Comparison of rhodamine WT and bromide in the determination of hydraulic characteristics of constructed wetlands

Hydraulic tracer tests were performed in the Prado Wetlands, Riverside County, California, USA. The goals of the tests were (1) to evaluate the suitability of rhodamine WT (RWT) as a tracer for wetlands studies, and (2) to determine the residence time distribution of the wetlands. The performance of RWT was evaluated by comparing the breakthrough curve (BTC) of RWT to that of bromide in a pilot-scale test. The BTCs of RWT and bromide indicated equal results. After the pilot test, a full-scale test was conducted by releasing a RWT pulse at the wetlands inlet and monitoring for RWT arrival near the wetlands outlet. The BTC indicated 10 and 90% (of the total mass recovered) breakthrough times of 25 and 112 h, respectively, but these must be considered approximations because only 29% of the injected RWT mass was recovered. Laboratory experiments suggest irreversible sorption to be the principal loss mechanism of RWT during transport through the wetlands. RWT is a suitable tracer in wetlands that are relatively small (less than 1 week residence time) and deep (at least 0.6 m) with limited sediment contact, but RWT yields only approximate results for the extended wetlands system. (C) 2003 Elsevier Science B.V. All rights reserved.