Environ. Sci. Technol. 2004, 38, 2785-2791
Transport of Suspended-Sediment-Bound Toxaphene in the Mississippi River JONATHAN D. RAFF AND RONALD A. HITES* School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405
Suspended sediment samples from ∼30 locations along the Mississippi River and six of its major tributaries were collected during July and August 2002 and March 2003 to investigate the distribution and transport of toxaphene in the Mississippi River. The concentration of toxaphene was measured, and the load of toxaphene carried by the river at each site was calculated using water discharge and suspended sediment concentrations. Results indicate that toxaphene is widespread throughout the Mississippi River Basin with the highest concentrations and loads observed for samples collected in the Lower Mississippi River. Among the tributaries, the Yazoo River carried the largest load, amounting to ∼21% of the estimated load carried by the Mississippi River just below Vicksburg, MS. The majority of the contamination is most likely from nonpoint source runoff from agricultural lands. We also found evidence of a localized toxaphene source in the vicinity of Memphis, perhaps coming from the Wolf River. From our data we estimate that between 200 and 1000 kg of toxaphene were released into the Gulf of Mexico in 2002 from the main stem of the Mississippi River.
Introduction Toxaphene is a complex mixture of chlorinated bornanes and camphenes that was primarily used as a replacement for DDT to kill pests on cotton. The U.S. Environmental Protection Agency (U.S. EPA) banned the use of toxaphene in 1990 because of concerns that it was a probable human carcinogen and endocrine disruptor (1). Because of its toxicity, persistence, and heavy use, toxaphene is one of 12 chlorinated compounds designated for international action by the United Nations Environmental Program and a priority pollutant in the United States under the Clean Water Act. Over 105 t of toxaphene was used in the United States between 1960 and 1990; worldwide usage between 1950 and 1993 was ∼1.2 × 106 t (2). In the United States, over 85% of all toxaphene was applied in the cotton-growing regions of the South, while only a few percent was used in the upper Midwest (3). Most monitoring efforts in North America have focused on the Great Lakes Basin, where toxaphene has been found in unusually high amounts in water, sediment, and biota (4-6). These studies have shown that atmospheric transport and deposition are responsible for the occurrence of toxaphene in these regions, which are far removed from toxaphene’s sources. Fewer studies have focused on the occurrence and transport of toxaphene by rivers. In fact, toxaphene is one of the least frequently detected orga* Corresponding author e-mail
[email protected]. 10.1021/es0351995 CCC: $27.50 Published on Web 04/20/2004
2004 American Chemical Society
nochlorine pesticides in rivers despite its widespread use (7). This is in contrast to DDT, for which the frequency of detection parallels its high use. It seems likely, however, that toxaphene is transported into rivers by erosion and transported by the Mississippi River to the Gulf of Mexico. Toxaphene is present in biota and surface water throughout the Mississippi River Basin. Schmitt studied toxaphene in fish collected from 47 sites along the Mississippi River and found the highest levels of toxaphene in fish from 5 sites located in the lower Mississippi River and Mississippi Embayment (8). Other studies have found toxaphene in biota (9-12), bed sediment (13), and water (14-15) from smaller watersheds within the Mississippi River Basin. An older study of whole-water samples collected from streams across the United States detected toxaphene most frequently in the streams of Arkansas, Louisiana, and Mississippi (16). Reports in the past, however, have probably underestimated the occurrence of toxaphene because of high detection limits. Advances in instrumentation have lowered toxaphene’s detection limits and made older data obsolete; this makes comparisons to those studies difficult. To our knowledge there are no contemporary studies of dissolved or suspendedsediment-bound toxaphene in rivers. Existing measurements are not easily adapted to quantifying the transport of toxaphene in the Mississippi River Basin. Load calculations, based on river discharge measurements, and contaminant concentrations in suspended sediment are needed to remove the effect of dilution at the point of sampling and to allow a direct comparison of the quantity of toxaphene transported at different sites along the river. Such calculations provide an estimate of the amount of toxaphene transported through the Mississippi River and ultimately to the Gulf of Mexico over a specific period of time. Thus, we collected suspended sediment samples from 25 locations along the Mississippi River and from several of its major tributaries to investigate the distribution and transport of toxaphene in the Mississippi River. The concentration of toxaphene was measured, and the load of toxaphene carried by the river at each site was calculated using measured water discharge and suspended sediment concentrations. By comparing concentrations and loads of toxaphene among the sampling sites, we can identify the sources of toxaphene to the river and estimate the discharge of toxaphene to the Gulf of Mexico.
Experimental Section Sampling. Suspended sediment samples were collected from 25 sites along the Mississippi River and from six of its major tributaries during July 5 to August 23, 2002, and March 1720, 2003. The locations and characteristics of the sampling sites are described in Table 1 and shown in Figure 1. In most cases public access boat ramps were used for sampling. On several occasions, docked ferryboats were also used as sampling platforms. Samples were collected between 3 and 20 m offshore. Suspended sediment samples were collected using a method similar to one recently described by Mahler and Van Meter (17). We drew water through aluminum tubing (1-cm i.d.) from a depth of 30 to 50 cm with a submersible pump (∼0.6 L/min) and passed the water through a 60-mesh stainless steel screen and then through a 293-mm-diameter glass fiber filter (Pall-Gelman A/E, 1-µm pore size) that was supported by a stainless steel filter holder. Between 60 and 300 L of water (depending on the concentration of suspended sediment) was filtered into glass carboys of a known volume. The filters were changed periodically and stored in preVOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Sampling Sites and Dates, River Location (d, in kma), River Discharge (Q, in m3/s), Toxaphene Concentration (Ctox, in ng/g of s.s.), Suspended Sediment Concentration (Css, in mg/L), and Toxaphene Load (Ltox in g/d) no.
sampling site
date
river
d
Q
Ctox
Css
Ltox
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
L. Itasca, MN Palisade, MN Little Falls, MN Minneapolis, MN Lake City, MN Prarie Du Chien, WI Savanna, IL Andalucia, IL Dallas City, IL Hannibal, MO Red Landing, IL Herculaneum, MO Cape Girardeau, MO Caruthersville, MO Ashport, TN Shelby Forest, TN Memphis, TN Memphis, TN Norfolk, MS Peters, AR Friars Point, MS Arkansas City, AR Le Tourneau, MS Natchez, MS St. Francisville, LA Baton Rouge, LA Boscobel, WI Kampsville, IL Hermann, MO Old Shawneetown, IL Pendelton, AR Yazoo City, MS
08/05/02 08/06/02 08/04/02 08/07/02 08/08/02 08/09/02 08/10/02 08/11/02 07/05/02 07/05/02 07/06/02 07/07/02 08/17/02 03/17/03 03/18/03 03/18/03 03/19/03 08/24/02 03/19/03 03/20/03 03/20/03 08/20/02 08/22/02 08/22/02 08/23/02 08/23/02 08/10/02 07/06/02 07/07/02 08/16/02 08/18/02 08/21/02
Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Mississippi Wisconsin Illinois Missouri Ohio Arkansas Yazoo
3780 3328 3137 2949 2827 2602 2445 2334 2210 2077 1988 1824 1665 1362 1282 1210 1188 1188 1144 1114 1049 911 686 586 428 370 2595 1930 1893 1580 935 703
n.a. 99 188 559 895 1511 1628 1829 3497 3514 3770 5663 3823 23 992 16 098 16 514 16 594 6966 16 287 15 863 15 348 7646 7957 7929 6201 6201 141 549 1356 1914 150 283
1.0 0.7 1.2 1.8 1.7 0.4 1.2 0.5 0.8 2.3 2.4 3.4 3.9 2.5 3.5 1.8 14 5.8 4.1 3.0 3.1 5.9 15 7.6 6.3 9.5 0.5 3.2 4.9 2.7 39 14
3.1 46.5 6.0 27.3 18.3 19.8 68.7 163 84.7 95.1 107 70.1 78.2 93.4 111 95.3 85.2 447 122 90.2 136 54.9 61.9 76.7 107 34.5 31.5 135 44.1 34.9 12.5 396
n.a. 0.3 0.1 2 2 1 11 14 21 67 85 118 102 487 533 250 1740 1560 700 376 564 214 647 401 364 175 0.2 21 25 15 6 134
a d is the upstream distance of each sampling site from Head of Passes; for tributaries, the reference location is the point of convergence with the Mississippi River.
cleaned glass jars with Teflon-lined caps. An additional 1 L of water was collected in a clean 1-L glass bottle to determine the suspended sediment concentration at each site. This sample was vacuum-filtered on site using a preweighed 67mm-diameter glass fiber filter (Pall-Gelman A/E, 1-µm pore size). Samples were protected from light and stored on ice until they were returned to the laboratory, where they were stored at -30 °C until analysis. Extraction and Cleanup. Each set of filters was thawed and placed on a Bu ¨ chner funnel to which a modest vacuum was applied to remove excess water. The filters were shredded, mixed with anhydrous Na2SO4, and transferred to Soxhlet extractors, where they were spiked with a known mass of the recovery standard, 13C10-γ-chlordane (Cambridge Isotope Laboratories, Andover, MA). Each sample (1-30 g of sediment) was extracted with 1:1 (vol/vol) acetone/hexane for 24 h in the dark, exchanged to hexane, and reduced to a volume of 20 mL via evaporation (RapidVap, Labconco, Kansas City, MO). The remaining aqueous layer was acidified with 0.5 mL of concentrated H2SO4, separated from the organic layer, and extracted with hexane. The combined organic extract was reduced to a volume of 1 mL under a gentle stream of dry N2 and loaded onto a column (1.5 cm i.d. × 30 cm length) containing a 6-cm upper layer of anhydrous Na2SO4, a 6-cm layer of silica (40% deactivated with concentrated H2SO4), a 2-cm layer of anhydrous Na2SO4, and a 1-cm bottom layer of copper beads (to remove elemental sulfur). The column was eluted with 100 mL each of hexane, 40% dichloromethane in hexane, and dichloromethane. The combined fractions were exchanged to hexane, reduced to a volume of 100 µL under a gentle stream of dry N2, transferred to GC autoinjector vials, and spiked with a known amount of the internal standard, 2786
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2,2′,3,4,4′,5,6,6′-octachlorobiphenyl (PCB 204; AccuStandard Inc., New Haven, CT). Analysis and Quantitation. Extracts were analyzed for toxaphene using an Agilent 5973 gas chromatographic mass spectrometer operating in the electron-capture negativeionization, selected-ion-monitoring mode (ECNI-SIM). The M- or (M - Cl)- ions of the hexa- through decachlorinated bornanes and camphenes were monitored using a method developed by Swackhamer et al. (18) and modified slightly for use with our standards. A 2-µL aliquot of each sample was injected into the gas chromatograph in splitless mode and eluted on a 60-m DB-5MS column (250-µm i.d.; 0.25-µm film thickness; J&W Scientific, Folsom, CA) using helium as the carrier gas. The injection port temperature was maintained at 285 °C to ensure complete volatilization of the sample. The temperature of the column was held constant at 80 °C for 1 min, ramped at 10 °C/min to 210 °C, then at 0.8 °C/min to 250 °C, and last at 10 °C/min to 310 °C, where it was kept for 10 min. The total run time was 80.5 min. The GC to MS transfer line was maintained at 280 °C. The temperature of the mass spectrometer ion source was held at 150 °C, and methane was used as the reagent gas at a manifold pressure of 2 × 10-4 Torr. The background-subtracted, selected-ion chromatograms were integrated using an Agilent data analysis program with a macro that generated peak areas and retention times of potential toxaphene peaks. A program developed in our laboratory then selected valid toxaphene peaks on the basis of expected chlorine isotope ratios and corrected for 13C and other interfering compounds (19). Because of the complex nature of the toxaphene mixture, the relative response factor (RRF) is not linear over the entire concentration range. Less abundant toxaphene peaks are not detected as the concen-
FIGURE 1. Map of study area indicating location of sampling sites on the Mississippi River and selected tributaries. Additional information about each site is given in Table 1. tration of toxaphene decreases. This causes the RRF to vary according to a power function with respect to the total toxaphene peak area in the standards. Therefore, the calculated RRFs from each standard were plotted against the total peak area for that standard. A power function was fitted to the data, from which an individual RRF was calculated for each extract on the basis of its total toxaphene peak area. In addition to toxaphene, the samples collected in March 2003 were analyzed for total hexachlorocyclohexanes (ΣHCH), total chlordanes (Σchlordanes), hexachlorobenzene (HCB), heptachlor, dieldrin, aldrin, endrin, p,p′-DDE, endosulfan I and II, endosulfan sulfate, and mirex. The Σchlordanes concentrations are the sum of the concentrations of R- and γ-chlordane, trans-nonachlor, cis-heptachlorepoxide, and oxychlordane. These analytes were analyzed on the same instrument using parameters similar to those described above. However, in this case the temperature of the GC column was held at 40 °C for 1 min, ramped at 30 °C/min to 130 °C, then at 3 °C/min to 241 °C, and at 30 °C/min to 285 °C, where it was held for 20 min. Finally, the temperature was ramped at 30 °C/min to 300 °C, where it remained for 20 min. The total run time was 83 min. The internal standard and recovery standard were PCB-204 and 13C10-γchlordane, respectively. The two most abundant ions in the mass spectra of the pesticides and internal standards were selected as SIM descriptors. Quantification was carried out using an external standard (Dr. Ehrenstorfer, Germany) with known amounts of all the target compounds, internal standard, and
recovery standard. Peaks in the chromatograms were identified and quantified only if the signal-to-noise ratio was greater than three, if the GC retention times matched ((0.05 min) those of the standard compounds, and if the isotopic ratio between the target and confirmation ions was within (15% of the theoretical value. Suspended sediment concentrations were determined in the laboratory as the difference between the preweighed filter and loaded filter after drying in an oven for 3 h at 105 °C. Loss-on-ignition (LOI) at 550 °C for 3 h was used to determine the total combustible carbon content of the suspended sediment samples. No effort was made to account for inorganic carbonates in these samples. Quality Control and Assurance. During sampling, the submersible pump was equilibrated with sample water for 10-20 min prior to the start of filtration. All filters were wrapped in aluminum foil to prevent contamination from ambient dust. A field blank, consisting of a glass fiber filter exposed to ambient air during the sampling interval, was collected every 8 sites; toxaphene was below the detection limit in all of these samples. In the laboratory, all solvents were of spectroscopic grade. All glassware was meticulously cleaned and heated to 450 °C for a minimum of 6 h. Glass fiber filters, glass wool, disposable pipets, and anhydrous Na2SO4 were heated under the same conditions; silica and the copper beads were pre-extracted with dichloromethane. A procedural blank, consisting of pure Na2SO4 and glass wool was extracted with every batch of 7 samples; toxaphene peaks were below the detection limit in these samples. A recovery blank consisting of pure Na2SO4 and glass wool spiked with a technical toxaphene mixture (Hercules Co.) was extracted every 5 samples; the average recovery of toxaphene from these samples was 85 ( 18% (N ) 9). The average recovery of the internal standard from all samples (including procedural blanks) was 91 ( 22% (N ) 55). The toxaphene detection limit for this method was 0.1 ng/extract (5). Our samples are not cross-sectionally or depth integrated, but we assume for the purpose of this study that our measurements of suspended sediment concentrations represent the cross-section of the river at each sampling site. This may be an oversimplification for some sampling sites because differences in river cross-sections and variations in water depth along the riverbank may or may not result in representative samples. However, we did find good agreement between the estimates of sediment load using our suspended sediment concentration measurements and those from five Mississippi River sites monitored by the U.S. Geological Survey’s National Streamwater Quality Network (NASQAN) (20). River Discharge Measurements. Instantaneous water discharge measurements at each sampling site were derived from either U.S. Geological Survey (USGS) or U.S. Army Corps of Engineers (USACE) data published online (21, 22) or requested. When possible, measurements from gauging stations at the same location as our sampling sites and at the same time of sampling were used. When this was not possible, data from the closest gauging station and time of sampling was chosen. For sampling sites located at a distance between two gauging stations, the discharge was determined by taking the average of the measurements at the two stations. The USACE considers on-line or requested data to be preliminary until given in the engineering pamphlet titled, “Stages and Discharges of the Mississippi River and Tributaries” published by the individual water control districts.
Results and Discussion Characteristics of the Study Area. The Mississippi River is the largest river system in North America, draining around 3.2 × 106 km2 or 41% of the conterminous United States. The source of the river is Lake Itasca, MN, from which the VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Concentrations of toxaphene (Ctox) as a function of upstream distance from the Gulf of Mexico; the line has an r2 value of 0.636. Mississippi extends southward 3780 km to the Gulf of Mexico. Along the way the Missouri and Ohio Rivers converge with the Mississippi and together contribute the greatest load of suspended sediment to the Mississippi River (23). The Ohio River alone adds the greatest volume of water of any tributary (∼50% of the Mississippi’s total flow) (24). Between its convergence with the Ohio River and the city of Vicksburg, MS, the average flow of the Mississippi increases by only 14% (24). Approximately 77 km south of Natchez, MS, a quarter of the flow is diverted west through the Old River Outflow Channel where it joins the Red and Ouachita Rivers to form the Atchafalaya River. Both outlets of the Mississippi River eventually discharge a combined average of 580 km3/ yr of freshwater and 200 million t/yr of sediment into the Gulf of Mexico (23). Suspended sediment concentrations are higher in the lower Mississippi because of the cumulative contributions of incoming tributaries. On the other hand, the organic carbon content of the suspended sediment is higher in the upper Mississippi River, where bio-productivity is higher because of conditions created by locks and dams. Rostad et al. reported that the organic carbon content of suspended sediment is consistent at around 4.8% in the lower Mississippi River (25). Although we did not measure organic carbon content directly in our samples, we found the total combustible carbon content in our samples (as determined by LOI) to be around 13% south of site 8 (river distance 2334 km); these high values for LOI may reflect the inclusion of inorganic carbonates in these samples. We have chosen to report toxaphene concentrations relative to suspended sediment (s.s.), but toxaphene concentrations reported relative to organic carbon or suspended sediment would show similar trends. Concentration and Load of Toxaphene. Toxaphene was detected in all river samples and in a control sample collected from Lake Itasca, MN. The sampling sites, dates of collection, estimated river discharge, and toxaphene and suspended sediment (s.s.) concentrations are given in Table 1. The toxaphene concentrations (in ng/g s.s.) in the Mississippi decrease exponentially as a function of distance from the Gulf of Mexico; see Figure 2. In other words, the concentrations on the suspended sediment increase rather rapidly as the Mississippi River flows through the cotton-growing regions of the southern United States. The concentration of toxaphene in the upper region of the Mississippi River is 0.5-1 ng/g s.s. and increases to ∼10 ng/g s.s. at Baton Rouge, LA. The highest toxaphene concentrations along the main stem of the Mississippi were observed at site 17 near Memphis, TN, and at site 23 just south of Vicksburg, MS. Among the tributaries, samples from the Arkansas and Yazoo rivers (sites 31 and 32) showed the highest toxaphene concentrations. 2788
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Toxaphene was also detected in the most remote location sampled, Lake Itasca (site 1 at the Itasca State Park, MN), which feeds the headwaters of the Mississippi River. We chose this site as the most pristine reference site associated with the Mississippi River system to measure background levels of toxaphene that may be present in the river at its onset. To our knowledge, there has been very little agricultural activity in the Lake Itasca watershed, which suggests that toxaphene found at this site likely comes from atmospheric transport from remote sources and deposition to the watershed. It is difficult to explain the variability in toxaphene concentrations in a system as complex and dynamic as the Mississippi River, especially when considering such a large study area. Concentrations are dependent on several processes that control the fate of particle-bound toxaphene in a river. Once washed into a river, several different processes may effect toxaphene’s concentration in the suspended sediment. Sorbed toxaphene may desorb and dissolve in the water column, mixing with cleaner particles may dilute the contaminated particles, or the dissolved toxaphene may vaporize into the air above the river’s surface. Resuspension of contaminated bottom sediment may also reintroduce more toxaphene into the system. Sedimentation, ingestion, or sorption by river biota and chemically or biologically driven degradation may also remove toxaphene from the water column. Given toxaphene’s persistence, the latter process is not expected to be important on the time scale of this study. Sedimentation and dilution probably dominate the loss processes, although rates and sediment fall velocities for the Mississippi River are not known and are expected to vary widely depending on the section of the river. The contribution of sedimentation to removing toxaphene from the river system is therefore difficult to estimate. Load calculations remove the effect of dilution at the point of sampling and allow a direct comparison of the quantity of toxaphene transported at different locations along the river. The load of toxaphene associated with suspended sediment was calculated as the measured toxaphene concentration times the suspended sediment concentration times the river flow at the time and location of sampling (see Table 1). Most of the uncertainty in this calculation comes from trying to predict the s.s. and toxaphene concentration in the entire cross-section of the river from a single sample taken near the shore (26). Suspended sediment concentrations may vary depending on whether cross-sectionally integrated or point samples are used. Short-term spatial and temporal variability in suspended sediment dynamics and water discharge may also compound errors, especially in point samples. However, because there are no new sources or applications of toxaphene, we expect similar trends in toxaphene levels along the river regardless of the time of year. Toxaphene’s load increases exponentially from the headwaters at d ) ∼3700 km until the Missouri-Arkansas border at d ) ∼1500 km; see Figure 3. After this point, the load is more or less constant at ∼500 g/d. Notably higher toxaphene loads were determined for samples taken near Memphis, TN (numbers 17 and 18). These results are compatible with previous measurements of toxaphene levels in tree bark along the Mississippi River corridor, which have suggested a possible regional or point source of toxaphene in the vicinity of Memphis (27). The load at Memphis may also be influenced by discharge from the Wolf River, which empties into the Mississippi ∼200 m upstream and on the same side of the river as our sampling site. Incomplete mixing of the suspended sediment over this short distance could lead to overestimated loads. The eventual dilution or settling-out of the contaminated sediment downstream could explain the apparent drop in toxaphene loads at sites south of Memphis. In the past, researchers investigating contaminant transport along the Mississippi River have used measurements
FIGURE 3. Loads of toxaphene (Ltox) at sampling sites along the Mississippi River. The following function was fit to the data: log(Ltox) ) 2.69 for d e 1500; log(Ltox) ) 5.37-0.00183 d for d > 1500. This fit has an r2 value of 0.934.
FIGURE 4. Processes effecting the current fate of toxaphene in North America; where M is the estimated remaining stock of toxaphene in North America, FA is the input rate from atmospheric deposition, FV is the volatilization rate, Fr is the output flow rate due to transport by the Mississippi River to the Gulf of Mexico, and Fd is the rate of all degradation processes. Estimates for M, FV, and Fd are from reference 31.
from sites in Southern Louisiana to estimate the pesticide load discharged into the Gulf of Mexico (28-30). From the data presented here, we can provide an estimate of the annual amount of toxaphene discharged into the Gulf of Mexico by the Mississippi River. The average toxaphene load at the mouth of the Mississippi is 490 g/d; see Figure 3. This terminal load suggests that ∼200 kg/year of toxaphene were released into the Gulf of Mexico in 2002 from the main stem of the Mississippi River. Alternatively, if we use the toxaphene concentration extrapolated to d ) 0 from Figure 2 (11 ng/g s.s.), the average annual suspended sediment concentration measured by the USGS (NASQAN) for the year 2002 at St. Francisville, LA (190 mg/L) (20), and the annual discharge of water at Tarbert’s Landing, MS obtained by integrating the average daily discharge over the year 2002 (4.7 × 1014 L/year) (22), we estimate that ∼1000 kg/year of toxaphene were discharged into the Gulf of Mexico in 2002. Because it is not possible to determine the accuracy of either of these two estimates, we will use them both, and we suggest that 200-1000 kg/year of toxaphene is discharged into the Gulf of Mexico from the Mississippi River. This calculation assumes that the toxaphene concentration on suspended sediment is constant throughout the crosssection of the river for the entire year and that the concentration does not appreciably change after Baton Rouge, because there are no inputs or losses of suspended sediment until the river reaches the Gulf. This calculation would not account for contributions from dissolved or colloid-bound toxaphene. Toxaphene certainly partitions into the water phase and thus may contribute to a significant dissolved phase load. It was not possible to measure the contribution to the total load from colloid-associated transport using the sampling methodology presented here. We hypothesize, however, that colloidal association could be an important consideration for at least two of our sampling sites (Yazoo City and Memphis in the summer of 2002) where we observed cloudy filtrates during sampling. We can compare the amount of toxaphene discharged into the Gulf of Mexico to other processes that effect the overall fate of toxaphene in North America; see Figure 4. Li et al. have recently estimated that as much as ∼20 000 t of toxaphene could be present in U.S. soil in 2004 (31). This is similar to the estimate of MacLeod et al. that put the amount of toxaphene remaining in “active circulation” in 2000 at ∼15 000 t (32). Li et al. have also estimated that ∼1200 t of toxaphene will have volatilized and ∼6800 t will have degraded between 2000 and 2004. From these values, we calculate that the flows of toxaphene due to volatilization and degradation are 300 and 1700 t/year, respectively. Our
calculated annual load would represent only 0.2% of the amount of toxaphene predicted to have been emitted into the atmosphere in 2004 and much less when compared to degradation. This observation suggests that degradation of toxaphene in the soil and volatilization of toxaphene from soil are more important loss processes than runoff in rivers (at least in the Mississippi). On a local scale, the Mississippi River system acts as an important sink and vector for the transport of toxaphene. This may have implications for water quality in the River and in the Gulf of Mexico, where toxaphene is eventually deposited. For example, if we assume the organic carbonwater partition coefficient (KOC) for toxaphene is ∼105 (33), and that the fraction of organic carbon in the suspended sediment is 0.05, then the dissolved toxaphene concentration at a site such as Memphis could be as high as 3 ng/L of water. This is higher than the U.S. EPA’s chronic exposure guideline for aquatic organism health of 0.2 ng/L of water (34). Because toxaphene tends to bioaccumulate (35) and sorption or ingestion is proportional to concentrations in water and sediment, aquatic organisms may be exposed to potentially high levels of toxaphene well into the future. Sources of Toxaphene. Although toxaphene was mostly used on cotton in the southern United States, it was also used to a lesser extent on other crops and livestock throughout the Mississippi River basin. We have used published estimates of existing toxaphene in soil in the conterminous United States (31) in combination with watershed boundaries (36) to show the total usage of toxaphene within each watershed subunit of the Mississippi River Basin; see Figure 5. Comparison of calculated loads to existing toxaphene in watersheds adjacent to the Mississippi River shows that the high local toxaphene loads correlate with contaminated watersheds. For example, the high loads measured at Le Tourneau and Natchez (sites 23 and 24) could be a result of contributions of contaminated sediment loads from the Yazoo and Big Black rivers, which are confluents of the Mississippi upstream of these sample sites. According to the toxaphene data compiled in Figure 5, these watersheds are among the 15 most toxaphene-contaminated watersheds in North America. Our estimate of the toxaphene loads from tributaries suggests that the Yazoo River delivers a large burden of toxaphene to the Mississippi River. Of the major tributaries sampled, the Yazoo River was estimated to carry 21% of the load measured for the Mississippi River at Le Tourneau (site 23). Fish tissue studies also confirm high levels of toxaphene in the Yazoo and Big Sunflower Rivers (8, 10) and regional fish advisories are in effect for this reason (37). It is difficult to locate point sources of toxaphene because this pesticide has been banned in the United States for more VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Map of the Mississippi River Basin and contributing watersheds. The color code indicates the existing mass of toxaphene (in t) remaining within each watershed derived from estimates by Li et al. (31): dark green, 0-6; light green, 6-16; yellow, 16-115; orange, 115-190; and red, 190-312.
FIGURE 6. Concentrations of toxaphene and other selected organochlorine pesticides in suspended sediment collected in the vicinity of Memphis, TN in March 2003. Error bars represent standard errors derived from duplicate samples at each site. than 15 years, and any remaining stocks in the area would be undocumented. There are, however, several indicators that suggest that the Wolf River, the mouth of which is ∼200 m upstream of our Memphis sampling site, may be a source of toxaphene and other organochlorine pesticides to the Mississippi River. When we revisited the river in the spring of 2003 we found that the toxaphene load measured at Memphis was comparable to the load we measured 7 months 2790
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earlier during low water discharge. Moreover, toxaphene concentrations at the Memphis site were again higher than at other sites sampled to the north and south of this site. The Wolf River is known to be contaminated with organochlorine pesticides by sites along its stretch that have been on the National Priorities List (NPL). One of these, the North Hollywood dump, is located several kilometers from the mouth of the Wolf River and has been identified as a
source of organochlorine pesticides, most notably chlordane, to the groundwater and adjacent river (38). In addition to relatively high levels of toxaphene, the samples collected from Memphis in the spring of 2003 (sample 17) also contained higher levels of Σchlordanes and heptachlor than all other sites sampled at this time (see Figure 6). Although we were not able to find any reports of toxaphene levels in the vicinity of Memphis, the correlation of elevated chlordane and heptachlor levels with high toxaphene concentrations at the Memphis site may suggest a common nearby source. A similar correlation is absent between toxaphene and p,p′-DDE, the recalcitrant degradation product of toxaphene’s predecessor, DDT. Whereas toxaphene concentrations were relatively higher at Memphis than at sites up and downstream, p,p′-DDE concentrations were about the same for all sites visited during spring sampling. If toxaphene had entered the river from agricultural runoff via the Wolf River, one would also expect p,p′-DDE to peak in concentration at the Memphis site. Although some toxaphene contamination of the Wolf River from regional sources is expected, this watershed experienced only moderate toxaphene application when compared to neighboring watersheds to the north along the Mississippi River (see Figure 5). If one assumes, as Larson and Gilliom have shown (39), that the intensity of pesticide use within a watershed is one of the most significant factors in predicting concentrations in streams, then one would expect the Wolf River to exhibit a lower relative concentration of toxaphene than samples collected near the outlet of more contaminated watersheds such as the Hatchie River (sites 15 and 16).
Acknowledgments We thank Debera Backhus for the use of sampling equipment and useful discussions; Maren Pink and William Hafner for their help with sample collection; and Hafford W. Barton and Wayland Hill from the USACE for providing us with discharge data for many of the sampling sites.
Literature Cited (1) U.S. Department of Health & Human Services, Agency for Toxic Substances and Disease Registry. Toxicological Profile for Toxaphene; U.S. Government Printing Office: Washington, DC, 1995. (2) Voldner, E. C.; Li, Y. F. Chemosphere 1993, 27, 2073-2078. (3) VonRumker, R.; Lawless, E. W.; Meiners, A. F.; Lawrence, K. A.; Kelso, G. L.; Horay, F. Production, Distribution, Use, and Environmental Impact Potential of Selected Pesticides; EPA-549/ 1-74-001; U.S. Environmental Protection Agency, Office of Pesticide Programs and Council on Environmental Quality; U.S. Government Printing Office: Washington, DC, 1974. (4) Swackhamer, D. L.; Pearson, R. F.; Schottler, S. P. Chemosphere 1998, 37, 2545-2561. (5) James, R. R.; Hites, R. A. Environ. Sci. Technol. 2002, 36, 34743481. (6) Jantunen, L. M.; Bidleman, T. F. Environ. Toxicol. Chem. 2003, 22, 1229-1232. (7) Nowell, L. H.; Capel, P. D.; Dileanis, P. D. In Pesticides in Stream Sediment and Aquatic Biota: Distribution, Trends, and Governing Factors; Nowell, L. H., Ed.; Lewis Publishers: Boca Raton, FL, 1999; pp 110-116. (8) Schmitt, C. J. Arch. Environ. Contam. Toxicol. 2002, 43, 81-97. (9) Winger, P. V.; Andreasen, J. K. Arch. Environ. Contam. Toxicol. 1985, 14, 579-586. (10) Ford, W. M.; Hill, E. P. Arch. Environ. Contam. Toxicol. 1991, 20, 161-167. (11) Knight, R. R.; Powell, J. R. Occurrence and Distribution of Organochlorine Pesticides, Polychlorinated Biphenyls, and Trace Elements in Fish Tissue in the Lower Tennessee River Basin, 198098; U.S. Geological Survey Water-Resources Investigations Report 01-4184; U.S. Government Printing Office: Washington, DC, 2001. (12) Tate, C. M.; Heiny, J. S. Arch. Environ. Contam. Toxicol. 1996, 30, 62-78. (13) Cooper, C. M.; Dendy, F. E.; McHenry, J. R.; Ritchie, J. C. J. Environ. Qual. 1987, 16, 69-72.
(14) Zimmerman, L. R.; Thurman, E. M.; Bastian, K. C. Sci. Tot. Environ. 2000, 248, 169-179. (15) Petty, J. D.; Poulton, B. C.; Charbonneau, C. S.; Huckins, J. N.; Jones, S. B.; Cameron, J. T.; Prest, H. F. Environ. Sci. Technol. 1998, 32, 837-842. (16) Gilliom, R. J.; Alexander, R. B.; Smith, R. A. Pesticides in the Nation’s Rivers, 1975-1980, and Implications for Future Monitoring; U.S. Geological Survey Water-Supply Paper 2271; U.S. Government Printing Office: Washington, DC, 1985. (17) Mahler, B. J.; Van Metre, P. C. Arch. Environ. Contam. Toxicol. 2003, 44, 288-297. (18) Swackhamer, D. L.; Charles, M. J.; Hites, R. A. Anal. Chem. 1987, 59, 913-917. (19) Glassmeyer, S. T.; Shanks, K. E.; Hites, R. A. Anal. Chem. 1999, 71, 1448-1453. (20) Kelly, V. J. The USGS National Streamwater Quality Network (NASQAN) published data for the Mississippi Basin (19962002): URL http://water.usgs.gov/nasqan/data/finaldata/miss.html. Accessed October 18, 2003. (21) USGS Surface-Water for the Nation; U.S. Geological Survey: URL http://waterdata.usgs.gov/nwis/sw. Accessed October 18, 2003. (22) Data are available from the water control pages of the respective districts within the Mississippi Valley Division of the U.S. Army Corps of Engineers: URL http://www.mvd.usace.army.mil/ main.php. Accessed October 18, 2003. (23) Meade, R. H. Contaminants in the Mississippi River, 1987-92; U.S. Geological Survey Circular 1133; Reston, VA, 1995. (24) Goolsby, D. A.; Batttaglin, W. A.; Lawrence, G. B.; Artz, R. S.; Aulenbach, B. T.; Hooper, R. P.; Keeney, D. R., Stensland, G. J. Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin: Topic 3 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico; NOAA Coastal Ocean Program Decision Analysis Series No. 17; NOAA Coastal Ocean Program: Silver Spring, MD, 1999. (25) Rostad, C. E.; Leenheer, J. A.; Daniel, S. R. Environ. Sci. Technol. 1997, 31, 3218-3225. (26) Horowitz, A. J.; Rinella, F. A.; Lamothe, P.; Miller, T. L.; Edwards, T. K.; Roche, R. L.; Rickert, D. A. Environ. Sci. Technol. 1990, 24, 1313-1320. (27) McDonald, J. G.; Hites, R. A. Environ. Sci. Technol. 2003, 37, 475-481. (28) Clark, G. M.; Goolsby, D. A.; Battaglin, W. A. Environ. Sci. Technol. 1999, 33, 981-986. (29) Pereira, W. E.; Hostettler, F. D. Environ. Sci. Technol. 1993, 27, 1542-1552. (30) Rostad, C. E. Arch. Environ. Contam. Toxicol. 1997, 33, 369377. (31) Li, Y. F.; Bidleman, T. F.; Barrie, L. A. J. Geophys. Res. 2001, 106, 17929-17938. (32) MacLeod, M.; Woodfine, D.; Brimacombe, J.; Toose, L.; Mackay, D. Environ. Toxicol. Chem. 2002, 21, 1628-1637. (33) Mackay, D.; Shiu, W.-Y.; Ma, K.-C. Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals; CRC Press LLC: Boca Raton, FL, 1997; Vol. V, p 554. (34) Nowell, L. H.; Resek, E. A. Rev. Environ. Contam. Toxicol. 1994, 140, 1-164. (35) Geyer, H. J.; Kaune, A.; Schramm, K.-W.; Rimkus, G.; Scheunert, I.; Bru ¨ ggemann, R.; Altschuh, J.; Steinberg, C. E.; Vetter, W.; Kettrup, A.; Muir, D. C. G. Chemosphere 1999, 39, 655-663. (36) Watermolen, J. 1:2,000,000-Scale Hydrologic Unit Boundaries, Version 2.3; U.S. Geological Survey: Reston, VA, 2002. URL http://nationalatlas.gov/atlasftp.html. (37) EPA Fact Sheet Update: National Listing of Fish and Wildlife Advisories; EPA-823-F-03-003; U.S. Environmental Protection Agency, Office of Water; U.S. Government Printing Office: Washington, DC, 2003. (38) Broshears, R. E.; Bradley, M. W. Hydrogeology, Water Quality, and Potential for Transport of Organochlorine Pesticides in Groundwater at the North Hollywood Dump, Memphis, Tennessee; U.S. Geological Survey Water-Resources Investigations Report 91-4022; U.S. Government Printing Office: Washington, DC, 1991. (39) Larson, S. J.; Gilliom, R. J. J. Am. Water. Res. Assoc. 2001, 37, 1349-1367.
Received for review October 28, 2003. Revised manuscript received March 3, 2004. Accepted March 5, 2004. ES0351995 VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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