عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Numerous articles printed in recent scientific journals about the heavy metals contamination of drinking water indicate its universal importance. These metals are commonly toxic at very low exposure levels and have acute and chronic effects on human health. Heavy metals are a multi-organ system toxicant that can cause neurological, cardiovascular, renal, gastrointestinal, hematological and reproductive effects. The type and severity of effects depend on the level, duration and timing of exposure. Heavy metals are released by various natural and anthropogenic sources to the aquatic and terrestrial environments and the atmosphere and there are fluxes between these compartments. One of the main sources of heavy metals in drinking water comes from lead service pipes and the lead pipes inside dwellings. In Iran, lead in petrol is still an important source of exposure. Other sources include lead in paint, low temperature-fired ceramics, informal sector recycling of cars batteries, mine tailings and the air, soil and dust in the vicinity of point sources (e.g., smelters). Recent studies in Iran show that the presence of lead in drinking water is mostly due to pipes, fittings, brass or bronze water service connection valves, faucets, fixtures, and other end-use devices.
In this research, the heavy metal contamination of drinking water in Zanjan City from the water distribution system is studied. Two district within the city; (I) an older district with lead alloyed service pipes, and (II) a newer district with plastic water mains (polypropylene) were selected. Random periodic spot samples were taken manually to investigate the pollution incidents. The samples were collected in 1000 ml polyethylene bottles. The polyethylene containers were washed with dilute detergent and distilled water, then rinsed thoroughly with tap water and at last washed with an aqueous 10% nitric acid solution, drained and immediately caped. A few drops of concentrated ultra-pure HNO3 were added to the samples to reach pH < 2 in order to prevent the loss of metals and bacterial and fungal growth. To ensure the removal of organic impurities from the samples and thus preventing interference in analysis, the samples were preserved and digested with concentrated nitric acid. A total number of 200 water samples were taken using three different sampling methods, namely; 1) Fully flushed sampling: The Fully flushed sample is a sample taken after prolonged flushing of the tap in a premise in such way that water stagnation in the domestic distribution system does not influence the concentration of lead. In practice, a sample is taken after flushing at least three plumbing volumes. 2) First draw sampling: This sample is taken first in the morning before the tap in the premise has been used for other purposes. During the over-night stagnation period no water should be drawn from any outlet within the property. 3) Random daytime sampling: This is a sample taken at a random time of a working day directly from the tap in a property without previous flushing. Physical properties of the water samples, notably; Temperature (T), pH, Dissolved Oxygen (DO) and Electrical Conductivity (EC) were immediately measured at sampling stations using a portable digital pH meter (HACH HQ 40D). The samples were analyzed for Cu, Cd, Zn and Pb using an anodic stripping voltammetry method. Voltammetry is an electrochemistry method based on the study of current vs. potential relationships. This method play an important role in analytical chemistry with a wide range of applications, especially metal analysis and their speciation.
The concentration of heavy metals in water samples, especially those from treatment plant (source water) and newer city neighborhoods with polypropylene service pipes are below the permissible limits set by WHO and ISIRI. In contrast, 75% of samples contaminated by cadmium and 60% of those contaminated with lead are belonged to the older districts of the city with lead alloyed service pipes. In the first draw sampling method, the concentrations of metal ions in water samples were much higher than their concentration in other sampling methods. This is more prominent in the older district of the city. This can be attributed to corrosion of old lead alloyed pipes due to sufficient stagnation of water in service pipes. The concentrations of metals are lowest using the fully flushed sampling method and highest in the first draw sampling. This further supports the fact that old lead alloyed service pipes are mainly responsible for the heavy metals contamination of water samples from the Zanjan City water distribution system.