Use of vegetated drainage ditches and low-grade weirs for aquaculture effluent mitigation: II. Suspended sediment

Abstract

Total suspended solids are a priority pollutant under the Clean Water Act and a point of concern for aquaculture facilities. The use of ubiquitous vegetated ditches on the aquaculture landscape may serve as an environmentally and economically sustainable practice for reducing suspended sediment contributions to downstream environments. This study assessed effects of consecutive low-grade weirs on suspended solids retention and settling rates of aquaculture pond effluent in a single drainage ditch. Two control and nine treatment discharges were conducted in September and October 2012 at the Mississippi State University South Farm Aquaculture Facility. All discharges decreased total and volatile suspended solid loads. Total suspended solids were decreased 72–94%, with a significant removal rate of 0.02±0.01mgL−1min−1 in both control (F=6.12, P<0.001) and treatment discharges (F=16.02, P<0.001). Volatile suspended solids comprised 2–80% of total suspended solids and had a significant removal rate of 0.02±0.001mgL−1min−1 in both control (F=10.46, P<0.001) and treatment discharges (F=6.28, P<0.001). There was no significant difference in overall settling rates between control and treatment discharges; however, prior to weir 1, both total and volatile suspended solid concentrations increased in control discharges. Treatment discharges decreased both total and volatile suspended solids significantly (P<0.001) prior to weir 1. Further analysis revealed flow rate to be a significant (P<0.001) variable in total suspended solid removal while initial concentrations affected reduction rates of volatile suspended solids significantly (P<0.001). These results suggest that the use of low-grade weirs could be a viable best management practice that easily integrates within the aquaculture landscape and creates hydraulic conditions conducive to sediment retention.