Contaminant Buildup and Distribution on Urban Impervious Surfaces at Parking Lots - page 3

Sulphates are moderately correlated to TS, TSS and HM, indicating that the solubility of hydroxides and sulphides and their precipitation (when they reach the critical concentration which causes the chemical precipitation) affects removal of heavy metals from stormwater.

In general, although two variables are strongly correlated, it cannot be concluded that there is a cause-and-effect relationship between the variables. Causation implies correlation, but correlation alone does not imply causation (Levine, 2006). The assumption of potential bonding of contaminants to the solid phase of urban runoff was further examined on a solid particle sample from the parking lot. The sample was dry sieved in five particle size fractions which were subsequently analyzed for heavy metal and nutrient content (Table 2). Results revealed that finer particles tend to contain higher concentrations of both heavy metals and nutrients: particles with a diameter less than 125 ┬Ám represented only 18.5% of the total solid phase mass but contained significant amounts of examined heavy metals and nutrients: 37.5% of TN, 71.0% of TP, 35.7% of Cr, 30.5% of Ni, 49.7% of Cu, 55.6% of Zn, 39.0% of Fe and 51.6% of Pb. This may be of particular importance for the selection and design of stormwater treatment facilities, especially if TSS or heavy metals are critical pollutants that have to be removed.

 

Tab02

 

The presence of heavy metals and nutrients in the solid phase of urban runoff confirms previous findings based on the correlation analyses. The highest concentrations of heavy metals are obtained for iron, zinc and copper which are the dominant contaminants that originate from vehicular traffic-related activities. The presence of TP in the solid phase is confirmed but the nature of its bond to other particulates, at this point, remains unknown (whether it is bonded to clay particles or iron). Further details in this respect need to be investigated by advanced characterization techniques such as scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDX) and X-ray diffraction (XRD), which are currently ongoing but not in the scope of the presented study.

The spatial variation of TSS, COD, TP, TN and heavy metals from the experimental catchment during the 17 day antecedent period is presented in Fig. 5.

All investigated parameters illustrate a large spatial variability, the results are largely scattered and do not exhibit any noticeable correlation trends. The highest surface loads were found for iron (386 mg/m2) and zinc (13.6 mg/m2) at the beginning of the experimental investigation due to conditions already explained in the previous sections. Correlation trends were not observed between the 6 examined sampling sites for any of the investigated contaminants. It is assumed that weather conditions, erosion and re-transportation of pollutants (particularly those that are particulate correlated) contributed to the large scattering of the experimental data.

 

Fig05
Figure 5: Spatial variation of selected urban runoff contaminants at different sampling sites.

 

Conclusions

Experimental investigation performed at a parking lot showed significant spatial variation and a declining trend of the surface contaminant loads during the 17-day antecedent period, due to a decrease in traffic and dry weather conditions. Results showed a strong correlation between solids (total solids and total suspended solids) and heavy metals and total P. Settleable solids, oils and grease, total N, BOD5 and chlorides were weakly correlated to other contaminants. Investigation of the contaminant content in solid particle fractions revealed that finer particles accumulated on the parking lot surface contained much higher concentrations of both heavy metals and nutrients.

 

Acknowledgements

The Authors are grateful to the Serbian Ministry of Education, Science and Technological Development for funding through projects nos.TR-37009 and TR-37010 (2012).