Tahoe Research Group : Annual Report

Atmospheric Lead and Mercury Deposition at Lake Tahoe

Alan C. Heyvaert *, John E. Reuter, Darell G. Slotton, and Charles R. Goldman

Department of Environmental Science and Policy, Tahoe Research Group, University of California—Davis, One Shields Avenue, Davis CA 95616.

Abstract

Evidence from this study suggests the existence of a significant modern source for atmospheric Hg deposition in the Sierra Nevada. Concentrations of both lead (Pb) and mercury (Hg) in the sediments of Lake Tahoe deposited prior to 1850 are similar to concentrations in the catchment bedrock, but their concentrations in modern sediments have increased six-fold for Pb (average 83 ppm) and five-fold for Hg (average 0.191 ppm). The lake occupies a relatively pristine, non-industrialized subalpine basin, with a watershed to lake surface ratio of only 1.6. Excess accumulation of trace metals in these sediments should closely reflect direct atmospheric deposition. On average, since 1980 there have been approximately 17 mg of Pb and 38 µg of Hg deposited annually per square meter in excess of the baseline flux. While Pb emissions have occurred locally in the Tahoe basin, from combustion of leaded gasoline until about 1985, the deposition of atmospheric Hg must represent a predominately regional to global source of contamination. Ratios of total modern flux to preindustrial flux are 29 for Pb and 24 for Hg. The flux ratio for Pb is somewhat higher than reported from the eastern USA and Canada, but is not atypical. The flux ratio for Hg is much higher than that observed in most other natural aquatic systems without point-source contamination. Both orographic scavenging and cold-condensation processes could enhance the deposition of Hg and other atmospheric pollutants over the Sierra Nevada.

Introduction

Modern industrial processes, product distribution, and material consumption patterns all disperse a wide variety of toxic metals into the environment. Of particular concern is the atmospheric emission of these metals, which can cause significant contamination over large areas. The introduction of alkyl-leaded gasoline in 1923, for example, ultimately produced a global anthropogenic Pb emission rate that exceeded the total contribution from natural sources by a factor of 28 (1). Mercury emission rates have also increased over modern times, and Hg is now listed as an EPA priority pollutant, in large part due to concerns about its biomagnification in aquatic food chains.

To date, there have been few studies of atmospheric deposition for trace metals on the U.S west coast. This study looks at the history of atmospheric Hg deposition over Lake Tahoe, a relatively pristine watershed in the Sierra Nevada mountains of California and Nevada. While there has never been any recorded use of Hg in the Tahoe basin, there was a substantial production and consumption of Hg in mining districts of California and Nevada adjacent to the Tahoe basin during the late 1800s. Our objective was to compare the modern rates of Hg deposition to the preindustrial (baseline) rates, as reconstructed from lake sediment cores. We also examined Pb accumulation rates, and compared the results for both Pb and Hg to sediment concentrations and to flux estimates from similar studies in other regions of North America. The sediment concentrations of titanium (Ti), a conservative reference element, were used as correction factors in reconstructing these trace metal deposition rates.

Study Site and Methods

Lake Tahoe occupies a graben in the northern region of the Sierra Nevada mountains, on the border between California and Nevada. Its surface area is 498 km², within a natural basin of 1,311 km². Less than 8% of the terrestrial area is urbanized. At its natural rim the lake is 1897 meters above sea level, but surrounding mountains extend to over 3000 meters. On its western boundary the Tahoe watershed is delineated by the north-to-south bearing crest of the Sierra Nevada range.

The sediment cores examined in this study were extracted with a Soutar box corer, deployed from the U.C. Davis Research Vessel John LeConte. Two box cores (LT-91-1 and LT-91-3) were extracted from opposite ends of the lake in the profundal zone below 400 meters. A third core (LT-91-4) was taken off the west shoreline on a deep shelf at 300 meters depth.

Concentrations of ²¹⁰Pb and ¹³⁷Cs were determined by alpha and gamma spectrometry, respectively, in the laboratory of Dr. David Edgington. The analyses for Pb and Ti were performed by energy dispersive x-ray spectrometry. Samples for Hg analysis were digested in nitric and sulfuric acids, under pressure, then subsequently analyzed for total Hg (THg) using a modified cold vapor atomic absorption (CVAA) micro-technique (2).

Results

The concentrations of Pb and Hg in each sediment section of the three cores from Lake Tahoe are shown below, along with the smoothed profiles produced by a three term moving average. The onset horizon of ¹³⁷Cs is indicated by a horizontal line at the bottom of the deepest sediment section in which ¹³⁷Cs was detected. This onset horizon is generally interpreted as representing the first appearance (1952—1954) of global fallout from the atmospheric testing of thermonuclear weapons. To facilitate interpretation, approximate dates of sediment deposition are also indicated on the vertical axis.

In all three cores, Hg concentrations increase substantially prior to the ¹³⁷Cs onset horizon, and prior to equivalent changes in Pb concentrations. Above the ¹³⁷Cs horizon, however, Hg concentrations increase more slowly, whereas Pb concentrations begin to increase rapidly until they stabilize somewhat in the surficial sediments. In contrast to Pb, the trend of increasing Hg content persists into surficial sediments, which are enriched about five-fold over the baseline concentrations (see sediment enrichment factors listed in Table 1).

Table 1.Concentrations of Pb, Hg and Ti in Lake Tahoe sediment cores; with mean values, relative standard deviations (RSD) and sediment enrichment factors (SEF) calculated for each element.
			Pb (ppm)					Hg (ppm)					Ti (wt %)					SEF
core	surficial				baseline	surficial			baseline	surficial			baseline	Pb		Hg		Ti
LT-91-1		84.7		12.2			0.223	0.030			0.278	0.225			6.0		6.4		0.2
LT-91-3		77.1		12.5			0.157	0.037			0.260	0.259			5.2		3.3		0.0
LT-91-4		85.9		10.5			0.193	0.033			0.306	0.284			7.2		4.8		0.1
mean		82.6		11.7			0.191	0.033			0.281	0.256			6.1		4.9		0.1
RSD (%)		6		9			17	10			8	12			17		32		< 1

Since it has been shown that redox conditions do not appreciably influence the structure of Pb or Hg stratigraphy in most lake sediments (3), we interpret these patterns in the Tahoe sediments as representing temporal changes in Pb and Hg loading rates. These patterns do not change significantly when corrected for the contribution of trace metals derived from watershed weathering (normalized by factoring to variation in the content of sediment titanium as a conservative lithogenic element in most depositional environments).

Sediment fluxes of Pb and Hg were calculated as the product of sediment concentration and mass sedimentation rate. These data are summarized for the modern (post 1980) depositional period in Table 2. For modern sediments, with equal weight given to each core, the estimate of excess (normalized) Pb flux is 17 mg m^-2 y^-1. A corresponding estimate for excess Hg flux is 38 µg m^-2 y^-1.

Concise representation of change in deposition rate over time within a system is given by the flux ratio. This is simply the modern flux divided by a baseline, or preindustrial (ante 1850) flux. Like SEF factors this flux ratio must be calculated from the total (i.e., non-normalized) concentrations. Flux ratios are independent of most factors that affect Hg concentrations, such as site conditions, sediment focusing, and site-specific differences in absolute rates of atmospheric Hg deposition. Thus, flux ratios provide a unitless measure for the comparison of changes in Hg deposition between sites and geographic regions. At 47 µg m^-2 y^-1 the average modern flux of Hg (uncorrected) to Lake Tahoe sediments is 24 times greater than the baseline flux was prior to 1850 (2.0 µg m^-2 y^-1). This flux ratio is substantially higher than observed in the eastern and midwestern U.S. or in Alaska and Canada (4). Neither the modern flux nor the preindustrial flux at Lake Tahoe, however, fall outside the range of results found in other studies. Thus, it appears that high flux ratios for Hg in the Tahoe sediments result from a combination of relatively low preindustrial flux and a comparatively high modern flux.

For Pb, the average modern flux (uncorrected) to Lake Tahoe sediments is 20 mg m^-2 y^-1, and the average preindustrial accumulation rate is 0.7 mg m^-2 y^-1. These values and the resulting flux ratio of 29 are similar to Pb accumulation rates found at other sites around the country (4).

Since Hg is known to bioaccumulate in aquatic food chains, and since Hg flux to the sediments of Lake Tahoe has increased substantially over the last 100 years, we obtained measurements of Hg content in the biota (4); specifically crayfish (Pacifastacus leniusculus), which has been recommended as a reliable indicator of trace metal contamination, and the Mackinaw trout (Salvelinus namaycush), which is a top aquatic predator and the basis of an important sport fishery at the lake. Several individuals of each species were collected from about one kilometer off the west shore, just south of Tahoe City. These concentrations are reported in ppm (µg g^-1), wet weight. The regressions show a trend of increasing Hg content with size of individuals for both Mackinaw trout and crayfish. All concentrations reported in this study, however, fall below the California state threshold of 0.5 ppm.

Discussion

One of the more interesting findings of this study is that Hg flux on the U.S. continental west coast near the crest of the Sierra Nevada mountains may be equivalent to or greater than rates of Hg deposition observed in the Midwest and eastern U.S. or in Alaska and Canada (4). Since there are no significant local sources of Hg emission within the Tahoe Basin, it would appear that air parcels coming off the Pacific Ocean must either carry Hg from distant sources or entrain Hg from regional sources on the west coast.

For Pb there has been a local source of historical emissions at Lake Tahoe, in the form of leaded gasoline consumption. Interestingly, this can provide some validation for the relatively high rate of modern Hg deposition estimated for this site. We have calculated automotive Pb emissions at Lake Tahoe for 1976, using fuel consumption records as estimated by in-basin gasoline sales (5). These calculations suggest that sufficient Pb was emitted locally to account for most of the Pb burden measured in recent sediments of the lake. Furthermore, our baseline flux of Pb to Tahoe sediments (0.7 mg m^-2 y^-1) is quite similar to Pb deposition measured at a remote Sierran site (6), and is just slightly greater than the flux of 0.5 mg Pb m^-2 y^-1 measured in bulk precipitation over the eastern central (33—48°N) Pacific Ocean (7).

The fact that we can accurately account for Pb burden in the Tahoe sediments, along with its general correspondence to loading rates and flux ratios observed in other studies, suggests that our reconstruction of historical sediment and trace metal deposition in this system is reliable. It is likely that Hg has been brought into the basin by prevailing westerly winds, but that Pb has been predominately contributed by automobile emissions distributed around the lake.

The unexpectedly high rate of Hg deposition observed at Tahoe in the modern sediments may occur as a result of efficient orographic scavenging by rain and snow as air parcels travel over the crest of the Sierra Nevada mountain range. Another factor that could significantly enhance Hg deposition over the Tahoe area is a process of cold-condensation, whereby temperature dependent partitioning and transport increase the concentrations of semi-volatile compounds over cooler environments (8). It has been shown that these processes and increased precipitation sharply enhance the accumulation of semi-volatile compounds at elevations above 2000 meters (9). This could increase the Hg accumulation rates over high altitude environments like Lake Tahoe, especially when there are regional downwind sources of Hg in a warmer climate at lower elevations.

Although USEPA region IX (California, Nevada and Arizona) is the second lowest of all regions in this country for estimated THg emissions (10), it is possible that air parcels traveling toward Tahoe could entrain Hg volatilized from the waste of historical gold and silver mining. A tremendous amount of elemental Hg was consumed during the late 1800s at several mining districts regionally close to the Tahoe basin. Somewhat surprisingly, these historical emissions from the western Sierra foothills and from Virginia City in Nevada did not produce an unequivocal signal in Lake Tahoe sediments. Elevated concentrations of Hg are found at depth in the west lake core, but do not appear in the south lake core and are significantly modulated in the north lake core. We suggest that high mass sedimentation rates from Comstock logging in the late 1800s diluted most of this historical Hg signal in the two midlake cores (4). For that reason we have focused this study on comparing the preindustrial Hg deposition rates to modern rates.

Much of the Hg lost to mining spoils or deposited locally during the mining era would continue to volatilize from depositional surfaces and may gradually be transported downwind across the landscape. Nriagu (11) suggested that re-emission of only 0.2% of Hg lost during the historical mining era in the Sierras would be equivalent to a substantial fraction of current annual anthropogenic emissions in the U.S. This continuous volatilization of Hg° from mining spoils and abandoned Hg mines in the Coastal Range, in conjunction with orographic precipitation, scavenging and cold-condensation, could be contributing to the relatively high rate of modern atmospheric Hg deposition at Lake Tahoe.

We still cannot say yet whether that Hg input derives predominately from regional, perhaps historical, sources on the west coast or from globally distributed atmospheric Hg, but the regional sources are suspect for up to 85% of THg deposition. Obviously, a series of sediment sampling transects or deposition monitoring stations are needed across both elevational and latitudinal gradients in the western U.S. to clarify the relative importance of these sources and processes.

Acknowledgements

Financial support for this research came from the Center for Ecological Health Research at the University of California—Davis (UCD) and from the Tahoe Regional Planning Agency. Peter Schiffman and Sarah Roeske in the Geology Department at UCD provided technical advice and assistance on XRF analyses. Bob Richards, Scott Hackley, Mark Palmer and Brant Allen of the UCD Tahoe Research Group helped with the sample collection. David Edgington at the University of Wisconsin—Milwaukee Center for Great Lakes Studies supplied the ²¹⁰Pb data and informative discussion on its interpretation. Shaun Ayers in the UCD Limnology Group performed the Hg analyses and QA/QC. We acknowledge the useful comments of three anonymous reviewers. Expanded text for most of this report can be reviewed as accepted for publication in the journal of Environmental Science and Technology (year 2000) under the title Paleolimnological Reconstruction of Historical Atmospheric Lead and Mercury Deposition at Lake Tahoe, California—Nevada.

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