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A study examining the occurrence and distribution of trace elements and organic compounds in southern Louisiana fish and streambeds

Study Description

Bed-sediment and fish tissue studies are the primary means by which trace elements and hydrophobic organic contaminants are initially assessed by NAWQA. Concentrations in bed sediments and fish and their areal distribution are assessed to identify sources and potential needs for more detailed study. Hydrophobic organic compounds preferentially adhere to bed sediments and fish (and other organisms) rather than occurring dissolved in water. Nevertheless, water is one of the primary means for these compounds to move around in the larger environment. Trace elements (i.e., heavy metals, either naturally occurring or man-made) are also moved by water from source areas through the larger environment. In the summer of 1998, bed sediments and fish tissues were collected from 21 sites around south Louisiana to determine the occurrence and distribution of a variety of organic compounds and trace elements present in freshwater environments.

Sites

Sites selected include our Fixed Sites and additional sites to form a broad picture for a one-time sampling. These sites cover a range of physiographic areas, land uses, and stream sizes and types. The Bed Sediment and Tissue section of the NAWQA design methods ("Design of the National Water-Quality Assessment Program: Occurrence and Distribution of Water-Quality Conditions, Circular 1112") describes how sites are selected, generalized methods for collection and analysis, and objectives in sampling.

Methods and Analysis

Fish were collected by electroshocking along a stretch of each waterbody, as close to or overlapping the water-sampling site. Fish species we collected included largemouth bass (Micropterus salmoides), longear sunfish (Lepomis megalotis), bluegill sunfish (Lepomis macrochirus), white crappie (Pomoxis annularis), warmouth sunfish (Lepomis gulosus), and common carp (Cyprinus carpio). A composite of six to eight fish of a given species were used for analyses to ensure a sufficient amount of material, particularly for the smaller species. Some sites had more than one species collected, and we submitted multiple composited tissue samples for comparison between species.

For all species, we analyzed trace elements (including mercury) in livers and in fillets, separately, as well as organic compounds in whole bodies. Trace elements are primarily stored in liver tissue in organisms, and the analysis of these tissues gives an idea of the total suite of metals that fish are exposed to. Fillets were collected to determine the amounts and kinds of metals that accumulated in edible portions of fish. Organic compounds, particularly hydrophobic ones, accumulate in fatty tissues of organisms, which are distributed across the whole body.

Bed sediments were collected according to methods published in "Guidelines for Collecting and Processing Samples of Stream Bed Sediment for analysis of trace elements and organic contaminants for the National Water-Quality Assessment Program, Open-File Report 94-458". Since trace elements and organic compounds are often moved through stream systems attached to sediment, areas of recent deposition indicate which chemicals are being transported in the present time. Deep cores of sediment can indicate a time series of deposition, and show changes in trace element concentrations over time.

Areas of recent sediment deposition in streams were identified for sources of samples. These areas tend to have backwater conditions or other kinds of slow flow that allow sediment to settle out, usually along streambanks or other shallow areas in Louisiana streams. An interpretive report will soon be available analyzing bed sediment occurrence and distribution. Raw data will also be available in our Annual Report.

• Trace Elements in bed sediments
• Organic Compounds in bed sediments
• Trace Elements in Fish Tissue
• Organic Compounds in Fish Tissue

For questions about our Bed Sediment and Fish Tissue Survey, please contact Dennis Demcheck [ddemchec@usgs.gov], 225-298-5481.

• U.S. EPA Fish Consumption Advisory page
• LDEQ Mercury programs includes fish consumption and swimming advisories, a summary of relevant issues and much more
• news bulletin from LDEQ
• TRI Summary of Mercury in Fish
• information pamphlets from LDHH: Eating Some Louisiana Fish and Mercury in Fish in Louisiana