R.K. Aryal*, M. Murakami*, H. Furumai*, F. Nakajima** and H.K.P.K. Jinadasa*** *Department of Urban Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan (E-mail:
[email protected]) **Research Center for Advanced Science and Technology (RCAST), University of Tokyo, 4-6-1 Komaba, Meguro, 153-8904 Tokyo, Japan ***National Water supply and Drainage Board, New Road, Bandarawela, Sri Lanka Abstract A field investigation of infiltration facilities, built two decades ago in Tokyo, was carried out and sediment samples were collected from 12 infiltration inlets of three different locations. Heavy metals contents in the inlet sediment, road dusts and soils samples were analysed and compared. The particle size distribution analysis showed its variation in depth as well as from inlet to inlet. The nature of organic substances present in sediment found changes in particle sizes as well as in depth. The heavy metals content in the sediment samples ranged from 6–143 (Cr), 1–84 (Ni), 49 –331 (Cu), 210 – 2186 (Zn) and 2–332 (Pb) mg/g. The heavy metal content ranges were similar to road dust, which indicated road dust as a possible source for sediment for the infiltration inlets. The lower heavy metals content in many sediment samples than the soil indicated possible release/desorption of heavy metals under newly created environments such as an anaerobic environment. Among the heavy metals there was a relatively good relationship between Cu and Zn, indicating the existence of their common sources. Keywords Heavy metals; heavy metals profiles; infiltration inlets; particle sizes; sediment
Introduction
Water Science & Technology Vol 54 No 6–7 pp 205–212 Q IWA Publishing 2006
Prolonged deposition of heavy metals in infiltration facilities and its possible threat to groundwater contamination
Stormwater infiltration facilities are in use in many cities to reduce the stormwater peak flow as well as ground water recharge. There are many types of artificial stormwater infiltration mechanisms that have been practiced in urbanising areas in order to decrease stormwater discharge to surface waters and help to preserve groundwater recharge (Furumai et al., 2005). However, there is considerable concern of groundwater contamination since these infiltration system structures are usually not designed with any consideration for pollutants retention (Mikkelsen et al., 1994). Road runoff pollutants, such as heavy metals and polycyclic aromatic hydrocarbons, are trapped at the infiltration facilities and their deposition occurs slowly with time (Mikkelsen et al., 1997). Most of the heavy metals are soluble in low pH and they are readily mobilised. For example, Zn and Cd are susceptible to low pH (Tyler and McBride, 1982; Lee et al., 1996). Similarly, the organic substances present in the sediment play an important role in the adsorption and desorption of heavy metals, especially Cu and Pb (Becker and Peiffer, 1997; de Matos et al., 2001). Within the sediment, different particle sizes have different heavy metals accumulating behaviour (Marsalek and Marsalek, 1997). The Tokyo Metropolitan Sewage Works experimentally applied infiltration facilities in some highly urbanised residential areas (300 hectares) two decades ago in order to reduce the runoff water peak flows (Fujita, 1986). The facilities include infiltration inlets, infiltration trenches and infiltration LU curbs and soakaways. Prolonged deposition of heavy metals in the infiltration inlets and changes in the environmental conditions with time doi: 10.2166/wst.2006.584
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may have helped to release some of the heavy metals to the soil. The implication of such heavy metals accumulation and their probable leaching has to be thoroughly investigated. The aim of our study is to understand the behaviour of heavy metals in the infiltration inlets, their possible leaching and its threat to groundwater contamination. In this paper, we focussed on the heavy metal content and organic matter characteristics in the inlet sediment, considering the particle size and the sediment depth. R.K. Aryal et al.
Materials and methods Study drainage area and sample collection/fractionation
The study drainage area is located in a highly urbanised residential area served by an Experimental Sewer System (ESS) 20 years ago. The area covers 8.28 ha and contains 244 infiltration inlets with infiltration trenches. The land use types are categorised into roof, road, park and agriculture, other pervious surfaces and other impervious surfaces including asphalt-paved car parking areas as shown in Figure 1 (left). Figure 1 (right) shows the sediment deposition conditions in different inlets. The numbers (1 and 171) in Figure 1 (left) are the sample ID numbers used in this paper. All 244 infiltration inlets were surveyed and the sediment depth in each inlet was measured. The sediment depth in the inlets showed a wide variation from almost no deposit to significant sediment accumulation covering the mouth of the infiltration trench (. 20 cm deep). The sediment seemed to originate not only from the road runoff particles but also from the park and agriculture soil and irregular inputs from the construction activities. A total of 57 sediment samples from 17 infiltration inlets around four different intersections were used for further investigation to understand the accumulation and release of heavy metals in different traffic conditions. Road dust and soil were also collected near the intersections. The sediment, road dusts and soil samples were collected in polypropylene tubes. The tubes were washed with HNO3 (1:1) before sampling to avoid any contamination. The core sediment samples were sliced in every centimeter and kept in a refrigerator until further analysis. The soil samples were collected from 10 cm below the surface to avoid any surface contamination. The sediment samples, the road dusts and the soil samples were sieved into four different particle sizes (, 45, 45 –106, 106– 250 and 250–1000 mm) with stainless steel sieves under wet conditions mixing with
No deposit
Partial deposit
Partial deposit
Trench blocked
1
171
Filled with water D = Sediment depth 206
Figure 1 Sampling area (left) and pictures of condition of infiltration inlets (right)
Filled with water
Milli-Q water. The fractionation was carried out by stepwise filtration with the sieve. The fractionated samples were recovered onto GF-75 (Advantec) glass fibre filters with a pore size of 0.3 mm and placed in desiccators for drying. Chemical analysis
Organic substance analysis by three dimensional fluorescence spectrometry. Since the mobility of heavy metals are also governed by organic substances present in sediment, we believed that better understanding of the structural and functional properties of organic substances may improve our understanding of the underlying mechanism responsible for the heavy metal mobility. Fluorescence spectrometry has been widely used in understanding the nature of organic substances present in natural as well as in wastewater, soil and sediment (Galapate et al., 1998; Kameda et al., 1999; Nakajima et al., 2003). We applied the method to understand the nature of organic substances in the inlet sediment. The sediment samples were treated with NaOH (0.1 M) and shaken in a shaker for 18 h (Michaelson and Ping, 1997). The extract was centrifuged at 3,400 rpm for 20 min and filtered using a glass fibre filter GF-75. The filtrate was used for the fluorescence analysis. Excitation emission matrices (EEMs) were obtained using a spectrofluorometer (Hitachi F4500) with a wavelength range of 200 to 600 nm (with 5 nm intervals) for excitation and emission. All slit widths were set to 5 nm. The fluorescence intensity was corrected by blank subtraction and was expressed in quinine sulphate unit (QSU), where 10 QSU was equivalent to the 450 nm fluorescence intensity of 10 mg/L quinine sulphate solution with an excitation wavelength of 345 nm.
R.K. Aryal et al.
Organic content analysis. The fractionated sediment and soil samples were heated in the furnace at 600 8C and organic content was measured as ignition loss.
Heavy metal analysis. Five heavy metals (Cr, Ni, Cu, Zn and Pb) were analysed. The samples were pretreated by microwave digestion with concentrated HNO3. USEPA method 3051 was followed for the extraction. The final extract was further diluted and finally quantified using ICP/MS (HP). Results and discussions Particle size distribution
Figure 2 shows the particle size distribution of two inlet sediments (abbreviated as SN1 and SN171). The particle size distribution showed that the sediment had a wide range of variation. It is seen that the contribution of the largest fraction (250–1,000 mm) is higher
SN1
>250 µm 1000 µm
250 µm 1000 µm
