Effect of dense vegetation on colloid transport and removal in surface runoff

Abstract

Widely distributed in aquatic environment, colloids can pose risks to water quality, particularly when loaded in surface runoff, which can facilitate fast distribution of contaminants. It is unclear whether densely vegetated areas, which are known to be effective to reduce solutes and sediment from runoff, can be used to remove colloidal contaminants. In this paper, laboratory experiments were conducted to investigate effects of dense vegetation on colloid transport and removal in surface runoff. First, batch experiments were conducted to obtain the sorption isotherms of colloids (fluorescent microspheres) onto different vegetation parts (grass leaf, stem, and root). Then, a laboratory rainfall/runoff system was used to compare the transport behaviors of a tracer (bromide) and colloids in overland and drainage flows during a simulated rainfall event (64 mm/h). The system was equipped with Bahia grass growing on sandy soil containers with one surface flow and four vertical drainage sampling outlets. We found that all grass parts can sorb aqueous colloids with the Langmuir maximum sorption capacity between 455.3 and 1188.3 mg kg−1. Runoff experimental results demonstrated that both the surface vegetation and the soil (including plant root) underneath can effectively remove colloids from surface and subsurface flows. Only about 33% of the applied colloids were recovered from the outflows, with about 29% and 4% from overland flow and drainages, respectively. Our results suggest that well designed and maintained areas of dense vegetation, like vegetative filter strips, can be used as an effective best management practice to reduce colloidal contaminants in surface runoff.