Sequential extraction of heavy metals from sorptive filter media and sediments trapped in stormwater quality improvement devices for road runoff

The performance of stormwater quality improvement devices (SQIDs), commonly based on sedimentation and media filtration, is generally examined either at lab scale or in field tests. However, there has so far been only little investigation into the remobilization of heavy metals retained in sediments and sorptive filter media. It is, moreover, unknown whether current lab-scale experiments are able to replicate the conditions of full-scale operation. To assess the potential impact of differences in conditions between lab-scale and field tests on the strength of association of heavy metals, filter media taken from three SQIDs were subjected to analysis by sequential extraction after prestressing both on a lab scale and in field tests. Sediments and filter cake trapped in the SQIDs were additionally analyzed. These displayed significantly higher heavy metal content than filter media that had been prestressed in the field. The zinc in the sediments and filter media displayed particularly high mobility. This study reveals a discrepancy between field and lab-scale conditions that creates differences in the content and strength of association of heavy metals. It is hence possible to show that previous lab-scale experiments potentially overestimated the risk of heavy-metal leaching, due to their predominant occurrence in more mobile fractions.