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]