Introduction
Medicine Lake is located within the summit caldera/basin of the Medicine Lake volcano, a Pleistocene and Holocene shield volcano located in northeastern California (Donnelly-Nolan et al., 1990). This study was initiated to establish the record of mercury and methyl mercury deposition in sediment cores from a lake developed within a young volcanic field where an active subsurface geothermal system is present. Numerous studies have been carried out to establish the record of mercury deposition in seepage lakes throughout the world (Porcella, 1996) but no data is presently available for a lake localized within a young volcanic field.
Selected sediment samples from two cores (core 3 and 6) were taken at Medicine Lake on September 16 and 17, 1999, for analysis of mercury and methylmercury. An unfiltered surface water sample in the central part of the lake was taken on September 17, 1999, and analyzed for total mercury and methyl mercury. The pH of the surface water was 4.5. The results of the sediment and water mercury analyses are listed in Table 1.
Sampling for mercury and methylmercury analysis followed ultra-clean sampling and handling protocols (Bloom, 1995, Gill and Fitzgerald, 1987) during the collection of field samples and analysis of samples in order to avoid introduction of mercury. Borosilicate I-CHEMTM glass with teflon-lined caps were used for water sampling. Water samples were preserved in a cooler at 1 to 4 degrees C until maintained under laboratory established conditions to preserve mercury retention and speciation. Sediment core samples were taken in precleaned amber borosilicate glass jars. Transportation to the laboratory was under conditions specified to maintain sample mercury retention.
Mercury and methyl mercury concentration in sediment from cores
Mercury and methyl mercury concentration in surface lake water
Table 2. Mercury and methyl mercury concentration in sediment from cores at Medicine Lake.
|
Number |
fraction |
Mercury wet weight ng/g |
dry weight ng/g |
wet weight ng/g |
dry weight ng/g |
Interval cm |
Depositional |
|
3ML0
|
0.71
|
0.034
|
0.048
|
25.8
|
36.3
|
3.0
|
Modern Lake
|
|
3ML0
replicate |
0.0
|
0.032
|
0.0
|
0.0
|
26.1
|
0.0
|
Modern Lake
|
|
3ML1
|
0.69
|
0.027
|
0.039
|
20.3
|
29.4
|
0.8
|
Modern Lake
|
|
3ML2
|
0.7
|
0.016
|
0.023
|
16.9
|
24.1
|
4.2
|
Modern Lake
|
|
3ML3
|
0.66
|
0.047
|
0.071
|
100.0
|
152.0
|
2.8
|
Terrestrial
|
|
3ML4
|
0.59
|
0.040
|
0.068
|
139.0
|
236.0
|
4.7
|
Terrestrial
|
|
3ML5
|
0.61
|
0.024
|
0.039
|
157.0
|
257.0
|
4.8
|
Terrestrial
|
|
3ML6
|
0.6
|
0.006
|
0.010
|
135.0
|
225.0
|
3.6
|
Terrestrial
|
|
3ML7
|
0.66
|
0.021
|
0.032
|
252.0
|
382.0
|
3.2
|
Lacustrine
|
|
3ML8
|
0.65
|
0.031
|
0.048
|
301.0
|
463.0
|
2.4
|
Lacustrine
|
|
3ML9
|
0.64
|
0.026
|
0.041
|
293.0
|
458.0
|
1.5
|
Lacustrine
|
Table 3. Sediment samples from core number 6
|
Number |
|
|
|
|
|
|
|
|
6ML1 |
0.69 |
.002 |
.003 |
10.5 |
15.2 |
1.0 |
|
|
6ML2 |
0.69 |
0.005 |
0.007 |
15.7 |
22.8 |
1.0 |
|
|
6ML3 |
0.57 |
0.011 |
0.019 |
11.8 |
20.7 |
1.0 |
|
|
6ML4 |
0.64 |
0.071 |
0.111 |
22.4 |
35.0 |
3.5 |
|
Table 4. Analysis of water from Medicine Lake
|
lake water |
mercury ng/L |
ng/L |
|
|
|
|
Mercury and methyl mercury concentration in sediment from cores
Analysis of the sediment cores indicates that the upper part of the cores consists of modern shallow lake sediments, the central part of the cores consist of terrestrial sediments, and the lower part of the cores consists of lagoon-marsh lacustrine sediments (see section on coring). The lowest mercury concentrations occur in modern shallow lake sediments. Mercury concentrations range from 24.1 to 36.3 ng/g (dry weight basis) in the upper 7 cm in core 3, and from 15.2 to 35.0 ng/g (dry weight basis) in the upper 7 cm of core 6 (Table 1). These mercury concentrations reflect the low level of clay and silt present in modern lake sediments which are primarily composed of coarse sand and gravel derived from pumice (see section on coring). Methyl mercury concentrations are similarly very low in modern lake sediments and range from .023 to .048 ng/g in core 3, and from .003 to .019 in core 6 (dry weight basis) (Table 1 above). The highest concentration of methyl mercury occurs in a fine sand layer at the transition from modern lake sediments to terrestrial sediment in core 6 (Sample 6ML4) where carbonaceous root fragments are present.
The terrestrial sediments in the central part of core 3 have considerably higher mercury concentration than modern lake sediments. Mercury concentrations range from 152 to 257 ng/g (dry weight basis) (Table 1 above). These sediments consist of lacustrine silt and clay that were exposed to the surface weathering during low stands of the lake resulting in soil development (see section on coring). Methyl mercury concentration in terrestrially modified sediments ranges from .01 to .071 ng/g.
Lacustrine environments represented in the lowest part of core 6 consist of shallow lagoon and marsh sediments consisting primarily of clay and silt. Mercury concentrations are the highest in these sediments, 458 to 463 ng/g, and methyl mercury concentration ranges from .041 to .048 ng/g (dry weight basis) (Table 1 above). A layer of more sandy sediment at the top of this section reflects shallow lake deposition and has somewhat lower mercury concentration, 382 ng/g, because of the lower fraction of silt and clay present in this layer.
Mercury and methyl mercury concentration in surface lake water
Mercury concentration in unfiltered surface water of the lake is very low, 0.74 ng/L, and methyl mercury is similarly low, 0.024 ng/L. These concentrations are comparable to other pristine lakes in the world where no mercury point sources of contamination are present. For comparative purposes, Lake Baikal, the largest pristine fresh water body in the world has mercury concentrations that range from 0.14 to 0.77 ng/L, and methyl mercury concentrations that range from .002 to .038 ng/L (Meuleman et al., 1995). Snow melt is the primary source water to Medicine Lake and the low level of mercury in lake water is consistent with this source.
The concentration of mercury in sediment cores from Medicine Lake is dependent on the grain size and depositional environment of the sediment. Modern shallow lake sediments have the lowest mercury concentration because these sediments are composed primarily of coarse sand and gravel with little or no silt or clay. These sediments contain little or no organic material and methyl mercury concentration is relatively low. The highest concentration of mercury occurs in the oldest marsh and lagoon sediments at the bottom of the cores because these sediments are composed primarily of fine silt and clay. Similar lacustrine sediments in the central part of the cores have lower mercury content because these sediments were exposed to the surface during low stands of the lake and mercury was apparently depleted during soil development.
Mercury concentrations in modern lake sediments at Medicine Lake are comparable to concentrations observed in sediments in other fresh water lakes (Lockhart et al. 1995). Lacustrine sediments in the lower part of the Medicine Lake core and in the terrestrially modified lacustrine sediments in the central part of the core are considerably higher than reported for sediments from other fresh water lakes by a factor of about 10 (Lockhart et al., 1995). Sediment cores from fresh water lakes commonly display an increase in mercury concentration from a baseline concentration established in sediments deposited prior to 1850, to higher values in more recently deposited sediments. The progressive increase in mercury concentration in lake sediments since 1850 is attributed to the increase in the global atmospheric flux of mercury resulting from processes associated with industrialization (Porcella, 1996). The sediment cores from Medicine Lake do not display this typical relationship because the highest concentrations of mercury are present in the oldest lacustrine sediments sampled. This inverse relationship and high mercury concentration in the older lacustrine sediments at Medicine Lake may be related to the volcanic setting and caldera environment in which the lake is localized.
Bloom, N. S., 1995, Mercury as a case study of ultra-clean sample handling and storage in aquatic trace metal research. Environ Lab 3-4, p. 20-25.
Bloom, N. S., 1989, Determination of picogram levels of methylmercury by aqueous phase ethylation, followed by cryogenic gas chromatorgraphy with cold vapor atomic fluorescence detection. Canadian Journal of Fish and Aquatic Sciences 46, p. 1131-1140.
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Donnelly-Nolan, J.M., Champion, D.E., Miller, C.D., Grove, T.L., and Trimble, D.A., 1990, Post-11,000-year volcanism at Medicine Lake volcano, Cascade Range, Northern Calfornia. Journal of Geophysical Research 95, p.19693-19704.
Gill, G. A. and Fitzgerald, W. F., 1987, Picomolar mercury measurements in seawater and other materials using stannous chloride reduction and two-stage gold amalgamation with gas phase detection. Marine Chemistry 20(3), p. 227-243.
Horvat, M., Bloom, N. S., and Liang, L.,1993, A comparison of distillation with other current isolation methods for the determination of methyl mercury compounds in low level environmental samples: Part 1, sediments. Analytica Chimica Acta 281, p.135-152.
Lockhart, W.L., Wilkinson, B.N., Billeck, B.N., Hunt, R.V., Wagemannn, R., and Brunskill, G.J., 1995, Current and historical inputs of mercury to high-latitude lakes in Canada and to Hudson Bay: Water, Air, and Soil Pollution, 80, p. 539-551.
Meuleman, C., Leermakers, M., and Baeyens, W., 1995, Mercury speciation in Lake Baikal: Water, Air, and Soil Pollution, 80, p. 539-551.
Porcella, D., 1996, Protocol for estimating historic atmospheric mercury deposition: Electric Power Research Institute Report TR-106768-3297, 56 p.
