تأثیر نوع پوشش سقف و فاصله از مرکز شهر بر کیفیت شیمیایی آب استحصالی از پشت‌بام در شهر اصفهان

Introduction and Objective
With the expansion of urbanization in arid regions like Isfahan, pressure on water resources has intensified, and urban runoff has emerged as a significant source of non-point source pollution. A substantial portion of this runoff originates from rooftops featuring various coverings—such as bituminous membranes (Isogam) and galvanized steeleach capable of introducing distinct ions and pollutants into the water flow. Beyond the material composition, proximity to urban pollution sources also influences runoff quality. Given the scarcity of studies simultaneously examining roofing materials and spatial location, this research investigates the impact of two types of roofing (bituminous membrane and galvanized steel) and three spatial distances in Isfahan on runoff quality parameters specifically EC, pH, and total calcium and magnesiumaiming to facilitate appropriate material selection and improved urban runoff management.
Materials and Methods
To achieve the objectives of the study, a factorial experimental design (pH, electrical conductivity, and the sum of calcium and magnesium) was implemented within a completely randomized design framework. This study was conducted in the city of Isfahan, which faces an arid climate and water scarcity challenges. To investigate the effect of spatial location, three zones were selected at distances of 5, 15, and 25 kilometers from the city center (an area characterized by high traffic density and intense urban activities. In each of these zones, samples were prepared from two types of roof coverings, including Isogam (representing bituminous membranes) and galvanized sheets (representing metallic coverings), with three replications (totaling 18 samples). The covering samples were prepared with specified dimensions and exposed to natural rainfall events (with an intensity of 0.5 mm/min or 30 mm/hr). Runoff sampling was carried out during several rainfall events and at predetermined time intervals (to examine the effect of time and the first-flush phenomenon). The measured quality parameters included electrical conductivity (EC) using a conductivity meter, pH using a pH meter, and the total concentration of calcium and magnesium ions via complexometric titration (using EDTA). The collected data were analyzed using SPSS statistical software, employing analysis of variance (ANOVA) within the factorial design framework to evaluate the main and interaction effects of the independent variables (spatial location, roof covering type, and sampling time) on the dependent variables (runoff quality). The significance level for all tests was set at 0.05.Results and DiscussionThe results of the data analysis revealed that runoff quality parameters are influenced by various factors. Regarding electrical conductivity (EC), which indicates the total ion concentration, both spatial location and sampling time exerted significant effects (p < 0.05), whereas roof covering type did not show a significant impact. This finding suggests that, in the study area, the soluble salts in runoff predominantly originate from atmospheric deposits and pollutants accumulated on surfaces, which vary according to spatial location and the intensity of air pollution in each zone. Mean comparisons using Tukey's test indicated that Zone 3 (the farthest zone, at a distance of 25 km) differed significantly from Zones 1 and 2 (closer to the center) and exhibited the lowest EC value, which may be attributed to the gradual reduction of airborne pollutants with increasing distance from pollution hotspots.
In contrast, pH was significantly influenced by both spatial location and roof covering type (p < 0.05), while sampling time had no significant effect on this parameter. This implies that the acidity or alkalinity of runoff depends both on the chemical nature of the covering material (e.g., the release of hydroxyl ions from certain materials) and on the chemical composition of particulates and gases absorbed from the air (such as sulfur and nitrogen oxides). The significant differences in pH observed among all studied zones (5, 15, and 25 km) further underscore the prominent role of air pollution in altering the chemical characteristics of rainfall and runoff.The most intriguing findings pertained to the total concentration of calcium and magnesium ions (Ca²⁺ and Mg²⁺). Unlike the other two parameters, roof covering type and sampling time exerted significant effects on the concentration of these ions (p < 0.05), whereas spatial location did not show a significant influence.
Conclusion Based on the findings of this study, it can be concluded that the quality management of urban runoff requires a comprehensive and multidimensional approach. The results clearly demonstrated that spatial location serving as a proxy for the intensity and type of regional air pollution is the primary determinant of variations in electrical conductivity (EC) and pH, thereby highlighting the significant role of ambient air quality in shaping the initial quality of runoff. Furthermore, the observed temporal variations in pollutant concentrations, particularly the decline in calcium and magnesium levels over time, confirm the occurrence of the first-flush phenomenon wherein pollutant concentrations are substantially higher at the onset of rainfall than in subsequent stages. This phenomenon represents a critical consideration for urban runoff management, as it implies that by separately collecting and diverting the initial runoff volume (which carries the highest pollutant load) to treatment facilities or quality control systems, a substantial portion of the pollution burden can be effectively prevented from entering receiving water bodies. Meanwhile, the main, less-contaminated runoff volume can be utilized for purposes such as green space irrigation or groundwater recharge. Overall, this research underscores the necessity of simultaneously considering both local (material-related) and regional (airborne pollution) sources of contamination in urban development planning and integrated water resource management in arid regions.