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2.1 Triangle Marsh
What is the impact of mining on Hg concentrations in estuary? What is “natural” concentration of Hg?
Hg-P
Hg-DOC
Project designed to measure historic Hg contamination in southern reach
Effects of the weathering of naturally rich Hg mineralization in the Santa Clara Valley?
2.2 Background
• Worldwide– 20 –200 ng/g “uncontaminated” soils
• San Francisco Bay Area– 50 –100 ng/g Northern Coast Range
streams (Domagalski 2001)
– 1 –20 ng/g Pacific Coast Ranges (Kerin 2002)
• Downstream of New Almaden
– 200 ng/g upstream of New Almaden
– “Modern” surface sediments of estuary 200–600 ng/g (Hornberger et al., 1999; Conaway et al., 2003, Marvin-DiPasquale et al., 2003)
2.3 Methods
• San Francisco Bay
– 3 cores in South Bay
• Triangle Marsh
– Tidal marsh where Coyote Creek and Guadalupe River enter estuary
Chronology
• AMS 14C dating of shell and organic material
• Appearance of non-native pollen
– Eucalyptus, Plantago lanceolata
• Abundance of Cyperaceae (sedge family) pollen related to ENSO events
2.4 Results
0
50
100
150
200
250
300
0 200 400 600 800 1000 1200 1400
mercury (ng g-1)
dep
th (c
m)
1983 ± 2 years: dramatic peak in Cyperaceae pollen and seeds corresponding to the 1982/83 ENSO event (35 cm)
1945 ± 10 years: beginning of increase in sediment lead concentration (125 cm)
1870 ± 10 years: first appearance of Eucalyptus and Plantago lanceolata pollen (140 cm)
1570 ± 70: 14C date for Spartina foliosa rhizome (240 cm)
Bay
M
ud
Inte
r-tid
al
peat
Tid
al m
arsh
Low
im
pact
Hig
h im
pact
2.5 Results
0 100 200
Ore processed
(kg X 106)
0 20
Subsidence rate
(cm yr-1)
0 10
Mercury production
(kg X 106)
1700
1750
1800
1850
1900
1950
2000
0 1000
Mercury concentration
(ng g-1)
Dat
e
Gap in time of greatest Hg production and deposition in core!
High impact means high sedimentation...
• Rising sea level
• Subsiding basin
• Land use
– Hydrologic modifications
– Dam building
…drive transport and deposition of sediment.
0.0 - 0.2 m
0.2 - 0.6 m
0.6 - 1.2 m
1.2 - 1.8 m
1.8 - 2.4 m
> 2.4 m
Subsidence (m)1934 to 1967
from Poland (1984)
2.6 ConclusionWhat is the impact of mining on background Hg? What is a “natural” concentration of Hg in SFB?
• Pre-mining [Hg] is similar in all SFB cores (~80 ng/g)
• Little evidence of contamination from natural weathering
• Contamination reflects mining activity and watershed hydrology
Hg-P
Hg-DOC
Bay mud
Intertidal peat
Tid
al m
arsh
Low impact
Hig
h im
pact
Rock
Peak [Hg]