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.
Additional Publication Details
USGS Numbered Series
Mercury concentrations in estuarine sediments, Lavaca and Matagorda bays, Texas, 1992
Water-Resources Investigations Report
U.S. Dept. of the Interior, U.S. Geological Survey ;
Branch of Information Services [distributor],
5 maps ;50 x 55 cm. or smaller, on sheets 68 x 64 cm. or smaller, folded in envelope 32 x 24 cm. +text (74 p. : tables, charts ; 28 cm.) [PGS - 74 p.]