PROCEEDINGS OF 1995
CANADIAN MERCURY NETWORK WORKSHOP
VOLATILIZATION OF Hg FROM LAKES MEDIATED BY SOLAR RADIATION
M. Amyot1,2, D. R. S. Lean3, G. Mierle4 & D. J. McQueen1
1Department of Biology, York University, 4700 Keele St., North York,
Ontario M3J 1P3,
2 Department of Marine Science, Texas A&M University, 5007 Avenue U,
Galveston, Texas 77551, USA, 3 National Water Research Institute,
Burlington, Ontario L7R 4A6, 4 Ministry of the Environment, Dorset
Research Centre, P.O. Box 39, Dorset, Ontario P0A 1E0
Elemental Hg (Hgo) plays a key role in the Hg cycle (Nriagu 1994).
It represents nearly 98% of the atmospheric Hg and 10-50% of the
dissolved Hg of lakes. The formation of Hgo can modify the fate of
this element in an aquatic system in two major ways. Firstly, due to
its low solubility and favorable Henry's Law constant, Hgo is a very
volatile species and is the principal form of dissolved gaseous Hg
(DGM). Thus, its formation will favor the removal of Hg from the
system, through gas evasion. Secondly, this volatilization of Hg
reduces the pool of reactive Hg in the water column and may thus
limit methylHg production and accumulation in fish (Nriagu 1994).
While photochemical and photobiological processes are thought to
alter the rate of volatile Hg production in lakes (Vandal et al.
1993; Xiao et al. in press), few field studies have directly
addressed this topic due to technical difficulties involved in
measuring DGM. In this context, the aim of this project is to
determine the importance of sunlight-induced processes on DGM
production in lakes, using ultra-clean protocols.
Experiments were conducted in four temperate lakes (L. Erie, Ranger
L., Plastic L., Fawn L.), three arctic lakes (Merreta L., North L. &
Amituk L.) and one arctic wetland. This set of aquatic systems
encompasses a large range of values of pH, dissolved organic carbon
(DOC), DOC fluorescence (a measure of the photochemical activity of
DOC) and total phosphorus (Table 1). In each case, surface water
samples were incubated in situ for 4 to 6 h at midday in transparent
and black Teflon bottles, and DGM levels were measured in each
sample. Also, to narrow down the possible mechanisms involved in the
photo-induced DGM production, we assessed the effect of UVA light
and UVB light (transparent bottles wrapped in UV Lee Filters and
Mylar), hydrogen peroxide spiking, Hg2+ spiking, filtration and
sterilization on DGM production. Furthermore, we determined the
feasibility of building light response curves of DGM production. A
daily pattern of DGM concentrations was also obtained in surface
waters. Depth profiles of DGM levels were measured in an attempt to
locate the sites of maximum DGM production in different lakes.
For DGM analysis, 500 mL of lake water sample was purged during 15
min with purified argon and trapped on gold-coated sand. The Hg was
then desorbed and measurements were made using an atomic
fluorescence detector (Merlin).
In all lakes studied, except L. Erie and Amituk L., DGM production
was photo-induced (Fig. 1 a and b). In lakes with higher DOC levels
(Ranger L.: 5.0 mg L-1 ; Fawn L.: 8.7 mg L-1 ), UVB light did not
contribute significantly to this production. In lakes with lower DOC
levels
Table 1. Chemical characteristics of study sites
LakespHDOC
(mgL-1)DOC fluorescence
(QSU)Total P
(=B5gL-1) Ranger6.1535.86 Jacks7.25.621.712 Fawn5.78.7N/A22
Erie8.32.63.38 Mouse5.74.427.17 North8.61.15.24.3 MerretaN/A2.35.26.2
(Merreta L.: 2.3 mg L-1 ; North L.: 1.1 mg L-1 ; Plastic L.: 2.2 mg
L-1 ), DGM photoproduction was mainly driven by UVA and UVB light.
We suggest that in high DOC lakes, UV light is absorbed mainly by
DOC and is thus less available for photochemical processes leading
to Hg reduction. In contrast, in low DOC lakes, UV light can
penetrate farther in the epilimnion and play a more active role in
the photoreduction of Hg.
[INLINE]
In two lakes in which sunlight-induced DGM production was described
(Ranger L and Jacks L.), peaks of DGM levels were measured in the
epilimnion, strongly supporting the results of the bottle
experiments. In L. Erie, a lake in which no effect of light was
found, no epilimnetic peaks was observed. However, a metalimnetic
peak was found, possibly resulting from the activity of Hg-reducing
bacteria.
A number of experiments were done on Ranger L. water to narrow down
the possible mechanisms of DGM production. Light response curves of
DGM production were obtained by incubating transparent bottles
wrapped in different thicknesses of screens. In Ranger L., light
response curves reached a plateau at 45-64 fM DGM h-1 during the
summer and at 20 fM DGM h-1 during the fall. In the two arctic
lakes, the light response curves were linear, never reaching a
plateau, with a maximum production < 17 fM DGM h-1 , suggesting that
light was a limiting factor. These light response curves are the
first ever generated for DGM production under field conditions, and
they allowed the formulation of a mechanistic model by one of the
authors (Mierle, in prep.), describing Hg volatilization from lakes.
Diel patterns in DGM levels were recorded in surface waters of
Ranger L. and will be used to validate the predictions of the model.
Filtration of lake water on 1 =B5m filters prior to incubation had no
effect on DGM production rates, suggesting that agents promoting DGM
formation were dissolved or associated with particles & 1 =B5m, such
as bacteria and colloids. Sterilization of the samples prior to
incubation slightly increased DGM production rates, indicating that
most of the DGM production is the result of photochemical processes.
Spiking with different levels of Hg2+ prior to incubation resulted
in increased DGM production in transparent bottles. The levels of
reducible Hg are thus limiting DGM production in Ranger L. Under
sunny conditions (June 22, 1995) approximately 27% of the added Hg
was transformed in DGM compounds after 4 h. Under cloudy conditions
(June 28), a time series of DGM production after spiking of 5 ng L-1
of Hg2+ revealed that 27% of the added Hg was transformed in 12 h.
DGM production rates thus vary from day to day, depending on the
weather.
In conclusion, sunlight-induced DGM production in the epilimnetic
waters of many lakes, both in temperate and Arctic regions. UVB
light contributed to the photoreduction of Hg only in lakes with
lower levels of DOC. In Ranger L., DGM production was mainly caused
by abiotic processes related to the dissolved and/or colloidal
phases. In this lake, DGM production was substrate-limited, and
reducible Hg compounds were probably the limiting substrate.
References
Amyot, M., G. Mierle, D.R.S. Lean and D.J. McQueen. 1994.
Sunlight-induced formation of dissolved gaseous Hg in lake waters.
Environ. Sci. Technol. 28: 2366-2371.
Barkay, T., R.R. Turner, A. VandenBrook and C. Liebert. 1991. The
relationships of Hg(II) volatilization from a frewshwater pond to
the abundance of mer genes in the gene pool of the indigeneous
microbial community. Microb. Ecol. 21: 151-161.
Nriagu, J.O. 1994. Mechanistic steps in the photoreduction of Hg in
natural waters. Sci. Tot. Env. 154:1-8.
Vandal, G.M., W.F. Fitzgerald, C.H. Lamborg and K.R. Rolfhus. 1993.
The production and evasion of elemental Hg in lakes: a study of
Pallette Lake, Northern Wisconsin, U.S.A. p.297-299. In R.J. Allan
and J.O. Nriagu [eds.], Heavy Metals in the Environment, vol. 2, CEP
Consultants Ltd., Edinburgh.
Xiao, Z.F., D. Stromberg and O. Lindqvist. (in press) Influence of
humic substances on photolysis of divalent Hg in aqueous solution.
Water Air Soil Pollut.
[INLINE]
back to intro next section
BACK TO
*********************************************************************