio of MMHg to THg up 1,000 % • FIRES INCREASE DELIVERY OF Hg TO RESERVOIRS

  32. Summary • Mercury loadings are elevated in alpine lakes geographically distributed throughout Colorado • Mercury loadings in all 5 lakes are more than 2x current global background • Mercury loadings in lake sediments are comparable to impacted states such as Minnesota and Wisconsin

  33. Summary 2 • Mercury accumulation in Front Range lakes is increasing with time • Mercury accumulation on Western Slope more difficult to evaluate

  34. Summary 3 • Snow melt runoff increases mercury and DOC transport to lakes • Nitrogen fertilization from atmospheric deposition increases lake production of DOC • DOC is coupled with mercury transport to lakes, mercury sequestration in lakes, and production of MMHg

  35. Summary 4: FIRE • Increases mercury availability • Produces DOC • Produces nutrients: N, P • Changes hydrologic flowpaths to increase transport of DOC, N, P, and Hg to lakes • Causes lake eutrophication, enhancing MMHg production and THg sequestration

  36. Suggested Future Research Objectives • More extensive investigation of atmospheric Hg deposition in lake sediment cores and fish bioaccumulation; SW Colorado emphasis • Whole lake and basin Hg accumulation rates from multiple sediment cores (Engstrom et al., 1994), compare these values to wet-fall collectors for determination of dry deposition • Analysis of spheroidal fly ash particles in sediments to assess power plant input: role of power plants

  37. More suggestions • Importance of TOC versus DOC for mercury transport • Hg and DOC during snowmelt • Hg measurements in burned basins • Amount of MMHg vs THg • Atmospheric sources of Hg

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