Sources and transport of mercury and methylmercury in rivers and streams of the Upper Mississippi River watershed

Transcription

1 NIMD Forum 21 February 4-5, 21 Sources and transport of mercury and methylmercury in rivers and streams of the Upper Mississippi River watershed Steven J. Balogh Metropolitan Council Environmental Services St. Paul, Minnesota, USA

2 Presentation outline: Orientation locating the Upper Mississippi River and relevant sub watersheds Description outlining important watershed characteristics Explanation how Hg enters these streams and rivers how watershed characteristics influence Hg concentrations, partitioning, and speciation in these streams Conclusion NIMD Forum 21 Balogh

3 Orientation: The Upper Mississippi River Upper Mississippi River Watershed area: 49 km 2 Mean discharge: 3576 m 3 /s Land use: 7% agriculture, 25% forest CANADA Mississippi River Watershed MEXICO Mississippi River Watershed area: 3 27 km 2 Mean discharge: 18 4 m 3 /s Gulf of Mexico NIMD Forum 21 Balogh

4 CANADA Mississippi River Watershed MEXICO Gulf of Mexico CANADA Lake Superior Mississippi River St. Croix River Minnesota River WISCONSIN MINNESOTA Research in three major sub watersheds in Minnesota: The Mississippi, Minnesota, and St. Croix Rivers NIMD Forum 21 Balogh

5 Ecological diversity makes Minnesota a very interesting place to study rivers! Land use/land cover in Minnesota Wetlands Forest Urban Intensive row-crop agriculture NIMD Forum 21 Balogh

6 Mercury (Hg) delivery to rivers and streams is primarily the result of watershed processing of both native and atmosphericallyderived Hg; delivery is driven by precipitation runoff events. THg (ng/l) discharge (m 3 /s) TSS (mg/l) Minnesota River at Fort Snelling 75.6 ng/l 6/19/ mg/l 6/19/ THg (ng/l) MeHg (ng/l) a discharge b Trott Brook unfiltered filtered Trott Brook. Apr Jun Aug Oct discharge (rel. units) discharge (rel. units) NIMD Forum 21 Balogh

7 Corollary: Point source inputs of Hg are small compared to nonpoint source (diffuse) inputs. Counterpoint: Point sources may be significant in developing nations. Comparing the Hg load (g/day) discharged by the Metropolitan Wastewater Treatment Plant to the instream load in the Mississippi River in St. Paul THg load ratio (%) discharge (m 3 /s) MeHg load ratio (%) MeHg/UF discharge discharge (m 3 s -1 ) Metro WWTP inputs to the Mississippi River: < 4% annual river loading for THg < 2% annual river loading for MeHg 8/14 8/28 9/11 9/25 1/9 1/23 11/6 11/2 27 NIMD Forum 21 Balogh

8 Historically, however, most major rivers in developed nations have experienced Hg inputs from point sources, including WWTPs and industries. Location of Lake Pepin Hg and sediment accumulation in Lake Pepin: 8 6 sediment (1 tons/yr) Hg (kg/yr) Sediment coring sites Sediment accumulation rate continues to increase, but accumulation rates of Hg and other trace metals are decreasing NIMD Forum 21 Balogh

9 Boreal rivers and streams (most studied) often see their highest Hg loads in spring with snowmelt (runoff from frozen surfaces); precipitation runoff determines timing of events throughout the rest of the year. 7 6 Mississippi River at Lock & Dam 1, St. Paul Hg load discharge 12 1 Hg load (g/day) discharge (m 3 /s) 1 2 Oct-94 Apr-95 Oct-95 Apr-96 Oct-96 Apr-97 NIMD Forum 21 Balogh

10 Important environmental controls on THg (and MeHg) in streams and rivers are: land use/cover, soils (wet vs. dry; organic vs. inorganic; fine grained vs. coarse), hydrology (flow paths, connectivity, drainage), climate (precipitation volume, intensity; temperature), topography Mississippi R. Watershed yield THg (g/km 2 -yr) 1.5 St. Croix R. Example of land use influence on THg export from watersheds: Much higher THg yield from areas of intensive row crop agriculture Minnesota R Soil enters streams from upland or bank erosion, carrying Hg with it.5 WISCONSIN MINNESOTA NIMD Forum 21 Balogh

11 Watershed yield THg (g/km 2 -yr) Minnesota (St. Peter) Blue Earth Le Sueur Mississippi (Anoka) Rum.1 Watershed yield MeHg (g/km 2 -yr) Minnesota (St. Peter) Blue Earth Le Sueur Mississippi (Anoka) Rum THg yields from intensively cultivated watersheds are much higher than from forested watersheds; however, MeHg yields are similar NIMD Forum 21 Balogh

12 Mean concentration THg (ng/l) Minnesota (St. Peter) Blue Earth Le Sueur Mississippi (Anoka) Rum Minnesota (St. Peter) Blue Earth Le Sueur Mississippi (Anoka) Rum Mean concentrations of MeHg are actually higher at outlets of the forested watersheds. Influences: hydrology, soils, topography Mean concentration MeHg (ng/l) NIMD Forum 21 Balogh

13 Fluvial THg outputs from watersheds are much less than atmospheric inputs; soils are a major store of THg. THg (g/km 2 -yr) Minnesota River watershed yield Mississippi River St. Croix River Atmospheric deposition input Atmospheric inputs of THg are much greater than the amount of THg exported from these watersheds NIMD Forum 21 Balogh

14 MeHg outputs from watersheds are generally less than atmospheric inputs; in some watersheds, however, wetland soils are significant sources of MeHg and may shift the balance..15 MeHg.1 (g/km 2 -yr).5 watershed yield Atmospheric inputs of MeHg are greater than the amount of MeHg exported from these watersheds. Minnesota (St. Peter) Mississippi (Anoka) Atmospheric deposition input NIMD Forum 21 Balogh

15 Important carriers of THg (and MeHg) in rivers and streams are DOC (allochtonous; quality matters) and suspended sediments (POC, clays, etc). Filtered MeHg is strongly correlated with DOC in many streams, especially those draining wetland areas [MeHg] F (ng/l) a) Trott Brook non-event event [MeHg] F (ng/l) b) Cedar Creek When DOC is flushed from wetlands, MeHg comes with it [MeHg] F (ng/l) c) Rum River [MeHg] F (ng/l) d) Mississippi River Wetlands and peatlands are MeHg, DOC, and POC sources, but THg sinks DOC (mg/l) DOC (mg/l) NIMD Forum 21 Balogh

16 1.5 Trott Brook 1.5 MeHg (ng/l) a b DOC (mg/l) DO (mg/l) Cedar Creek.5.1 SRP (mg/l) c d Fe (mg/l) MeHg (ng/l) f g h i.2. DOC (mg/l) DO (mg/l) SRP (mg/l) Fe (mg/l) Water flushed from wetlands is anoxic, chemically reduced, and high in MeHg. NIMD Forum 21 Balogh

17 THg (ng/l) THg (ng/l) Minnesota River at Jordan slope = 45 ng/g r 2 =.98, n= TSS (mg/l) Minnesota River at Fort Snelling Unfiltered Filtered TSS (mg/l) THg is strongly correlated with TSS in the Minnesota River and its tributaries THg (ng/l) TSS (mg/l) Minnesota River km 144 St. Peter Minnesota River km 193 Judson Blue Earth River km 19 Le Sueur River km 2 Cultivated, disturbed (mining, logging, etc.), or drained landscapes can be significant sources of soil and THg inputs to rivers and streams NIMD Forum 21 Balogh

18 The physical partitioning (dissolved vs. particulate) of inorganic Hg (IHg) and MeHg is controlled by DOC and particulate (TSS) levels. IHg F/UF MeHg F/UF DOC (mg/l) 1. IHg F/UF MeHg F/UF TSS (mg/l) NIMD Forum 21 Balogh

19 Runoff from upland systems: flow paths matter- surface, interflow, riparian influence; flow paths dictate runoff water chemistry. Connectivity: are THg and MeHg sources hydrologically connected to streams? Particularly important for MeHg source areas such as wetlands. 1/alkalinity (L/mg) a) Trott Brook MeHg.4 Apr Jun Aug Oct 2 1 [MeHg] (ng/l) 1/alkalinity (L/mg) b) Cedar Creek MeHg.6 Apr Jun Aug Oct 2 1 [MeHg] (ng/l) Trott Brook and Cedar Creek drain watersheds containing wetlands; when streamflow increases, wetlands are connected and flushed NIMD Forum 21 Balogh

20 Climate drives temporal changes in river THg and MeHg loads; higher loads observed in discrete hydrologically (i.e. runoff) driven episodes; with changes in source areas, chemistry. Flooding, floodplain effects, floodplain lakes, rewetting effects: flooding results in changed redox conditions in flooded landscapes, possibly stimulating methylation; flooding also changes hydrologic connectivity, bringing new sources into the river network. In Minnesota, climate change and increased precipitation volumes and intensity will increase flooding, affecting redox cycles, perhaps stimulating methylation. Elsewhere, climate change may result in drought, with effects on water chemistry and Hg speciation. MeHg, THg (ng/l) Jun Jul Aug Sep Oct Nov discharge (m 3 /s) Rum River 22: Two major flood events accompanied by high MeHg concentrations (>1 ng/l) and high MeHg/THg ratios (28 39%) NIMD Forum 21 Balogh

21 Conclusion: To a large degree, it s about the watershed! The inputs, speciation, and partitioning of Hg in rivers and streams in the Upper Mississippi River (and elsewhere) are primarily influenced by watershed characteristics, especially the interaction of precipitation runoff with landscape features. Many thanks to colleagues who joined me in this work: Ed Swain, Dan Engstrom, Jim Almendinger, Mike Meyer, Kent Johnson, Heather Offerman, Jie Hu, Yabing Nollet, Jill Coleman Wasik, Martin Tsui, and others. Thank you to NIMD, and to our host, Dr. Matsuyama, for the opportunity to participate in Forum 21. NIMD Forum 21 February 4-5, 21 NIMD Forum 21 Balogh

22 Sources and transport of mercury and methylmercury in rivers and streams of the Upper Mississippi River watershed Steven J. Balogh Metropolitan Council Environmental Services, St. Paul, Minnesota, USA In the absence of significant point sources, mercury (Hg) and methylmercury (MeHg) enter rivers and streams primarily during precipitation runoff events, carried from terrestrial source areas by moving water. Atmospheric deposition to the watershed provides much of the Hg transported to streams, but native geological sources may be important in some cases. Land use/land cover, soils, hydrology, climate, and topography are important characteristics of watersheds that determine the physical and chemical speciation and the quantities of Hg and MeHg that are delivered to streams. In runoff and in rivers and streams, Hg and MeHg are generally associated with soil particles or particulate (POC) or dissolved (DOC) organic carbon, and source areas of soil-derived suspended sediments, POC, or DOC will be source areas of Hg or MeHg, or both. Soils and landscapes disturbed by cultivation, mining, logging, or other means are potentially significant sources of Hg to waterways. Wet organic soils provide conditions conducive to the bio-mediated production of MeHg, and wetlands are often the most significant sources of MeHg to streams. The Minnesota and St. Croix Rivers are major tributaries to the Upper Mississippi River, joining the mainstem in or near the Twin Cities of Minneapolis and St. Paul, Minnesota, USA. Despite being relatively near to one another, these two rivers drain watersheds that are markedly different in terms of land use/land cover, soils, hydrology, and climate. The headwater Mississippi River and its watershed, extending from the Twin Cities to the headwaters in Lake Itasca, physically occupy the middle ground between the Minnesota and St. Croix Rivers, and literally represent the middle ground in terms of watershed and water quality attributes. This presentation will summarize studies of Hg and MeHg in streams and rivers in this part of the Upper Mississippi River watershed, beginning with early efforts to characterize the relative importance of point source inputs; touching on studies in streams heavily impacted by modern industrial agricultural practices; and examining the influence of wetland inputs in less-disturbed watersheds. Finally, recent efforts to understand the influence of sulfate-deposition on MeHg production in boreal wetlands and to characterize mechanisms of in-stream Hg- methylation in streams where wetland inputs of MeHg are absent will be discussed.