Geochronology of Pb and Hg pollution in a coastal marine environment using global fallout 137Cs

Journal of Environmental Radioactivity 69 (2003) 145–157 www.elsevier.com/locate/jenvrad

Geochronology of Pb and Hg pollution in a coastal marine environment using global fallout 137 Cs S.K. Jha ∗, S.B. Chavan, G.G. Pandit, S. Sadasivan Environmental Assessment Division, Bhabha Atomic Research Centre, Mumbai, India

Abstract Global fallout 137Cs was used for dating sediment cores and estimation of recent sedimentation rates (up to 1 cm/y) in the Thane Creek, which lies in the southern part of the Deccan belt of India. The residence time of 210Pb in the Thane Creek water was calculated to be 0.7 years. Further, the concentrations of Pb (up to 70 µg/g) and Hg (up to 10 µg/g) in sediment profiles were measured to assess the anthropogenic input of contaminants due to large-scale industrialization, which has taken place in this area over the last two decades. The depth-wise concentration profile of Hg shows positive evidence of continued fresh input into the Creek.  2003 Elsevier Science Ltd. All rights reserved. Keywords: 137Cs;

210

Pb; Dating techniques; Hg and Pb pollution; Creek ecosystem

1. Introduction Due to population growth, large-scale industrialization and new farming methods, water bodies are continuously being loaded with large amounts of elements, which were either absent earlier or present in small quantities. The understanding and the evaluation of the possible interaction of various anthropogenic contaminants with the near-shore environment off the Indian sub-continent assumed significance about three decades ago with the release of industrial and sewage wastes in Thane Creek

∗

Corresponding author. Tel.: +91-22-255-92916; fax: +91-22-255-05151. E-mail address: [email protected] (S.K. Jha).

0265-931X/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0265-931X(03)00092-4

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(Fig. 1). About 35 tons of suspended solids, 20 kg of Hg and 10 kg of Pb are discharged by the chemical and leather industries (Patel et al., 1985) into the Creek each year. Significantly high concentrations of mercury in surface sediment and biota have been reported (Pandit et al., 1999). The presence of anthropogenic lead in sediment near the locations of industrial effluent inputs has been reported by Jha et al. (1999). Sediments provide a basis for reconstructing many aspects of the impact of anthropogenic activity on the coastal marine environment. Significantly high concentrations of Hg in surface sediment at biota have been reported by Pandit et al. (1997, 1999). The presence of Pb in some industrial effluents is reported by Mirajkar et al. (1995). The objective of the present study is to utilize the vertical distribution profile of global fallout 137Cs in a sediment core for the estimation of recent sedimentation rates based on peak concentrations and fallout history. This will help in understanding the behaviour of heavy metals over time through the investigation of vertical concentration profiles in sediment.

2. A geochronology approach The above-ground testing of nuclear explosives, carried out mainly from 1945 to 1964, resulted in radioactive global fallout consisting of approximately 200 radioactive species including 137Cs (Comar, 1963). 137Cs has been used as an indicator for the amount of global fallout deposited in soil (Beck and Krey, 1983) and in sediment (Krey et al., 1990). The use of 137Cs as an indicator of sedimentation processes is effective because it attaches firmly to soil particles (Ritchie et al., 1971) and has a long half-life (30.2 years). Sediment cores also provide the detailed depth profiles needed to infer fallout history as 137Cs concentrations in sediment layers are not easily disturbed (Whicker and Schultz, 1982; Whicker et al., 1994). 137 Cs reaches lentic sediment mainly through two processes: (i) direct deposition on the estuary surface where it ultimately accumulates in sediment, (ii) erosion of 137Cs-contaminated soil particles by input from the surrounding catchments into the estuary. Global fallout levels in the specific regions vary with climatic conditions, distance from detonation, device type and size, and latitude (Whicker and Schultz, 1982). The history of 137Cs fallout deposition on an undisturbed water body can be inferred by assessing the years from 1945 to 1964 against the peak concentration in the sediment depth profile. Finally the mean sedimentation rate can be estimated by dividing the depth of the sediment, where the peak 137Cs concentration is located, by the proper time elapsed. Once the profile is dated it is possible to understand the behaviour over time of heavy metals from the vertical concentration profile.

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Fig. 1.

Map of Thane Creek showing sampling locations.

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3. Study area

The study area, Thane Creek (Fig. 1), lies on the southern part of the Deccan belt of India between latitude 18° 53’ to 19° 04’ N and longitude 72° 48’ to 72° 53’ E and includes the Ulhas River estuaries. It is a land-locked water mass connected to the Arabian Sea at the south. The Creek is funnel-shaped with a narrow end into which a fresh water river, namely the Ulhas, enters from the north. The broad southern end of the Creek receives tidal water from the Arabian Sea. The Creek is subjected to semidiurnal tides which are asymmetrical in both their periods and ranges. The water movements are mainly governed by tide. During the southwest monsoon period there is also an influx of about 1.0 to 1.5 × 106 m3 of fresh water from the rivers per tidal cycle, affecting the circular pattern considerably. The Creek is thus subjected to the inflow of water from the Arabian Sea during most of the year. The surface water temperature varies between 22.5 and 32.4° C. The depth of the water column varies from 8 m at Location S1 to 15 m at Location S3 (Fig. 1).

4. Sample collection and processing

Core samples were collected at different locations in Thane Creek (Fig. 1) during 1996 using a gravity corer (the inner and outer diameters were 5.2 and 6.0 cm, respectively). The sampling strategy followed the procedure given by Carpenter et al. (1985). The length of the core collected for the current study using an adjustable piston rod with silicone packing was found to vary from 29 to 53 cm. The gravitycoring unit was lowered as slowly as possible into the sediment to avoid the lateral movement which would be caused by the pressure wave created by the descent of the corer. Care was taken during coring to ensure minimum disturbance of the sediment–water interface. The sampling location was carefully chosen based on the work carried out by Desai and Pillai (1982), which shows that the bottom sediments in the north of the Creek, where it is joined by the Ulhas River, are comprised mainly of silt and clay. S1, S2 and S3 are sampling points in the Thane Creek area (Fig. 1) which covers the newly developed chemical zone by the Maharashtra Industrial Development Corporation, India. It contains a large number of major and smallscale industries such as Standard Alkali, electroplating, alloys making, petrochemical industries, lead smelting, etc., where heavy metals such as Pb and Hg are used in various forms. The Creek is the ultimate recipient of all treated and untreated liquid effluents from these industries. The Creek is not used for maritime transport and only small fishing boats operate in the area. The collected cores were extruded vertically and sliced at 2 cm intervals. The sliced cores were homogenized, freeze-dried and stored in the laboratory for subsequent measurements.

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Table 1 Quality assurance of the measurement techniques Sr. no.

Nuclide

Reference material

Certified value (Bq Our value (Bq kg⫺1) kg⫺1)

1 2 3

137

IAEA-375 (soil) QAP USDOE (soil) IAEA-327 (soil)

5281±80 810±40 25.1±2.1

Cs Cs 137 Cs 137

5174±100 786±30 25.6±0.6

5. Methods 5.1. Analysis of

137

Cs

137

Cs in the sediment sample was analysed by placing the sample in a plastic vial 2.5 cm in diameter and 5 cm in height on an HPGe detector with a relative efficiency of 30% (with respect to 7.5 × 7.5 cm, NaI(Tl)) housed in a 7.5 cm thick lead shield, coupled to an 8K PC based MCA card and associated electronics. Soil reference materials, IAEA-327 and IAEA-375, were used for quality assurance/quality control. The detection limit for 137Cs was 0.25 mBq g⫺1 and the standard counting uncertainty varied from 2 to 8% in different core sections (counting time 60 000 s). Details of the reference materials used are given in Table 1. To obtain supporting geochronological information, the excess 210Pb in the different core sections was measured using the standard procedure described by Carpenter et al. (1981, 1982). 210Po activity, which is in secular equilibrium with 210Pb, along with 208Po tracer were measured by alpha-spectrometry using an Si-surface barrier detector connected to a multi-channel analyzer. Duplicate measurements of wellhomogenized samples showed an analytical precision of 4–6%. The spike tracer 208Po recovery was in the range of 85–95%. The quality assurance of measurements was assessed using the reference material, IAEA-135 Irish Sea sediment sample (Table 2). 5.2. Determination of mercury Approximately 1.2 to 2.0 g (wet weight) of each segment of the core sample was digested in a soxhlet extractor employing a mixture of 10 ml HNO3, 1.0 ml HClO4 and 0.5 ml H2SO4. The digested mixture was centrifuged and the acid extract was Table 2 Analysis of

210

Pb in a reference material

Sr. no.

Element

Reference material

Observed value Reference value Accepted range (Bq kg⫺1) (Bq kg⫺1) (Bq kg⫺1)

1

210

IAEA-135 Marine sediment

44.8±3.5

Pb

48.0

42.2–54.1

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made up to 50 ml volume. The mercury concentration was estimated in an aliquot volume (10 ml) by cold vapor Atomic Absorption Mercury Analyzer using SnCl2 for the reduction step. The corresponding volumes of acids were processed in the same way as samples for blank subtraction. The sensitivity of this method was 1 ng/ml. 5.3. Determination of lead The Energy Dispersive X-Ray Fluorescence (XRF) technique was used to analyze Pb in sediment. The powdered 500 mg sediment samples were mixed with 500 mg of cellulose powder and the mixture was pressed using a 20 tons hydrolytic press to convert it to pellets. The XRF results were obtained by exciting the pellets with an X-ray tube with 36 kV and 0.68 mA beam. The fluorescent X-rays from the elements were detected by an X-ray spectrometer consisting of an Si(Li) detector (Ortec) coupled to a multi-channel pulse height analyzer. The system resolution was 200 eV for 5.9 keV X-rays. The concentrations were evaluated by comparing with the peak counts of Pb L X-rays of known concentrations in IAEA-356 marine sediment reference material. The concurrent analysis of primary reference material, Soil5, was used to test the reliability. 6. Results and discussion 6.1.

210

Pb profiles in sediment cores

Fig. 2 gives the typical plot of excess 210Pb versus depth (cm) at Thane Creek Location S2. The linear sedimentation rate calculated using the 210Pb method was

Fig. 2.

Profile of excess

210

Pb versus depth in the sediment core from Location S2.

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0.77 cm y⫺1, 0.92 cm y⫺1 and 0.34 cm y⫺1 at Locations S1, S2 and S3, respectively (Table 3). The mean global atmospheric 210Pb fallout was about 0.0165 Bq cm⫺2 y⫺1 (Krishnaswami and Lal, 1978), therefore the mean atmospheric 210Pb inventory should be about 0.53 Bq cm⫺2. The atmospheric deposition flux of 210Pb at Thane Creek was estimated to be 0.025 Bq cm⫺2 y⫺1 (Joshi et al., 1969) and the inventory supported by this flux, Iatm, is about 0.80 Bq cm⫺2. The surface inventories of 210Pb in the Creek varied from 0.77 to 0.80 Bq cm⫺2 at different locations. The mean value of surface inventories of unsupported 210Pb was found to be 0.78 Bq cm⫺2. The residence time of particles in the water column was calculated to understand the sedimentation rate of particles on which 137Cs is absorbed. If jw and js (Bq cm⫺2 y⫺1) are the fluxes of 210Pb in water and sediments, respectively, then their corresponding inventories Iw and Is (Bq cm⫺2) can be derived using the first order kinetic relation

再冋 冉 冊册 冎 再冋 冉 冊册 冎

1 dIw ⫽ jw⫺ lPb ⫹ I , dt Tw w dIs ⫽ js⫺ dt

lPb ⫹

(1)

1 I , Ts s

(2)

where lPb is the radioactive decay constant [lPb=0.0311 y⫺1] and Tw and Ts are the residence times of 210Pb in creek water and sediments, respectively. At steady state, the inventory of unsupported 210Pb in creek water, Iw, is the difference between the atmospherically supported inventory Iatm (0.80 Bq cm⫺2), and the mean sedimentary unsupported 210Pb inventory of the lake Is (0.79 Bq cm⫺2). Thus the relationship for the derivation of residence time of 210Pb in creek water, Tw can be obtained from Eqs. (1) and (2), with dIw/dt and dIs/dt=0, as: Iatm⫺Is . Tw⬇ lPbIs

(3)

Since lPb1/Tw, lPb1/Ts, and Iw=Iatm⫺Is. The unsupported 210Pb inventory, Is in the creek was found to be 0.78 Bq cm⫺2. The residence time of 210Pb in the Thane Creek water was calculated to be 0.7 years. Table 3 Radionuclide inventories and sedimentation rates in Thane Creek Sampling location

Linear sedimentation rate by 210Pb (cm y⫺1) Mass sedimentation rate by 210Pb (g cm⫺2 y⫺1) Linear sedimentation rate by 137Cs (cm y⫺1) Mass sedimentation rate by 137Cs (g cm⫺2 y⫺1) Mean residence time (y)

S1

S2

S3

0.77±0.06 0.20±0.018 0.81±0.07 0.21±0.02 0.7

0.92±0.07 0.21±0.02 0.98±0.06 0.22±0.015

0.32±0.02 0.08±0.005 0.45±0.036 0.10±0.007

152

6.2.

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Cs profiles in sediment

The history of 137Cs fallout deposition in an undisturbed water body can be inferred by assigning these years to the peak concentration in the undisturbed sediment depth profile to the year when the fallout deposition was maximum. In the present study, to estimate the recent sediment accumulation rate, the concentration of 137Cs was measured in each core segment collected at Thane Creek. Very little post-depositional mixing has been reported in the salt marsh in the floodplain. 137Cs binds effectively with clay and silt particles (MacKenzie et al., 1994). The composition of the core samples collected from S1, S2 and S3 sites was mainly clay and silt. The total clay material was 48 to 65% with a mean grain size of 2–9 µm. Mineralogical and major elemental analysis of Thane Creek sediments (Rath and Sahu, 1994) indicate domination of montmorillonite, degraded chlorite and illite. The concentrations of major elements, Si, Ca, Mg and K, in the different core sections vary for dry sediment from 25 to 35 mg/g, 20–25 mg/g, 20–29 mg/g and 10–13 mg/g, respectively. Fig. 3 represents a typical plot of 137Cs deposition in core sections at different depths of the core obtained from Location S2. The concentrations in the uppermost segments were assigned to the year of sample collection, 1996. In India the maximum fallout activity in soil was observed in the spring of 1963 (Mishra et al., 1975). A

Fig. 3. Observed 137Cs activity in each section of the Thane Creek sediment core and marker dates assigned to specific discontinuities in the depth profile.

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53 cm long Thane Creek core segment showed maximum 137Cs peak concentration at a depth of 33 cm, which was assigned to the year 1963. A smaller peak at 39 cm depth in the core was assigned to 1959. The peak concentration at 23 cm depth was assigned to 1973, the year of atmospheric nuclear weapons testing conducted by the Chinese and the French (Mishra et al., 1975). The different maxima in the depth profile of 137Cs measured in the core segment were used for measuring the average sedimentation rate. The initial activity was observed at 49 cm depth, which corresponded to 1945. In this core, which assigned four dating markers, a linear regression (Whicker et al., 1994) of marker depth on marker age was performed (Fig. 4). The resulting gradient (0.98 cm y⫺1) was inferred as the estimated sedimentation rate. Table 3 gives the sedimentation rate obtained at the three locations by 137Cs and 210Pb dating techniques. At all locations the shortterm rates (last three decades) derived from 137Cs have been observed to be marginally higher compared to long term (last 120–150 years) rates deduced from 210Pb. 6.3. Hg and Pb vertical profiles The depth profiles of Hg concentrations in the sediment cores collected at S1, S2 and S3 are shown in Figs. 5a, b and c, respectively. The surface concentrations of Hg at S1 are different from surface concentrations at S2 and S3. The concentrations of Hg at S1 remained constant between 35 to 55 cm depth. High surface concentration of Hg at S1 can be attributed to the newly developed chemical zone of the Thane Belapur industrial belt. The increase in Hg concentrations was observed from

Fig. 4. Analysis of data shown in Fig. 3 with the depth of the dating marker in the sediment core plotted as a function of the age of the marker. The gradient of regression was taken as the estimate of the average sedimentation rate.

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Fig. 5.

Vertical concentration profiles of Hg (ng/g) in sediment cores from Thane Creek.

1957 onwards at both S1 and S2 and peak concentrations were reached around 1973– 1975 during which pre-operational conditioning of different plants was carried out in the newly developed Thane-Belapur industrial belt. However, the concentration distribution with depth in the upper 10 cm strongly suggests fresh input of Hg at S3 compared to S1 and S2. There is an indication that S1 and S2 sites may have received a higher input of Hg two decades earlier based on 137Cs chronology. A similar study carried out by Glass et al. (1999) in the lower portion of the St. Louis River, between Cloquet and Duluth, Minnesota, USA, shows the highest Hg concentration (3500 ng/g) occurred at a depth of 90 cm in a sediment deposition zone of the Scanlon Reservoir. The depth profiles of Pb concentrations in the sediments collected at three locations are shown in Figs. 6a, b and c. The Pb vertical profile revealed higher concentrations (30–74 µg/g) in the initial 25 cm compared to the bottom section concentrations (20–37 µg/g) reflecting contributions from industrial effluents. In addition, the magnitude of Pb concentration (18–28 ppm) in the core collected at S3 remained more or less the same through the entire depth of the core (Fig. 6c). Wright and Mason (1999) measured the vertical concentration profile of Pb in sediments collected from two adjacent estuaries, namely the Orwell and Stour, in eastern

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Fig. 6.

155

The vertical concentration profiles of Pb (µg/g) in sediment cores from S1, S2 and S3.

England. Their study showed higher concentration of elements in the first 10 cm section of the core compared to 22 cm value obtained in the present study at Thane Creek. This could be attributable to higher sedimentation rate in Thane Creek.

7. Conclusions Depth profiles of 137Cs in sediment core samples collected at Thane Creek successfully identified the main atmospheric nuclear weapons tests carried out in the past. The high sedimentation rates observed (up to 1 cm/y) helped to resolve the concentration peaks at all locations. Depth profiles of Hg (concentrations up to 10 µg/g) and Pb (concentrations up to 70 µg/g) show the anthropogenic input into the coastal environment around Thane Creek over the years. The positive evidence of continued input of Hg at S3 has been confirmed by this study. The vertical concentration profiles of Pb in the sediment cores collected at S1 and S2 revealed earlier input of Pb there compared to S3.

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