Environ. Sci. Technol. 2010, 44, 6570–6575
Recent Changes in Atmospheric Mercury Deposition Recorded in the Sediments of Remote Equatorial Lakes in the Rwenzori Mountains, Uganda H A N D O N G Y A N G , * ,† DANIEL R. ENGSTROM,‡ AND NEIL L. ROSE† Environmental Change Research Centre, University College London, Pearson Building, Gower Street, London WC1E 6BT, United Kingdom, and St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, Minnesota 55047
Received October 27, 2009. Revised manuscript received July 20, 2010. Accepted July 22, 2010.
We analyzed sediment cores collected from three equatorial zone lakes in the Rwenzori Mountains of Uganda for Hg and dated them using 210Pb. The results show that the lakes have been contaminated by anthropogenic Hg from atmospheric deposition and that the onset of Hg pollution in the region began at least by the late 19th century. Mercury accumulation in all sediment cores increased by about 3-fold since the mid19th century, a similar increase to that shown in other remote regions worldwide. These results from tropical high-elevation sites are the first for this region and contribute to our understanding of global Hg pollution trends. The atmospheric boundary layer is at a higher altitude in equatorial areas than at midlatitudes, and therefore, Hg deposition in these regions may not be enhanced by diurnal penetration of tropospheric air and associated reactive gaseous mercury as has been reported for mountain lakes at higher latitudes. Furthermore, the relatively low abundance of atmospheric oxidants may limit the amount of gaseous elemental mercury oxidized to the reactive gaseous form in equatorial Africa. These Rwenzori Hg records therefore have important implications for the understanding of Hg dynamics at high elevations in equatorial regions.
Introduction The Industrial Revolution brought about an unprecedented increase in the intensity of anthropogenic mercury (Hg) emissions through processes such as coal burning, waste incineration, and ore refining (1), and atmospheric deposition of Hg increased as a consequence. It has been estimated that current global atmospheric concentrations of Hg are three times higher than preindustrial levels (2, 3). Most Hg emitted from anthropogenic and natural sources is elemental Hg (4), the dominant chemical form in the atmosphere (5). Elemental Hg is only slowly oxidized into more soluble species and hence is available to be transported and deposited far from emission sources, resulting in widespread environmental * Corresponding author phone: + 44-20-7679-0544; fax: + 4420-7679-0565; e-mail:
[email protected]. † University College London. ‡ Science Museum of Minnesota. 6570
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contamination. Atmospheric inputs of anthropogenic Hg are thus significant even in areas remote from direct industrial emissions. Natural archives such as lake sediments have been used to reveal long-term changes in Hg contamination around the world, and many of these archives, especially from remote sites, also demonstrate about a 3-fold increase in Hg deposition since preindustrial times (Table 1) (5-15). Although some arctic sites show considerable departure from the 3-fold increase (16), these studies did not take into account erosional inputs of Hg, which can be large relative to atmospheric deposition at high latitudes (8). Hence, these data sets may not depict true Hg deposition trends in these study areas. Furthermore, records from peat bogs tend to overestimate Hg deposition derived from modern Hg accumulation rates (17) and therefore are not considered further here. The 3-fold increase in Hg deposition since preindustrial times would thus appear to be a global phenomenon (18), but Hg records in the southern hemisphere and in the tropics are scarce. Therefore, confidence in historical Hg accumulation trends is low. Additional empirical data, especially in remote areas, are therefore needed to confirm current estimates, provide ground truth for global models, and extend our knowledge. The Rwenzori Mountains in equatorial Africa are far removed from direct human impact, and therefore, sediments from lakes in this area potentially contain reliable archives of long-range atmospheric pollution for the region. Historical Hg records from the Rwenzoris provide valuable new data beyond the geographic bounds of currently available data sets on Hg accumulation in lakes, which have a strong bias toward Europe and North America. Hence, they are important for the understanding of global Hg pollution trends. Furthermore, because the lakes of the Rwenzoris are at high altitudes (2700-4000 m a.s.l.), where atmospheric Hg dynamics may differ from those at lower elevations, they could be affected by diurnal penetration of tropospheric air and associated reactive gaseous mercury (RGM). This altitudinal effect may result in enhanced Hg deposition (19, 20) compared with that of lower-elevation lakes and hence increased Hg accumulation in high-altitude lake sediments as seen in North America and Tibet (21, 22). However, the elevation of the atmospheric boundary layer is higher in the tropics than at midlatitudes (23, 24), so the elevation at which RGM concentrations are enhanced by free tropospheric air is also higher. Oxidants in the atmosphere over equatorial Africa are also less abundant compared with midlatitudes (25), and this can affect oxidation of gaseous elemental mercury (GEM) to RGM and consequently Hg deposition (20, 26). Therefore, historical Hg records in lake sediments from the Rwenzori Mountains have important implications for the understanding of Hg dynamics at high elevations in equatorial regions and can help our understanding of Hg cycling on regional and global scales. In this paper, we use 210Pb-dated sediment cores taken from three lakes in the Rwenzori Mountains to reconstruct secular trends in Hg deposition and examine the emerging paradigm that Hg deposition has increased by a factor of 3 since the onset of the industrial period over equatorial Africa and worldwide. We consider these sediment records with respect to current understanding of Hg behavior at high elevations and discuss possible factors affecting Hg dynamics and deposition over equatorial areas. 10.1021/es101508p
2010 American Chemical Society
Published on Web 08/03/2010
TABLE 1. Mercury Flux Ratios (Recent: Preindustrial Hg Accumulation) in Sediment Cores from Various Lake Regions region
reference
mean
s.d.
# lakes
W. Greenland SE Alaska Newfoundland N. Minnesota Arctic Alaska Vermont/New Hampshire New Zealand Sweden and Finland New York N. Quebec Arctic Canada Subarctic Canada Midlatitude Canada N. Minnesota, Wisconsin Total
Bindler et al. (5) Engstrom et al. (6) Engstrom et al. (6) Engstrom et al. (7) Fitzgerald et al. (8) Kamman and Engstrom (9) Lamborg et al. (10) Landers et al. (11) Lorey and Driscoll (12) Lucotte et al. (13) Muir et al. (14) Muir et al. (14) Muir et al. (14) Swain et al. (15)
2.90 2.70 2.39 3.46 3.24 3.89 3.00 3.60 3.50 2.25 2.16 2.50 3.56 3.67 3.06
2.60 0.40 0.68 1.42 1.20 1.83 0.50 2.60 1.66 0.56 0.58 0.75 1.00 0.60 0.58
21 4 4 20 5 10 4 20 8 11 18 14 18 7 164
a
multiple coresa 24 24 55
81
Flux ratios derived from multiple cores per lake (number indicates total cores in study).
FIGURE 1. Topographic map of part of the Rwenzori Mountains showing the locations of the lakes (shown black) for this study. The location of the Rwenzori in Africa is marked with a star in the inset map.
Materials and Methods Study Area. The Rwenzori Mountains lie just north of the Equator and straddle the border between the Democratic Republic of Congo and the Republic of Uganda. They form part of the East African Rift System and are composed of an uplifted block of Precambrian rock that rose about 4000 m above the surrounding African plateau during the late Pliocene. Our study sites, Lake Bujuku, Lake Mahoma, and Lower Lake Kitandara are located in high-elevation glaciated areas of the Rwenzoris (Figure 1), where rainfall is high (2-3 m yr-1) and daily temperature variations are large (-5° to +20 °C (27)). Lower Lake Kitandara was formed during the most recent Omurubaho glaciation, about 7000 years B.P. (28), and most of its catchment area is covered by alpine vegetation. Lake Bujuku was created by landslides from the slopes of Mount Baker following the Omurubaho glaciation
about 3000 years B.P. (28). Lake Mahoma was formed during the retreat of a local glacier, and radiocarbon dating suggests an age of 15-20 thousand years (29). The Kitandara and Bujuku catchments are large relative to the surface area of the lakes, although Upper Lake Kitandara, a short distance upstream, acts as a sediment trap for the lower lake (30). The Mahoma catchment is small and forms the headwaters of the River Mahoma, a tributary of the River Mubuku (27, 28). Locational and morphological information for the lakes is shown in Table 2. Sediment Sampling. Sediment cores were taken from deep flat areas of each lake in June 2003 using a Glew gravity corer (31) fitted with a 7.1 cm internal diameter polycarbonate core barrel. Cores were extruded vertically in the field and sectioned using a stainless steel slicer at 0.25 cm intervals from the surface to 2 cm and then 0.5 cm intervals to the VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Geographic Information for the Rwenzori Study Lakes lake
latitude (N)
longitude (E)
altitude (m)
catchment type
lake area (ha)
maximum depth (m)
catchment area (ha)a
Bujuku Kitandara Mahoma
0° 22′ 0° 21′ 0° 21′
29° 54′ 29° 53′ 29° 58′
3960 3990 2690
alpine alpine forest/bamboo
7.87 2.86 4.76
14.5 9 25
510 51 21
a Catchment areas from Russell et al. (30); the catchment area for Kitandara is that below the outlet of its upstream lake basin (upper Lake Kitandara).
FIGURE 2. Mercury concentration profiles (filled circles) and sediment accumulation rates (line) for Lake Bujuku, Lower Lake Kitandara, and Lake Mahoma. The horizontal dashed lines indicate 1860 A.D. calculated by 210Pb data (errors for 1860 are (28 for Bujuku, (15 for Kitandara, and (11 for Mahoma). base. Samples were stored in polyethylene sediment sample bags. All the sediments were kept cool prior to further processing. Sample Analyses. Lithostratigraphy. Standard methods were employed for these analyses (32, 33). Sediment samples from each core interval were analyzed for water content by measuring weight loss after drying at 105 °C for 24 h. Losson-ignition (LOI), a proxy for organic content in the sediments, was measured by weight loss following heating of dried samples at 550 °C for 4 h. Wet density measurements were undertaken on every fifth sample by evenly filling wet sediment into a 2 cm3 measurement vial and weighing on an electronic analytical balance. Dry density was calculated from wet density and water content. Radiometric Dating. Lead-210 in the sediments was measured through its granddaughter product 210Po, with 209Po added as an internal yield tracer. The Po isotopes were distilled from 0.1 to 1.5 g freeze-dried sediment at 550 °C following pretreatment with concentrated HCl and plated directly onto silver planchets from a 0.5 N HCl solution (34). Activity was measured for (1-5) × 105 s with ion-implanted surface-barrier detectors and an Ortec alpha spectroscopy system. Unsupported 210Pb was calculated by subtracting supported activity from the total activity measured at each level; supported 210Pb was estimated from the asymptotic activity at depth. Sediment chronologies and accumulation rates were calculated using the constant rate of supply (CRS) model (35). Basal sediment accumulation rates (based on the earliest reliable 210Pb data) are consistent over a period of decades in the mid-late 19th century, and we have extrapolated these rates to provide tentative estimates for preindustrial dates. Mercury Analysis. Mercury was determined in freeze-dried sediment samples from each core. A 0.2 g subsample was extracted using 8 mL of conc. HNO3 (Aristar) at 100 °C for 1 h in rigorously acid-leached 50 mL Teflon beakers (36). Mercury concentrations were measured by cold vaporatomic absorption spectrometry (CV-AAS) following reduction with 2 mL SnCl2 (10% in 20% (v/v) HCl) (36, 37). The 6572
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mean Hg concentration in standard reference material, stream sediment GBW07305, was determined at 93 ng g-1 (n ) 12; relative standard deviation (RSD) ) 8.6 ng g-1) compared with a certified value of 100 ( 10 ng g-1. Analyses within each digestion batch included at least one sample digested in duplicate along with a blank and a sample of reference material GBW07305.
Results and Discussions Mercury Concentrations and Accumulation Rates. In general, Hg concentrations increase in all the sediment cores from lowest levels to the surface. In the core taken from Lake Bujuku (Figure 2), Hg concentrations are relatively constant in the lower levels and then increase from around 17 cm (1930 ( 6) to the surface, while in the cores taken from Lake Mahoma and Lower Lake Kitandara, Hg concentrations increase from the base of the cores (pre-1860). Mercury concentrations increase continuously to the sediment surface in the Kitandara core but decline above about 12 cm (1946 ( 3) in the Mahoma core (Figure 2). Some changes in Hg concentration are derived from variation in sediment accumulation rate. Local sediment sources are likely depleted in Hg such that increased sedimentation can further dilute the Hg concentration in the core (7, 38). For example, in the Bujuku core, relatively low Hg concentrations in the sediments at 7-10 cm and 17-20 cm correspond to relatively high sediment accumulation rates, and in the Mahoma core, a decline in Hg concentrations from 12 cm to the sediment surface corresponds to a concurrent increase in sediment accumulation rate over the last 20 years. Differences in accumulation rates may also affect Hg concentrations among cores. For example, sediment accumulation rates are generally higher in the Bujuku core, and Hg concentrations are correspondingly lower. While Hg concentration profiles show different temporal patterns among cores, Hg fluxes, calculated by multiplying the sediment accumulation rate (g sediment m-2 yr-1) by the Hg concentration (ng Hg g-1), show a more consistent pattern
FIGURE 3. Mercury accumulation rates in sediment cores from Lake Bujuku, Lower Lake Kitandara, and Lake Mahoma in the Rwenzori Mountains (M/P ) ratio of modern Hg flux over preindustrial Hg flux to the sediments). and indicate a gradually increasing Hg burden in the sediments since around the 1860s (Figure 3). Although the absolute Hg fluxes for the different cores are quite different within the same period (e.g., they range between 6 and 35 µg m-2 yr-1 in the 1860s and between 20 and 110 µg m-2 yr-1 in the 2000s), each core shows a 3-fold increase in Hg fluxes since preindustrial times. Ratios of modern to preindustrial (pre-1860) Hg flux are 2.93, 2.86, and 2.53 for Bujuku, Lower Kitandara and Mahoma, respectively (Figure 3). Mercury fluxes to lake sediments are derived from a combination of atmospheric deposition and in-wash from the catchment (15, 36). Thus, the much higher Hg fluxes in the Bujuku core are likely a consequence of Hg inputs from its relatively large catchment (catchment area:lake area [C/L] ) 65; Table 2). Assuming a global average preindustrial Hg flux of 2 µg m-2 yr-1 (39) and export of 25% of catchment deposition (15), we would expect an annual Hg load of 34 µg m-2 yr-1 to Bujuku, very similar to the measured sediment flux of 31 µg m-2 yr-1 prior to about 1860. Similar calculations for Mahoma (C/L ) 4) yields 4 µg m-2 yr-1 as compared to a measured flux of 7.5 µg m-2 yr-1. Expected values are the average for the lake basin; however, measured Hg fluxes result from the influences of many factors, including sediment focusing. Relative focusing is the ratio of the measured flux value to the basin-wide average; so for the Bujuku core, it is 31 µg m-2 yr-1/34 µg m-2 yr-1 ) 0.91 and for the Mahoma core is 7.5/4.0 ) 1.87. The ratio of the focusing between the two cores, Mahoma/Bujuku, is therefore 1.87/0.91 ) 2.1, which is in a good agreement with the ratio of unsupported 210 Pb flux for the cores [999 Bq m-2 yr-1 (Mahoma)/377 Bq m-2 yr-1 (Bujuku) ) 2.6]. This also implies that the assumption of 25% catchment input is reasonable. In the case of Kitandara, Hg inputs from much of its catchment are likely trapped in the Upper Kitandara Lake basin. Considering only direct catchment inputs (C/L ) 18 below the upper lake), preindustrial Hg loads to Lower Lake Kitandara would be around 10 µg m-2 yr-1, very similar to the measured core flux. Ratios of the focusing between the cores can thus be estimated. However, as we do not know the atmospheric 210 Pb deposition for the study area, the calculations of catchment contribution to the coring locations are not corrected for sediment focusing, which would tend to overestimate lake-wide Hg inputs based on fluxes measured in single deep-water cores. The core Hg fluxes also do not take into account losses via evasion and outflow, both of which could be substantial (36, 40). While such mass-balance
considerations are beyond the scope of the current study, our rough estimates of catchment Hg loads do explain much of the difference in Hg accumulation rates among the three lakes and also indicate that the fluxes are not substantially different from those measured in low-elevation lakes from temperate regions (Table 1). Engstrom et al. (7) demonstrated that human impact in lake catchments can significantly increase Hg accumulation in sediments. Cooke et al (41) reported a modern to background Hg flux ratio of 4.6, higher than 3-fold, in an equatorial high-altitude site in the Peruvian Andes of South America. Although RGM enhancement cannot be ruled out here, the higher than 3-fold ratio is more likely derived from catchment Hg inputs, as the region has been affected by pre-Columbian and early Spanish mining, and the Hg flux profile of the core follows the same trends as those from sites more proximal to local mining activity. However, in the remote Rwenzoris, where human intensification of catchment erosion can be ruled out, changes in sediment Hg fluxes mainly reflect changes in Hg deposition. The nearly 3-fold increase in modern Hg fluxes over preindustrial levels in these Rwenzori lakes therefore implies that Hg deposition in the area has increased by about a factor of 3 in about the last 150 years. Mercury emissions to the atmosphere have greatly increased since the Industrial Revolution. It is estimated that Hg in the global atmospheric pool has increased from a preindustrial level of 2050 Mg to 5600 Mg at the present day, an increase of 2.7 times (3). Direct anthropogenic sources, land surface emissions, and emissions from ocean surfaces each contribute about one-third to total present day Hg inputs to the atmosphere (2). A fraction of the Hg emitted from land and ocean surfaces is recycled from previously deposited Hg originally released from anthropogenic sources (2, 18). In general, increased Hg input to the atmosphere results in greater atmospheric Hg concentrations and subsequently an increase in Hg deposition. Increased Hg deposition has been reported in many remote lake sediments around the world, though mainly in the northern hemisphere (Table 1). Most historic records show a 3-fold increase in Hg fluxes since preindustrial times, and hence, our data indicate that Hg deposition has accelerated in equatorial Uganda at about the same rate as other regions of the world, providing a wider picture of Hg deposition trends and supporting the hypothesis that this increase is a global signal. Mercury Fluxes in High-Altitude Lakes. Although sediment records of Hg deposition to high-elevation lakes are VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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sparse, a small group of well-dated cores from western North America, specifically California and adjacent Nevada, and cores taken from a series of Tibetan lakes indicate a larger secular change in Hg deposition than the 3-fold increase recorded in other remote areas of the globe (21, 22, 42, 43). The reasons for this larger than expected increase in western North America are discussed in detail by Drevnick and coauthors (21), who conclude for Lake Tahoe, a deep alpine basin on the California/Nevada border, that a 7.5-10-fold increase in Hg accumulation from preindustrial to modern times is the result of either decreased Hg evasion from the lake surface (caused by a decrease in lake clarity) or an increase in atmospheric deposition related to enhanced production of RGM in the free troposphere. They argue that Hg deposition to Lake Tahoe and similar alpine sites may be elevated by virtue of nighttime penetration of free tropospheric air as observed at the Mount Bachelor Observatory (2700 m a.s.l.) by Swartzendruber et al. (20). There is good evidence that atmospheric RGM increases with altitude (19, 26), the cause of which appears to be in situ oxidation of gaseous elemental mercury (GEM) principally by ozone (20). Drevnick et al. (21) suggest that the nightly downslope flows of RGM-rich tropospheric air accounts for the high modern flux of Hg in Tahoe sediments as well as the large increase over preindustrial times. In order for the latter to be true, they argue that there must be some regional increase in atmospheric oxidants in the airsheds of these lakes, presumably from emission sources in the highly populated urban centers to the west. Similarly, lake sediment records from the Tibetan Plateau, which also show a greater than 3-fold change in Hg accumulation in lakes above 3000 m a.s.l., suggest that increased atmospheric deposition may be related to RGM-enriched tropospheric air as well as increasing atmospheric oxidants derived from rising industrial activity in China and India (22). Absolute Hg deposition at high elevation may thus be enhanced, and the degree of change may be substantially larger than three because of high-altitude RGM inputs and an overall increase in the oxidative power of the atmosphere when compared to lower altitudes. Lake Bujuku and Lower Lake Kitandara are located at nearly 4000 m a.s.l., while Lake Mahoma is at 2700 m a.s.l., similar to the Mt. Bachelor Observatory, so we might expect to observe such an enhancement at these sites. However, as argued above, the preindustrial Hg fluxes to these Ugandan lakes are not out of line with those expected for lakes with similar-sized catchments in other areas of the globe. Furthermore, we see only a 3-fold increase in Hg accumulation in the sediment cores, similar to that reported from other remote and lowaltitude locations worldwide. Therefore, it would appear that the high elevation of the Rwenzori lakes does not affect actual rates of Hg deposition or their increase from preindustrial to modern times. There are two possible reasons for this. First, as the atmospheric boundary layer is typically at a higher elevation nearer the equator (23, 24), it may lie above the altitude of these Rwenzori lakes, so that there is no RGM enhancement from free tropospheric air. Second, atmospheric oxidants (e.g., ozone) are less abundant in equatorial regions compared to midlatitudes. Almost all ozone production in the upper stratosphere occurs in the tropics as this is where the solar radiation is most intense (44). As the newly produced ozone moves downward to the lower stratosphere, it also moves outward toward the polar regions of both hemispheres. Hence, more ozone is stored near the poles than near the equator (45). In the troposphere, ozone concentration increases generally from the equator to the polar regions (25), and Egorov (46) demonstrated a significant latitudinal decline in ozone in the atmospheric boundary layer from midlatitudes to the equator. Relatively low ozone in equatorial regions may result in lower concentrations of 6574
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RGM in the atmosphere above the sampled lakes, comparable with that at other relatively low-altitude remote locations in the midlatitudes. Moreover, the 3-fold increase in Hg accumulation in the Rwenzori lakes implies that atmospheric oxidants have increased in the tropics at roughly the same rate as those worldwide. The remoteness of the Rwenzori Mountains from large and industrialized population centers offers a study in contrast with those regions of western North America where larger secular increases in Hg accumulation have been reported. Mercury accumulation in lake sediments of high-elevation sites in equatorial Africa may therefore be affected by the region’s unique meteorology and ozone distribution. It seems that sediment records from highaltitude lakes away from the tropics may reveal different atmospheric Hg dynamics than those recorded in the tropics such as the Ugandan lakes and may help explain the mechanisms of Hg deposition at high elevation in equatorial areas.
Acknowledgments Field work in the Rwenzori Mountains was conducted with the permission and cooperation of the Uganda National Council for Science and Technology (ref EC583) and the Uganda Wildlife Authority. It was supported primarily by grants from The Royal Geographical Society (Ralph Brown Award). We thank Richard Taylor and Virginia Panizzo (UCL), Andrew Muwanga, Immaculate Ssemmanda, Bob Nakileza, Nelson Kisaka, Alex Mbonimba, Allen Ndayanabo (Makerere University), the Water Resources Management Division, and the Rwenzori Mountaineering Service for their assistance in the field, and Carl Lamborg for his comments on the paper.
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