Trace elements and organic compounds in bed sediment from selected streams in southern Louisiana, 1998

Bed-sediment samples from 21 selected streams in southern Louisiana were collected and analyzed for the presence of trace elements and organic compounds during 1998 as part of the U.S. Geological Survey National Water-Quality Assessment Program. Concentrations of selected trace elements and organic compounds were compared on the basis of sediment-quality criteria, land use, and grain size; concentrations of selected trace elements also were compared with concentrations from previous studies. Concentrations of seven selected trace elements and 21 organic compounds were evaluated with sediment-quality criteria established by the Canadian Council of Ministers of the Environment. Concentrations of selected trace elements and organic compounds were highest at sites draining urban and agricultural areas and may result from cumulative effects of relatively high percentages of fine-grained material, iron, and organic material. Concentrations exceeding sediment-quality criteria for the protection of aquatic life occurred most frequently at Bayou Grosse Tete at Rosedale and Bayou Lafourche below weir at Thibodaux. Exceedance of Interim Sediment Quality Guidelines occurred most frequently for arsenic and chromium. Trace-element concentrations in fine-grained samples were compared with concentrations in bulk samples and were determined to be significantly different, and concentrations were generally higher in finegrained sediment. Shapiro-Wilk, paired t-test, and Wilcoxon rank sum statistical procedures, with an alpha of 0.05, were used to compare concentrations of 21 trace elements, total organic carbon, and total carbon in finegrained and bulk sediment samples for 19 sites. Significant differences were determined between fine-grained and bulk sediment samples for aluminum, barium, beryllium, chromium, copper, iron, lithium, nickel, phosphorus, selenium, titanium, and zinc concentrations. Of 133 paired concentrations, 69 percent were greater in fine-grained samples, and 23 percent were greater in bulk samples. Comparisons with data from previous studies indicate increases by more than 20 percent in concentrations of antimony at Bayou Lafourche below weir at Thibodaux, arsenic and chromium at Tickfaw River at Liverpool, lead at Bayou Lafourche below weir at Thibodaux, and zinc at Bayou Lafourche below weir at Thibodaux and Vermilion River at Perry. Historic comparisons also indicate decreases by more than 20 percent in concentrations of chromium at Bayou des Cannes near Eunice and mercury at Mermentau River at Mermentau.