EPA/600/A-93/212 DEVELOPMENT AND VALIDATION OF METHODS FOR REAL-TIME MEASUREMENT OF POLLUTANT TRANSPORT FROM AN URBANIZED ESTUARY Gregory A. Tracey1 Charles James1 Gerald Hatcher1 William Nelson2 'Science Applications International Corporation Marine Services Branch c/o U.S. Environmental Protection Agency 27 Tarzwell Dr. Narragansett, RI 02882 2U.S. Environmental Protection Agency Environmental Research Laboratory- Narragansett 27 Tarzwell Dr. Narragansett, RI 02882 ABSTRACT The potential remobilization of pollutants during dredging and disposal of contaminated sediments is of principal concern to environmental managers in the selection of dredging methods and disposal options. In this study, we describe methods for estimation of pollutant transport within New Bedford Harbor (NBH), an urbanized estuary between the cities of New Bedford and Fairhaven, MA. On eight separate dates corresponding to various dredging activities, current velocity and total suspended solids (TSS) profiles were taken across a narrow constriction (separating the upper and lower harbor) in conjunction with chemical sampling for dissolved and paniculate PCB analyses. Three methods of estimating PCB transport were compared. In the "6-point" method, current measurements and water samples were taken at three discrete depths for two cross-channel locations. In the "transect" method, continuous profiles of currents and suspended particulates were collected at five cross-channel locations. In the "hypsographic" method, depth-volume relationships were derived from NOAA bathymetry and USGS topography data bases, and changes in tidal height were used to estimate transport. Event-based (6-point and transect) methods yielded comparable estimates of water transport, indicating that over the duration of the ebb or flood exchange, the flow field was adequately characterized by a limited number of sampling locations. However, estimates of magnitude and direction of net transport were observed with these methods differed from that of the hypsographic method, indicating that event-based methods did not adequately integrate the temporal variation in the flow field. Significant differences were observed between discrete- (6-point method) and profile-based (transect method) TSS flux estimates, indicating that the 6-point method did not adequately characterize the spatial variation in TSS distribution under the Coggeshall St. Bridge. These results suggest that better estimation of water transport and TSS flux would be obtained by more elaborate sample compositing procedures during each sampling event and more frequent sampling events over the tidal cycle. The direction of net PCB flux was consistent among methods on the majority of dates and predicted magnitudes varied less than two-fold. Estimates of net corrected PCB flux for various dates suggested significant temporal variation independent of tidal asymmetry. Wind-driven tidal asymmetry may be an important factor contributing to this effect, and could alter transport by up to 80%. Best estimates of net TSS flux, based on the transect method, ranged from +3500 (up-harbor) to -3800 (down-harbor) kg cycle'1. Best estimates of the net fluxes of particulate, dissolved and total PCBs were based on the hypsographic method, and were observed to range from -31 to -98, +196 to -216, and +150 to -293 g cycle'1, respectively. INTRODUCTION The potential remobilization of sediment-associated contaminants during dredging and disposal of sediments is of principal concern to environmental managers in the selection of dredging methods and disposal options. Predictive evaluation of associated environmental impacts requires adequate assessment of the amounts and types of contaminants escaping the dredging and disposal site. In this study, we examine methods for estimation of particulate and contaminant transport within New Bedford Harbor, an urbanized estuary between the cities of New Bedford and Fairhaven, MA. Sediments of upper New Bedford Harbor are highly contaminated (e.g. 500-100,000 ppm) with polychlorinated biphenyls (PCBs) due primarily to the discharge from electrical component manufacturing facilities (Weaver, 1984). Accordingly, high contamination levels have been observed in commercially important fin- and shellfish species, resulting in closure of the fishery in the vicinity of the estuary. As a result of EPA Superfund site designation in 1982, several options for remediation of PCB's from New Bedford Harbor were investigated. These involved the construction of containment facilities in the upper harbor and the movement of harbor sediments by various types of dredges. During the study, current velocity and total suspended solids (TSS) data were collected in conjunction with chemical sampling for dissolved and particulate PCB analyses in order to assess dredging efficiency and to monitor the loss of PCBs into the lower harbor environment. The objectives of the present study are 1) to assess various measurement methods and make recommendations for estimating the flux of TSS and PCBs out of the upper harbor, 2) to provide 0-7803-0202-8/91/0000-0501$1.00©1991 IEEE ------- estimates of the amount and type of PCBs being transported, and 3) to assess the extent to which wind-related events may contribute to these losses. It is hoped that these results will be useful in the design of monitoring programs for estimation of paniculate or chemical fluxes in general, and for the remediation of the New Bedford Harbor Superfund Site in particular. MATERIALS AND METHODS Data collection The Coggeshall St. Bridge spans approximately 36 meters across a relatively narrow and shallow constriction of Upper New Bedford Harbor (Fig. 1). This constriction served as a natural boundary across which the flux of PCBs and TSS could be measured. On 8 separate dates corresponding to various phases of the Pilot Dredging Project, sampling events were carried out under the bridge at 5 equally-spaced time intervals between slack tides. Each hourly sampling event commenced approximately 10 minutes prior to the hour and was completed about 10 minutes after the hour. Figure 1. Reference map for New Bedford Harbor, MA identifying the remediation ("hot spot") and sampling (Coggeshall St. Bridge) locations. Figure 2. Sampling locations under the Coggeshall St. Bridge for 6-point and transect methods. Arrows indicate approximate depths for surface, middle and bottom discrete samples. Round Hill Pol Wilbur Roint Continuous profiles of current velocity and transmissivity were taken at 5 separate transect points positioned approximately 6 meters apart on the bridge (Fig. 2). A SEABIRD CTD instrument equipped with a Marsh-McBirney electromagnetic current meter (model 501) and a SEATEC 25-cm transmissometer was used. The current meter was laboratory calibrated against a laser doppler velocimeter (LDV) installed in a 20-m flume operated by Woods Hole Oceanographic Institution, and found to be accurate to + 1.0 cm sec"1 over a range of 0 - 75 cm sec"1. In order to determine a relationship between transmissivity and total suspended solids (TSS), the transmissometer was placed in a stirred container with 0.45-^m filtered seawater and its response to the addition of sediment collected from upper New Bedford Harbor was measured. As sediment was added to the sea water, the reduction in transmissivity was observed and recorded. The observed linear relationship (TSS = -0.19(% Trans.) - 16.5; r2 = 0.97) was used to convert field-measured transmissivity data to total suspended solids concentration. The SEABIRD system was deployed over the south side of the bridge with a davit attached to a motor vehicle equipped with 5 10 15 20 25 30 35 Distance from Cost Bank of Channel (m) a power winch and camper. Real-time data collection was achieved by connecting the SEABIRD data cable and deck unit to a personal computer. The instrument sampling rate was pre-set at 12 samples sec"1, and it was lowered at the rate of 0.25 m sec"1. Transect data were stored in the form of raw data files containing the water depth, north and east current vectors, and percent transmissivity. Because the transmissometer required complete submergence to give accurate results, it was necessary to neglect the first 50 cm of transmissivity data for each cast. Similarly, the positioning of current, transmissivity and pressure probes on the CTD housing restricted deepest measurements to within 30, 10 and 20 cm of the bottom, respectively. Data were adjusted for positions of the probes relative to the pressure sensor. Water samples for suspended particulate and chemical analyses were taken at the second and fourth transect location. A 1/2-inch teflon hose was attached to the CTD cage with inlet near the current and depth probe. Samples were manually pumped with a Viton-lined diaphragm pump (Guzzler model, Ryan-Herco). Previous studies of dissolved-particulate partitioning of metals had shown this pump did not significantly alter particle size-frequency distribution (Hunt, Battelle, pers. comm.). Three depths were sampled at each transect location. "Surface" and "bottom" samples were collected at 0.5 m below the surface and 1.0 m above the bottom, respectively. The "mid-depth" sample was collected at a depth equal to 1/2 of the total water column height at that time. Chemical analyses A composite sampling strategy was used to reduce the number of samples for chemical analyses. Potential spatial variation in PCB distribution was incorporated by compositing of water samples taken at the 6 discrete locations each hour. The six samples were flow-weighted to adjust for potential depth-dependent differences in transport according to the formula: 1.) V,,d = x 6000 ml; where V, „ is the volume of water from a given transect location (t) and depth (d) added to the composite, Su is the current velocity at a given transect (t) and depth (d), S^ is the sum of the 6 current velocity measurements and 6000 ml was the desired composite sample size. In the rare case where current flow was opposite of the expected direction, e.g. to the north during the ebb tide, Su was set = 0. In these cases, Vu would equal 0, and no water from the sample would contribute to the composites. After a sub-sample of 500 ml was taken from the composite for TSS measurement, 502 ------- 200 ml of the remaining sample was passed through a 0.22-^m filter. The procedure was repeated each hour until samples from the 5 hourly composites had been filtered, thus integrating temporal variation. At the end of the sample day, the filtered material and filtrate were analyzed for paniculate and dissolved PCS concentration by methods outlined in Palmquist, ej ii. (1987). Total PCB concentration was quantified as the total of Aroclors 1242 and 1254 in the dissolved and paniculate fractions, and expressed in units of fig, L"1. Transport analyses Three methods for estimating the net tidal transport cycle"1 were employed. A tidal cycle is defined here as one consecutive ebb and flood tide. The methods differ primarily in the relative characterization of spatial and temporal variability in the flow field under the bridge. Six-point method. In the "6-point" method, water transport (V) per tidal cycle was estimated from the sum of products of mean north current velocity (SJ and channel cross-sectional area (A,) measured each hour (t) according to the formula: 2.) V = S,(S, x A,) where (t = 1,2,..,,5) The cross-sectional area was found to increase linearly with water depth (y = 34.4X-53.3; r3 = 0.99). Only the north current velocity was used since the harbor at the bridge is oriented north- south, and thus any easterly component of the current would not contribute to net transport. By convention, northerly currents (flow to the north and into the upper harbor) were taken to be positive. Total TSS flux (T) for the ebb or flood tide was determined in a manner similar to transport: 3.) T = S,(V, x T,) where (t = 1,2,...,5) where V, is the hourly transport estimate, and T, is the mean hourly TSS concentration determined from water samples collected at each of the 6 discrete depths. Transect method. In the "transect* method, hourly profile data were used to create cross-sectional contours of north current velocity and transmissivity. A vendor-supplied software package (Surfer, Golden Software *) was used to transform the data into a 25 by 25 grid. A kriging technique was selected for final gridding as the method tended to be less influenced by noise or wide fluctuations in data than other methods (e.g. inverse distance). Within the kriging routine, an octal search method was used to select profile data points for grid value calculation. The octal method forces the gridding program to search for data both vertically within the profile and horizontally between adjacent profiles in the data set. This approach was necessary to remove spatial bias which would otherwise arise because the profile data were far more concentrated vertically than horizontally. The Surfer program automatically scales the grid dimensions to the maximum depth and channel width observed. A "blanking" file was used to exclude grid values outside of the channel banks and thus restrict grid dimensions to the actual channel bathymetry. Representative contour plots of current velocity and transmissivity obtained from gridded files are found in Figs. 3 and 4, respectively. The weighted mean transport (m3 sec"') through all grid cells in the vertical cross-sectional plane was calculated as the sum of products of current speed (m sec"1) x Area (m1) through each grid cell. The calculation was based on Simpson's 3/8 Rule using the Util program included in the Surfer package. Transport estimates were subsequently converted into units of L hr'. Similarly, the weighted average TSS for each hour was derived by calculating the sum of products of percent transmissivity and grid cell area for all grids and dividing by the channel cross-sectional area. Transmissivity data were converted into TSS concentration (mg L"1) using the calibration equation. As with the 6-point method, water transport and TSS flux were summed by hour to estimate total transport on the ebb or flood tide. Figure 3. Contour plot of current velocities (cm sec"1) under the Coggeshall St. Bridge on 13 June 1988, one hour after high tide. Figure 4. Contour plot of transmissivity (%) under the Coggeshall St. Bridge on 13 June 1988, one hour after high tide. Cr- a m m CHon n w Hypsographic method. In the "hypsographic" method, tidal transport was calculated from tide gage data and derived hypsographic (tidal height - tidal volume) relationships. Harbor bathymetry was obtained from the NOAA National Ocean Service database system. Additional 7.5 minute (1:24,000) digital elevation data were obtained from the USGS National Geographic Data Center to provide coverage from mean low water up to high tide, These databases were merged and gridded to 0.5 m contour intervals using the SEABEAM mapping programs at the Seafloor Mapping Development Center, University of Rhode Island (Fig. 5), Gridded data files were imported into the Surfer program to determine water volumes associated with each 0,5 m change in tidal height in order to construct a cumulative volume - tidal height relationship (Fig. 6), A second order quadratic curve was fit to the relationship. 503 ------- Figure 5. Three-dimensional representation of upper New Bedford Harbor bathymetry up to 3 m above mean low water. --*>. Figure 6. Water depth-tidal volume relationship for New Bedford Harbor north of the Coggeshall St. Bridge. £2.0 Y = 7.7x10' + 6.9x10°X + 8.4x1oV o X -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Tidal Height Relative to Mean Low Water (m) PCS flux calculations Total suspended solids flux was determined from the sum of products of hourly transport and TSS concentration as taken from either discrete water samples ("discrete" method) or from hourly transport and TSS concentration derived from continuous profile data ("profile" method). Dissolved, paniculate and total PCB flux was estimated from the product of total tidal transport by either the 6-point, transect or hypsographic method and the composite sample PCB concentration for each tidal phase. In all cases, net flux was determined as the difference between ebb and flood fluxes. Net PCB flux estimates were also corrected for differences in tidal asymmetry according to the formula of Teeter (1987): 4.) Fc = (Ce - Cf) x ((T. where Fc is the corrected flux, Ce, Cf are the ebb and flood PCB concentrations (/ig L'1), respectively, and Te, Tf are the ebb and flood transport estimates (L cycle"1), respectively. Climatological analyses To assess the potential effect of wind events on water transport, wind data during a typical winter period were obtained from a weather station maintained at the New Bedford Harbor hurricane barrier. For this period, tide gage records were obtained and decomposed into tidal and non-tidal components using standard tidal analysis routines and NOAA tidal coefficients applicable for the lower harbor. The non-tidal component or tidal "residual" is that fraction of the tidal amplitude not due to astronomical influences. Tidal residuals were compared against the wind data to elucidate climatological effects on tidal height, and hence, transport. RESULTS PCB and TSS concentrations Ebb and flood tide dissolved, paniculate and total PCB concentrations are summarized in Table 1. The observed range of paniculate PCB concentrations on the ebb tide (0.07 - 0.32 fig L'1) Table 1. Summary of chemistry, transport and total suspended solids (TSS) measurements for 8 sampling dates during the New Bedford Harbor pilot dredging study. Date 6/13/88 6/27/88 9/1/88 11/13/88 12/5/88 12/12/88 12/20/88 1/9/89 Tide Ebb Flood Ebb Flood Ebb Flood Ebb Flood Ebb Flood Ebb Flood Ebb Flood Ebb PI ortH PCB Concentration, lua I/'l Diss. 0.15 0.17 0.44 0.05 0.28 0.06 0.22 0.16 0.03 0.00 0.02 0.29 0.16 0.11 Part. 0.19 0.12 0.32 0.12 0.27 0.20 0.15 0.03 0.13 0.04 0.07 0.02 0.24 0.16 Total 0.35 0.29 0.76 0.18 0.55 0.26 0.36 0.19 0.16 0.04 0.08 0.31 0.39 0.28 Water Transport, (L x 10f) e^Pt, -973 787 -464 703 -880 1001 -751 597 -492 862 -1360 692 -1266 799 -842 Q Tl Trsct -929 860 -745 681 -844 841 -720 520 -650 820 -1193 640 -1209 807 -835 flST . HVPS. -808 985 -614 967 -850 961 -821 825 -461 667 -954 724 -943 826 -1128 1QA Total sus (ma Discrete 15.7 7.4 15.5 13.1 8.1 6.2 4.1 2.4 6.2 2.0 2.6 1.7 4.6 6.1 10.4 i A n . solids Profile 9.0 10.9 8.9 8.4 9.9 7.3 4.4 4.7 5.4 3.6 6.2 8.2 8.4 9.9 10.7 i n R 504 ------- for each date exceeded flood concentrations (0.02 - 0.20 j*g L4) in all seven cases. For dissolved PCBs, however, flood concentrations (0.00 - 0.29 jig L"1) exceeded the ebb concentration (0.02 - 0.44 ng L4) on three of seven dates. Total PCB concentrations on the flood tide (0.04 - 0.31 £ig L"1) were less than that of the ebb tide (0.08 - 0.76 pg L;1) on six of seven dates. Mean TSS concentrations measured from discrete water samples (1.7 to 15.5 mg L4) varied by a factor of two to three from those estimated from transmissivity profiles (4.7 to 10.9 mg L4). The coefficient of variation (CV) between estimates was 50.2%. This difference most likely reflects varying degrees in spatial integration of TSS distribution under the bridge by the two methods. Water transport The transport of water under the Coggeshall St. Bridge for 8 dates was observed to range from -1360 x 106 L on the ebb tide to +1000 x 10* L on the flood tide (Table 1). The coefficient of variation between 6-point and transect method predictions was small (13.9%), indicating close agreement among methods. The coefficient of variation between 6-point and hypsographie methods was 21.8%. A comparable variation coefficient was observed between the transect and hypsographie methods (23,4%). Thus, better agreement was observed between event-based (6-point and transect) methods than was observed between event-based methods and the hypsographie method. TSS fluxes The net flux of total suspended solids was calculated from the product of hypsographie method-based water transport and TSS concentration. Using discrete depth-derived (6-point) TSS data, net TSS flux was found to be out of the upper harbor in six of eight cases, and ranged from +3200 to -5500 kg cycle'1 (Fig. 7). In contrast, results using the profile-based (transect) method indicated net TSS flux was into the upper harbor in five of eight cases, and ranged from +3500 to -3800 kg cycle'1. Figure 7. Net total suspended solids flux under the Coggeshall St. Bridge during the NBH Pilot Dredging Study. net flux for this fraction was consistent among methods, being out of the upper harbor on the majority (6 of 7) of dates, and predicted magnitudes varied less than 50% among methods. Total PCB flux, calculated as the sum of dissolved and particulate fluxes, ranged from +150 to -440 g cycle"1 for the three methods (Fig. 10). The direction of net transport was again out of the upper harbor and consistent among methods on six of seven dates, while two-fold variation in magnitude among methods was observed. Net PCB fluxes based on hypsographie method transport estimates were corrected for tidal asymmetry in order to assess non-hydrologic related variability among sampling dates (Fig. 11). Corrected dissolved PCB flux was up-harbor on two of seven dates, ranging from +20 to +50 g cycle'1, and on five of seven dates, was down-harbor (-20 to -300 g cycle"1). Corrected particulate PCB flux was always down-harbor, ranging from -30 to -150 g cycle"1. Corrected total PCB flux was down-harbor on six of seven dates, and ranged from +20 to -450 g cycle"1. Figure 8. Comparison of net particulate PCB flux under the Coggeshall St. Bridge during the NBH Pilot Dredging Study as estimated by 6-point, transect, and hypsographie methods. 30Q _ 200 < 100 -2OO -30O Figure 9. Comparison of net dissolved PCB flux under the Coggeshall St. Bridge during the NBH Pilot Dredging Study as estimated by 6-point, transect and hypsographie methods. PCB fluxes The net fluxes of dissolved, particulate and total PCBs were calculated from the product of water transport for the three methods and PCB concentration. The direction of net particulate PCB flux was consistently out of the upper harbor, and ranged from -30 to -170 g cycle4 (Fig. 8). Factor of two to three variation was found among methods for a given date. Net dissolved PCB flux ranged from +200 to -290 g cycle'1 (Fig. 9). The direction of Climatological effects Relationships between wind stress and residual tidal height were examined. Data were extracted from a July 1988 time series for which both upper and lower harbor tide gage data were available to investigate the possibility of tidal amplification in the upper harbor relative to the lower harbor. Differences in tidal range between upper and lower harbors were minimal (Fig. 12). Thus, the data indicate that no differential tidal forcing effects were occurring in the harbor. Tidal residuals for a winter time 505 ------- Figure 10. Comparison of net total PCB flux under the Coggeshall St. Bridge during the NBH Pilot Dredging Study as estimated by 6-point, transect and hypsographie methods. Figure 11. Comparison of net corrected dissolved, paniculate and total PCB flux under the Coggeshall St. Bridge during the NBH Pilot Dredging Study by the hypsographie method. series were compared against wind data obtained during a period for which both northerly and southerly winds were apparent (January 7-14, 1988; Fig. 13). With the exception of the first 24 hr period, results show that wind events originating from the north (i.e. upward on plot) resulted in negative tidal residuals of up to -0.5 m, while southerly winds resulted in positive tidal residuals. DISCUSSION In this study, various methods for estimating PCB and TSS fluxes out of upper New Bedford Harbor were developed and compared. This effort was undertaken in order to validate previous flux estimates, as well as to recommend new approaches to estimating fluxes, if appropriate, when the remediation plan for the harbor is developed. Figure 12. Differences in tidal amplitude between upper and lower New Bedford Harbor during July, 1988. -0.1 -I D 5 1O 55 Figure 13. Wind stress-tidal residual relationships for an eight day winter storm period in New Bedford Harbor JT 20 H 0 -o J -20 - tf> -40 -J 7 >J 24 I | I «8 •" I ' ' ' I I ' ' I ' ' 72 96 120 Time (hoys) T1 H* •p- 166 Transport analyses Studies by Teeter (1987) observed net transport on three dates to range from -250 to +210 x 10* L. In the present study, net transport as estimated by the hypsographie method were somewhat similar (-330 to + 350 x 10* L) but covered a broader range in values. In both studies, depth-volume relationships were used to calculate transport from tide height data. The difference in observed ranges could be partly due to a greater number of days sampled in the present study, but also due to shape and slope of the hypsographie curve itself. Based on limited data, Teeter's study found a linear depth-volume relationship, whereas in the present study, a second-order quadratic provided the best fit to the data. With regard to slope, Teeter's study found tidal volume to change by 800 x Itf L m4 of tide height, whereas in the present study the volume is predicted to change by 688 - 1110 x 106 L rrr1, depending on tidal height. These differences could account for discrepancies in net transport estimates between studies. TSS flux analyses The transect-based net fluxes of TSS, which ranged from +3500 to -3800 kg cycle"', are comparable in magnitude with studies by Teeter (1987) who found the net flux of TSS on two dates to be about +2100 kg cycle4. Differences in the magnitude of observed TSS flux between studies may differ due both the method of water sampling and water transport estimates as discussed above. Our study, however, has found that net TSS transport may be either direction, a finding which may be due to a greater number of sampling days incorporated into the present study. PCB flux analyses Fluxes of PCBs beneath the Coggeshall St. Bridge were first investigated in 1983 by an EPA response team (USEPA 1983). From data obtained in their study, the net total PCB flux was calculated to be -0.83 kg per tidal cycle, with a range of -0.62 to -1.20 kg (Teeter, 1987). Additional studies were conducted in 1986 by the U.S. Army Corps of Engineers (Teeter, 1987). In that study, PCB fluxes of -0.32 and -1.27 kg per tidal cycle were observed. In a study by Battelle (1990), a model was used to simulate PCB flux at the Coggeshall St. Bridge and predicted -0.22 kg cycle"1. Fluxes for total PCBs found in the present study (+0.15 to -0.29 kg cycle"1) are low relative to previous findings, despite the fact that various dredging and construction activities were occurring. The current data suggest that no significant 506 ------- dredging-induced fluxes were occuring on these seven dates, and that these sampling periods may be representative of baseline conditions. Despite the correction for tidal asymmetry, net total PCB fluxes were still observed to vary by up to an order of magnitude among dates. In addition, paniculate PCB fluxes were always down-harbor while dissolved PCB fluxes were often up-harbor, suggesting that down-harbor dissociation of PCBs from particulates may have occurred and contributed to the up-harbor dissolved PCB flux. Thus, factors controlling PCB concentration and partitioning behavior would appear to have a greater effect on net PCB flux than does tidal asymmetry as previously reported (Teeter, 1987). Climatological effects The extent to which wind events affect net PCB flux out of upper New Bedford Harbor by altering transport was examined. From a data set collected during a winter storm period, north wind events were observed to cause lower tides in the region, while south winds caused higher tides in the region. This effect is most likely caused by seiching, or pushing of surface waters down wind, and appeared to cause tidal height at slack water to vary by up to 0.5 m. It is estimated that this residual could alter tidal transport on a given ebb or flood tide by an additional 320-580 x 106 L, or 80% of the tidal volume on a given tide. Comparison of methods In this study, it was assumed that net water transport was controlled primarily by changes in tidal height while the magnitude of transport is dependent on the depth-volume relationship. The upper harbor does receive some freshwater input from the Acushnet River, however even under high flow conditions the flow is a small fraction (e.g. 4.7%, Teeter, 1987) of the tidal volume. Thus, both the magnitude and direction of net tidal transport can be known with certainty given accurate tide gage and bathymetry data. Tide gage data were available from the study site for each sampling date to provide estimates of tidal height differences to ± 0.1 m. The accuracy of these data was verified by comparisons of soundings taken at the Coggeshall St. Bridge and with two tide gages maintained at the hurricane barrier in the lower harbor. Depth-volume relationships were derived from controlled bathymetric survey data with a single point precision of ± 0.3 m. It is assumed that the associated accuracy is substantially better since the grid data were obtained from multiple soundings. In addition, high resolution (1:24,000 scale) USGS topographic data specific for the study area were available to extend the bathymetric data set from the mean low water to the observed tide height. Thus, we are confident that the derived hypsographic curve provides the best estimates of net transport, and these data can be used to evaluate 6-point and transect predictions. Good agreement was observed between the 6-point and transect method predictions for both the direction and magnitude of transport, suggesting that the discrete sampling approach should provide adequate spatial integration of the flow field during a given sampling event. However, a similar comparison of TSS flux estimates revealed substantial differences in both direction and magnitude between methods, indicating the limited number of sampling locations incorporated into the 6-point method did not adequately integrate the spatial differences in suspended paniculate concentrations in the channel. This finding is consistent with occasional observations significant spatial variability in TSS concentration (e.g. Fig. 4), with highest concentrations being restricted to surface waters and/or sides of the channel. These results suggest future sampling designs should include broader spatial coverage than was done by the 6-point method in this study. Net transport predictions by the 6-point and transect methods were consistent with the hypsographic method only about half of the time, indicating that the 6-point and transect methods lack sufficient temporal coverage to adequately characterize net transport. Future discrete sampling programs should be intensified to include, at a minimum, hourly sampling through the complete tidal cycle. Order-of-magnitude variation observed in net corrected PCB flux between dates suggests that temporal variability may be important, and should also be considered in future sampling programs. CONCLUSIONS 1.) Comparisons of water transport by the 6-point and transect-based methods exhibited good agreement, with the coefficient of variation between methods being less than 14%. This suggests that the flow field under the bridge at a given time may be adequately characterized by the 6-point sampling strategy. 2.) Discrete- (6-point method) and profile-based (transect method) estimates of TSS concentration differed greatly; a coefficient of variation of 50% was observed between methods. Given that greater spatial measurement of TSS distribution under the Coggeshall St. Bridge was provided by the profile-based method, the results suggest that broader spatial coverage than that obtained by the 6-point method will be required in future sampling programs to estimate TSS and paniculate PCB net flux. 3.) The direction of net transport predicted by event-based (6-point and transect) methods and were often opposite to that known to occur from tidal height observations. This error is attributed to poor temporal sampling of the tidal cycle and suggests that broader temporal coverage should be incorporated into event- based sampling methods for elucidation of TSS and PCB fluxes. 4.) Profile-based estimates of net TSS flux ranged from +3500 to -3800 kg cycle'1. Hypsographic-based net fluxes of paniculate, dissolved and total PCBs were observed to range from -31 to -98, +196 to -216, and +150 to -293 g cycle'1, respectively, and did not indicate any dredging-induced effects. 5.) Wind-driven tidal asymmetry was observed to cause up to 0.5 m variation in tidal height. This tidal residual may cause transport on the ebb and flood tides to increase by up to 80%, and thus could also similarly affect net PCB flux. 6.) Hypsographic-based estimates of net corrected PCB flux, ranged from +20 to -450 g cycle"1 over seven dates. This suggests that factors other than tidal asymmetry may play a significant role in PCB flux dynamics, and assessment of day-to- day variation in PCB fluxes should be an important component of future sampling programs. 507 ------- ACKNOWLEDGEMENTS This manuscript is ERLN contribution No. 1326. The research was supported through EPA contract 68-C1-Q005 to Science Applications International Corporation, Ms, Patricia Cant, Project Officer. The contents of the manuscript do not necessarily reflect views or policies nor does mention of trade names or commercial products constitute endorsement or recommendation for use by the U.S. Environmental Protection Agency. LITERATURE CITED Battelle, 1990. Modeling of the transport, distribution and fate of PCBs and heavy metals in the Acushnet River/New Bedford Harbor/Buzzards Bay System. Volume II: Final Report to EBASCO Services, Boston, MA. Palmquist, R., K. Schweitzer, R. Bowen and R. Pruell, 1987. New Bedford Harbor Pilot Study Pre-operational Monitoring- Progress Report: Chemical analysis from the two pre- operational water samplings conducted in New Bedford Harbor during July, 1987. EPA Environmental Research Laboratory- Narragansett Internal Report. Teeter, A. 1987. Baseline conditions for contaminant and sediment migration, New Bedford Harbor, Massachusetts. USACE-Watenvays Experiment Station. Publ. No. WESHE-P, 1/26/87. USEPA, 1983. Aerovox PCB Disposal Site- Acushnet River and New Bedford Harbor, MA: Tidal cycle and PCB mass transport study, January 10-12, 1983. USEPA -Edison Laboratory, Edison, NJ Final report 3/4/83 Weaver, G. 1984. PCB contamination in and around New Bedford, Massachusetts. Environ. Sci. Technol. 18:22A-27A. 508 ------- TECHNICAL REPORT DATA (Please read Instructions on the reverse before completing) 1. REPORT NO. 2. EPA/600/A-93/212 - 4. TITLE AND SUBTITLE DEVELOPMENT AND VALIDATION C FOR REAL-TIME MEASUREMENT OF POLLUTANT TRANS URBANIZED ESTUARY F METHODS 5. REPORT DATE PORT FROH AN "' *" ' "" 6. PERFORMING ORGANIZATION CODE 7^UTHOR(S). _ „,,,„,, 8. PERFORMING ORGANIZATION REPORT NO, Uregory A. Tracey, Charles James, Gerald Hatcher, William Nelson ERLN-1326 9. PERFORMING ORGANIZATION NAME AND ADDRESS US EPA Environmental Research Laboratory 27 Tarzwell Drive Narragansett, RI 02882 12. SPONSORING AGENCY NAME AND ADDRESS 10. PROGRAM ELEMENT NO. 11. CONTRACT/GRANT NO. 60-C1-QOQ5 13. TYPE OF REPORT AND PERIOD COVERED Proceedinqs-peer reviewed 14. SPONSORING AGENCY CODE 15. SUPPLEMENTARY NOTES is. ABSTRACT The p^,^ remobilization of pollutants during dredging and disposal of contaminated sediments is of principal concern to environmental managers in the selection of dredging methods and disposal options. In this study, we describe methods for estimation of pollutant transport within New Bedford Harbor (NBH), an urbanized estuary between the cities of New Bedford and Fairhaven, MA. On eight separate dates corresponding to various dredging activities, current velocity and total suspended solids (TSS) profiles were taken across a narrow constriction (separating the upper and lower harbor) in conjunction with chemical sampling for dissolved and paniculate PCB analyses. Three methods of estimating PCB transport were compared. In the "6-point" method, current measurements and water samples were taken at three "~ discrete depths for two cross-channel locations. In the "transect" method, continuous profiles of currents and suspended particulates were collected at five cross-channel locations. In the "hypsographic" method, depth-volume relationships were derived from NOAA bathymetry and USGS topography data bases, and changes in tidal height were used to estimate transport. 17_ KEY WORDS AND DOCUMENT ANALYSIS a. DESCRIPTORS pollutant transport New Bedford Harbor estuary Massachusetts measurement dredging total suspended solids TSS PCB particulates 18. DISTRIBUTION STATEMENT RELEASE TO PUBLIC b.lDENTIFlEHS/OPEN ENDED TERMS C. COSATI Field/Croup UNCLASSIFIED K 20. SECURITY CLASS (This page > 22. PRICE UNCLASSIFIED EPA Foirn 2220-1 (R«». 4-77) PREVIOUS EDITION is OBSOLETE ------- |