Conclusions-Total Hg concentrations in the Apalachicola, Sabine and Brazos were higher during the Fall - when discharge was slightly lower -LogKd values decreased as total Hg concentration decoupled from TSS-MeHg LogKd values were lower than for total Hg illustrating the importance of the dissolved fraction-MeHg generally made up less than 5% of the total Hg concentration

Mercury Partitioning and Speciation in Gulf of Mexico Estuaries

092-209 092-209

Clifton Buck1,4, Katlin Bowman2, Gary Gill3, Chad Hammerschmidt2, William Landing4, Alexandra Landing4

1Skidaway Institute of Oceanography, 2Wright State University, 3Pacific Northwest National Laboratory, 4Florida State University

BackgroundWe conducted seasonal estuarine transects in 11 of the largest systems on the northern Gulf coast to better understand the biogeochemistry of Hg in Gulf of Mexico estuaries. Both total and mono-methylmercury (MeHg) concentrations were quantified in the dissolved and particulate fractions. Samples were collected at 9 stations located along the salinity gradient from the freshwater to marine end members by peristaltic pump from a nominal depth of 1 meter.

The study's goal was to provide improved data to help constrain estuarine mercury inputs in biogeochemical models.

Seasonal sampling was intended to provide data from high and low discharge periods. The discharge data are daily averages of USGS river guage station data for the time period of each sampling trip.

Suwannee

Apalachicola

Pascagoula

Escambia

MobilePearl

L. Atch

afalaya

Sabine

Mississ

ippi

Trinity

Brazos

SpringFall

Dis

char

ge(m

3 /day

)

105

106

107

108

109

These boxplots show the range of particulate + dissolved total Hg concentrations observed at 9 stations across the salinity gradient in each estuary and the relative difference in Spring and Fall concentrations. Total Hg concentrations tended to encompass a wider range during the Fall but were only significantly higher in the Apalachicola, Sabine and Brazos.

Kd gives an indication of the extent to which the total Hg concentration will be a function of the concentration of total suspended solids (TSS). Partitioning showed some seasonality in some of the estuaries. LogKd values <5 indicate that the particule phase was less likely to dominate. Values <4 indicate dominance of the dissolved phase. The higher Fall concentrations of total Hg in the Apalachicola, Sabine and Brazos were driven by an increase in dissolved Hg concentrations.

Estuary LogKd Reference

Chesapeake Bay Tributaries

5.1 - 5.5

San Francisco Bay

San Francisco Bay

4 - 7

5.4 - 5.6

Lawson et al. Water Research

2001

Conaway et al. Marine Chemistry

2003

Choe et al. L&O2003

Apalachicola

L. Atch

afalaya

Suwannee

Pascagoula

Escambia

MobilePearl

Sabine

Mississ

ippi

Trinity

BrazosPar

ticul

ate

+ D

isso

lved

Tot

al H

g

SpringFall

(pM

)

0

10

20

30

40

50

60

70

TSS (mg/L)0 20 40 80

Par

ticul

ate

+ D

isso

lved

Tot

al H

g(p

M)

0

20

60

80

Suwannee Apalachicola Mobile Pascagoula L. AtchafalayaMississippiSabine TrinityBrazos

Spring Fall

TSS (mg/L)0 10 20 30 40 50 60

Par

ticul

ate

+ D

isso

lved

Tot

al H

g(p

M)

0

10

20

30

40

50

60

SuwanneeApalachicolaEscambia MobilePascagoulaPearl L. Atchafalaya Mississippi Sabine Brazos

Apalachicola

TSS

(mg/

L)

0

10

20

30

40

50

Brazos

Sabine

FallSpring

TSS and total Hg showed a strong linear correlation during the Spring sampling. Each system showed good internal agreement though the slope of the relationship varied among the estuaries.

%M

eHg

0

2

4

6

8

10M

eHg

Log

Kd

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Suwannee

Apalachicola

Pascagoula

Escambia

MobilePearl

L. Atch

afalayaSabine

Mississ

ippi

Trinity

Brazos

Suwannee

Apalachicola

Pascagoula

Escambia

MobilePearl

L. Atch

afalayaSabine

Mississ

ippi

Trinity

Brazos

Suwannee

Apalachicola

Pascagoula

Escambia

MobilePearl

L. Atch

afalayaSabine

Mississ

ippi

Trinity

BrazosP

artic

ulat

e +

Dis

solv

ed M

eHg

(pM

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

SpringFall

The range in particulate + dissolved mono-methylmercury concentrations was wider during the Fall for most estuaries. The greatest range and highest concenrations were observed in the Escambia River estuary which was not sampled during the Spring. LogKd values were generally lower than for total Hg, indicative of the tendency for the dissolved fraction to dominate methylmercury partitioning.

The percentage of total Hg present as mono-methylmercury was not generally different between the Spring and Fall samplings and were generally <5%. The highest percentage was in the Mobile River estuary where samples collected at the riverine end member had high concentrations of mono-methylmercury.

The association weakened during the Fall. Two of the systems with significant changes in logKd, Apalachicola and Sabine, con-tinued to have positive correlations albeit with increased scatter in the data. The Brazos showed the greatest divergence in Spring and Fall logKd values and total Hg was inversely correlated with TSS in the Fall. These changes do not appear to tbe driven by changes in the TSS load within the systems.

6.0

Suwannee

Apalachicola

Pascagoula

Escambia

MobilePearl

L. Atch

afalayaSabine

Mississ

ippi

Trinity

Brazos

3.5

4.0

4.5

5.0

5.5

Kd = ([Hg] Part/[TSS])[Hg] Diss

Tota

l Hg

log

Kd