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Data Evaluation and Interpretation Report

Executive Summary
February 13, 1997

The U.S. Environmental Protection Agency is conducting a study of the Hudson River PCBs Superfund site, reassessing the interim No Action decision the Agency made in 1984. The goal of the Reassessment study is to determine an appropriate course of action for the PCBcontaminated sediments in the Upper Hudson River in order to protect human health and the environment.

During the first phase of the Reassessment, EPA compiled existing data on the site, and conducted preliminary analyses of the data. As part of the second phase, EPA conducted field investigations to characterize the nature and extent of the PCB loads in the Upper Hudson and the importance of those loads to the Lower Hudson. EPA also conducted analyses of data collected by the New York State Department of Environmental Conservation, the U.S. Geological Survey, and the General Electric Company (GE), as well as other private and public agencies.

This report is the third of a series of six volumes that make up the Phase 2 Report. This volume, the Data Evaluation and Interpretation Report, provides detailed descriptions and in-depth interpretations of the water column and dated sediment core data collected as part of the Reassessment. The report helps to provide an improved understanding of the geochemistry of PCBs in the Hudson River. The report does not explore the biological uptake and human health impacts, which will be evaluated in future Phase 2 volumes.

The conclusions presented herein are based primarily on direct geochemical analyses of the data, using conceptual models of PCB transport and environmental chemistry. The geochemical analyses will be complemented and verified to the extent possible by additional numerical analysis via computer simulation. Results of the numerical simulations will be reported in subsequent reports, primarily the Baseline Modeling Report.

Major Conclusions

The analyses presented in the Data Evaluation and Interpretation Report lead to four major conclusions as follows:

1. The area of the site upstream of the Thompson Island Dam represents the primary source of PCBs to the freshwater Hudson. This includes the GE Hudson Falls and Ft. Edward facilities, the Remnant Deposit area and the sediments of the Thompson Island Pool.

2. The PCB load from the Thompson Island Pool has a readily identifiable homologue pattern which dominates the water column load from the Thompson Island Dam to Kingston during low flow conditions (typically 10 months of the year).

3. The PCB load from the Thompson Island Pool originates from the sediments within the Thompson Island Pool.

4. Sediment inventories will not be naturally "remediated" via dechlorination. The extent of dechlorination is limited, resulting in probably less than a 10 percent mass loss from the original concentrations.

A weight of evidence approach provides the support for these conclusions, with several different lines of investigation typically supporting each conclusion. The subordinate conclusions and findings supporting each of these major findings are discussed below.

1. The area of the site upstream of the Thompson Island Dam represents the primary source of PCBs to the freshwater Hudson. This includes the GE Hudson Falls and Ft. Edward facilities, the Remnant Deposit area and the sediments of the Thompson Island Pool.

Analysis of the water column data showed no substantive water column load increases (i.e., load changes were less than ten percent) from the Thompson Island Dam to the Federal Dam at Troy during ten out of twelve monitoring events. These results indicate the absence of substantive external (e.g., tributary) loads downstream of the Thompson Island Dam as well as minimal losses from the water column in this portion of the Upper Hudson. These results also indicate that PCB transport can be considered conservative over this area, with the river acting basically as a pipeline (i.e., most of the PCBs generated upstream are delivered to the Lower Hudson). Some PCB load gains were noted during spring runoff and summer conditions, which were readily attributed to Hudson River sediment resuspension or exchange by the nature of their homologue patterns. These load gains were notable in that they represent sedimentderived loads which originate outside the Thompson Island Pool, indicating the presence of substantive sediment inventories outside the Pool. The Mohawk and Hoosic Rivers were each found to contribute to the total PCB load measured at Troy. The loading from each of these rivers during the 1993 Spring runoff event could be calculated to be as high as 20 percent of the total load at Troy. However, these loads represent unusually large sediment transport events by these tributaries since both rivers were near or at 100-year flood conditions.

A second line of support for the above conclusion comes from the congener specific analyses of the water column samples which show conformity among the main stem Hudson samples downstream of the Thompson Island Dam and distinctly different patterns in the water samples from the tributaries. These results indicate that the tributary loads cannot be large relative to the main stem load since no change in congener pattern is found downstream of the tributary confluences.

This conclusion is also supported by the results of the sediment core analyses which showed the PCBs found in the sediments of the tributaries to be distinctly different from those of the main stem Hudson. As part of this analysis, two measurement variables related to sample molecular weight and dechlorination product content were shown to be sufficient to clearly separate the PCB patterns found in the sediments of the freshwater Hudson from those of the tributaries, indicating that the tributaries were not major contributors to the PCBs found in the freshwater Hudson sediments and by inference, to the freshwater Hudson as a whole.

When dated sediment core results from the freshwater Hudson were examined on a congener basis, sediment layers of comparable age obtained from downstream cores were shown to contain similar congener patterns to those found in a core obtained at Stillwater, just 10 miles downstream of the Thompson Island Dam. Based on calculations combining the homologue patterns found at Stillwater with those of other potential sources (e.g., the Mohawk River) it was found that no less than 75 percent of the congener content in downstream cores was attributable to the Stillwater core. This suggests that the Upper Hudson is responsible for at least 75 percent of the sediment burden, and by inference, responsible for 75 percent of the water column load at the downstream coring locations. Only in the cores from the New York/New Jersey Harbor was substantive evidence found for the occurrence of additional PCB loads to the Hudson. Even in these areas, however, the Upper Hudson load represented approximately half of the total PCB load recorded by the sediments.

The last line of evidence for this conclusion was obtained from the dated sediment cores wherein the total PCB to cesium137 (137Cs) ratio was examined in dated sediment layers. Comparing sediment layers of comparable age from Stillwater (10 miles downstream of the Thompson Island Dam) to Kingston (100 miles downstream of the Thompson Island Dam), the data showed the sediment PCB to 137Cs ratios at downstream cores to be readily predicted by those at Stillwater, implying a single PCB source (i.e., the area above the Thompson Island Dam) and quasiconservative transport between Stillwater and locations downstream. These calculations showed downstream ratios to agree with those predicted from Stillwater to within the limitations of the analysis (+/- 25 percent).

2. The PCB load from the Thompson Island Pool has a readily identifiable homologue pattern which dominates the water column load from the Thompson Island Dam to Kingston during low flow conditions (typically 10 months of the year).

Evidence for the first part of this conclusion stems largely from the Phase 2 water column sampling program which provided samples above and below the Thompson Island Pool. In nearly every water column sampling event, the homologue pattern of the water column at the Thompson Island Dam was distinctly different from that entering the Thompson Island Pool at Rogers Island. In addition, the Phase 2 and GE monitoring data both showed increased water column PCB loads at the downstream station, relative to the upstream station, particularly under low flow conditions. Based on the monitoring data collected from June 1993 to the present, water column concentrations and loads typically doubled and sometimes tripled during the passage of the river through the Pool. Thus, a relatively large PCB load originating within the Thompson Island Pool is clearly in evidence in much of the Phase 2 and GE data. This load was readily identified as a mixture of less chlorinated congeners relative to those entering the Pool.

The importance of this load downstream of the Thompson Island Dam is demonstrated by the Phase 2 water samples collected downstream of the Dam. These samples indicate the occurrence of quasiconservative transport of water column PCBs (i.e., no apparent net losses or gains) throughout the Upper Hudson to Troy during much of the Phase 2 sampling period. This finding is based on the consistency of homologue patterns and total PCB load among the downstream stations relative to the Thompson Island Dam load. Thus, the region above the Thompson Island Dam is responsible for setting water column concentrations and loads downstream of the Dam to Troy. During the lowflow conditions seen in the Phase 2 sampling period, as well as in most of the postJune 1993 monitoring data collected by GE, the Thompson Island Pool was responsible for the majority of the load at the Dam. Thus, the Thompson Island Pool load represents the largest fraction of the water column load below the Dam during at least 10 months of the year, corresponding to low flow conditions.

The importance of this load for the freshwater Lower Hudson is derived from a combination of the water column and the sediment core results discussed above. Specifically, the water column results show the Thompson Island Pool to represent the majority of the water column load during much of the year throughout the Upper Hudson to Troy. The dated sediment core results show the Upper Hudson to represent the dominant load to the sediments of the Lower Hudson and, by inference, to the water column of the Lower Hudson. Since the majority of the Upper Hudson load is derived from the Thompson Island Pool, the Thompson Island Pool load represents the majority of the PCB loading to the entire freshwater Hudson as well.

3. The PCB load from the Thompson Island Pool originates from the sediments within the Thompson Island Pool.

The PCB homologue pattern present in the water column at the Thomson Island Dam is distinctly different from that which enters the Thompson Island Pool at Rogers Island. This change in pattern was nearly always accompanied by a doubling or tripling of the water column PCB load during the Phase 2 sampling period and subsequent monitoring by GE. This pattern change and load gain occurred as a result of passage through the Pool. With no known substantive external loads to the Pool, the sediments of the Pool were considered the most likely source of these changes. Upon examination of the PCB homologue and congener patterns present in the sediment cores collected from the Thompson Island Pool and elsewhere it became clear that the sediment PCB characteristics closely matched those found in the water column at the Thompson Island Dam and sampling locations downstream during most of the Phase 2 sampling period. On the basis of this PCB "fingerprint" it was concluded that the Thompson Island Pool sediments represented the major source to the water column throughout much of the year as discussed above.

Two possible mechanisms for transfer of PCBs to the water column from the sediment were explored and found to be consistent with the measured water column load changes. The first mechanism involved porewater exchange, i.e., the transport of PCB to the water column via the interstitial water found within the river sediments. This mechanism was examined using sedimenttowater partition coefficients developed from the Phase 2 water column samples. These coefficients were used to estimate the homologue patterns found in porewater from the Thompson Island Pool sediments. These patterns were then compared with the measured water column patterns at the Thompson Island Dam. On this basis it was demonstrated that this mechanism is generally capable of yielding the water column homologue patterns seen. This analysis suggested that if porewater exchange is the primary exchange mechanism, then sediments with relatively low levels of dechlorination are the likely candidates for the Thompson Island Pool source.

The alternate mechanism, resuspension of Thompson Island Pool sediments, was also shown to be capable of yielding the water column patterns seen. Since this mechanism works by directly addingsediments to the water column, sediment homologue patterns were directly compared to those of the water column at the Thompson Island Pool. The close agreement seen between the sediment and water column homologue patterns demonstrated the viability of this mechanism. If resuspension is the primary sedimenttowater exchange mechanism, then the responsible sediments must have comparatively high levels of dechlorination, since the water column homologue pattern at the Thompson Island Dam contains a relatively large fraction of the least chlorinated congeners.

As part of the investigation of Hudson River sediments, a relationship between the degree of dechlorination and the sediment concentration was found such that sediments with higher PCB concentrations were found to be more dechlorinated than those with lower concentrations, regardless of age. This relationship had important implications for the nature of the sediments involved in the sedimentwater exchange mechanisms. For porewater exchange, which indicated a low level of dechlorination in the responsible sediments, the sediment concentrations had to be relatively low, although no absolute concentration could be established. For resuspension, the sediment concentrations had to be relatively high (i.e., greater than 120,000 ug/kg (120 ppm)) in order to attain the level of dechlorination necessary to drive the Thompson Island Pool load. This in turn suggested that older sediments, particularly the relatively concentrated ones found in the previously identified hot spots, are the likely source for the Pool load via the resuspension mechanism. Given the complexities of sedimentwater column exchange, it is probable that the current Thompson Island Pool load is the result of some combination of both mechanisms.

Recent large releases from the Bakers Falls area may have also yielded sediments with sufficient concentration so as to undergo substantive alteration and potentially yield some portion of the measured load via resuspension. However, the mechanism for rapid burial and subsequent resuspension is unknown. It is also conceivable that these materials could be responsible for a portion of the load if porewater exchange is the driving mechanism. However, the presence of such deposits is undemonstrated and must still be viewed in light of the prior, demonstrably large PCB inventory.

In this assessment, neither porewater exchange nor resuspension was evaluated in terms of the scale of the flux required to yield the measured Thompson Island Pool load. Such an evaluation will be completed as part of the Baseline Modeling Report.

4. Sediment inventories will not be naturally "remediated" via dechlorination. The extent of dechlorination is limited, resulting in probably less than a 10 percent mass loss from the original concentrations.

Evidence for this conclusion is principally derived from the dated sediment core data obtained during the Phase 2 investigation. These data show that dechlorination of PCBs within the sediments of the Hudson River is theoretically limited to a net total mass loss of 26 percent of the original PCB mass deposited in the sediment. This is because the dechlorination mechanisms which occur within the sediment are limited in the way they can affect the PCB molecule, thus limiting the effectiveness of the dechlorination process. In fact, although theoretically limited to 26 percent , the actual estimated mass loss is much less, in the range of only10 percent based on the sediment core results (the mean mass loss for the high resolution sediment core results was eight percent).

A second finding was obtained from the core data which supports this conclusion as well. In core layers whose approximate year of deposition could be established, no correlation was seen between the degree of dechlorination and the age of the sediment. If dechlorination were to continue indefinitely, such a correlation would be expected, with the oldest sediments showing the greatest degree of dechlorination. Instead, a relationship was found between the degree of the dechlorination and the PCB concentration in the sediment, such that the most concentrated samples had the greatest degree of dechlorination. Also, sediments below 30,000 ug/kg (30 ppm) showed no predictable degree of dechlorination, suggesting that the PCBs in sediments with less than 30 ppm are largely left unaffected by the dechlorination process. These findings indicate that the dechlorination process occurs relatively rapidly, within perhaps five to ten years of deposition but then effectively ceases, leaving the remaining PCB inventory intact. These results also indicate that the dechlorination process is generally limited to the areas of the Upper Hudson where concentrations are sufficient to yield some level of dechlorination. For those areas characterized by concentrations less than 30 ppm, dechlorination is not expected to have any effect at all. Thus, dechlorination cannot be expected to yield further substantive reductions of the Hudson River PCB inventory beyond the roughly ten percent reduction already achieved.

An important related finding concerning the Upper Hudson sediments was obtained from the geophysical survey completed during the Phase 2 investigation. This survey showed a general correlation between areas of finegrained sediment and the hot spot areas previously defined by NYSDEC. Since PCBs have a general affinity for finegrained sediments, it can be assumed that the finegrained sediment areas mapped by the geophysical survey represent the same PCBcontaminated zones mapped by NYSDEC. This indicates that the hot spot areas previously mapped by NYSDEC are largely still intact and have not been completely redistributed by high river flows.

Ancillary Conclusions

In addition to the conclusions described above there are several additional findings which have important implications for the understanding of PCB transport in the Hudson River. These are discussed briefly below. More extensive discussions of these conclusions can be found in the summary discussions contained within each chapter.

In conclusion, the sediments of the Thompson Island Pool strongly impact the water column, generating a significant water column load whose congener pattern can often be seen throughout the Upper Hudson. The Phase 2 investigation has also found a number of sediment structures via the geophysical investigation which closely resemble the hot spot areas defined previously by NYSDEC. These hot spotrelated structures appear to be intact in spite of the time between the Phase 2 and NYSDEC studies. Given the strong linkage between sediment and water, the large inventory of PCBs in the Upper Hudson, and the apparent lack of significant reduction in PCB concentrations via in situ degradation, it is unlikely that the water column PCB levels downstream of the Thompson Island Dam will substantially decline beyond current levels until the active sediments are depleted of their PCB inventory or remediated. The time for depletion appears to be on the scale of a decade or more and will be investigated further through the planned computer simulations.
 

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