Risk assessment of heavy metal pollution in alluvial soils and sediments of the Grote Beek river (Belgium)

Consoil 2003 RISK ASSESSMENT OF HEAVY METAL POLLUTION IN ALLUVIAL SOILS AND SEDIMENTS OF THE GROTE BEEK RIVER (BELGIUM) Valérie Cappuyns, Rudy Swennen and Katrien De Nil Katholieke Universiteit Leuven, Fysico-chemische Geologie, Celestijnenlaan 200C, 3001 Heverlee, Belgium Tel. +3216327297, Fax. +3216327981, e-mail: Valerie.Cappuyns@geo.kuleuven.ac.be 1. Introduction Wastewater discharge from the processing of phosphate ores has contributed to pollution by heavy metals and As in soils adjoining the Grote Beek river (15km long) (Central Belgium). Moreover, elevated chloride concentrations comparable to concentrations in seawater are discharged into the river. The study area is characterized by sandy soils and underlain by the Diestian Formation, containing between 30-40% glauconite. Organic- and iron-rich wetland soils have developed along this stream. The river follows a very meandering path and is characterised by several flooding zones that are inundated a few times a year. The analysis of soil and porewater samples from the area indicated a severe contamination of the floodplain soils and sediments with Cd, Cu, Ni, Zn, Ba and As (up to 276, 531, 172, 7507, 523 and 496 mg/kg respectively) (Cappuyns et al, 2002). The porewater contained elevated concentrations of Cd, Cu, Ni, Zn and Ba (up to 43, 187, 138, 1034 and 4432 μg/l respectively), while As did not seem of immediate environmental concern. However, porewater composition only gives an indication on the availability and mobility of heavy metals in soils on one specific moment, yet fluctuations in porewater compositions often occur. To perform a risk assessment, predictions about the long-term behaviour of pollutants are necessary, which cannot only rely on porewater analysis. Also the capacity controlling parameters (CCP’s) have to be taken into account since they control geochemical and microbiological processes that determine the fate of pollutants in soils and sediments (Stigliani et al., 1991). CEC, pH, redox potential, soil organic matter, salinity and microbiological activity are the CCP’s of soils and sediments for heavy metals. In this study, different extractions and leaching tests were used to assess the influence of CCP’s on heavy metal behaviour and to estimate the potential (long-term) mobility of heavy metals. Present discussion focuses on the results obtained for two samples (an overbank sediment rich in Fe and organic matter and a dredged sediment that was disposed on the riverbank) representative for the studied area. 2. Material and methods Physico-chemical analysis, extractions and leaching tests were performed on two oven-dry samples, one representative for overbank sediments (O) and the second for dredged sediments (D). pH(H2O) was measured in a soil/water suspension (1/2.5). Organic carbon was determined according to the Walkey and Black method (Nelson and Somers, 1982); effective cation exchange capacities (ECEC) were analyzed applying the ‘silver thiourea method’ (Van Reeuwijk, 1992). Total element concentrations (Al, As, Ba, Cd, Co, Cr, Cu, Ni, Pb, Zn, Fe, Mn, K, P and Ca) were determined after dissolution of the samples with a mixture of 3 concentrated acids (4 ml HClconc, 2 ml HNO3conc and 2 ml HFconc). These solutions were analyzed by AAS (VarianÒ Techtron AA6) for Ca, Fe, K and Al. For As, Ba, Cd, Co, Cr, Cu, Ni, Pb, Zn, Mn and P a multi element analysis by ICP-MS (HP 4500 series) was carried out. A certified reference material (Montana Soil 2710) and sample duplicates were used for quality assurance of the analytical data. A mineralogical sample characterization was conducted by X-ray diffraction. The influence of reducing conditions on heavy metal mobility was assessed by making use of a reducing agent (NH2OH.HCl in 25% CH3COOH) at different concentrations (Davranche and Bollinger, 2002). 30 ml of NH2OH.HCl in 25% CH3COOH (0.01, 0.05, 0.1, 0.2 and 0.5 M) was added to 0.2 g of oven dry soil in a 50 ml centrifuge tube. The experiment was conducted at 96 °C with a 5 h equilibration time as determined by Tessier et al. (1979). After reaction, the suspension was centrifuged (2500 r.p.m., 10 min.), decanted off and filtered (Millipore 0.45 μm). 2 To investigate the impact of elevated Cl- concentrations in the floodwater on heavy metal mobility, extractions with Cl- solutions at different concentrations (0, 250, 500, 1000, 2000, 4000 and 6000 mg/l) were performed. 20 ml of a NaCl solution was added to 1 g of sample in a polyethylene centrifuge tube, shaken on a reciprocal shaker during 10 h, centrifuged (3500 r.p.m, 10 min.), decanted off and filtered (Millipore 0.45μm). The samples were acidified and stored at 4°C until their analysis (ICP-MS) A modified BCR-extraction scheme was applied (Table 1). Because of the elevated Fe-content of the samples, a reducing extraction with NH2OH.HCl 0.5 M was added to the original sequence. pHstat leaching tests with continuous setpoint titration (pH 2, 4, 6, 8 en 10) were used to assess long-term effects of pH on heavy metal mobility and predict possible chemical time bombs (cfr. Van Herreweghe et al., 2002). The pHstat tests were conducted during 96 h. However the pHstat test for sample O was prolonged to 176 h because a sudden increase in the BNC curve at pH 10 after 71 h. Reaction kinetics was also considered by mathematical fitting of leaching curves as a function of time (Schwarz et al., 1999). A Cascade Leaching Test (NEN 7341) was used to estimate the actual leachability of heavy metals in the samples. The extractions were carried out in triplicate in acid rinsed 50 ml polyethylene centrifuge tubes with screw caps. 30 ml of distilled water, acidified to pH 4 with ultrapure HNO3 was added to 1.5 g of dry sediment sample. The suspension was shaken during 22 h on a reciprocal shaker, centrifuged (3000 rpm, 10 min), decanted off and filtered (0.45 μm). This extraction was repeated until five fractions, with a solid/liquid ratios ranging from 20 to 100 were obtained. The cascade leaching test was also performed on Ca3(PO4)2 (sample C) en Fe-oxide (sample F) subsamples that were separated from the dredged sediment (sample D). Element concentrations in the leachates of the pHstat and cascade leaching tests were measured with ICP-MS. SO4 2- was determined by turbidimetry (Vogel, 1961). Table 1: Modified BCR extraction scheme Fraction Chemical agents Duration Step 1 Acid-extractable CH3COOH 0.11M 16 h Step 2a Reducible NH2OH.HCl 0.1M, pH 2 16 h Step 2b Reducible NH2OH.HCl 0.5M in CH3COOH 25%, 90°C 5 h Step 3 Oxidisable H2O2 15%, pH 2, 80°C; CH3COONH4 2x evaporate; 16 h Step 4 Residual HNO3/HCl/HF conc 3. Results Total concentrations of heavy metals are shown in Table 2. Intervention Values for Soil Contamination (Anonymous, 1995) were exceeded for Cd, As (both samples) and Zn (sample D). Quartz, hematite, amorphous Fe-oxides, pyrrhothite and glauconite were identified in the overbank sediment (O) by XRDanalysis. The dredged sediment (D) contained amorphous Fe-oxides, quartz, glauconite and Ca3(PO4)2. Ca3(PO4)2 and Fe-oxide grains were separated manually from the dredged sediment sample (D), characterised by XRD and analysed for major and trace elements. Amorphous Fe-oxides (F) were enriched in As and Ni. Small white particles which consisted of Ca3(PO4)2 (C) contained elevated concentrations of Cd, Zn, Cu and As . Table 2: Concentrations of heavy metals, As, Fe, organic carbon (org. C), CEC, pH (H2O) in the 2 samples (D= dredged sediment, O= overbank sediment, C= Ca3(PO4)2, F=Fe-oxide). NA = Not Analysed Cr Ni Cu Zn As Cd Ba Pb P Ca Fe Org. C CEC pH mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg % % % cmol/kg D 84 108 182 4083 254 213 753 84 20521 1.67 8,2 8,8 19.2 6,8 O 44 30 10 265 176 40 292 67 3920 0.53 14.7 9,0 31.2 6,3 C 119 78 462 8025 305 374 516 23 122659 3.43 0.47 NA NA NA F 41 183 67 1773 446 210 350 27 12336 0.93 23.5 NA NA NA 3 3.1 Single Extractions EDTA was capable of extracting between 50 to 100 % of the total concentrations of Cd, Ni, Zn, Cu an Pb. Only negligible amounts of Ba, As and Cr were released (Fig. 1). Cl- had the most significant effect on the leaching of Cu, Cd (Fig. 2a), Zn and Ni. In absolute concentrations the metal leachability decreased in the order Cu > Zn > Cd > Ni. Relative to their total concentrations in soil the order was: Cu > Ni, Cd > Zn. Stronger reducing conditions induced a significant increase of the release As (Fig. 2b), Pb, Ba and Cr. 3.2 Sequential extractions The fractionation of trace elements was very similar for both samples. Zn, Ni and Cd were principally released during the CH3COOH (Step 1) and the NH2OH.HCl 0.1M (Step 2) extractions. For Cr an As, the residual fraction prevailed, while Pb was mostly recovered in Step 2b and 3. In the dredged sediment, a low but significant As-concentration (6 mg/kg) was extracted with CH3COOH (Step 1) and EDTA. Cu and Ba are generally characterized by a considerable reducible fraction (Step 2a) and a significant amount of Cu was also released during the oxidising extraction (Step 3). Figure 1: Heavy metal and As fractionation according to the BCR extraction (Table 1) in sample D (a) and O (b) Figure 2: (a) Leaching of Cd and Cu as a function of Cl- concentration (b) Leaching of As and Fe as a function of NH2OH.HCl concentrations. 3.3 Leaching tests Heavy metal concentrations in the leachates of the cascade test performed on the dredged sediment (D) were significantly higher than for the overbank sediment (O) (Table 3). Although Cu, Ni, Zn, As, Cd and Baconcentrations were in the μg/l range for sample D, only Zn, Ni and Ba were leached from sample O in significant amount. The pH remained more or less constant during the leaching experiment. Nevertheless, sample O had a lower acid neutralizing capacity than sample D, as the pH of the extracts was on the average 0.5 units lower in the overbank sediment than for the dredged sediment. 4 Element concentrations in the leachates of the pHstat tests after 96 h (D) and 167 h (O) at different pH values are given in Table 4. The dredged sediment is characterised by an elevated acid neutralising capacity (ANC). In general, the highest pollutant concentrations are released at the lowest pH values, except As in sample O, which is only leached at pH 10. In this sample also a considerable SO4 2- and DOC release was observed at pH 10. Table 3: Concentrations of selected trace elements (μg/l) and pH of the leachates of the cascade leaching test (mean ± standard deviation of three replicates). DL = below detection limit L/S 20 40 60 80 100 D pH 7 ± 0,08 6,97 ± 0,04 7,06 ± 0,11 7,05 ± 0,01 6,95 ± 0,13 Ca 18837 ± 1107 6158 ± 456 5215 ± 505 4658 ± 57 3859 ± 122 Fe 107 ± 58 447 ± 95 151 ± 43 351 ± 126 320 ± 15 P 1290 ± 5 2271 ± 130 1692 ± 153 1805 ± 47 1654 ± 46 Ni 51 ± 6,0 35,2 ± 2,5 18,8 ± 1,2 16,1 ± 0,5 11,9 ± 0,3 Cu 59 ± 3,1 43,6 ± 4,1 20,9 ± 6,2 15,9 ± 1,6 17,3 ± 3,4 Zn 60 ± 11 53 ± 14 76 ± 32 25 ± 5 28 ± 9 As 22 ± 0,5 37,9 ± 1,8 28,2 ± 2,5 29,4 ± 0,9 26,8 ± 1,0 Cd 2 ± 0,49 1,65 ± 0,93 0,18 ± 0,16 0,60 ± 0,67 DL Ba 10 ± 1,46 6,82 ± 0,69 3,75 ± 0,55 3,89 ± 0,87 3,61 ± 0,24 O pH 6,07 ± 0,04 6,43 ± 0,07 6,62 ± 0,14 6,57 ± 0,22 6,51 ± 0,21 Ca 35060 ± 465 5908 ± 153 3328 ± 141 3222 ± 191 2368 ± 98 Fe 113 ± 7 743 ± 189 505 ± 216 765 ± 114 721 ± 80 P DL DL DL DL DL Ni 18 ± 1,7 15,7 ± 1,0 8,0 ± 1,2 8,0 ± 1,1 5,7 ± 3,4 Cu 0,1 ± 0,1 DL DL DL 1,0 ± 1,8 Zn 27 ± 1 18 ± 13 20 ± 4 4 ± 1 13 ± 11 As DL 0,2 ± 0,2 DL 0,1 ± 0,2 0,1 ± 0,1 Cd 0,86 ± 0,03 DL DL DL DL Ba 29,04 ± 0,80 8,00 ± 0,82 3,52 ± 0,80 4,63 ± 1,36 3,31 ± 0,66 Table 4: pHstat leaching of sample D and O. ‘Time’ gives the time to reach a certain ANC assuming a worst case scenario. DL = below detection limit D O pH2 pH4 pH6 pHsoil pH8 pH10 pH2 pH4 pH6 pHsoil pH8 pH10 Ca mg/kg 22841 8537 1827 401 113 92 6712 4256 1401 779 299 250 Fe mg/kg 206 25 4 3 7 160 876 10 <1