Environ. Sci. Technol. 1999, 33, 3077-3085
Modeling Variability in 210Pb and Sediment Fluxes Near the Whites Point Outfalls, Palos Verdes Shelf, California SUSAN C. PAULSEN* AND E. JOHN LIST Flow Science Incorporated, 723 East Green Street, Pasadena, California 91101 PETER H. SANTSCHI Department of Oceanography, Texas A&M University, 5007, Avenue U, Galveston, Texas 77551
Many researchers have had difficulty interpreting sediment data collected from the Palos Verdes Shelf, southern California. Factors that have been difficult to reconcile include the distribution of 210Pb and metals, the depth and extent of bioturbation, and the rate of sedimentation. This paper presents a simple model that includes these elements and simulates the flux of 210Pb, sediment, and metals to the sea floor near the Whites Point wastewater outfalls. The model uses known particle and metals emission rates from the outfalls and 210Pb fluxes to the sediments that vary in proportion to the flux of sediment mass to the sea floor. Model-predicted metals and 210Pb concentration profiles in the sediments agree well with data from cores collected at three locations on the Palos Verdes Shelf between 1972 and 1997. The implication of the model results is that 210Pb fluxes to the sediments in this area have varied greatly over the past 60 years. The model suggests that subsurface 210Pb maxima and uniform 210Pb concentrations to depths within the sediments of roughly 30 cm have resulted from time-variable 210Pb fluxes to the sediments and relatively shallow bioturbation and that natural sedimentation rates are relatively high (roughly 500-1000 mg cm-2 yr-1, or about 0.7-1.3 cm yr-1 of unconsolidated sediment).
Introduction 210Pb dating has been applied extensively to sediments in the deep basins off the coast of southern California (1-6), and interpretation of data from these cores has been relatively straightforward. In contrast, data from cores collected on the Continental Shelf are much sparser, and interpretation has been much more difficult. 210Pb concentrations have been reported in cores collected on the Palos Verdes Shelf in about 1972 (2, 3), 1991 (7), 1992 (8), and 1996 and 1997 (9). Using 210Pb profiles in sediment cores, several authors have recently attempted to draw conclusions about the extent of bioturbation on the Palos Verdes Shelf. These sediments on the Shelf lie in the “shadow” of major municipal wastewater outfalls and have been subjected to significant anthropogenic influences since the late 1930s, including increased sediment, contaminant, and
* Corresponding author phone: (626)304-1134; fax: (626)304-9427; e-mail:
[email protected]. 10.1021/es990026u CCC: $18.00 Published on Web 08/04/1999
1999 American Chemical Society
organic carbon fluxes. Because of these influences, interpreting 210Pb profiles in cores has been problematic, and several authors have proposed that enhanced 210Pb scavenging in particles falling to the sea floor in the outfall shadow may be a factor (3, 9, 10). Nonetheless, Swift et al. (7) modeled sediment cores using constant 210Pb fluxes to the sediments and deep bioturbation, but analysis reveals large inconsistencies in their data, which are addressed later in the paper. In addition to modeling the sediments on the Palos Verdes Shelf using traditional 210Pb models, many of the estimates of sedimentation rates by others (particularly those presented in Table 2) were made on the assumption that p,p′-DDE (a congener of DDT) behaves as a conservative tracer and, thus, that DDE profiles within the sediments can be interpreted to give a sedimentation rate (8, 11). However, new research has shown that DDE biodegrades in the sediments of the Palos Verdes Shelf (12) and that degradation processes are enhanced as the redox potential of the sediments decreases (i.e., at depth). Therefore, DDE is decidedly not a conservative tracer, and sedimentation rates cannot be determined with any accuracy using DDE profiles in the sediments. Furthermore, many of the estimates of sedimentation rates on the Palos Verdes Shelf were made using cores collected with gravity coring devices (13), which may lose up to 18 cm of the surface sediments (see ref 13) and thus make it difficult to estimate the true depth to key features in the concentration profiles. The extensive record of detailed physical and chemical analysis results from sediment cores collected from the Palos Verdes Shelf provides a unique opportunity to evaluate and model the fluxes of sediment, 210Pb, and other tracer constituents in the sediments. The model presented in this paper was developed to reproduce tracer profiles observed in sediments and to provide thereby estimates of the rate of sediment deposition and the importance of bioturbation in the development of sediment tracer concentration profiles.
Study Area and Available Data The Palos Verdes Shelf is located off the coast of southern California (approximate latitude 33°42′ and longitude 118°22′). Because sediments on the Palos Verdes Shelf have been affected by treated wastewater discharged from the Whites Point Outfalls, these sediments have been routinely sampled over the last 30 years, and the data record for a variety of chemical constituents in these sediments is extensive. The flux of suspended solids from the outfalls is known with some degree of certainty from 1971 to the present and has been estimated for the period 1937-1970 (14). The flux of metals, including zinc and copper, from the outfalls is also known from 1971 to the present (14). Zinc profiles are presented for cores collected in 1970 (15), and zinc and copper profiles are presented for cores collected in 1993 (LACSD, unpublished data) and in 1996 and 1997 (16). The available data demonstrate quite clearly that zinc and copper, once deposited to the sediments, behave conservatively. For example, peak zinc emissions occurred in 1971 (see Figure 1); zinc concentrations just below the peak concentration (and therefore corresponding to deposition in about 1970) in a core collected at station 6C in 1987 differ by less than 3% from concentrations measured at the surface in core 21B (near station 6C) in 1970. Indeed, others have found that profiles of zinc in sediment cores correspond well to known mass emission rates and patterns (5, 17, 18). These results also call into question the deep bioturbation hypothesis, as deep bioturbation would result in a significant smoothing of metals profiles and a decrease in peak metal concentrations. VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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then moved into a layer below the mixed (bioturbated) layers. This layer is then advected downward one layer for each time step, and there is no further mixing. Bioturbation is therefore assumed to cause uniform mixing only in the top layers, with no mixing into or between any layers below the bioturbation zone (see additional discussion below). No decay or mass loss is incorporated into the model for sediment mass, zinc, or copper. Decay of the mass of 210Pb is simulated using a half-life of 22.3 years. Each layer within the model is described by the total mass of sediment contained within that layer. Sediment fluxes are determined in terms of a surface mass loading rate (mg cm-2 yr-1). A time step of 1 year was chosen. To compare model results with measurements from core samples, the cumulative mass in the simulated sediment core is converted to an equivalent depth; each layer has a thickness that may vary from 0.5 to 2.0 cm. For cores collected in 1970 and 1972, this conversion is accomplished using the measured sediment wet density in core P6C collected in about 1972 by Myers (19) at station 6C. For cores collected in 1987, 1992, 1996, and 1997, the observed sediment porosities in cores 6C, 5C, and 3C, as measured in 1996 and 1997 (9, 16), were used to convert the cumulative mass to an equivalent core depth.
FIGURE 1. (a) Emissions of total suspended solids (TSS) from the Whites Point Outfalls (14). (b) Emissions of zinc (Zn) and copper (Cu) from the Whites Point Outfalls. Data on zinc and copper emissions from 1971 to present from ref 14; zinc and copper emissions from 1937 to 1970 are estimated so as to produce modeled zinc and copper profiles that match those measured in cores collected from LACSD stations 3C, 5C, and 6C at depths in cores corresponding to this time period (see text for details). In this paper, 210Pb profiles are presented for cores collected in 1972-1973, 1991, 1992, 1996, and 1997 (see Figures 6 and 7). In the core collected in 1972 or 1973 (3), the supported 210Pb concentration was calculated from the measured total 210Pb concentration at depth in the core. [It should be noted that 210Pb concentrations in this core are very similar to the uniform 24 dpm/g throughout a 26-cm core length reported by Bruland et al. (2) for a core collected at roughly the same time at a location about 2 km from the outfalls.] Little information is given in Swift et al. (7) about the method of measurement of total 210Pb in cores collected from stations 3C and 6C in 1991, but it appears that supported 210Pb was measured for each core interval. The data from the 1992 cores (8) are particularly reliable, as both supported and total 210Pb were measured repeatedly in each core section using several highly sensitive detectors and measurements were repeated for four cores collected at each location, with little spread in the results. The data from the 1996 and 1997 cores were collected using subcores of box cores from stations 3C and 5C (October 22, 1996) and 6C (September 17, 1997). These analysis results are also considered highly reliable, and a full account is provided in Santschi et al. (9).
Description of the Model To simulate sediment, metals, and 210Pb fluxes to the ocean floor on the Palos Verdes Shelf, a model that uses annual depth increments was developed. The sediment input for a single year is represented in the model as a single well-mixed layer. The top sediment layers at the end of each year are mixed uniformly to form a well-mixed zone that simulates the effects of bioturbation over that year. The bottom-most layer of the mixed zone (representing a year’s time step) is 3078
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Natural background sedimentation rates must be assumed, as described below, to implement this model. Fitted parameters in the model are as follows: (i) the fraction of outfall solids emitted that settles to the sea floor at each location considered here; (ii) the fraction of zinc (or copper) that is associated with the particles settling at each specific location considered; (iii) zinc (or copper) fluxes from the outfalls prior to 1971; and (iv) an outfall enhancement factor that describes 210Pb fluxes to the sediments.
Model Calibration Trace metal profiles were used to calibrate the model to determine sedimentation rates, which were then used to simulate the 210Pb profiles. Since sediment mass fluxes (Fparticles, in metric tons per year) from the outfall are known from about 1937 and metals fluxes (Fmetals, also in metric tons per year) from 1971 (14), these parameters were used to determine the sediment and metals deposition fluxes to the Palos Verdes Shelf at stations 6C, 5C, and 3C. Bioturbation profiles were determined by comparing measured and modeled zinc and copper concentration profiles. At station 6C, this was accomplished by matching zinc profiles in a core collected in 1970, zinc and copper profiles from a core collected in 1993, and zinc and copper profiles in a core from 1997. At stations 5C and 3C, zinc and copper profiles from cores collected in 1996 were used. The zinc (or copper) concentration in the effluent from 1937 to 1971 was then estimated by determining the best fit to the core concentration below the peak metals concentration, which corresponds to the year 1971. Figure 1 shows the outfall emissions of suspended sediments (from LACSD data), zinc, and copper. From 1971 to the present, LACSD zinc and copper emissions and core sample data are used. Prior to 1971, discharge emission rates of zinc and copper are estimated from observed zinc and copper concentrations in sediment cores. These fitted zinc and copper emission rates are then used to model zinc and copper concentrations prior to 1971 for each of the three stations at which sedimentation was modeled. The purpose of the modeling of trace metal profiles is the calibration of a numerical model to determine sedimentation rates that are then used to simulate the 210Pb profiles. Therefore, it should not matter which metal is used for this purpose. While it would be more logical to use Pb to test the model, lead profiles in the sediments do not match outfall
TABLE 1. Sedimentation Rates Derived from Modeling Metals Profiles (Presented on a Mass Basis) and Comparisons with Estimates by Others
station
natural sedimentation ratea (mg cm-2 yr-1)
effluent-contributed sedimentation rate per 100 000 t of TSS emittedb (mg cm-2 yr-1)
max modeled sedimentation rate (1971) (mg cm-2 yr-1)
av sedimentation rate, 1937-1997 (mg cm-2 yr-1)
This Model 6C 5C 3C
500 700 900 500 700 900 700 900 1000
670 430 200 910 630 500 430 260 140
1610 1430 1230 2020 1760 1740 1430 1340 1240
980 1010 1040 1150 1150 1260 1000 1080 1100
Estimates by Others 6Cc
9-500
499-637
a
Note that the natural sedimentation rate was held constant for all years modeled. Three separate possible scenarios are presented (e.g., for station 6C, natural sedimentation rates of 500, 700, and 900 mg cm-2 yr-1 were modeled as three separate scenarios). b Note that the effluentcontributed sedimentation rate varies in proportion to the outfall TSS emission rate (see Figure 1). The three outfall-contributed sedimentation rates presented correspond to the three natural sedimentation rates. c See ref 10 for a data summary.
emission rates as closely as either zinc or copper. Consequently, the model fits to the lead profiles (not shown) are not as good as for copper and zinc. The reasons are likely related to the fact that lead in the sewage outfall was associated with different types and sizes of particles than zinc or copper (20). This would have produced fractionation of the metals (21), leading to different input functions into the sediments. The total amount of sedimentation from 1971 to present is fixed by the depth to the peak zinc (or copper) concentration. This sedimentation process is represented in the model as a combination of background (natural) sedimentation and effluent-enhanced sedimentation (including the enhancement of natural sedimentation by flocculation processes). The background sedimentation rate, Snat (g cm-2 yr-1), is the sedimentation rate that would occur in the absence of the outfall. The effluent-enhanced sedimentation rate, Seff (g cm-2 yr-1), is assumed to be proportional to the flux of particles (i.e., TSS, in metric tons per year) from the outfall, Fparticles, and includes both the direct sedimentation of outfall-derived particles and flocculation-enhanced sedimentation of natural particles, so that the total sedimentation rate, Stot, is given by
Stot ) Snat + Seff ) Snat + RFparticles Thus, when a value is assumed for Snat (in g cm-2 yr-1), the constant of proportionality [R, with units of g cm-2 (t of TSS)-1] is fit so that the depth to the peak zinc (or copper) concentration matches that observed in core samples. The concentration of zinc (or copper) that arrives on the sea floor at any location is represented in the model as the sum of the metal that arrives from the outfall and the metal that would arrive in the absence of the outfall. The metal concentration added to the sediments by the outfall at a particular location, [Me]effluent-derived (g g-1), is given by the following expression:
[Me]effluent-derived ) Fmetal flux of metal from outfall )β β flux of particles from outfall Fparticles In this expression, β is a unitless factor of proportionality that is determined for a particular location and is assumed constant with time. It is dependent upon the outfall dynamics, the particle size distribution in the effluent, and other factors.
The concentration of a metal arriving on the sea floor, [Me], is determined by the following expression:
[Me] )
)
[Me]effluent-derivedSeff + [Me]backgroundSnat Stot RβFmetal + [Me]backgroundSnat Snat + RFparticles
In this equation, Fmetal, [Me]background, and Fparticles are known. Both Fmetal and Fparticles are determined from LACSD data, as described above, and [Me]background is determined from measured zinc (or copper) concentrations below the effluentaffected layer in core samples; [Me]background is assumed to be constant with time (see additional discussion below). Thus, when Snat is assumed, the constants R and β are used to fit the depth to the peak metal concentration and the magnitude of that peak concentration, respectively. For each measured metal profile in the sediments, a number of combinations of Snat, R, and β were determined. Each combination produced a profile of metal concentration with a different shape, and the “best fit” profiles were selected and are presented here. A reasonable fit to core sample data can be obtained with a range of natural sedimentation rates, as presented in Table 1. Results for stations 6C and 3C are also shown graphically in Figures 2 and 3. Sedimentation rates significantly lower or higher than those shown in the table and figures produce worse model fits to the measured metals profiles in core samples. Note that the natural sedimentation rates calculated here are significantly higher than those given in Santschi et al. (9). This effect occurs because the slope to the pre-1950 210Pb data results in a lower limit (see ref 9) to the presentday contribution of natural sediment supply. In addition to enhanced natural sedimentation due to flocculation after outfall emissions began, additional sources of natural sediment (e.g., Portuguese Bend Landslide) may have been important after the mid-1950s, resulting in enhanced natural sedimentation rates. Table 1 also presents a comparison with available estimates by others and the modeled maximum and average sedimentation rates (on a mass basis) for the period during which the outfalls have been operational. Table 2 presents the range of sedimentation rates used in the model on a depth basis; these data are also compared to estimates by others, all of which were derived, as noted in the table, using profiles of either 210Pb or DDE, which was assumed to behave conservatively. VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Modeled and measured Zn and Cu concentrations in cores collected at or near station 6C. (a) Measured Zn concentrations from core 21B (near station 6C) collected in 1970 (15). (b and c) Measured Zn and Cu concentrations are the composite results of Zn and Cu concentrations measured in three gravity cores collected at station 6C in 1993 by LACSD (unpublished data). Depth of core has been offset by 7 cm to account for loss of surface sediments by gravity coring device. (d) Measured Cu concentrations from a core collected at station 6C in 1997 (16). It is noted that in all cases the modeled concentrations of metals at the sediment surface (corresponding to very recent inputs) are lower than concentrations measured in core samples (see Figures 2 and 3). This observation indicates that the outfall is not the only significant source of metals to recently deposited surface sediments. This phenomenon is discussed extensively in Santschi et al. (16) and prevents the evaluation of sedimentation rates using cumulative metals inventories from the peak concentration depth to the sediment surface. [Note that if it is assumed in the model that the concentration of metals on “natural” particles, [Me]background, has increased over time, a much better fit to the metals data at the sediment surface is obtained. The required increase in the concentration of metals on the natural particles ranges from a factor of about 2.5 (at station 3C) to about 10 (at station 5C) over pre-1937 levels. This observation is not surprising given the increase in the mass of metals that is associated with stormwater runoff and the development of the Los Angeles Basin since 1937 (22). Because the depth to the peak concentration is not changed by this modification, the fitting parameter R does not change. Similarly, because the concentration of metals on particles emitted in the early 1970s was much higher than the background concentration of metals, the parameter β does not change. Thus, this change has a significant effect only on the surface concentrations predicted by the model; it does not change the sedimentation rates developed using the model. Since there exist no hard data to support the magnitude of the increase in background metals concentrations, the results of this modification are not presented here.] The number of annual surface layers that were assumed to 3080
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be mixed by bioturbation processes was determined by using the model to obtain the best fit to the shape of the measured zinc and copper concentration profiles in the cores. With no bioturbation, concentration peaks are very narrow; with deep bioturbation (i.e., many layers mixing), concentration peaks are wider. The best fit to the zinc and copper profiles was obtained by mixing only the top two layers of sediment, roughly equivalent in 1971 to about the top 6 cm at station 6C, the top 7 cm at station 5C, and the top 4 cm at station 3C and equivalent in 1997 to the top 2-3 cm at all stations. This is in agreement with mixed layer depths measured using 234Th penetration depths, which were 2-6 cm at stations xs 6C, 5C, and 3C (9, 23). Finally, the zinc and copper emission rates prior to 1971 were estimated by fitting the modeled concentrations to measured concentrations at depth in the cores. Zinc profiles in sediment cores collected in 1970, zinc and copper profiles in cores collected in 1993, and Cu in a core collected in 1997 at station 6C are shown in Figure 2 (note that for the 1997 box core, results for zinc and copper were substantially similar, and only results for copper are shown here). Zinc and copper profiles in cores collected in 1996 at station 3C are shown in Figure 3. Results for station 5C (not shown) are similar. All except the 1993 cores were collected with box coring devices, which maintain the integrity of surface sediments. Data from cores collected in 1993 were offset by 7 cm to compensate for the loss of surface material by the gravity coring device used to obtain these cores. The value of 7 cm achieved the best fit between modeled and measured results.
assemblage. The enrichment of 210Pbxs in sewage particles is a consequence of a dilution effect, i.e., the sewage discharge plume generates an entrainment demand that is filled by a large influx of seawater (see ref 24). Sewage-derived particles are able to continue to scavenge 210Pb from the water column, which is continuously supplied by coastal upwelling at a concentration of about 0.1 dpm/L to the region (6). In the model, it is assumed that oceanborne particles are already at equilibrium with the 210Pb in the ocean water and that outfall-derived particles adsorb much of the remaining 210Pb from the water column. The model calculates the concentration of 210Pb to the sea floor using the following equation:
[210Pb] )
)
FIGURE 3. (a and b) Modeled and measured Zn and Cu concentrations in cores collected at or near station 3C in 1996 (16).
Application of the Model to Sediments
210Pb
Distribution in the
The sedimentation rates and bioturbation depths that were determined by matching the zinc and copper concentration profiles in the sediments were used to model the distribution of 210Pb in the sediments. Because many workers have noted a 210Pb anomaly in the Palos Verdes Shelf sediments, likely due to enhanced 210Pb fluxes caused by outfall dilution water fluxes and enhanced scavenging by effluent particles, it was assumed that 210Pb fluxes are a direct function of the enhanced particle sedimentation rate. Because enhanced sedimentation is very localized downcurrent from the ocean outfalls, the addition of effluent-derived particulate matter serves to “sweep” 210Pb from the large flux of relatively particle-free ocean waters that are drawn into the effluent mixing zone by the dilution demand of the outfall plume, which is of the order of 100-200 times the outfall flow rate. 210Pb concentrations in present-day surface sediments xs are lower than those of the early 1970s, when peak sewage particle loadings occurred, by a factor of about 3 (see Figure 4). The decrease since the 1970s has been accompanied by a concomitant decrease in the organic carbon content of the sediments. Although there are indications of a strong correlation (see Figure 4), the significance of this correlation depends on one data point (the 1972 value). The slope of the line shown in Figure 4 is about 300 dpm/g of OC, or 100 dpm/g of sewage matter [sewage matter is about 30% OC or 70% organic matter (19)], which agrees with our model assumptions (discussed below). While the enrichment at peak effluent particle emission rates might be partly due to a grain size effect (caused by enhanced 210Pb adsorption to finer effluent-derived particles), it suggests a mixing of a natural seawater particle population (with a 210Pb concentration of about 5-10 dpm/g) with a more enriched sewage particle
γSeff + [210Pb]naturalSnat Stot RγFparticles + [210Pb]naturalSnat RFparticles + Snat
All variables in this equation (except γ) have been determined from existing data or from the analysis of sedimentation rates using metals concentrations, as described above. The constant γ represents the “concentration factor” for 210Pb sedimenting to the sea floor at each location. It is determined for each location by matching the surface sediment concentration of 210Pbxs to concentrations measured in cores. This constant is then used to fit all cores at a given location. Typical concentrations for newly deposited particles in nepheloid layers overlying the sediments can be estimated as follows. Assume a constant particle-water partition coefficient, Kd, of about 106 cm3/g (25, 26), a sewage-derived 210Pb particle concentration of about 9 dpm/g at typical nearoutfall suspended particulate matter (SPM) concentrations of 10 mg/L, and about 50 dpm/g at SPM concentrations of 1 mg/L. The SPM concentrations of 10 and 1 mg/L correspond to an effluent dilution of about 10:1 and 100:1, respectively, and are in the range of suspended sediment concentrations observed above the bottom on the Palos Verdes Shelf (27). The sediment concentration on outfall-derived particles, calculated as [Pbp]/[SPM], can then be calculated from the total 210Pb concentration of 0.1 dpm/L and the following equation:
Kd )
[Pbp] [Pbd][SPM]
where [Pbp] is the particulate 210Pb concentration (dpm/L), [Pbd] is the dissolved 210Pb concentration (dpm/L), and [SPM] is the suspended particulate matter concentration (mg/L). Clearly, the relationship between the concentration of 210Pb xs in the sediments and the concentration of outfallderived particles is complex and will depend on the level of dilution in the wastewater plume, the concentration of particles in the wastewater plume, and the sedimentation characteristics of the outfall-derived particles, all of which will vary with time. To understand the individual effects of these factors, consider three cases. First, consider the hypothetical case of a constant mass flux of particles from the outfall but a changing effluent dilution. At low dilution levels, the mass of outfall-derived particles per unit volume of entrained seawater will be high, and the 210Pb content of those particles will be relatively low (e.g., about 9 dpm/g at 10 mg/L SPM concentrations). Thus, the 210Pb content of the sediments will be 10 dpm/g (an approximate upper bound on the current measured concentration of 210Pb in the sediments deposited at the surface in the absence of the outfall) or lower. At higher outfall dilutions, the mass of outfall-derived particles per VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Sedimentation Rates Derived from Modeling Metals Profiles (Presented on a Depth Basis)a and Comparisons with Estimates by Others station
natural sedimentation rate (cm yr-1)
effluent-contributed sedimentation rate per 100 000 t of TSS (cm yr-1)
max modeled sedimentation rate (cm yr-1)
av unconsolidated sedimentation rate (cm yr-1)
This Model 6C 5C 3C
0.68 0.95 1.22 0.70 0.98 1.26 0.88 1.13 1.26
0.91 0.58 0.27 1.28 0.88 0.70 0.54 0.33 0.18
0.1
0.34 (1994)
2.19/7.7b 1.94/6.8b 1.67/5.9b 2.83/9.6b 2.47/8.4b 2.44/8.3b 1.80/6.8b 1.69/6.4b 1.56/5.9b
1.33 (1937-1997) 1.37 (1937-1997) 1.41 (1937-1997) 1.61 (1937-1997) 1.61 (1937-1997) 1.77 (1937-1997) 1.26 (1937-1997) 1.36 (1937-1997) 1.39 (1937-1997)
Estimates by Others 6Cc 6Cd 6Ce 6Cg 5Cg 3Cc 3Cd 3Ce 3Cf 3Cg
0.70 (1994) 2.0
0.2 0.3 0.1
0.17 (1994)
0.2 0.35
0.40 (1994) 1.0
0.47 (1983-1993) 2.0 (1970-1983) 1.4-1.9 (1974-1994) 1.1-1.4 (1971-1997) 1.8 (1971-1996) 0.44 (1981-1991) 1.2 (1945-1970) 2.0 (1981-1992) 1.2 (1971-1997)
Modeled sedimentation rates are converted from the values given in Table 1 (in mg cm-2 yr-1) using station-specific surface porosity values measured in 1997 by Santschi (9, 16). b Maximum modeled sedimentation rates are presented in two forms: as equivalent depths based upon 1997 measured porosity profiles and as equivalent depths based upon 1972 measured wet density in core P6C, collected by Myers (19). Note that the wet density measured by Myers was exceptionally and atypically low at 0.21 g dry sediment cm-3 wet sediment at the sediment surface.c Natural sedimentation rates were estimated “by dividing the thickness of the Holocene transgressive sand deposit by the length of time since sea level rose to its current elevation (approx. 10,000 yr)” (33). The effluent-contributed sedimentation rate is the total amount contributed per year in 1994 and is not based upon known TSS emission rates from the outfall. Average sedimentation rates are based upon the movement of the lower shoulder of the DDE profile at station 3C for the period 1981-1991 and the movement of the peak of the DDE profile at station 6C for the period 1983-1993 (see ref 33). d The maximum modeled sedimentation rate represents the maximum likely current sedimentation rate (1994). The average sedimentation rate was calculated based on historical changes in p,p′-DDE profiles and the thickness of the effluent-affected layer. See ref 11. e The natural sedimentation rate at station 6C was estimated using four 210Pb measurements including an anomalous peak and three points below the peak in a single core. Values for maximum modeled sedimentation rate were estimated using the change in the depth to peak DDE concentrations between 1972 and 1981. Values for average sedimentation rate were estimated using 210Pb profiles in the sediments. See ref 8. f This sedimentation rate was estimated using molecular markers (alkylbenzenesulfonates and similar compounds) in sediment cores collected at or near station 3C in 1981 and 1992 (34). g Sedimentation rates estimated using various radionuclides. See ref 9, which also contains estimates of sedimentation rates for other locations and time periods. a
FIGURE 4. Relationship between surface 210Pbxs and organic carbon content of the surface layer of the sediment. Data for 1972 from refs 3 and 35; data for 1992 from ref 8; data for 1996 from ref 9. unit volume of entrained water will be much lower, and the 210Pb concentrations on those particles will be much higher (e.g., about 90 dpm/g at 0.1 mg/L SPM). In this case, the 210Pb content of the surface sediments will be between about 10 and 90 dpm/g, depending upon the ratio of the mass of natural to outfall-derived particles that sediment out onto the sea floor. A second case to consider is that of a constant wastewater flow rate from the outfall and a changing outfall particle flux. If the concentration of particles in the wastewater emitted from the outfall is initially high but decreases over time (say 3082
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from 300 to 50 mg/L), the 210Pb content of the outfall-derived particles will increase at a given location in the plume. The amount of entrained seawater (and 210Pb) is a function of the wastewater emission rate, which in this case remains constant at a given location. However, the number of particles available to capture 210Pb decreases, so there are fewer particles ‘competing’ for the available 210Pb in the seawater. Finally, consider the case of a change in the characteristics of the particles emitted from the outfall. For example, improvements in wastewater treatment will remove many of the larger, coarser particles, resulting in generally finer particles emitted from the outfall. Finer particles will sediment toward the sea floor more slowly than larger particles, so that even if the 210Pb content of the particles is higher, they may remain in the water column longer before settling out, and the 210Pb content of the sediments at a given location may change as the outfall particle characteristics change. Despite this complexity, the analysis presented here represents a first-order approximation of the physical processes that occur on the Palos Verdes Shelf. At moderate to high values of dilution of the wastewater plume, outfallderived particles will have significantly enhanced 210Pb concentrations, which will result in an increase in the 210Pb concentrations in the sediments over the natural background levels that would occur in the absence of the outfall. The possibility that 210Pb might already be enriched in sewage particles was also considered. Groundwater from Metropolitan Water District of Southern California supplies in the Chino Basin, which is recharged from the Jurupa and San Gabriel Mountains, has average radon-222 concentra-
210
FIGURE 5.
210Pb
and total sediment fluxes used in the model at
tions of about 1000 dpm/L (28). This concentration of radon is sufficient to result in 0.1 dpm/L of 210Pb after 1 day of storage and about 1 dpm/L after a 10-day storage time [210Pb concentrations grow as [210Pb] ) [222Rn](1 - exp(-λ210∆t)), where λ210 ) 0.031 yr-1]. Although this water source could result in slightly elevated 210Pb concentrations in sea floor sediments, the effect is not large enough to account for the enhanced 210Pbxs concentrations observed in cores collected since the early 1970s. The sediment and 210Pbxs fluxes to the sea floor at stations 6C and 3C as used in the model are shown in Figure 5. Measured 210Pbxs profiles in cores collected at or very near to station 6C in about 1972, 1991, 1992, and 1997 are presented along with model-predicted results in Figure 6. Modeled
Pbxs profiles and measurements made at station 3C in 1991, 1992, and 1996 are plotted in Figure 7. Agreement between measured and modeled 210Pbxs results is excellent in the more recent (>1960) sediments, particularly for stations 5C (not shown) and 3C. Total sedimentation rates at stations 5C and 3C were determined using zinc and copper profiles in sediment cores collected using a box coring device, which retains surface sediments. Metals concentrations in 1970 and 1993 at station 6C, by contrast, were measured in core samples collected using a modified gravity corer (see ref 29 for a description), which likely lost the top 5-10 cm of sediments in the sampling process. Although an attempt was made to correct for the loss of surface sediments by the gravity corer (by assuming a uniform 7 cm loss), the amount of the loss can vary from core to core, and sedimentation rates are estimated less accurately at station 6C than at stations 5C and 3C. Total sedimentation rates were also estimated using copper concentrations measured in a box core collected from station 6C in 1997 (16). Note that this core does not show a distinct peak (see discussion in ref 16), as do the other cores from this location, leading again to difficulty in predicting sedimentation rates as reliably at station 6C as at stations 5C and 3C. Despite the changes that have occurred since the start of outfall operations, agreement is good at stations 5C (not shown) and 3C and reasonable at station 6C. The location and configuration of the outfalls has changed several times since outfall operations began in 1937. The initial discharge was from an open pipe in water of about 34 m (110 ft) depth, but the outfall configuration was changed several times, ultimately to a multiport diffuser in water 60 m (197 ft) deep in the mid-1960s. Sedimentation dynamics have likely also
stations 6C and 3C. FIGURE 6. Modeled and measured sediment 210Pbxs profiles at or near station 6C. (a) Measured data collected from station m6 (near station 6C) in 1972 or 1973 (3). (b) Measured data collected from station 6C in 1991 (7). (c) Measured data in cores collected from station 556 (near station 6C) in 1992 (8). Note that all cores except 179-G1 were collected in a box coring device and likely include surface sediments. Core 179-G1 was collected with a gravity corer and likely lost a significant portion of the surface sediments, perhaps as much as 18 cm (see ref 13). (d) Measured data collected from station 6C in 1997 (9). VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Modeled and measured sediment 210Pbxs profiles at or near station 3C. (a) Measured data from station 3C in 1991 (7). (b) Measured data box cores collected from station 522 (near station 3C) collected in 1992 (8). (c) Measured data from station 3C in 1996 (9). changed significantly. Prior to 1971, emissions of total suspended solids from the outfalls are known with less certainty, bioturbation depths and zinc and copper emission rates are only estimated, and treatment processes were somewhat basic, resulting in the discharge of particles with very different characteristics from modern discharges. The good agreement of model results with observed concentrations indicates that these effects (not incorporated into the model) are second-order effects and that the model incorporates the primary factors of importance. Assumptions commonly used by 210Pb dating models, such as constant 210Pb fluxes or constant 210Pb concentrations on settling particles, were also tested in the model. These assumptions produced 210Pb profiles that (below the top 6 cm or so) more strongly resembled profiles reported for 3084
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undisturbed sediment cores (e.g., those collected in deep, anoxic basins that are relatively unaffected by sewage discharges) and that were unrelated to the 210Pb profiles measured in cores from stations 6C, 5C, and 3C. On the Palos Verdes Shelf, estimates of 210Pb fluxes to the sediments have been made only by Murray (3) for core m6 (near station 6C) collected in about 1972. In this core, the 210 Pb flux to the sediments was calculated from the 210Pb profile over the length of the core and the average density and water content in the core. Murray’s value of 11.0 dpm cm-2 yr-1 is somewhat lower than the average annual 210Pb flux of about 20.0 dpm cm-2 yr-1 derived from the model presented here for the period 1945-1972, covering roughly the same period of time as contained in Murray’s core. (The model predicts a range of 8.1-28.4 dpm cm-2 yr-1 for the time period 1945-1972.) Note that the measured sediment wet density in core P6C, collected in 1972 (19), varies greatly (approximately 6-fold) in the top 20 cm of the core. The use of average density and water content in this core by Murray likely underestimated the 210Pb flux to the sediments because surface sediments were significantly less dense than underlying sediments in 1972 (19). Even so, this flux is about 80 times the atmospheric flux of 210Pb to this general area (30). Although it is well-documented that this area is a site of natural radionuclide focusing due to enhanced particle fluxes sweeping through advected waters originating from coastal upwelling (e.g., ref 31), 210Pb dating using traditional, constant-flux models is appropriate only if this enhancement effect is constant over time (e.g., ref 32). On the Palos Verdes Shelf in the shadow of the Whites Point Outfalls, this assumption is obviously violated due to additional enhancement by the time variable sewage particle input to the Palos Verdes shelf. For this reason, the 210Pb profiles cannot be treated with standard models, as was done by Swift et al. (7). These authors modeled sediment cores using constant 210Pb fluxes to the sediments (210Pb data replotted here in Figures 6b and 7b for stations 6C and 3C, respectively) and concluded that the top 18-28 cm were nearly fully mixed by bioturbation and that bioturbation occurred to lesser extents to depths of greater than 40 cm. However, this interpretation leads to serious inconsistencies with the model of Niedoroda et al. (10) and with the dates for the ΣDDT (sum of o,p′-DDT, o,p′DDD, o,p′-DDE, p,p′-DDT, p,p′-DDD, and p,p′-DDE) peak positions at sites 6C and 3C, described by the same authors. For example, based on the 210Pb profiles, Swift et al. (7) assign the peak concentrations of DDT in the sediments to 1950 rather than to 1970, when peak emissions occurred. The deep bioturbation hypothesis is also inconsistent with the concentration profiles of metals such as zinc, copper, cadmium, and lead.
Acknowledgments This work was supported financially by the Montrose Chemical Corporation of California, Stauffer Management Company, Rhone-Poulenc, and Chris-Craft Industries.
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modeling of the natural recovery of the contaminated effluentaffected sediment, Palos Verdes Margin, Southern California; WHOI Expert Report, 1994. Santschi, P. H.; Guo, L.; Asbill, S.; Allison, M.; Perlet, B.; Wen, L.-S. Environ. Sci. Technol., in review. Niedoroda, A. W.; Swift, D. J. P.; Reed, C. W.; Stull, J. K. Sci. Total Environ. 1996, 179, 109-133. Drake, D. E.; Sherwood, C. R.; Wiberg, P. L. Predictive modeling of the natural recover of the contaminated effluent-affected sediment, Palos Verdes Margin, Southern California; USGS Expert Report, 1994. Quensen, J. F.; Mueller, S. A.; Jain, M. K.; Tiedje, J. M. Science 1998, 280, 722-724. Lee, H. J. The distribution and character of contaminated effluentaffected sediment, Palos Verdes Margin, Southern California; USGS Expert Report, 1994; 237 pp + appendices. County Sanitation Districts of Los Angeles County (LACSD). Palos Verdes Ocean Monitoring Annual Report for 1996, report and appendices; LACSD Annual Report, 1997. Galloway, J. N. Ph.D. Thesis, University of California, San Diego, 1972; 143 pp. Santschi, P. H.; Wen, L.-S.; Asbill, S.; Guo, L. Environ. Sci. Technol., in review. Stull, J. K.; Baird, R. B.; Heesen, T. C. J. Water Pollut. Control Fed. 1986, 58, 985-991. Finney, B. P.; Huh, C.-A. Environ. Sci. Technol. 1989, 23, 294303. Myers, E. P. Ph.D. Thesis, California Institute of Technology, Pasadena, CA, 1974, p 179. Faisst, W. K. Ph.D. Thesis, California Institute of Technology, Pasadena, CA, 1976, 193 pp. Katz, A.; Kaplan, I. R. Mar. Chem. 1981, 10, 261-299. Air-water-land relationships for selected pollutants in Southern California; Huntzicker, J. J., Friedlander, S. K., Davidson, C. I., Eds.; Final Report, W. M. Keck Laboratories, California Institute of Technology: Pasadena, CA, 1975; p 219. Wheatcroft, R.A.; Martin, W. R. J. Mar. Res. 1996, 54, 763-792. Fischer, H. B.; List, E. J.; Koh, R. C. Y.; Imberger, J.; Brooks, N. H. Mixing in Inland and Coastal Waters; Academic Press: Orlando, FL, 1979; p 483.
(25) Bacon, M. P.; Belstock, R. A.; Bothner, M. H. Deep-Sea Res. II 1994, 41, 511-536. (26) Santschi, P. H.; Guo, L.; Walsh, I. D.; Quigley, M. S.; Baskaran, M. Cont. Shelf Res. 1999, 19, 609-636. (27) 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 Predictive modeling of the natural recovery of the contaminated effluent-affected sediment, Palos Verdes Margin, Southern California; USGS Expert Report, 1994. (28) Berelson, W. M.; Hammond, D. E. A technique for the rapid extraction of radon-222 from water samples and a case study. In Radon, Radium, and other Radioactivity in Groundwater. Hydrogeologic Impact and Application to Indoor Airborne Contamination; Graves, B., Ed.; Proceedings of NWWA Conference, April 7-9, 1987; Lewis Publ.: Somerset, NJ, 1987; pp 271281. (29) Bascom, W.; Mardesich, J.; Stubbs, H. An improved corer for soft sediments; Southern California Coastal Water Research Project (SCCWRP) Biennial Report; 1981-1982, pp 267-271. (30) Fuller, C.; Hammond, D. E. Geophys. Res. Lett. 1995, 10, 11641167. (31) Bruland, K. W.; Franks, R. P.; Landing, W. M.; Soutar, A. Earth Planet. Sci. Lett. 1981, 53, 400-408. (32) Appleby, P. G.; Oldfield, F. Catena 1978, 5, 1-8. (33) Sherwood, C. R.; Drake, D. E.; Wiberg, P. L. Additional results from the one-dimensional model of bed-sediment contamination profiles; Supplement to USGS Expert Report Predictive modeling of the natural recovery of the contaminated effluent-affected sediment, Palos Verdes Margin, Southern California; 1996. (34) Eganhouse, R. P. ACS Div. Environ. Chem. Prepr. Ext. Abstr. 1996, 36, 145-148. (35) Eganhouse, R. P.; Kaplan, I. R. Mar. Chem. 1988, 24, 163-191.
Received for review January 11, 1999. Revised manuscript received June 9, 1999. Accepted June 21, 1999. ES990026U
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