Environmental Engineering Reference
In-Depth Information
CHAPTER 8
Mercury Cycling in Terrestrial Watersheds
JAMES B. SHANLEY and KEVIN BISHOP
et al., 2004) and North America (Lucotte et al., 1995). High
levels of Hg in freshwater fi sh have been a concern for more
than three decades (Johnels et al., 1967; Håkansson et al.,
1990). More recently, bio-accumulation in the terrestrial
ecosystem has come to the fore (this topic, chapter 16), and
even the possibility that soil microbiota are directly affected
(Bringmark and Bringmark, 2001). When the problem was
originally addressed in the 1970s and 1980s, researchers
focused on Hg deposition and the role that acidifi cation
might play in mobilizing Hg from catchment soils to surface
waters (Johansson et al., 1991). This focus has since given way
to an emerging awareness of the importance of catchment
processes in controlling the loading of Hg to aquatic ecosys-
tems, and especially loading of the most bio-available form of
Hg, methylmercury (MeHg) (Rudd, 1995; St. Louis et al., 1996;
Meili et al., 2003; Drevnick et al., 2007). As understanding
has grown, so too has an appreciation of the magnitude of
the Hg problem.
Some of the earliest investigations of Hg cycling in ter-
restrial watersheds were carried out in boreal landscapes,
and many of the fi ndings were presented in special issues
of
Water, Air and Soil Pollution
in 1991 and 1995. These
were followed by studies aimed at quantifying fl uxes of
Hg from the terrestrial landscape in small forested streams
(Krabbenhoft et al., 1995; Lee et al., 1995; Allan and Heyes,
1998; Scherbatskoy et al., 1998; Schwesig and Matzner,
2000; Allan et al., 2001; Shanley et al., 2002) to larger rivers
of mixed land use (Hurley et al., 1995; Babiarz et al., 1998;
Balogh et al., 1998a, 2005). The majority of the literature is
weighted toward midlatitudes and especially high latitudes
in the northern hemisphere, primarily North America and
Europe (Figure 8.1). The more limited literature on tropical
systems and the southern hemisphere is nearly exclusively
from mining-impacted areas of the Amazon (Hacon et al.,
1995; Malm, 1998; Wasserman et al., 2003).
MERCURY INPUTS
MERCURY STOCKS
Vegetation
Soils
MERCURY OUTPUTS
Volatilization
Stream Mercury Export
ROLE OF DISSOLVED AND PARTICULATE ORGANIC CARBON
IN-STREAM MOBILITY AND BIO-AVAILABILITY
MERCURY MASS BALANCES
MERCURY MOBILITY AND FLOW PATHS
Uplands
Wetlands
Riparian Zones
TRANSFORMATION/METHYLATION PROCESSES
EFFECTS OF PERTURBATION
CONCLUSIONS AND FUTURE SCENARIOS
Long-range atmospheric transport of anthropogenic mer-
cury (Hg) emissions has led to Hg deposition well in excess
of natural levels throughout much of Europe, North America,
and other areas (Iverfeldt, 1991; Mierle and Ingram, 1991;
Fitzgerald et al., 1998; Slemr et al., 2003). This deposition has
contributed to the accumulation of Hg in the aquatic food
chain (Lindqvist et al., 1991; Kamman et al., 2005). It is now
common for some species of fi sh to contain Hg near or above
levels deemed harmful to human health, even in relatively
remote areas such as boreal areas of Fenno-Scandia (Munthe
