Environ. Sci. Technol. 1994, 28, 1987-1991
RESEARCH COMMUNICATIONS Clean Technique Measurement of Pb, Ag, and Cd in Freshwater: A Redefinition of Metal Pollution Gaboury Benoit Yale School of Forestry and Environmental Studies, 370 Prospect Street, New Haven, Connecticutt 06511
Introduction Ultraclean sampling, handling, and analysis techniques necessary to obtain reliable trace metal data for aquatic environments. Failure to follow appropriate clean protocols calls into question much of the previous research on metal cycling in freshwaters (1-4), just as virtually all marine trace metal data from before about 1975 are now considered invalid. Reported here are data for Pb and Cd in freshwaters measured using clean techniques and some of the first such data for Ag. Samples were collected from the length of the Quinnipiac River, Connecticut, and its tributary streams. At all locations, Pb, Ag, and Cd were below the detection limits of routine monitoring measurements by governmental agencies. In spite of these lower than expected levels, the metals are almost 2 orders of magnitude higher in the industrialized portions of the river than in the undeveloped headwater streams in this watershed or elsewhere in New England, and clear reproducible trends are evident. Only through the use of clean techniques and sample preconcentration is it possible to detect the enormous difference in metal concentration levels between clean and contaminated portions of the river. Likewise, only if these methods are strictly applied will it be possible to monitor trends in toxic trace metals over time. This study and other recent work suggest that we may need to redefine the level at which a river is are
considered “polluted” with heavy metals.
Methods The 188-km2 watershed of the Quinnipiac River is highly industrialized and contains the municipalities of Southington, Meriden, Wallingford, and North Haven and their sewage treatment plants (Figure 1). This site is of special interest because it is highly contaminated. Therefore, metal levels even lower than those reported here are likely to prevail at most other sites. Two sets of samples were collected under high and low flow conditions in March and May 1993, when discharge was 4.4 and 0.5 times the mean annual value, respectively. All samples were collected upstream from the zone of influence of tidal salt water.
Filtered and unfiltered surface water samples were measured for Pb, Ag, and Cd. Clean techniques (5) were a crucial part of this study and were based on the experience of oceanographers (6-9) and others working on pristine samples (like arctic ice) from remote locations (10, 11). These same methods have been applied only sporadically 0013-936X/94/0928-1987$04.60/0
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1994 American Chemical Society
Figure 1. Quinnipiac River, Connecticut, and its tributaries. Letters A-F Indicate the locations of Southington (A) and Its sewage treatment plant (B), Meriden (C) and Its plant (D), and Wallingford (E) and Its plant (F). There are also several active industrial facilities in the area from E to F.
to freshwaters (1-3,12-20), mainly the Great Lakes and the Mississippi River. These investigations uniformly reveal trace metal levels that are much lower than those measured using routine methods. Clean procedures used in this study depend on three guiding principles: (1) samples contact only surfaces consisting of materials that are intrinsically low in trace metals (Teflon or low-density polyethylene) and that have been extensively acid-cleaned in a filtered air environment, (2) samples are collected and transported taking extraordinary care to avoid contamination from field personnel or their gear, and (3) all other sample handling steps take place in a filtered air environment and using ultrapure reagents. In the field, extraordinary precautions were taken to prevent contamination of water column trace metal Environ. Sci. Technol., Vol. 28, No. 11, 1994
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samples. Samples were collected downwind and downcurrent by personnel wearing clean-room polyethylene gloves, which were changed frequently during the collection of each sample to avoid contamination (2). Water was collected by peristaltic pumping through acid-cleaned Teflon tubing into acid-cleaned low-density polyethylene bottles. Acid cleaning consists of 48-h soaking in 33% HC1 at 60 °C, followed by soaking in hot, ultrapure 1% HN03 (Seastar brand) for 48 h, and rinsing with >18 water (tested for trace metals). Acid soaking took place in a HEPA filter-supplied, all polypropylene, Class-100 clean fume hood in a positive-pressure, Class-100 clean
Baths were tested weekly for contamination. Sample bottles were stored and transported to the field filled with 0.03 M ultrapure HNOg as a final wash/storage solution, and contained in double plastic bags. Water was filtered during collection by passage through acid-cleaned 0.45-µ Millipore Durapore filters. Filter membranes were contained in acid-cleaned 47 mm diameter Teflon filter holders that were loaded within a laminar flow clean bench and stored in double plastic bags. Separate filter assemblies were prepared in advance for each sample, since changing filters cleanly in the field is impossible. Using this protocol, our filter blanks were consistently below our detection limit (see below). For some samples, in addition to the filter-retained and filtrate fractions, a total metal sample was collected by room.
pumping directly into a bottle without filtration. This allowed a mass balance to be performed to monitor quality control (i.e., total = filtrate + filter retained) and to test for losses due to sorption to filters. Trace metal water samples were returned to the laboratory in the original double bags, stored on ice. Samples were acidified in a clean bench within a few hours of collection using 2 mL of ultrapure HNO3/L of sample. Filters were unloaded from their sample holders in a clean bench and transferred to acid-cleaned Teflon beakers for digestion. Water samples were preconcentrated by evaporation with nitric acid in Teflon beakers within a filtered-air clean fume hood (Class-100) constructed entirely of polypropylene. Filters were heated with 20 % ultrapure HNO3 in Teflon beakers for 1 h before evaporation. Metals were measured by background-corrected graphite furnace atomic absorption spectroscopy. Significant matrix intereferences were judged to be absent because selected samples gave similar results whether measured by the method of additions or by the external standard series. Blanks and spikes were carried through all stages of sample collection, pretreatment, and analysis to evaluate recovery, accuracy, and potential contamination. Field blanks consisted of preanalyzed distilled water that were “collected” in the field by the standard protocol. Recovery blanks consisted of standard reference river water SLRS-2 (National Research Council of Canada) collected in the normal way except for the addition of a known spike. During GFAAS analyses, samples were measured in triplicate and repeated if relative standard deviation exceeded 15%. Midrange control standards were analyzed every six samples, and the instrument was recalibrated if they were not measured within ±10% of the nominal value. A blank and an aliquot of SLRS-2 were analyzed with every batch of 10 samples. Blanks always fell within the instrumental standard deviation, and we routinely measured SLRS-2 within its 95% confidence limits (Pb = 129 ± 11 ppt, Cd = 28 ± 4 ppt). Temperature, conductivity, pH, alkalinity,turbidity, 1988
Environ. Sel. Technol., Vol. 28, No. 11, 1994
Figure 2. Lead in the Quinnipiac River, March 26 and May 25, 1993. Note that total (open symbols) and dissolved (filled symbols) Pb are expressed on different scales for clarity. Dissolved metal is operationally defined as that which passes a 0.45-µ pore-size filter and, therefore, includes both colloidal and truly dissolved fractions. Letters along the x-axls match those in Figure 1.
suspended particulate matter (SPM), and total organic carbon (TOC) were measured at the same time by standard methods (21). Detection limits were calculated as the product of three times the standard deviation of the blank and the slope of the calibration curve. They varied depending on the original volume taken for evaporation and the stability of the graphite furnace when analuzed. Using 100-mL original sample volumes, detection limits near 1 ppt were possible for the three metals. This was necessary only for the cleanest headwater samples. Evidence for the reliability of analyses conducted by these methods includes: (1) the average relative standard deviation of all duplicate water samples is 10%, = 19, (2) recovery is better than 95 % based on measurements of a standard reference river water (SLRS-2, National Research Council of Canada), (3) the sum of individually measured filtrate and filterretained fractions match analyses of whole water samples (Pb = 98%, Ag = 101 %), and (4) the results follow smooth trends when plotted as a function of distance downstream, i.e., they are geochemically coherent (22).
Results and Discussion
Nearly all metal measurements in the current study (Figures 2-4) were in the parts per trillion range, compared to ppb levels in previous U.S. Geological Survey (USGS) data for the same sites. For example, USGS data for dissolved Pb and Cd in the Quinnipiac River near Wallingford during the period 1981-1991 are illustrated in Figure 5. (Ag was not measured by the USGS.) The plotted results are annual averages of monthly measurements. In certain years, some of the data were below the detection limit (1 ppb for both metals). Averages for this left-censored data were calculated by the method of Schneider (23) and confirmed by the iterative technique of Gleit (24). In contrast, clean technique measurements of dissolved metals in the main stem of the Quinnipiac
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YEAR Figure 5. Dissolved Pb and Cd as measured by the USGS between 1981 and 1991 and in this study in 1993. For the USGS data, each point represents an average of 12 monthly measurements from a site near Wallingford. Several of the years' data included some nondetects (200, and 90, respectively. The smaller range for Pb, compared to Ag and Cd, may be due to its many diffuse sources. Lead has been dispersed throughout the environment by burning of gasoline that contained lead antiknock compounds. Use of leaded gas has declined considerably in recent years (25), but lead incorporated in watershed soils in the past is now being released gradually to streams via erosion or other remobilization processes. Thus lead comes from nonpoint sources that can contaminate water at nearly any location within the watershed. When Pb in the main Quinnipiac River is compared to a stream in a remote part of New England (Bear Brook, NH, unpublished data) or the Sierra Nevada Mountains in California (26, 27), the ratio rises from 14 to greater than 90. On both sampling dates, lead had its greatest proportional increase between the first and second stations (Figure 2). The first station (River Road) represents the upper, relatively undeveloped portions of the river, while the second location (Old Turnpike Road) is downstream of nonpoint urban runoff from the city of Southington. Similarly, the upper reaches of three Quinnipiac tributaries (Roaring Brook, Tenmile Brook, and Spring Brook) all had much lower lead levels than the main river itself. This is true even though the headwaters are contaminated by regional atmospheric inputs. Farther downstream, dissolved lead exhibited rather small variations. Increases occurred at locations corresponding to sewage treatment plants and sources of urban runoff (i.e., B-F). The pattern of changing Pb concentration with distance was not exactly the same on the two dates (e.g., the maxima in total Pb were displaced farther upstream in May than in March). This may be attributable to different relative contributions by point (e.g., sewage) and nonpoint (e.g., urban run-off) sources under the changing discharge conditions represented by these two dates. Below each major source, Pb declined approximately exponentially with distance, reflecting dilution and sedimentation losses. Total lead and SPM were both about a factor of 2 greater in March, when the flow was 10 times higher than in May. Silver concentrations were less than those of lead. This reflects the lower utilization of the former metal in Environ. Sel. Technol., Vol. 28, No. 11, 1994
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industrial processes and the lack of ubiquitous nonpoint sources (Figure 3). Like lead, most silver was associated with particles. Silver showed a much larger proportional increase between tributary and river sampling locations (River Road and Hall Street) on both sampling dates and in both dissolved and particulate fractions. This is consistent with mainly point sources for this metal. Unlike lead, silver did not contaminate tributaries through the route of atmospheric deposition. Silver in the Quinnipiac River can be divided into three zones according to concentration and presumed source: (a) headwater streams, where total silver is
