Abstract

The results of this study highlight the sorption potential of mature wetland-bed gravel and attached biomass for soluble phosphorus in trout farm effluents, the effectiveness of conventional lab studies in predicting field results, and the potential pitfalls associated with the use of RWT as conservative tracer. Gravel media extracted from a pilot-scale, horizontal subsurface flow (HSSF) wetland was evaluated for its phosphorus sorption affinity and influence on Rhodamine WT (RWT) transport. After receiving effluent from a flow-through trout farm for about one year, the gravel media exhibited moderate removals of soluble phosphorus (PO_4-P) in batch and column sorption experiments. Partition coefficients (Kd) from batch sorption tests ranged from 45-90 mL/g, with lower initial concentrations resulting in increased Kd values. Low (60 mL/min) and high (165 mL/min) flow column experiments resulted in partition coefficients of 0.9 and 0.8 mL/g, respectively. Removal of PO4-P from initial solutions was also generally greater in batch experiments with 69-82% removal, as compared to 40-50% removal for column experiments. The conservative nature of RWT in subsurface media has been called into question by many authors. Tracer response curves from tests conducted in a pilot-scale HSSF wetland exhibited elongated tails and dual peaks, in addition to mean tracer retention times far exceeding the theoretical value. Laboratory column testing of RWT and the more conservative NaCl tracer indicated that RWT was more reactive within the wetland media, even with good mass recovery (96%). First moment analysis revealed that the mean retention times were 4.6 and 7.4 hours for NaCl and RWT tracers, respectively, in the lab testing.