Correspondence Comment on “In Situ Measurements of Chlorinated Hydrocarbons in the Water Column off the Palos Verdes Penninsula, California” SIR: Zeng et al. (1) recently concluded that DDT compounds measured in waters overlying the Palos Verdes Shelf indicated a significant net flux of ∑DDT from the sediments and away from the Shelf (more than 400 kg/yr). Assuming that Zeng et al.’s data are reliable, this conclusion is not supported by these data and current meter records for the Shelf (2), which imply a lower flux of DDE leaving the Shelf at the NW boundary (station 0C) than entering at the SE boundary (station 9C) and little or no cross-shelf flux. The logical fallacy in Zeng et al.’s argument about the flux from the sediment is illustrated by a consideration of the suspended sediment concentrations in the water column (3). Applying Zeng et al.’s approach to the sediment concentrations would lead to the conclusion that the Shelf is eroding at a prodigious rate, when in fact the opposite is true (4-6). Zeng et al. discarded the alternate hypothesis that new DDT is being deposited on the sediments. As will be demonstrated below, this alternate hypothesis is actually very consistent with all of the available data. Shelf Sedimentation Dynamics. Current sedimentation rates on the Palos Verdes Shelf are approximately 800 mg cm-2 yr-1 as shown by radioisotope data (4, 5) or up to 2 cm yr-1 as shown by molecular markers (6). If this new sediment contained no DDT and there was a net loss of 400 kg/yr through the sediment surface, as Zeng et al. conclude, surface sediment ∑DDT concentrations should be declining. However, along the 60-m isobath, surface sediment ∑DDT concentrations have remained largely constant since 1981 except at station 7C (7), and concentrations are nearly uniform in the top 15-25 cm of cores (see Figure 1) (7), well below the 2-6-cm bioturbation depth (4). Repeated measurements have shown that metals inventories and peak metals concentrations at depth in sediment cores have remained approximately constant, especially near the outfall (LACSD, unpublished data; see also ref 5). Thus, movement of sediment particles within the sediment bed cannot explain the observed decline in peak DDT concentrations (7) and DDT inventories, because the peak particle-bound metals concentrations, which are coincident with the peak DDE concentrations, have remained in place. No mechanism that could cause a net transfer of DDT from the sediments to the overlying water (e.g., molecular diffusion, resuspension and desorption, transfer by bioturbation or irrigation, or sediment erosion) is at all consistent with observed sedimentation rates and sediment DDT and metals concentrations. Thermodynamic Driving Force. Zeng et al. argue that if the equilibrium water column concentration (based on literature Kow values) exceeds the measured water column concentration of ∑DDT compounds, a thermodynamic driving force exists for diffusion of ∑DDT out of the sediments. However, measured sediment porewater p,p′-DDE concentrations are higher near the sediment surface (∼80-100 ng/ L) than below about 5-10 cm depth (∼10 ng/L) in three cores collected by the USGS near site 6C, despite the fact that the p,p′-DDE concentration on sediment particles is higher deeper into the sediments (Figure 1) (8). These data indicate an apparent driving force for diffusion in porewater from the sediment-water interface into the sediments (9). 10.1021/es990699c CCC: $18.00 Published on Web 09/23/1999
1999 American Chemical Society
FIGURE 1. Porewater profile of p,p′-DDE from core 157-W1 at a site near station 6C (8). Note that the more distant site 3C did not show any measurable porewater concentrations (i.e., e 10 ng/L). Sediment profiles of p,p′-DDE are presented for the same core (157-W1) and for a nearby core (147-B3) that extends to the depth of maximum p,p′-DDE concentration (5). Furthermore, these measured porewater concentrations imply values of the partition coefficient (Kp ) focKoc) for Palos Verdes sediments (4.91 e log Kp e 6.38 at station 6C) (8) that agree with values from independent diffusion tube experiments for these sediments (log Kp ∼ 5.83) (10). Higher than expected Kp values are possibly related to the substantial amount of soot carbon in these sediments (11), which, like activated carbon, can sorb planar aromatic molecules much more strongly than sedimentary organic matter (12). Using the Karickhoff et al. formulation (see ref 1), the values of log Kow for p,p′-DDE implied by the porewater and diffusion tube experiments lie in the range of 6.15-7.40. These values are much closer to the Kow measured by others for strongly hydrophobic compounds (e.g., log Kow ∼ 6.96 for p,p′-DDE; see ref 13) than to the value used by Zeng et al. (log Kow ) 5.83). Use of this higher log Kow for DDE reduces the value of log (Cw,e/Cw) in Zeng et al.’s Figure 4 to close to zero, negating the thermodynamic driving force argument for loss of DDT from the sediments. Alternate Sources of DDT. Zeng et al. (1) cite sediment PCB/DDT ratios measured by Eganhouse et al. (14) in support of the proposition that land-derived runoff is not a significant source of DDT. However, Eganhouse et al. evaluated only sediment from inner Los Angeles Harbor, which receives little land-based runoff and contains industrial sources of PCBs. Major rivers in the region (e.g., the Santa Clara, Santa Ana, Los Angeles, and San Gabriel Rivers) supply large quantities of wash load sediment to the Southern California Bight, drain agricultural soils that contain high concentrations of DDT (15), and discharge water with significant DDT concentrations (16, 17). Turbid plumes generated by river drainage after major storm events spread sediment over large areas of the Bight (18), and the heavy mineral fraction of the sediments on the Palos Verdes Shelf is rich in hornblende (19), a mineral that originates primarily from regional rivers. Additionally, it has been demonstrated that the mixing of the outfall plume with surrounding seawater scavenges 210Pb from seawater, concentrating it in the sediments in the “footprint” of the Whites Point outfalls (5). A similar mechanism is likely responsible for elevated surface sediment concentrations of ∑DDT. This hypothesis is further supported by the data of Zeng et al. (1), which show that water column DDT concentrations are low immediately upstream of the VOL. 33, NO. 21, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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outfall (station 9C), highest in the high deposition zone downstream of the outfall (station 6C), and decrease with distance downstream of the high deposition zone (i.e., 6C g 5C > 3C > 0C), as the effluent particles that have scavenged DDT from the seawater settle to the sea floor. Thus, it is likely that external sources supply significant amounts of ∑DDT to Palos Verdes Shelf sediments and that the flux of DDT leaving the Palos Verdes Shelf is 2 orders of magnitude lower than the 400 kg/yr suggested by Zeng et al. Zeng et al.’s theoretical flux therefore cannot account for the observed losses of 5-10 ton/yr of ∑DDT from these sediments (see ref 1). Zeng et al.’s flux estimate clearly conflicts with available evidence on sedimentation rates, thermodynamic driving forces, and the observed distribution of ∑DDT in the water column. Biodegradation therefore cannot be discounted as a major loss mechanism in these sediments (see ref 20). Indeed, it appears from the available evidence that biodegradation is almost certainly responsible for the observed DDT mass losses and that external sources are the cause of persistent, elevated ∑DDT concentrations in surface sediments.
Literature Cited (1) Zeng, E. Y.; Yu, C. C.; Tran, K. Environ. Sci. Technol. 1999, 33 (3), 392-398. (2) Noble, M. Circulation patterns over the Palos Verdes Shelf and upper slope. Appendix H to Drake, D. E.; Sherwood, C. R.; Wiberg, P. L. Predictive modeling of the natural recovery of the contaminated effluent-affected sediment, Palos Verdes Margin, Southern California; USGS Expert Report; 1994. (3) Drake, D. E. Geoprobe system description, instrument calibrations, data processing, and field results for deployments on the Palos Verdes Margin in winter 1992/1993. Appendix G to Drake, D. E.; Sherwood, C. R.; Wiberg, P.L. Predictive modeling of the natural recovery of the contaminated effluent-affected sediment, Palos Verdes Margin, Southern California; USGS Expert Report; 1994. (4) Santschi, P. H.; Guo, L.; Asbill, S.; Allison, M.; Perlet, B.; Wen, L.-S. Accepted for publication in Environ. Sci. Technol. (5) Paulsen, S. C.; List, E. J.; Santschi, P. H. Environ. Sci. Technol. 1999, 33 (18), 3077-3085. (6) Eganhouse, R. P. ACS, Environ. Chem. Div. Abstr. 1996, 36 (1), 145-148. (7) Lee, H. J. The distribution and character of contaminated effluent-affected sediment, Palos Verdes Margin, Southern California; USGS Expert Report; 1994. (8) Eganhouse, R. P. Deposition testimony on May 20, 1998, and October 6, 1998, in United States of America et al. vs Montrose Chemical Corporation of California et al.
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(9) Berg, P.; Risgaard-Petersen, N.; Rysgaard, S. Limnol. Oceanogr. 1998, 43 (7), 1500-1510. (10) U.S. Army Corps of Engineers. Options for In-situ Capping of Palos Verdes Shelf Contaminated Sediments; Draft Report submitted to US EPA; September 1998. (11) Gustafsson, O ¨ .; Gschwend, P. M. Geochim. Cosmochim. Acta 1998, 62, 465-472. (12) Gustafsson, O ¨ .; Haghseta, F.; Chan, C.; Macfarlane, J.; Gschwend, P. M. Environ. Sci. Technol. 1997, 31, 203-209. (13) DeBruijn, J.; Busser, F.; Seinen, W.; Hermens, J. Environ. Toxicol. Chem. 1989, 8 (6), 499-512. (14) Eganhouse, R.; Gossett, R.; Hershelman, G. P. Congener-specific characterization and source identification of PCB input to Los Angeles Harbor; Report to California Regional Water Quality Control Board, Monterey Park, CA, 1990; 51 pp; Contract 70184140-0. (15) Mischke, T.; Brunetti, K.; Acosta, V.; Weaver, D.; Brown, M. Agricultural sources of DDT residues in California’s environment; Report prepared in response to House Resolution No. 53 (1984) by Environmental Hazards Assessment Program, California Department of Food and Agriculture, Sacramento, CA, 1985. (16) Cross, J. N., Francisco, C., Eds. In Southern California Coastal Water Research Project, Annual Report 1990-91 and 1991-92; SCCWRP: Westminster, CA, 1992. (17) State Water Resources Control Board. California State Mussel Watch Ten Year Data Summary 1977-1987; Water Quality Monitoring Report 87-3; SWRCB: May 1988. (18) Hickey, B. M.; Kachel, N. B. Cont. Shelf Res. In review. (19) Wong, F. Heavy-Mineral Provinces of the Palos Verdes Area, Southern California. Appendix J to Drake, D. E.; Sherwood, C. R.; Wiberg, P. L. Predictive modeling of the natural recovery of the contaminated effluent-affected sediment, Palos Verdes Margin, Southern California; USGS Expert Report; 1994. (20) Quensen, J. F., III; Mueller, S. A.; Jain, M. K.; Tiedje, J. M. Science 1998, 280, 722.
S. C. Paulsen* and E. J. List Flow Science Incorporated 599 North Fair Oaks Avenue Pasadena, California 91103
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