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Final Progress Memorandum:Investigation of PCBs on Suspended Sediment

In Donna Canal, Texas

December 14, 2000

Barbara MahlerUS Geological Survey

Introduction

The U.S. Geological Survey was asked to investigate the occurrence of PCBs in fish inDonna Canal by collecting and analyzing suspended sediment in the canal and the nearbyportions of the Arroyo Colorado, and from sediment cores taken from Donna Reservoir.PCBs in Donna Canal have been under investigation by various government agenciessince 1993, when a carp caught in the canal was found to be highly contaminated. Sincethat time, additional fish contaminated with PCBs have been found in the canal, resultingin a ban on fish possession, yet analysis of numerous water and bed sediment samplesfrom the canal failed to detect any PCBs. In 1998, the U.S. Geological Survey was askedto use an alternative approach-suspended sediment sampling-in an attempt todetennine the occurrence of PCBs in the canal and, ifpossible, locate the source.

Four sampling trips were undertaken from February 1999 to July 2000. Existing data onPCB concentrations in fish and field reconnaissance were the basis for choosing fourinitial sampling sites. Sampling sites for subsequent trips were chosen based on theanalytical results of the previously collected samples. The initial sampling was designedto determine in which portions of the canal PCBs occurred at detectable concentrations,and subsequent sampling was designed to narrow down the possible location of a PCBsource.

Methods

Large-volume suspended-sediment sampling consists of pumping a large volume of waterthrough a glass fiber filter to concentrate sufficient sediment on the filter for analysis.Approximately 2 to 4 grams of solids are required by the U.S. Geological Survey'sNational Water Quality Laboratory (NWQL) to perform the analysis. The detection limitvaries from sample to sample, based on the amount of material submitted for analysis, theconcentration of suspended sediment in the water, and the presence of any material suchas algae or organic acids that might interfere with the analysis. Sampling was carried outby pumping water from the canal through a 298-mm diameter, 0.5 J..lm pore-size pre­baked glass fiber filter. The filter was held in a stainless-steel filter support. Water waspumped through the filter until it clogged-from 12 to 16 liters were required. The filterwas then stored in a baked glass jar and chilled on ice. Three filters were collected ateach site. Between sites, the pwnp tubing and filter support were washed with liquinoxdetergent, rinsed with distilled water, then rinsed with methanol and allowed to air dry.

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The samples were submitted for analysis to the NWQL where they were analyzed forPCBs by gas chromatography/mass spectrometry. The results are reported by the NWQLas mass of PCB per liter of water filtered. This concentration is then converted to massof PCB per mass of sediment using the total suspended sediment concentration (TSS­mass of sediment per liter of water) of the sample. TSS is measured in a 250-ml rawwater sample collected at the same time as the filtered sample and analyzed the the USGSLouisiana Sediment Laboratory. Two TSS samples were collected at each site and theresults averaged.

Sampling trips and results

Sampling sites and results for the four sampling trips are shown in Figures 1 and 2 and inTable 1.

February 1999: The initial sampling trip was carried out in February 1999. Four siteswere chosen, based on where the fish with the highest PCB concentrations had beencaught. The highest concentrations of PCBs had been found in fish closest to the RioGrande, the Donna Canal pwnphouse, and the siphon with concentrations in fish ingeneral decreasing closer to Donna Reservoir. Relatively high concentrations of PCBs insome fish in the Arroyo Colorado suggested a hitherto undetected hydraulic connectionbetween the canal and the arroyo. The field reconnaissance confinned the connectionbetween the two: the approximately 200 m stretch of terrain where the Donna Canal isburied was extremely wet with numerous seeps and springs, suggesting that the concretebox containing the buried canal is leaking. Where the Donna Canal passes underneaththe Arroyo Colorado, the top of the concrete canal liner was visible in the bed of thearroyo. Direct leakage from the canal into the arroyo may provide a mechanism for PCBcontamination to pass from one water body into the other.

Based on the data and field observations, suspended sediment samples were collected atfour sites:

• In the canal just downstream of the pumphouse at the transition from lined tounlined canal ("Pumphouse");

• In the canal just upstream of the siphon ("Above Siphon");• In the canal just downstream of the siphon ("Below Siphon");• In the Arroyo Colorado approximately 50 m downstream of the point where

the arroyo crosses over the buried canal ("Arroyo Colorado")

Sediment from all four sites were analyzed for PCBs; at the request of Roger Miranda,sediment from the Arroyo Colorado was also analyzed for organochlorine pesticides. NoPCBs were detected at any of the sites. Detection limits ranged from 14 to 32 j..lg of PCBper kg of suspended sediment. At the Arroyo Colorado site, p,p' -DOE was detected at2.5 j..lg/kg; no other organochlorines were detected. The lab also detected low-level p,p'­ODE at the site below the siphon, although the amount was not quantified because thisanalysis had not been requested.

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July 1999: Based on the series of non-detections from the first sampling trip, the next setof suspended sediment samples were collected and analyzed from four sites in the canal.The sites were located beginning downstream of the previous sample sites and continuingto the reservoir:

• Just downstream of the 90° bend, downstream of the siphon (''90° Bend");• At the 3rd bend after the siphon ("3rd Bend");• From the paved-road bridge crossing ("Bridge");• Just upstream of the reservoir (''Near Reservoir").

PCBs were detected on suspended sediments from all four sampling sites. The highestconcentration, 102 J,Lglkg, was detected at the most upstream site, ''90° Bend", locatedjust downstream of the 90° bend in the canal downstream of the siphon. Concentrationsat the three sites downstream from there were reported by the laboratory as "estimated"(the compound was detected but quantification uncertain) to be 41.8 J,Lglkg, 35.1 J-lglkg,and 36.8 J,Lglkg, respectively. All PCBs detected were Aroclor 1254.

In addition, sediment cores were collected from the east and west sides of DonnaReservoir to determine if there had been a historical release of PCBs and/or if there was acontinuing PCB source. The core collected from the west side was 15 cm long and wassubdivided into 3-cm sample intervals. Relatively low concentrations (4-7 J,Lglkg) ofPCBs were detected in all 5 samples. There was no evidence of a historical release ofPCBs. Sediment samples from this core were also analyzed for organochlorinepesticides. Low concentrations of DDT (0.6-1.6 J-lglkg) and its metabolites DDE (5.2­7.2 J,Lglkg) and DDD (0.94-1.4 J,Lglkg) were detected in the core. No PCBs were detected(reporting level of 5 J,Lglkg) in the core taken from the east side of the reservoir.

January 2000: Four sites were chosen to attempt to bracket the possible location of thePCB source (see Figure 1):

• Above Siphon;• Upstream of Bridge;• Upstream of 90° Bend; and

• 90° Bend.Above Siphon and 90° Bend were chosen for resampling to corroborate previous results.

No PCBs were detected above the siphon, consistent with the results of the first samplingtrip. PCBs were detected at the other 3 sites, with the highest concentration detected atUpstream of 90° Bend. All PCBs detected were ArocIor 1254 except for those detectedat the 90° Bend, where an estimated 8.8 J-lglkg of the total PCB detected was ArocIor1260. These results tentatively bracketed the source of the PCBs between the dischargingend of the siphon and the bridge located between the siphon and the 90° bend, a distance ­of approximately 250 meters. The occurrence of the highest concentration of PCBsbelow another detection was thought to signal one or both of two possibilities: that thereare two (or more) PCB sources, and/or the source may be located on one bank or theother and complete mixing is not occurring.

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July 2000: A final sampling trip was designed to further define the source of the PCBs.One sample was collected from Below Siphon (just in front of the siphon discharge) and4 samples were collected along each of the banks along the 250 meter stretch atapproximately 50 meter intervals, for a total of 9 samples (Figure 2):

• Below Siphon• Station I, right bank• Station 1, left bank• Station 2, right bank• Station 2, left bank• Station 3, right bank• Station 3, left bank• Station 4, right bank• Upstream of Bridge (corresponds to Station 4, left bank)

The results of the analyses of these samples indicate a potential PCB source located onthe right bank of Donna Canal between the siphon outlet and the first sampling point.The detection of PCBs at Below Siphon at a concentration of 11.5 /-lg/kg is consistentwith the earlier non-detection at a detection level of 18.9 /-lglkg. It is likely turbulence atthe siphon outlet causes some back eddies, making possible a detection there even if thesource is downstream The results for the sample from Station 3, left bank areunavailable as the sample was ruined by glassware failure during analysis at the NWQL.

Aroc1or 1254 was detected in all 9 samples; Aroclor 1260 was detected in 5 of thesamples, but in no clear spatial pattern. Intriguingly, the only site at which theconcentration of Aroclor 1260 exceeded that of Aroclor 1254 was at Station 1, right bank,the site with the highest total PCB concentration on that sampling trip and probably thatclosest to the PCB source.

The results of the analyses of the four trips suggest that measured concentrations areconsistent during anyone sampling trip, but may not necessarily be comparable from onesampling trip to the next. Possible reasons for this include variation in the mass ofsuspended sediment from the Rio Grande "diluting" the PCB signal, and a varying rate oferosion of the contaminated sediment from the PCB source, either of which could causetemporal variability in PCB concentrations. Overall, however, the non-detection of PCBsabove the siphon and the occurrence of PCBs beginning at or just downstream from thesiphon is consistent among all four rounds of sampling.

Conclusions

The PCBs contaminating fish in Donna Canal are also detectable in suspended sedimentsin Donna Canal. A series of four sampling trips comprising a total of 71 samplesnarrowed down the location of a probable source of PCBs to a 35-m long stretch on theright bank of Donna Canal just below the siphon outlet. In general, the results of eachsampling trip showed a consistent spatial pattern of PCB detections, althoughconcentrations between trips probably cannot be compared. The possibility of more thanone PCB source cannot be ruled out. A relatively high detection of PCBs at Upstream of

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90° Bend suggests that a second PCB source may be located between that location andthe Upstream of Bridge site, but additional sampling would be required for confinnation.

The success of the large-volume suspended sampling method in detecting the PCBs whenthey were not detected in water or bed sediment samples may be due to one or morecauses. First, the bed sediment samples were submitted to a different laboratory than thatused in the suspended sediment study, and analytical procedures may not be consistent.Second, even 60 samples collected along a several-kilometer long stretch leaves manymeters of canal bank unsampled. If there is indeed a point source, the probability ofcollecting a bed sediment sample close enough to the source for a detection may be small.However, as contaminated sediment from near the point source erodes, it creates a plumeof contaminated sediment in suspension downstream, effectively integrating the signalover a long distance. By sampling the suspended sediment the occurrence of the PCBscan be detected and the location of the source estimated.

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1 km 0.5 km 0 km- 1 km 2km 3km 4km 5km

Figure 1.Donna Canal

Sample Locations

Concentrations givenin micrograms totalPCB per kilogramof sediment.

Area in squareenlarged inFigure 2

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Figure 2. Donna Canal Sample Sites • July 2000

100m Om 200m

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