Summary-- Trace Elements in Bed Sediment from 1991 NAWQA Study Units

Occurrence and Distribution Assessment

Information from 1991 Study Unit summary reports indicates that urbanization was the primary cause of elevated trace element concentrations in bed sediment in at least 12 of the 1991 Study Units . Trace elements reported as being associated with urbanization were (number of Study Units in parentheses): Pb (11); Zn (9); Hg (6); Cd (4); Cr (4); Cu (3); Ni (2); As (1). In addition, the WILL Study Unit reported elevated Ag concentrations in urban basins.

The nine trace elements of interest have known industrial uses, and they can be found in municipal sewage sludge (Rice, 1999). Thus, point sources can be important contributors of trace elements to bed sediment in specific basins (ALBE and POTO). Combustion of fossil fuels and solid wastes can elevate airborne trace element concentrations and can contribute to nonpoint source loads in urban basins (ACFB) or to elevated background concentrations in undisturbed watersheds (GAFL). It follows that large basins with mixed land use are likely to have high trace element concentrations in bed sediment if they include important urban areas, point sources, or nearby atmospheric sources (ACFB, CONN, HDSN, WMIC).

A number of Study Units reported quantitative relationships between trace element concentrations in bed sediment and some measure of urbanization:

• Cd, Pb, Hg, and Zn concentrations increased in direct proportion to the percentage of industrial and transportation land use in the ACFB Study Unit .
• Pb, Hg, and Zn concentrations increased in direct proportion to the percentage of urban land in the HDSN Study Unit.
• Cd, Pb, Ag, and Zn concentrations were significantly higher in urban basins than in forested, agricultural, mixed land use, or mined basins in the WILL Study Unit.

Rice (1999) provided the first integrated evaluation of the nine trace elements of interest in bed sediment from 1991 Study Units. A total of 541 samples were included in her analysis.

• Normalization to Al, Fe, Ti, or organic carbon provided no interpretive advantage for the entire 1991 NAWQA data set. This apparently resulted because the process of sieving to <63 microns yielded samples of relatively homogeneous composition with regard to silts and clays.
• The sum of Cu, Hg, Pb, and Zn concentrations correlated positively with population density.
• Cd, Cr, Cu, Pb, Hg, Ni, and Zn concentrations from urban indicator basins were significantly greater than those from nonurban (agricultural plus reference) basins. Only As and Se were similar for the two basin types.

Several Study Units compared trace element concentrations in bed sediment to guidelines for protection of aquatic life; however, the approach was inconsistent because different States use different guidelines, and no guidance had been provided with regard to a preferred set of guidelines. The Canadian government recently has published updated sediment quality guidelines (SQGs) for bulk sediment samples (Canadian Council of Ministers of the Environment, 1999 ¹). These guidelines provide a consistent and current reference for evaluating effects on aquatic life. Seven of the nine trace elements of interest (As, Cd, Cr, Cu, Pb, Hg, and Zn) have published SQGs. For the 1991 NAWQA data set, one or more of the 7 trace elements with Canadian SQGs exceeded the Probable Effects Level at 45 of 91 urban indicator sites (49%) and at 176 of 541 sites overall (33%) (Rice, 1999).

Although urbanization was the primary cause of elevated trace element concentrations in bed sediment of 1991 Study Units, natural geologic sources and sources related to mining and agriculture also were identified in some Study Units.

• Natural geologic sources were found to contribute elevated concentrations of trace elements in the LSUS, NVBR, RIOG, USNK, WILL, and WMIC Study Units. All trace elements of interest were represented among the six Study Units.
• Coal (LSUS, POTO), complex sulfide ore (NVBR, RIOG, SPLT), cinnabar (NVBR, WILL), and Pb and Zn sulfides in sandstone, shale, or carbonate rock (OZRK) were mined. As, Hg, and Zn were most often identified as elevated downstream from mined areas.
• Seiler and others (1999) have described areas of the Western United States where a combination of geologic sources and high evaporation rates are likely to yield elevated Se concentrations in water and biota, especially under the stress of irrigation. Elevated concentrations of Se in bed sediment from these areas would not be unexpected. Three 1991 Study Units (RIOG, SANJ, and SPLT), two 1994 Study Units (SCTX and UCOL), and four 1997 Study Units (CHEY, HIPL, SANA, and YELL) fall within the areas identified.

Reservoir Coring Studies

A NAWQA study to investigate the utility of lake and reservoir sediment cores for evaluating historical trends in trace elements has been underway since 1992 (P.C. VanMetre and E. Calendar, written commun., April 2, 1997). At least three 1991 Study Units participated in this study. Results for two Study Units corroborate the association of urbanization and elevated trace element concentrations in bed sediment.

• In the ACFB Study Unit, Cr, Cu, Pb, and Zn concentrations in reservoir cores increased from the mid-1940s to the mid-1970s in parallel with growth of upstream metropolitan Atlanta. Concentrations generally decreased after enactment of clean air and clean water legislation in the mid-1970s.
• In the TRIN Study Unit, Pb concentrations in a reservoir core increased with urbanization in the Dallas metropolitan area from the mid-1950s to the mid-1970s; concentrations then decreased as leaded gasoline was phased out (VanMetre and Callender, 1997). Large increases in As and Zn concentrations and small increases in Cr, Cu, and Ni concentrations occurred from the mid-1950s to the time of core collection in 1994.
• In the RIOG Study Unit, increasing concentrations of As, Cr, Cu, Pb, Hg, Ni, V, and Zn from 1969 to the mid-1990s in a core from a reservoir draining largely undeveloped land seem to be associated with fossil fuel combustion and solid waste incineration (VanMetre and others, 1997).

References

Canadian Council of Ministers of the Environment, 1999, Canadian sediment quality guidelines for the protection of aquatic life--Summary tables: In Canadian environmental quality guidelines, 1999, Canadian Council of Ministers of the Environment, Winnipeg (http://www.ec.gc.ca/ceqg-rcqe/sediment.htm).

Rice, K.C., 1999, Trace-element concentrations in streambed sediment across the conterminous United States: Environmental Science & Technology, v. 33, no. 15, p. 2499-2504.

Seiler, R.L., Skorupa, J.P., and Peltz, L.A., 1999, Areas susceptible to irrigation-induced selenium contamination of water and biota in the Western United States: U.S. Geological Survey Circular 1180, 36 p.

VanMetre, P.C., and Callender, E., 1997, Water-quality trends in White Rock Creek Basin from 1912-1994 identified using sediment cores from White Rock Lake reservoir, Dallas, Texas: Journal of Paleolimnology, v. 17, p. 239-249.

VanMetre, P.C., Mahler, B.J., and Callender, E., 1997, Water-quality trends in the Rio Grande/Rio Bravo Basin using sediment cores from reservoirs: U.S. Geological Survey Fact Sheet FS-221-96, 8 p.