The characterization of stormwater runoff from road surfaces.

Abstract

The water quality of stormwater runoff from various road surfaces in Durban, KwaZulu- Natal was characterised by monitoring programmes established at four study areas. The investigation focussed on obtaining representative stormwater samples in order to quantitatively identify pollutant constituents transported within stormwater runoff from road surfaces during rainfall events. Stormwater runoff is a leading cause of degradation in the water quality of receiving water bodies. A review of sampling methods and equipment, informed by the objectives and resources of this investigation, resulted in the development of a stormwater runoff sampler. An economical flow weighted composite sampler was designed, developed and tested. The sampler produces one representative composite event mean concentration sample. Controlled laboratory experiments, computer simulations and field tests were conducted in order to test and calibrate the sampler. The product of the event mean concentration and total runoff volume estimated provide an estimation of the total pollutant loading from a particular drainage area. Twenty stormwater runoff event mean concentration (EMC) and atmospheric deposition samples were collected over an eighteen month period. The characterisation of stormwater runoff for this investigation included heavy metals, oxygen demanding substances, sediments and physico-chemical analysis for pH, conductivity and water hardness. Chemical analysis indicates that the pollutant levels of constituents sometimes exceeded EMCs reported internationally. The majority of contaminant EMCs exceeded the South African wastewater discharge general and special limits. The results also indicate a 60% reduction in lead (Pb) levels since the introduction of lead-free fuel in South Africa in January 2006. The findings from this investigation will provide decision-makers with an improved local data base for estimating the impacts of road transportation systems on water quality. A comparison between the four study areas also provides guidance concerning the variability of stormwater quality from different road surfaces and usage patterns.