Highway runoff and potential for removal of heavy metals in an infiltration pond in Portugal

Highway runoff from IP 4, a mountain road in the north-east of Portugal, has been monitored using a system integrating a raingauge, a flowmeter and an automatic water sampler. Average daily traffic (ADT) is 6000 and the study catchment has 5970 m2 of total area and 2500 m2 of road pavement. A single stormwater outlet discharges into an infiltration pond with overflow to a creek. Sampling was carried out before the runoff water entered the pond. Among the parameters analysed in the runoff water, the heavy metals cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn) were emphasised because of their toxicity. Concentrations of Cd and Cr were usually lower than the detection limit (1 microgram/l). Copper levels found were between 1 and 54 micrograms/l; lead from 1 to 200 micrograms/l and zinc from 50 to 1460 micrograms/l. A first flush effect was observed, meaning that the first 50% of the runoff volume for each event typically transported between 61 and 69% of the total suspended solids, Zn, Cu and Pb loads. Runoff water is totally infiltrated into the pond and heavy metals are being sorbed to the soil. Soils used in infiltration ponds should have specific characteristics in order to perform effectively and ensure groundwater protection. Not only well-known soil texture and composition characteristics are relevant: the soil sorption capacity--the extension and reversibility of the processes--is of main importance in this kind of highway runoff treatment. Experiments concerning the sorption of Zn, Cu and Pb to soils, followed by desorption at pH values of 2, 4 and 6 were conducted in the laboratory. These experiments were performed with the soil existing at the highway IP 4 infiltration pond and with two other common types of Portuguese soils. The three types of soil showed different behaviours, which must be related to their characteristics; the soil pH seemed to play a significant role in controlling the Zn, Cu and Pb sorption processes. As expected, as lowering of the pH value increased the desorption rate. The infiltration pond soil is the one with the lowest sorption capacity, however, it showed a relatively high sorption strength which means that it is considered reliable, concerning highway runoff treatment and groundwater protection.