Environmental Engineering Reference
In-Depth Information
short episodes of high concentrations and loading (Bishop
et al., 1995a, 1995b; Allan and Heyes, 1998; Hurley et al.,
1998; Mason and Sullivan, 1998; Scherbatskoy et al., 1998;
Whyte and Kirchner, 2000; Allan et al., 2001; Shanley
et al., 2002, 2008; Schuster et al., 2008). The episodic load-
ing of Hg during brief periods may exacerbate its toxicologic
impact on downstream water bodies relative to the same
load distributed more evenly in time. MeHg export may also
be episodic, but its behavior is more variable.
During events, dissolved THg (and to some extent MeHg)
is mobilized from surface soil by rising water tables and
shallow hydrologic fl ow paths. Particulate THg and MeHg
enter streams during events by erosion of soil organic mat-
ter or streambanks. Some particle Hg may also be resus-
pended from streambed sediments (Hurley et al., 1998).
Dissolved THg concentrations increase during storms by
typically twofold to fi vefold (Schuster et al., 2008; Shanley
et al., 2008) and rarely exceed 20 ng L
-1
in unpolluted
landscapes. HgP concentrations, on the other hand, are
negligible at base fl ow but increase more dynamically
during storms, often exceeding 50 ng L
-1
during high fl ow
(Figure 8.3). Selected measured peak THg concentrations
are presented in Table 8.1. Progress on unattended, auto-
mated sampling for Hg may help researchers collect impor-
tant high-fl ow samples (Riscassi et al., 2010).
Landscape characteristics, to a much greater extent than
Hg deposition, control the amount and form of stream
Hg export (Schelker et al., 2011). Hg on the landscape is
strongly associated with organic matter, and thus water-
shed features that regulate the accumulation and release
of dissolved organic carbon (DOC) and particulate organic
carbon (POC) will have a strong bearing on the mobility of
Hg and MeHg. The mechanism of Hg-DOC and Hg-POC
associations will be discussed later in the section “Role of
Dissolved and Particulate Organic Carbon.”
In a Vermont stream, Scherbatskoy et al. (1998) found
that annual THg export was about two thirds particulate
and one third dissolved. Particulate THg export occurred
primarily during high-fl ow events. Dissolved THg export,
though its concentration also increased with discharge,
occurred primarily as cumulative release during low-fl ow
periods. A single event may dominate the annual stream
fl ux of particulate THg (Babiarz et al., 1998; Scherbatskoy
et al., 1998). Hurley et al. (1995) found that THg export in
agricultural and developed watersheds was dominated by
particulate Hg, but 80% of the THg was in the dissolved
fraction in a forested watershed with numerous wetlands.
2.0
100
THg
1.5
10
Filtered THg
1.0
1
MeHg
0.5
0.1
0.01
0.0
6
7
8
9
10
11
12
13
Hour of day
FIGURE 8.3
Hydrograph showing fi ltered and unfi ltered THg and
unfi ltered MeHg in response to an intense rain storm (51 mm in 1 hr)
at the 41-ha W-9 catchment at Sleepers River, Vermont, September 15,
2002. Modifi ed from Schuster et al., 2008.
table 8.1
Selected Maximum Stream Total Mercury Concentrations from Various Studies
Location
Land cover
Hydrologic condition
Peak THg (ng/L)
Source
Wisconsin
Forested
Snowmelt
21
Hurley et al., 1995
Vermont
Forested
Snowmelt
80
Scherbatskoy et al., 1998
Vermont
Forested
Summer storm
96
Schuster et al., 2008
Puerto Rico
Forested
Tropical downpour
112
Shanley et al., 2008
Georgia
Forested
Heavy rain
55
Shanley et al., 2008
Minnesota
Forest/agriculture
Summer storm
76
Balogh et al., 1998b
Wisconsin
Urban
Summer dam release
182
Hurley et al., 1998
Vermont
Urban
Summer storm
129
Shanley and Chalmers, 2012
Washington, D.C.
Urban
Spring rain storm
205
Mason and Sullivan, 1998
California
Mine area
Intense rain storm
1,040,000
Whyte and Kirchner, 2000
NOTE
: All values are for unfi ltered samples and refl ect dominance by particulate mercury.
