Environ. Sci. Technol. 2003, 37, 3413-3421
Modeling the Transport of PCDD/F Compounds in a Contaminated River and the Possible Influence of Restoration Dredging on Calculated Fluxes OLLI MALVE,* SIMO SALO, AND MATTI VERTA Finnish Environment Institute, P.O. Box 140, FIN-00251, Helsinki, Finland JOHN FORSIUS Fortum Engineering Ltd, P.O. Box 10, 00048 Fortum, Finland
River Kymijoki, the fourth largest river in Finland, has been heavily polluted by pulp mill effluents as well as by chemical industry. Loading has been reduced considerably, although remains of past emissions still exist in river sediments. The sediments are highly contaminated with polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated diphenyl ethers (PCDEs), and mercury originating from production of the chlorophenolic wood preservative (Ky-5) and other sources. The objective of this study was to simulate the transport of these PCDD/F compounds with a one-dimensional flow and transport model and to assess the impact of restoration dredging. Using the estimated trend in PCDD/F loading, downstream concentrations were calculated until 2020. If contaminated sediments are removed by dredging, the temporary increase of PCDD/F concentrations in downstream water and surface sediments will be within acceptable limits. Long-term predictions indicated only a minor decrease in surface sediment concentrations but a major decrease if the most contaminated sediments close to the emission source were removed. A more detailed assessment of the effects is suggested.
Introduction River Kymijoki, the fourth largest river in Finland, has been heavily polluted by pulp mill effluents as well as by the chemical industry. Loading has been reduced considerably, although remains of past emissions still exist in river sediments. The objective here was to model the transport of sediments and dioxins and to assess the impact of restoration dredging on sediment and contaminant transport. During the 1990s, the sediments were recognized to be highly contaminated with polychlorinated phenols (PCPs), polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated diphenyl ethers (PCDEs), and mercury (Hg) originating from production of the wood preservative Ky-5, chloralkali processes, and other sources (1, 2). High toxicity of sediment to exposed microorganisms and high frequencies of mentum deformities in * Corresponding author e-mail:
[email protected]; phone: +358-9-40300359; fax: +358-9-40300-391. 10.1021/es0260723 CCC: $25.00 Published on Web 07/01/2003
2003 American Chemical Society
midge (Chironomus spp.) larvae populations were measured in areas with high pollutant concentrations in sediment (2). Certain PCDE congeners as well as hepta- and octachlorinated dibenzofurans, all typical for Ky-5, predominated in river sediments indicating that the production of this fungicide was the main source of contaminants in the river system (1, 3). Production of Ky-5 in Kuusankoski (Figure 1) was begun in 1939. In all 24 000 t of Ky-5 was manufactured from 1940 to 1984, from which an unknown amount of the product and impurities entered the river and finally the Gulf of Finland. The composition of the product and its impurities have been analyzed (4-7). The product consisted mainly of PCPs, PCDDs, and PCDFs; heptachlorinated dibenzofurans especially occurred as impurities. Toxic substances were released into the river in connection with washing of production instruments and with an explosion accident and resulting fire fighting in the plant at 1960. Estimates of the total amounts of combined PCDDs and PCDFs in river and marine sediments, based on sampling and echo sounding of loose contaminated sediments, range from 4 000 to 5 000 kg of PCDD/Fs [16-21 kg as international toxicity equivalents (ITEQ); the toxicity of a mixture of various PCDD/F congeners is expressed as the toxicity equivalent of 2,3,7,8-tetrachlorodibenzo-p-dioxin] in the contaminated area (3). This corresponds to the amount of 2,3,7,8-tetrachloro-p-dibenzodioxin emitted into the atmosphere over Seveso in 1976 (8). Recently, it was estimated that the clearly river-impacted sedimentation area in the Gulf of Finland stretches for a distance of 75 km from the estuary (9). The total load to the Gulf of Finland attributed to the Ky-5 source was 1770 kg of PCDD/Fs or 12.4 kg WHO-TEQ. The surface sediments in the impacted area still contained 24-66% of the maximum concentrations present in the 1960-1970s depending on the site and sediment profile, showing that the river remains a significant PCDD/F source (9).
Materials Study Area. The study area was a 130-km-long river stretch with branches between Lake Pyha¨ja¨rvi and the Gulf of Finland,with the lake occupying only 2.1% of the area (Figure 1). There are 11 power plants and 6 rapids in the river stretch. The total drop is 50 m, and the mean bottom slope is small (0.0006). The drainage area of the Kymijoki River is 37 200 km2 (lake percentage 18%) with only 3% (1 100 km2) running directly into the studied river stretch. Accordingly, 97% of the water running in the river stretch comes from upstream sources. The mean discharge at the downstream end of the river was 330 m3 s-1. Loading of plant nutrients and suspended solids (SS) originates from 8 industrial wastewater treatment plants, some tributaries, and diffuse nonpoint sources. Physical Properties of Bottom Sediments. The river bottom in the area consisted mainly of transport or erosion sites, which contain noncohesive soil or solid clay and silt. In the expansions of the river there were sedimentation pools, which were the main traps of PCDD/F compounds. Their combined area is small as compared with that of the transport or erosion site area. The bottom material in sedimentation areas was usually gyttja, clay, and silt with varying composition. Abundant wood debris and fibers originating from the pulp and paper industry have been found in the most contaminated areas in the upstream and central parts of the river stretch. Large amounts of timber were earlier transported in the river and kept for landing, causing unknown amounts of wood debris to enter the system. The loading of organic particulate material has decreased notably from the level VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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explosion accident and fire in the Ky-5 plant occurred in 1960, which probably caused exceptionally high spillage of impurities at that time and subsequently contributed to the contaminant profile in river and estuarine sediments. After the closing down of the plant in 1984, the PCDD/F concentration in the sediment has remained almost unchanged with only a slight decrease. At present, the PCDD/F concentration in the surface sediment at this site is only about 25% lower than the maximum concentration in the sediment layer corresponding to the average for 1969. Evidently PCDD/F compounds are continuously transported downstream from the most contaminated area in Kuusankoski. This is also indicated by the PCDD/F concentrations in sediment traps collected in 1997-1998 from four sites along the river showing almost equal concentrations of contaminants as in the surface sediment at each site (3). Location of the Most Contaminated Bottom Sediments. The most contaminated sediments with the maximum surface (0-3 cm) concentration of 193 000 ng g-1 (350 ng g-1 I-TEQ) for PCDD/Fs, 1017 ng g-1 for PCDEs, and 13.8 ng g-1 for Hg in the dry sediment are located between Kuusankoski and Keltti (Figures 1 and 2). From Kuusankoski to Anjalankoski (33 km), the maximum concentration lies between 3400 and 190 000 ng g-1 (9.6-350 ng I-TEQ g-1). Further downstream the concentration ranges from 120 to 1200 ng g-1 (0.5-4.3 ng I-TEQ g-1). In the estuarine and coastal areas, the range is 1-53 ng g-1 (0.01-0.2 ng I-TEQg-1). The estimated total amount of contaminated sediments and PCDD/F compounds between Kuusankoski and Anjalankoski is 1 052 170 m3 and 2377 kg, respectively (i.e., roughly half of all PCDD/Fs in the river). The measured PCDD/F concentration in SS accumulated in sediment traps decreases exponentially from 21 900 ng g-1 (dry weight) to 228 ng g-1 in a longitudinal direction from Kuusankoski to the Gulf of Finland (3).
FIGURE 1. Map of the study area. that prevailed from the 1950s to the early 1980s. It appears that contaminated organic particulate materials accumulated earlier in the main sedimentation pool in Kuusankoski. At present, sediment is slowly decomposing, eroding, and migrating downstream. Transported sediment with highest settling velocity has accumulated into downstream sedimentation pools whereas smaller particles have migrated to the estuarine and the sea area. Because of implemented hydrological regulation at power plants, sediments have not been exposed to high floods, and discharges have not increased. In the future, water construction projects and changes in river regulation can cause a risk of mobilization of PCDD/F compounds. Historical Loading Records of PCDD/F Compounds. Variation in PCDD/F concentration in an age-determined (210Pb dating) sediment profile from the river estuary (Ahvenkoskenlahti) and in historical production records of Ky-5 followed similar trends (unpublished data). PCDD/F concentrations in the sediment layers corresponding to the years from 1959 to 1969 increased 4-fold while Ky-5 production increased only 3-fold during the same period. Maximum PCDD/F concentration in the estuary occurred in a sediment layer corresponding to the years 1966-1972, whereas maximum Ky-5 production occurred during the 1970s. The 3414
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Analysis of Water and Sediments and Processing of Hydrological and Chemical Background Data Sets. Daily discharge, water level, runoff, and concentrations of suspended solids (SS) from several points on the river were available in a hydrological and water quality database of the Finnish Environment Institute (SYKE). Monthly values for SS from the industrial effluent point loading were collected from sewage treatment plants. Nonpoint loading was estimated from continuous runoff data and weekly sampled water quality data from two small representative catchments (30 and 178 km2) in the drainage area (1100 km2). A mean discharge in the upstream end of the modeled river stretch was 336 m3 s-1. Runoff from the drainage basin was 9 l s-1 km-2. The mean concentrations of SS in the upstream and downstream ends of the modeled river stretch were 2.4 and 5.0 mg L-1, respectively. Average SS fluxes from upstream, from industrial plants, from the drainage basin and to Gulf of Finland were 24 000, 7700, 29 400, and 61 100 t a-1, respectively. Variation in fluxes was considerable in some years due to rainfall (Figure 3).
Methods Modeling of Sediment and PCCD/F Transport in the Kymijoki River. River hydraulics and sediment transport of the 130 km long river stretch with 21 branches between Lake Pyha¨ja¨rvi and the Gulf of Finland (Figure 4) were modeled with a one-dimensional (1-D) river model. The model was used to calculate time-series and longitudinal profiles of SS and PCDD/F concentrations in river water and bottom sediment. The results were used for evaluating the impact of dredging on transport of PCDD/F compounds. In the 1-D unsteady river flow model, the full de Saint Venant equations (eq 1) were solved numerically with a finite
FIGURE 3. Fluxes of suspended solids in the modeled river stretch in 1980-1995.
FIGURE 2. Sediment quality between Kuusankoski and Keltti power plants and horizontal distribution of contaminated mud sediments and measured flow fields in the upstream heavily contaminated area. difference method (10) in which Verwey’s variant of the Preissmann implicit discretization scheme was used. The equations were solved with the double-sweep method, and the resistance term was calculated using the Manning approach, with the Manning number as an empirical constant. External boundary conditions that could be applied included discharge and water level as tabulated functions of time and discharge as a tabulated function of water level (a Q-y relationship). The model can be applied to a river with several dams or power plants in a row and to a tree-like or looped branching system. Inflow can consist of main river,
FIGURE 4. Schematic map of modeled river stretch. tributaries and lateral inflow:
∂y 1 ∂Q + )q ∂t b ∂x ∂Q ∂ βQ2 ∂y Q|Q| + + gA + gA 2 ) 0 ∂t ∂x A ∂x K
(1)
where y ) water level (m) measured from a reference height, VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Scientifically Reported Sediment Parameter Values and Ranges Used in the Modela value parameter
unit
avg
ws
m
s-1
10-5 b
τd K2 τe 1, layer 2, layer 3, layer 4, layer TC,i 1, layer 2, layer 3, layer 4, layer
N m-2 g m2 s-1 N m-2
min 1×
10-4
max 1×
ref
10-5
5× 1 × 10-5 c 0.084 0.1
0.1 0.01
9 × 10-4 (7 × 10-3)d 0.5
14-16, 18 14, 18
0.5 1.0 2.0 10
3 3 3 10
1 × 10-3 (8 × 10-3)d 1 × 10-3 (8 × 10-3)d 1 × 10-3 (8 × 10-3)d 10
14-16, 18 14-16, 18 14-16, 18
12, 13, 18
d 1 30 365 >365
a Dispersion coefficient (D ) was 20 m2 s-1, and calculation time step was 12 h. Parameter ranges were arranged to produce minimum and x maximum SS concentration in the river water. b Calibrated value. c Calibrated settling velocity in Lake Tammija¨ rvi. d The values in brackets were used to evaluate the sensitivity of calculated PCDD/F concentration in bottom sediments (Figure 12c).
Q ) discharge (m3 s-1), b ) width (m) of the river channel, q ) direct inflow from the catchment area to the river channel (m3 s-1), A ) cross section area (m2) of the river channel, g ) the acceleration of gravity (m s-2), K ) water-carrying capacity of the river channel ) n-1 AR2/3, n ) Manning coefficient, R ) hydraulic depth (≈A/b), and x ) longitudinal coordinate. The 1-D sediment and contaminant transport model was used to calculate the convection, dispersion, sedimentation, and erosion of SS and PCDD/F with unsteady flow (10); the model was linked with the flow model. Mass flow of SS and pollutants into the river could be added as point or diffusive nonpoint loading. Convection, dispersion, sedimentation, and erosion of sediment and contaminant were solved numerically from the convection and dispersion equation (eq 2) with a double-sweep method. The boundary condition required was the known concentration value at the upstream end of each river stretch. The concentration in tributaries and in point and nonpoint loading was given as a tabulated function of time:
∂C Q ∂C ∂C q D E ∂ D ) (CL - C) - + + ∂t A ∂x ∂x x ∂x A h h
(
)
(2)
where C ) concentration of suspended solids (SS) (mg L-1) in the river water, Dx ) dispersion coefficient (m2 s-1), CL ) concentration of SS (mg L-1) in the direct inflow from the catchment area, D/h ) sedimentation term (mg L-1 s-1), E/h ) erosion term (mg L-1 s-1), D ) sedimentation rate (mg m-2 s-1), h ) water depth (m), and E ) erosion rate (mg m-2 s-1). Sedimentation rate of SS in river water was calculated as a function of shear stress and settling velocity (eq 3):
D/h ) -(1 - τ/τd)wsC/H
if τ τd), ws is settling velocity (m s-1), H ) h/2 is the mean height of falling (m), h is the water depth (m), D is the sedimentation rate (g m-2 s-1), and C is the concentration of SS (mg L-l) or other concentration. Erosion of bottom sediments was calculated as a function of shear stress (eq 4):
E/h ) K2(1 - τe/τ)
if τ > τe
(4)
where τe is the critical shear stress (there is no erosion if τ < τe), K2 is the erosion coefficient, and E is the erosion rate 3416
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(g m-2 s-1). Sedimentation and erosion cannot occur simultaneously (τd < τe). Shear stress was calculated using
τ ) Fgv2/(M2h1/3)
(5)
where F is density of water (1000 kg m-3), g is the acceleration of gravity 9.81 m s-2, v is the flow velocity (m s-1), h is the water depth (m), and M is the Manning number. The bottom sediment was divided into four layers with differing values of consolidation time TC and τe. The values for these constants were selected according to analyzed sediment properties. The calculated value of shear stress τ and selected values for critical shear stress τe and τd were used to determine the presence of erosion or sedimentation. The mass of sediment in layers i ) 1, ..., 4 and in cross section j was integrated from the mass balance equation (eq 6). If the mass in the topmost layer decreased, the model determined the level of activity from the next lower layer. It was assumed that the material in a layer was transferred to a lower layer with a rate corresponding to the consolidation time:
dmi,j 1 m ) D j - Ej dt TC,i i,j
i)1
dmi,j 1 1 m m ) Dj - Ej + dt TC,i-1 i-1.j TC,i i,j
(6a)
i ) 2, 3, 4 (6b)
mi g 0 always, Ei+1 ) Di+1 ) 0, if mi > 0 where mi ) mass of sediment (g m-2) in sediment layer i, TC,i ) consolidation time of a sediment layer i (s), i ) sediment layer number, and j ) cross section number. Govers and Krop (11) have used a thermodynamic lattice model to determine subcooled liquid vapor pressure and aqueous solubility, Henry law constant, n-octanol-water and sediment-water partition coefficients, and lipid weight bioconcentration factors for 210 PCDD/Fs. The results confirmed that PCDD/Fs are poorly water soluble and that the solubility decreases with increasing chlorination level. The substances are increasingly sediment bound with increasing chlorination level. With these statements, PCDD/F compounds were assumed to migrate adsorbed on particulate matter, and the same convection and dispersion equation (eq 2) and sediment mass balance equation were used to calculate the transport of PCDD/F compounds.
FIGURE 6. Estimated loading of PCDD/F compounds (kg d-1, total mass) from Kuusankoski-Keltti area in 1969-1994 and exponential regression curve with 95% confidence limits as an estimate for extrapolated evolution of loading. FIGURE 5. Rate of sedimentation ) S/G1 (cm a-1) and erosion ) E/GI (cm d-1) calculated with mean hydraulic dimensions of the Kymijoki River as a function of flow velocity. Density of the sediment layer G1 ) G2 ) 1330 kg m-3, G3 ) G4 ) 1990 kg m-3.
TABLE 2. Calibrated Manning Coefficients (n) in River Branches (no.) no.
n
no.
n
no.
n
no.
n
no.
n
1 2 3 4 5
0.020 0.050 0.040 0.040 0.050
6 7 8 9 10
0.053 0.043 0.037 0.036 0.050
11 12 13 14 15
0.050 0.036 0.033 0.033 0.100
16 17 18 19 20
0.100 0.143 0.040 0.040 0.040
21
0.040
Parameter Ranges. Transported sediment consists mainly of clay, silt, wooden debris, cellulose fibers, and humus. Average value and range for settling velocity (Table 1) were obtained from laboratory experiments conducted with samples taken from Myllykoski (Figure 1), and they were further specified with the literature (12, 13, 18) and calibration. Average values of critical shear stress (τe and τd) and their possible ranges were also obtained from the literature (1416, 18). The τd value selected was 0.084 N m-2. Erosion coefficient K2 was fixed to 0.1 g m-2 s-1 (14, 17, 18). The consolidation time TC,i in the first to fourth layers were 3, 30, 365, and >365 d, respectively. The selected τe values of consolidating sediment were 0.5, 1.0, 2.0, and 10.0 N m-2, respectively. Flow velocities corresponding to selected τe and τd values were found to be normal. When considering the morphological dimensions and sediment characteristics of River Kymijoki and the given values of critical τ, calculated sedimentation was above zero if flow velocity was
