An Evaluation Of Two Tracers In Surface-Flow Wetlands: Rhodamine-Wt And Lithium

• Published in: Wetlands (Wilmington, N.C.) N.C.), 2005-03, Vol.25 (1), p.8-25
• Main Authors: Dierberg, F E, DeBusk, T A
• Format: Article
• Language: English
• ISSN: 0277-5212
• DOI: 10.1043/0277-5212(2005)025<0008:AEOTTI>2.0.CO;2

Rhodamine-WT and LiCl were compared for their suitability as hydraulic tracers in wetlands. Using outdoor mesocosms, we found lithium to be more conservative than rhodamine-WT when initial concentrations were 4.9 to 64 mu g/L for rhodamine-WT and 28-516 mu g/L for Li super(+) (1:6 to 1:8 (wt/wt) ratio of rhodamine-WT to Li super(+)). At higher initial concentrations (i.e., 100 mu g/L for rhodamine-WT and 1000 mu g/L for Li super(+)), both tracers returned more than 95% of the injected amount in submerged aquatic vegetation-dominated mesocosms; rhodamine-WT was returned at only 74-75% in cattail-dominated mesocosms. Batch studies using different sediment substrates exposed to direct sunlight and shade indicated that Li super(+) was less affected by adsorption and microbial processes than was rhodamine-WT at low initial tracer concentrations of each (4.9 and 19.4 mu g/L for rhodamine-WT and 28 and 106 mu g/L for Li super(+)). Both rhodamine-WT and Li super(+) desorb back to the water column in small amounts. The extent of adsorption losses by rhodamine-WT depended on the organic matter content of the sediment and the extent of photolysis. Even though rhodamine-WT was not as stable as LiCl at initial concentrations less than 60 mu g/L, the reduction in the recoveries did not affect the accuracy of key hydraulic parameters (hydraulic retention time, dispersion, tanks-in-series, and wetland dispersion numbers) derived from the method of moments analysis as long as a discernible concentration-time response still existed. This is because the tracer losses were approximately zero-order and irreversible.