SOIL AND STORMWATER QUALITY IN A TROPICAL URBAN AREA OF INDIA

Abstract

Urbanization replaces the natural vegetative cover in a watershed with impervious surfaces and causes runoff to be the dominant hydrologic factor (Bedient and Huber, 2002). Urbanization is usually associated with the pollution of surface water as urban runoff contains many polluting materials (Wanielista, 1978). With the progress of urbanization, many types of generated contaminants are accumulated on the soil surface, move down to deeper layers and eventually change the soil physicochemical properties directly or indirectly (Kim et al., 2002). Heavy metals and organic toxicants are the most common contaminants, mainly due to the emissions of industrial plants, thermal power stations and traffic and transportation system (Li et al., 2001; Banerjee, 2003). Urban soil and streets dusts have been observed to have elevated concentrations of Cd, Cu, Pb, and Zn coming mainly from traffic emissions and industrial activities (Paterson et al., 1996; Li et al., 2001). Rain water composition plays an important role in scavenging soluble components from the atmosphere and helps us to understand the relative contribution of different sources of atmospheric pollutants. The variation in chemical composition of rain water is mainly due to influence of local sources (Kulshrestha et al., 2003). A non-marine contribution for most of the rain water components has been observed by many researchers in India (Kulshrestha et al., 1996 and 2003). Surface runoff produced by rainfall events has been found to be a major carrier of variety of contaminants accumulated on roads, parking lots, roofs, open areas etc. and perceived as a major contributor to the degradation of many urban stream and rivers (Pitt and Bozeman, 1982).Urban runoff comprises many separate flow components originating from various urban sources areas, contributing different quantities of runoff and pollutants depending on their specific characteristics. The concentration of pollutants in generated runoff has been observed to vary throughout the hydrological event, as do the flows. It is commonly thought that the concentration of pollutant is substantially higher at the beginning of storm runoff (called first flush) than the rest of the event and varies between event and monitored sites (Deletic, 1998). The first flush has been observed to be influenced by many parameters such as watershed area, rainfall intensity, impervious area, antecedent dry weather period etc. (Gupta and Saul, 1996). The first flush phenomenon has been defied and analyzed by various methods (Lee et al., 2002). Noting the absence ofcomprehensive information on the above aspects about urban areas in India, the present work was planned with an aim to study the soil, dry dust, stormwater runoff and drain water quality in an integrated manner in a tropical urban area ofIndia. The study area, Roorkee town, is a medium sized 'town group" comprising of two units, Roorkee municipal board and Roorkee cantonment board. It lies at 29°51" N latitude and 77°63" E longitude on the right bank of Solani River and on the left and right banks of the Upper Ganges Canal. The coldest months in general are December and January, when the temperature attains a minimum around 2°C, while the summer season witnesses minimum humidity around 10% during day time and a maximum temperature around 40°C. The rain occurs mainly during 15th Jun to 15th September. Annual average rainfall is near 110 cm. The study area was divided in three main types of land use viz. residential, commercial and industrial; and two main source areas within each land use type viz. roadside and open areas. To study the characteristics of urban soil and dry road dust, soil samples were collected and dry road dust was sampled by sweeping the road surface from different land use types. Rain water samples (wet deposition) were also collected. Soil, dry road dusts and rain water samples were analyzed for EC, pH, major ions, nutrients, TOC and metals. Metal leaching (bioavailability) studies were also conducted on soil and dry dust. The results show that urban soil pH varies from 7.40 to 7.82. HC03" is the dominant ion followed by CI". Major ions show significant increase in the concentration in comparison to the rural soil (control) samples. High values of TOC have been observed in commercial roadside soils as well as in industrial open area soil. For the roadside soils, high values of Cd, Cu, Ni and Zn have been observed in industrial area, Cr in residential area and Mn and Pb in commercial area. For open area soils, the high value of Cd, Cr, Ni and Mn have been observed in the industrial area, while higher value of Cu, Pb and Zn have been observed in the commercial area. Analysis of dry road dust indicates that high values of Cd, Cr, Cu, Mn, and Pb have been observed from highways whereas high value of Ni and Zn has been observed from residential and commercial streets respectively. Decreasing levels of TOC have been observed from highways>commercial streets>residential streets>industrial streets.. Traffic and transportation system has emerged as a major source of heavy metals and organics. Enrichment factors (EFs) for urban soil and dry road dusts have been calculated taking geochemical background concentration of rural (control soil) area as reference. Moderate to high values of EF have been displayed confirming an important role of anthropogenic pollution. Further, soil metal extractability studies have indicated the ranges of bioavailability in roadside and open area soils for Cd, Cr, Cu, Mn, Ni, Pb and Zn as 2-20%, 3.79-15%, 8.75- 30.82%, 21.60-44.73%, 5.11-14.09%, 7.35-27.09% and 7.31-30.65% respectively. Studies on dry road dust have indicated that most of the metals related with vehicle inputs (Cd, Cu, Ni, Pb and Zn) have the highest extractable percentage (HCl/Total) in the highway dust. The heavy metal pollution index for soil in the study area suggests multi-element contamination and in some cases recommended for treatment. The heavy metal pollution index for dry road dust is the highest in the highways and commercial streets suggesting the influence of emissions from vehicles and commercial activities rather than the residential and industrial activities. The concentration of ions in rain water have been observed to follow the pattern Ca2+>HC03>Cr>N03">Na+>Mg2+>S042">K+. In order to estimate the marine and nonmarine contribution, sea salt fraction has been calculated taking Na+ as reference. All ionic ratios have been found to be higher than the recommended sea water ratios in all three types of land use, suggesting a significant contribution of non-marine origin for these components. In order to study of pollutants in runoff emanating from different urban sources areas, simple sampling devices were fabricated to contain polyethylene and glass bottles and installed in various source areas to collect runoff samples. The collected samples were analyzed for pH, EC, solids, major ions, nutrients, organics and metals. The values of pH have been observed to vary from 7.35 to 7.90 indicating neutral to sub-alkaline nature. High concentration of solids has been observed in the commercial street runoff with possible sources of suspended solids being dry deposition, wear of road surfaces and vehicles, commercial operations, and eroded roadsides etc. Ca +and HC03"have been the dominant cation and anion respectively. Runoff from industrial streets has displayed high value of TKN, whereas runoff from commercial open areas has displayed high value of NO3- N and PO4-P. In general, the runoff generated from streets and highways has displayed higher values of oil and grease, BOD, COD and TOC than runoff generated from open area. Among metals, the largest values of Cu, Mn and Zn have been observed from industrial street runoff. On the other hand, highway runoff has displayed large values of Cr and Pb, whereas Ni has been observed in the commercial streets runoff. In order to explain the change in quality of rain water as it got transformed to runoff, runoff-rainfall quality ratios have been calculated for all constituents. The results indicate that concentration of most measured constituents gets iii significantly enhanced. Cluster analysis has also been employed to classify the constituents to understand the presence of manmade influences and pollutant sources. The quality of dry and wet weather flows in drains was assessed. Six major drains were investigated viz. C.B.R.I, (purely residential catchment), Adarsh Nagar (mixed residential and commercial catchment), IIT Roorkee (mixed with high percentage of residential catchment), Industrial area (purely industrial catchment), Amber Talab (commercial catchment with high percentage of paved surface) and Ramnagar (mixed residential and commercial catchment). In the dry weather condition, flows were monitored for a full 24 hour period two times a year (summer and winter). Physicochemical analysis for characterization of quality was performed on composite discharge-weighted samples. During the monsoon season, seventeen events were monitored (entire hydrograph) over a two year period, and discrete water samples were analyzed for quality to describe complete pollutographs. The dry weather flows clearly illustrate the influence of drainage areas on the magnitude and pattern of flows in drains. Higher flows have been observed in the summer. With few exceptions, all the constituents have displayed higher concentration in summer season. Dry weather flows in drains from mixed residential and commercial catchments (Ramnagar) and commercial catchment (Amber Talab) have displayed high TSS, nutrients and organics, while in the flows from industrial and residential catchments along with main highway (Adarsh Nagar), high concentration of heavy metals has been displayed. For the storm events, the dry weather flow was separated and the volume of direct storm runoff (DRO) obtained. These direct runoff data along with, the measured rainfall data has been used to estimate rainfall-runoff relationships and the volumetric and peak discharge coefficients. During the storm events, the residential and commercial catchments (Ramnagar) have been observed to contribute more TSS, P04-P,TP, TKN, oil and grease and Zn, whereas commercial catchments (Amber Talab) have been observed to contribute major ions, NO3- N,BOD, COD, TOC, and Mn. Ni and Pb have been observed at Adarsh Nagar Site only. Regression analysis relating storm characteristics and event mean concentration (EMC) for various constituents has indicated significant relationships between factors like peak discharge, dry weather since last storm, duration of storm and time to peak (on one hand) with constituents like TSS, TKN, BOD, COD, TOC, oil and grease, Mn and Zn. The total load of selected water quality constituents has been observed to be very large during storm events and varied considerably between study sites, further relationship between the pollutant load and runoff volume has also been explored employing power relationship i.e. IV Load =a(Runoff)p. Analysis for first flush estimation has been performed by three methods, viz. first flush coefficient method, cumulative curve ratio method and maximum difference (between cumulative runoff mass and cumulative volume) method. The first flush results suggest a first flush effect in most of the cases.