Evaluation of the runoff water quality from a tunnel wash

Tunnels are specific sections of roads, where the rainfall runoff process does not take place. Nevertheless, they are washed out periodically mainly for traffic safety reasons. The Portuguese National Laboratory for Civil Engineering (LNEC) monitored twice (in May and December 2003) the washout process of the 1570 in long IP2 Gardunha tunnel. The objective was to evaluate pollutant accumulation in the road pavement. Due to specific requirements from LNEC the wash operation was performed without the common detergents used to enhance the cleaning process. The annual average daily traffic (AADT), in 2003, was 9923. Measurements of flow and the collection of manual samples took place at the inlet of an oil separator tank that receives all the runoff from the tunnel, and discharges it into the local creek. The first wash (1st wash) took place over two nights, with a total duration of 345 min, and an average flow of 1.6 L s(-1). The second wash (2nd wash) operation was shorter, with the duration of 103 min and an average flow of 4.2 L s(-1). The annual pollutant loads for metals, total suspended solids (TSS), oil, and grease and polycyclic aromatic hydrocarbon (PAH) were estimated based on wash volumes and pollutant concentrations. Compared to other monitoring studies of highway runoff in Portugal, Gardunha tunnel wash water presented low pollutants content. Only an evaluation of the results taking into consideration the flow processes inside the tunnel atmosphere allowed an understanding of the data, being concluded that tunnel pavements accumulate much less traffic pollutants than ordinary road pavements, under similar operation conditions. Furthermore, pollutants emitted inside the tunnel will tend to build up outside, therefore increasing runoff concentrations in the road sections flanking tunnels. These results are important for the control of highway runoff impacts.