Storm water infiltration is a cause of concern in urban areas. It mobilises, migrates, and accumulates in groundwater as a result of infiltration. A study was carried out to identify and quantify possible pathways of trace elements in the hydrological system and their distribution pattern. Samples collected prior to and after the monsoon period were analysed to find out the impact of storm water infiltration on groundwater pollution. In recent years, attention on the increasing ionic concentration of traces metals in groundwater as a result of storm water infiltration has been studied by various researchers. This has been attributed to human interference, proliferation of industries, and recent agriculture practices in urban areas, where storm water flow recharges the aquifer.
It has become necessary to assess, quantify, and monitor the quality of groundwater influenced by non-point source pollution as a result of storm water infiltration in the urban environment of Karachi. Infiltration experiments were undertaken to establish the suitability of soils type in terms of infiltration rates among the various soils and to investigate the potential effects of the infiltration media on the water quality by the storm water runoff. The city has a large number of industrial units of different sectors, such as textile, pharmaceutical, engineering, paint, paper, chemical, detergents, vegetable oils, beverages, food products, etc.
These industrial units generate approximately 1.3 million litres of effluent per day. Most of them discharged their effluent directly to the hydrological system thus causing an imbalance to the environment. There are three major waterways passing through the mega city, namely the Hub, Lyari, and Malir rivers, which have confluence with the Arabian Sea. These waterways serve an important function of carrying away storm water, particularly during the monsoon period. In all, from 33 shallow wells, 132 groundwater samples were collected during the monitoring period, starting from February 2007 to November 2007.
The samples cover the catchments of the Hub, Lyari, and Malir rivers and were collected at considerable distances from each other in order to find the impact of pollutants on hydrological system. The results showed that determinants such as cadmium, copper and total dissolved solids (TDS) exceed the limits established by the WHO (1993) for drinking water. However, other trace metals, such as arsenic, are almost negligible, while lead, iron, and zinc are well within the permissible limits prescribed by the WHO.
Certain locations close to industrial area are still unaffected due hydrological gradients, topography and local geological structure such as porosity, permeability, joints and physiochemical process in the soil. The migration and distribution patterns of determinants such as copper, iron, cadmium, lead, and zinc with groundwater down stream is quite significant as it increases possibility of accumulation of pollutants in the Arabian Sea. The study showed that the degree of pollution in the groundwater of Karachi is not the results of storm water infiltration. This also is in conformity with another study in which it was disclosed that the groundwater of Karachi is not significantly affected by total dissolved solid and trace metal.
Extract from the research paper ‘Toxic Trace Element Pollution in Storm Water of Karachi: A Graphical Approach’ authored by A Zubair, MA Farooq and HN Abbasi.
