Vertical distribution and speciation of heavy metals in stormwater infiltration facilities: possible heavy metals release to groundwater
Water sensitive urban designs such as stormwater infiltration facilities play a key role in the water cycle of urbanized area with high coverage of impervious surface. The infiltration inlet sediment, accumulated during the stormwater runoff, is known to be contaminated by heavy metals but information on the mobility/speciation of them is limited. Some previous research reports have pointed out the considerable concern of groundwater contamination since these infiltration facilities were usually not designed with view of non-point pollutants retention (Mikkelsen et al., 1994). The sources of heavy metals in the urban stormwater runoff are numerous and the metal release mechanisms are complex. Some of the heavy metals are soluble in low pH and they are readily mobilized while some are bound to organic substances and susceptible to move in reducing environment. Thus, heavy metals accumulation and their release should be regularly monitored in such infiltration facilities.
The Tokyo Metropolitan Government Bureau of Sewerage constructed infiltration type sewage system in highly urbanized residential areas (1400 hectares) in Tokyo two decades ago as a pilot work in order to reduce the runoff water peak flows (Fujita, 2002). The facilities include infiltration inlets, permeable pavements, infiltration trenches, infiltration LU curbs, and soakaways. There has been no investigation so far in pollutants retention in the infiltration facilities and possible release from the long term deposited sediment to the soil. The aim of this research work is to obtain the field data on vertical distribution and speciation of heavy metals in the infiltration inlet sediment. Based on this observation, possible release of heavy metals from the infiltration inlet sediment is discussed.
Sediment core samples from 4 infiltration inlets in a residential area in Tokyo, Japan were collected and sliced in 1cm interval. Seven heavy metals (Cr, Mn, Co, Ni, Cu, Zn and Pb) were measured. The heavy metals were extracted according to USEPA method 3051. A sequential extraction procedure, three-step protocol proposed by the Standards, Measurements and Testing programs (SM & T- formerly BCR) of the European Union was applied to sediment samples to determine heavy metals (Tokalioglu et al., 2003). This procedure provides measurement of extractable metals from the sediment with the media such as acetic acid (0.11mol/L), hydroxylammonium chloride (0.1mol/L) and hydrogen peroxides (8. 8mol/L) plus ammonium acetate (1 mol/L), which are exchangeable, reducible and oxidizable metals respectively. The extracted heavy metals were quantified using ICP/MS.
Possible releases of heavy metals from the infiltration inlet were studied by using the sediment samples of infiltration inlet having depth more than 12cm. The particle size distribution in the sediment was almost similar among the layers in each inlet. It indicates regular sources of sediment for a specific subcatchment. Figure 1 shows the vertical profiles of oxidation reduction potential (ORP) and heavy metal contents in sediment (inlet no. 172). The heavy metals contents were found basically decreasing with depth. The decreasing trend of heavy metal contents with depth indicated possible release/desorption of heavy metals from the sediment possibly due to reducing environment in the lower layer as the ORP decreased with depth.
The distribution of heavy metals in the different fractions of sediment sample allows us to predict their mobility. Figure 2 shows the extractability of heavy metals in acid exchange (a), reducible/metal oxide bound (b) and oxidizable/ organic bound fractions. The order of the mobility of heavy metals in the acid exchangeable fraction was Mn>Zn>Co>Ni>Cu>Cr>Pb. Furthermore the mobility of metals in the reducible/metal oxide bound fraction was Mn>Zn>Pb>Cr>Co>Cu>Ni and in oxidizable/organic bound fraction was Pb>Cr>Cu>Ni>Zn>Co>Mn. The higher extractability of heavy metals in the acid exchangeable fraction from the bottom layer sediment than the top indicated their pH susceptibility. In opposite, the lower extractability in the oxidizable/organic bound fraction in the bottom sediment possibly due to enrichment of humic substances by the decomposition of organic substances in reducing environment. The different extractability of heavy metals in reducible/metal oxide bound fraction indicated metal specificity in mobilization. The overall mobility of heavy metals in the order are Pb>Mn>Zn>Cr>Co>Cu>Ni. The mobility is supported by the Figure 1 also.
Finally, the basically decreasing of heavy metal contents in the lower sediment layer of infiltration facilities indicated possible release of them to the soil. Among the heavy metals the mobility from the lower sediment was higher in the acid exchangeable fraction while lower in the oxidizable/organic bound fractions. The mobility in the reducible/metal oxide bound fraction was metal specific. The overall mobility in the sediment was Pb>Mn>Zn>Cr>Co>Cu>Ni.
References:
Fujita , S. (2002). Proc. 9th Int. Conf. on Urban Drainage, Portland, Oregon, 8-13 September 2002. CD-ROM, ASCE Publications.
Mikkelsen, P. S, Hafliger, M., Ochs, M., Tjell, J. C., Jacobson, P., Boller, M. (1996). Science of Total Environment, 189/190, 341-347.
Tokalioglu, S., Kartal, S and Birol, G. (2003). Turk. J. Chem., 27, 333-346
