Environ. Sci. Techno/. 1995, 29, 1313-1317
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Comparison Study: Implications for Water balm Monitoring- of Dissolved Trace -Elements HOWARD E . T A Y L O R * US.Geological Survey, 3215 Marine Street, Boulder, Colorado 80303 A L A N M. S H I L L E R t Center for Marine Sciences, University of Southern Mississippi, Stennis Space Center, Mississippi 39528
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Recent reports have questioned the validity of dissolved trace element concentrations reported by the U.S. Geological Survey's National Stream Quality Accounting Network (NASQAN) as well as by other water-quality monitoring programs. To address these concerns and to evaluate the NASQAN protocols, the US. Geological Survey undertook the Mississippi River Methods Comparison Study. We report here the major results and implications of this study. In particular, we confirm the possible inaccuracy of previous NASQAN dissolved trace element data. The results suggest that all steps of the NASQAN protocol (sampling, processing, and analysis) require revision, though the sample filtration step appears to be of pa rtic uIar concern.
Measurements of dissolved trace elements in rivers are of substantial interest to researchers examining basic scientific questions related to geochemical weathering and transport and to scientists involved in pollution control evaluation and monitoring of water quality, including studies of contamination at low-concentration levels of many dissolved trace elements, such as lead, cadmium, and copper, for human health and biotoxicity purposes. Two different approaches are commonly used: researchers using stateof-the-art techniques who report their findings in the reviewed scientific literature and personnel of agencies responsible for water-quality monitoring who follow standardized protocols and tabulate their results in agency data bases and reports. A major water-quality monitoring network in the United States is the National Stream Quality Accounting Network (NASQAN),designed, managed, and implemented by the U.S. Geological Survey (1,2). Determination of dissolved trace elements has been a major component of NASQAN since its inception. Starting in 1972 with 50 stations at outlets of major drainage basins, the network has been expanded to encompass quarterly surface water collections at about 300 river and stream sites. The primary purpose of collecting data from a fixed-station network such as NASQAN is to permit the interpretive study of long-term trends of water-quality constituents and to establish their dependence on the variation of physical and chemical parameters (2, 3). The pioneering work of Patterson and Settle ( 4 ) demonstrated that establishing the reliability of results is of major importance in any study of trace elements. Recently, concerns have been expressed regarding the accuracy of NASQAN dissolved trace element data (5-7) particularly that contamination during sampling and analysis may invalidate NASQAN results. These concerns are not unique to the NASQAN program: for example, Lum et al. (8) speculate on the potential of contamination contributing to high dissolved lead, cadmium,and copper concentrations reported for the Rhone (9, 101 and Rhine Rivers (11). To address these concerns and as part of a continuing effort to improve data quality, the U.S. Geological Survey conducted a comparison experiment in 1990 to define potential problems in collecting, processing, and analyzing surface water samples for dissolved trace elements. This experiment was performed at 10 stations on the lower Mississippi River and its tributaries. In this paper, we describe the results and implications of this intercalibration study. Furthermore, we discuss the problems and approaches associated with obtaining reliable dissolved trace element data and describe considerations that need to be taken into account in designing a monitoring program.
Study Area and Methods The Mississippi River Methods Comparison Study (MRMCS) was devised to identlfy possible contamination in fluvial dissolved (nonparticulate) trace element sampling, processing, and analysis. The Mississippi River and its ~
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This article not subject to U.S. Copyri ht Published by the American Chemical Eociety.
VOL. 29, NO. 5, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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tributaries were chosen because of the river’s importance to the United States (draining 66% of the nation’s geographical area) and the several previous studies of its dissolved trace element content (5, 12-14). Additionally, by studying a river system, the results can be evaluated for internal consistency within a geochemical context (5,15). The high suspended solids load and the alkaline pH of water in the Mississippi River make for a rigorous test of sample processing artifacts (16). Specifically, this geochemical situation results in many trace elements occurring predominantly in the suspended phase (13). For example, the Mississippi River carries 1000 times more suspended iron than dissolved iron (5, 13). Thus, the slight passage of particles smaller than the pore diameter of the filter into the filtrate or particle contamination from poor handling techniques can result in a significant increase in the measured dissolved iron concentration, especially when the filtrate is acidified for sample preservation. Because of the importance of fdtration in obtaining reliable results, some discussion of fdters and filtration is necessary in order to understand the methods and results of MRMCS. Two types of fdters are in common use: depthtype tortuous path filters in which particles are trapped within a matrix of fibers and screen-type polycarbonate membranes in which particles are trapped on the filter surface. Tortuous path filters have commonly been used in monitoring programs because of their high particle load capacities and ease of use. However, the lack of rigidly defined pore size and the potential for contamination from the high surface area of the fibers have resulted in many researchers usingpolycarbonate screen filters. The screentype filters have a well-defined pore size, and acid-rinsed polycarbonate membranes are generallyregarded as having a low potential for trace element contamination. For both types of fdters, the pore size can decrease during filtration as small particles clog the filter; screen-type filters, due to their trapping of particles only on the filter surface, clog more rapidly than tortuous path filters. In addition to filter type, there is another important factor to consider. Whereas some workers try to filter only small amounts of water or use large area filters in order to minimize clogging, other workers deliberatelypreload their fdters, resulting in a lower effective pore size. This preloading can occur during the rinsing of the filter and fdtration apparatus with part of the sample. Indeed, many workers consider the rinsing of filter and apparatus with sample as a normal part of trace element “contamination consciousness” and may not note the prerinsing in their published methods. We have used the term “exhaustive filtration” to denote the technique in which the filter is preloaded with sample particles, the initial filtrate being discarded. The sampling, processing, and analytical methods used in the MRMCS have been described in detail elsewhere ( 1 7), and we outline only the most pertinent facts here. The field work was carried out in June 1990 and involved three separate groups of scientists. The sampling sites were as follows: Illinois River at Valley City, IL; Mississippi River below Grafton,IL; MississippiRiver at Thebes, 11; Ohio River at Olmstead, IL; Mississippi River below Memphis, TN; Mississippi River below Arkansas City, AK; Yazoo River below Steele Bayou, MS; MississippiRiver belowvicksburg, MS; Mississippi River near St. Francisville, LA; and Mississippi River below Belle Chasse, LA. Additional grab samples were collected on the Mississippi River above the 1314 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 5,1995
confluence with the Illinois and on the Missouri River at St. Charles, MO. Scientists from U S . Geological Survey collected and processed samples in accordance with the routine NASQAN protocols (18). Samples were collected by standard discharge-weighted depth and width integration techniques (19) using a variety of sampler types, composited in a polyethylene chum splitter, subsampied, and filtered through a 0.45-pm pore size, 142-mm diameter, tortuous pdth membrane filter prior to preservationwith high-purity HN03. Sampleswere then sent to the U.S. Geological Survey Branch of Analytical Services laboratory where dissolved trace elements were determined by inductively coupled plasmalatomic emission spectrometry (20) and atomic absorption spectrometry (18). Research scientists from the U.S. Geological Survey National Research Program (NRF’) collected samples using a discharge-weighteddepth and width integrated sampling technique. This technique employed a collapsible-bag sampler (21, 22) modified to minimize trace element contamination (23) (i.e., extensive use of Teflon and other nonmetallic materials). Composited samples were exhaustively filtered with an enclosed, all-Teflon apparatus using a 0.4-pm pore size, 47-mm diameter, Nuclepore polycarbonate membrane filter. The initial 50 mL of filtrate collected was discarded after thoroughly rinsing the receiving bottle. After filtration, samples were preserved at pH < 2 by the addition of ultra-high-purity “OB. In the laboratory, all samples were analyzed without further pretreatment using an inductively coupled plasmalmass spectrometric technique specifically optimized for the determination of trace elements in natural waters (24). University of Southern Mississippi (USM) researchers collected surface water samples at the centroid of flow of the river using acid-cleaned linear polyethylene bottles attached to a Plexiglass holder fixed to the end of a long nonmetallic pole. Replicate samples were exhaustively filtered using 0.4-pm pore size, 47-mm diameter, Nuclepore polycarbonate membranes held in acid-cleanedTeflon filter holders. The initial 60 mL of filtrate was discarded. The filtration was carried out in a HEPA-filtered laminar flow bench, and samples were preserved by acidification to pH
