Comparison of temporal trends in ambient and compliance trace element and PCB data in Pool 2 of the Mississippi River, USA, 1985-1995

The Intergovernmental Task Force on Monitoring has suggested studies on amb ient tin-stream) and compliance (wastewater) data to determine if monitorin g can be reduced locally or nationally The similarity in temporal trends be tween retrospective ambient and compliance water-quality data collected fro m Pool 2 of the Mississippi River, USA, was determined for 1985-1995. Const ituents studied included the following trace elements: arsenic (As), cadmiu m (Cd), chromium (Cr), hexavalent chromium (Cr6+), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), selenium (Se), zinc (Zn), and polychlorinated bi phenyls (PCBs). Water-column, bed-sediment, and fish-tissue (fillets) data collected by five government agencies comprised the ambient data set; efflu ent data from five registered facilities comprised the compliance data set. The nonparametric Mann-Kendall trend test indicated that 33% of temporal t rends in all data were statistically significant (P < 0.05). Possible reaso ns for this were low sample sizes, and a high percentage of samples below t he analytical detection limit. Trends in compliance data were more distinct ; most trace elements decreased significantly, probably due to improvements in wastewater treatment. Seven trace elements (Cr, Cd, Cu, Pb, Hg, Ni, and Zn) had statistically significant decreases in wastewater and portions of either or both ambient water and bed sediment. No trends were found in fish tissue. Inconsistency in trends between ambient and compliance data were o ften found for individual constituents. making overall similarity between t he data sets difficult to determine. Logistical differences in monitoring p rograms, such as varying field and laboratory methods among agencies, made it difficult to assess ambient temporal trends.