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In cooperation with the National Oceanic and Atmospheric Administration,
U.S. Fish and Wildlife Service,
Texas General Land Office,
Texas Parks and Wildlife Department, and
Texas Natural Resource Conservation Commission

Mercury Concentrations in Estuarine Sediments, Lavaca and Matagorda Bays, Texas, 1992

By David S. Brown, Grant L. Snyder, and R. Lynn Taylor

U.S. Geological Survey
Water-Resources Investigations Report 98–4038


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Contents

Abstract

Introduction

Purpose and Scope

Description and Hydrologic Setting

Previous Investigations

Acknowledgments

Sampling

Selection of Sample Areas

Sampling Methods

Sample-Numbering System

Mercury Concentrations in Estuarine Sediments

Distribution and Variability in Sediment Environments

Open-Water Sediments

Ship-Channel, Dredged-Spoil, Oyster-Reef, and Salt-Marsh Sediments

Relations Between Total Mercury Concentration and Grain Size and Total Organic Carbon Concentration

Applicability of Kriging to Estimate Mercury Concentrations at Unmeasured Sites

Evaluation of Field and Analytical Methods

Field Methods

Analytical Methods and Possible Refinements

Summary and Conclusions

References Cited

Plates

1–5. 
Maps showing location of data-collection sites:
1. 
Lavaca, Matagorda, and Carancahua Bays, Texas
2. 
Upper Lavaca Bay, Texas
3. 
Middle Lavaca Bay, Texas
4. 
Lower Lavaca/upper Matagorda Bays, Texas
5. 
Carancahua Bay, Texas

Figures

1. 
Map showing location of study area
2–6. 
Boxplots showing range and distribution of:
 
2. 
Total mercury concentrations by sediment environment in the surface sample-depth zones, Lavaca, Matagorda, and Carancahua Bays, Texas, 1992
 
3. 
Total mercury concentrations in the open-water sediment environment, by sample area and sample-depth zone, Lavaca and Matagorda Bays, Texas, 1992
 
4. 
Total mercury concentrations in the ship-channel sediment environment, by sample area, combining sample-depth zones 0 to 2, 10 to 20, and 20 to 50 centimeters, Lavaca and Matagorda Bays, Texas, 1992
 
5. 
Grain size in the open-water sediment environment in percent, by weight, of total sample with grain-size diameter less than 74 micrometers, Lavaca and Matagorda Bays, Texas, 1992
 
6. 
Total organic carbon concentrations in the open-water sediment environment, by sample area and sample-depth zone, Lavaca and Matagorda Bays, Texas, 1992
7. 
Graph showing isotropic total mercury concentration variogram for the 0- to 2-centimeter sample-depth zone, combining all eight open-water sample areas, Lavaca and Matagorda Bays, Texas, 1992

Tables

1. 
Description of measured sections of dredged-spoil sediment cores, Lavaca and Matagorda Bays, Texas, 1992
2. 
Selected field observations of data-collection sites, Lavaca, Matagorda, and Carancahua Bays, Texas, 1992
3. 
Geographic location of data-collection sites, Lavaca, Matagorda, and Carancahua Bays, Texas, 1992
4. 
Summary of concentrations of total mercury and total organic carbon and grain-size distribution, Lavaca, Matagorda, and Carancahua Bays, Texas, 1992
5. 
Summary statistics for total mercury concentrations in the open-water sediment environment, Lavaca and Matagorda Bays, Texas, 1992
6. 
Summary statistics for grain-size distribution in the open-water sediment environment in percent, by weight, of total sample with grain-size diameter less than 74 micrometers, Lavaca and Matagorda Bays, Texas, 1992
7. 
Summary statistics for total organic carbon concentrations in the open-water sediment environment, Lavaca and Matagorda Bays, Texas, 1992
8. 
Rank correlation coefficients between total mercury concentrations and grain-size fractions and total organic carbon concentrations in the open-water sediment environment, Lavaca and Matagorda Bays, Texas, 1992

Abstract

A preliminary assessment of the distribution and variability of total mercury concentrations in five sediment environments—open water, ship channel, dredged spoil, oyster reef, and salt marsh—of the Lavaca-Matagorda Bays estuarine system along the central Texas Gulf Coast shows that the largest total mercury concentrations in the bays are in the 10- to 20-centimeter sample-depth zone in 2 of the 3 sample areas (1 open water and 1 salt marsh) closest to Point Comfort. The concentrations range from 137 to 1,270 micrograms per kilogram in the open-water environment and 73.8 to 1,900 micrograms per kilogram in the salt-marsh environment. In the surface-sample-depth zones among all sediment environments, total mercury concentrations typically are largest in the open-water environment and smallest in the dredged-spoil and salt-marsh environments.

Open-water sample areas 1–01, 1–02 (middle Lavaca Bay), and 1–06 (upper Matagorda Bay) have median total mercury concentrations in all three sample-depth zones (0 to 2, 10 to 20, and 20 to 50 centimeters) greater than detection limits. Median concentrations for the different depth zones in the three sample areas range from 30.5 to 705 micrograms per kilogram.

Statistical tests indicate that in all three sample-depth zones in open-water sediments, median total mercury concentrations in some sample areas are significantly different from median total mercury concentrations in other sample areas. Another statistical test indicates that the variance in concentrations of open-water samples collected within 10 meters of each other is the same as the variance in concentrations of samples collected randomly within each 1 square kilometer. However, the degree to which a probable lack of independence among the closely spaced data affects the test result is not known.

Rank correlation coefficients between total mercury concentration and grain-size fractions (percentages of sand, silt, clay, and silt plus clay) and between total mercury concentration and total organic carbon concentration for open-water sample areas indicate that total mercury concentration has a significant positive correlation with clay percentage in 4 of the 8 open-water sample areas. In 6 of the 8 open-water sample areas, total mercury concentration has a significant positive correlation with silt-plus-clay percentage and total organic carbon concentration.

The use of a technique known as kriging to estimate total mercury concentrations at unmeasured sites on the basis of sampling sites where mercury concentrations are measured in open-water sediments in the 0- to 2-centimeter sample-depth zone was explored. Kriging to estimate concentrations in the areas between clusters of sample data points is not a practical solution for obtaining a distribution of concentrations in the bays.

U.S. Environmental Protection Agency Method 7471 (Cold Vapor Atomic Absorption) was an acceptable analytical method for determining the total mercury concentrations in the Lavaca-Matagorda Bays estuarine sediment samples. Measurement of additional trace metals would aid in the characterization of total mercury concentrations and in the identification of concentrator/collector relations that are principally responsible for the adsorption of mercurous compounds to particulates in the bottom sediments.

 

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