Submitted to: Wetlands Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2014
Publication Date: 7/29/2014
Citation: Kaplan, D., Bachelin, M., Yu, C., Munoz-Carpena, R., Potter, T.L., Rodriguez-Chacon, W. 2014. A hydrologic tracer study in a small, natural wetland in the humid tropics of Costa Rica. Wetlands Ecology and Management. DOI 10.1007/s11273-014-9367-1]. Interpretive Summary: Wetlands provide a diverse set of ecosystem services including flood control and flow regulation, attenuation and retention of sediments and other pollutants, aquifer recharge, and maintenance of water levels and flows in receiving water-bodies during dry seasons. Ecological functions tied to these processes and include habitat and biodiversity conservation, biomass and nutrient transformation and transport, and water quality improvement. Natural and constructed wetlands are increasingly being used to remove contaminants from municipal, agricultural, and stormwater runoff. A critical first step in evaluating their ability to serve these functions is to determine water flow paths, velocities, and hydraulic retention times. This study examined these processes in a small tropical wetland in Costa Rica by evaluating the transport of two tracers that were injected into water at two points. The work was pioneering since there have been few studies of type conducted in the tropics. Data showed, as is typical of wetlands, that flow paths were complex indicating both fast and slow transport zones. The study contributed essential information for evaluating the potential for this and other wetlands to retain and attenuate a broad range of contaminants that may be entrained in runoff from adjacent banana plantations. Results are expected to support public decision-making regarding wetland use, conservation, and preservation.
Technical Abstract: Growing populations combined with increasing food demand are leading to increased environmental pressures on tropical wetland ecosystems, including a greater reliance on natural wetlands for water quality improvement. Effective assessment of their performance requires an improved understanding of hydraulic and biogeochemical factors that govern contaminant behavior; however, detailed studies of flow through natural, tropical wetlands are scarce. We performed a dual tracer study using a conservative salt (bromide, Br-) and a dissolved gas (sulfur hexafluoride, SF6) to examine the hydraulic behavior of a small, natural wetland in the Costa Rican humid tropics. Br- was an effective tracer under the study’s shallow, slow-flow conditions, allowing an analysis of wetland hydraulics and water quality enhancement potential. The non-conservative behavior of SF6 limited its usefulness as a surface water tracer under these conditions. Water velocities calculated from the Br- tracer ranged from 3.7-35.3 m d-1 and were distributed across several flowpaths, illustrating a heterogeneity of flow and velocity between “slow” and “fast” flow regions that likely supports a number of wetland functions. Calculated residence times suggested high pollutant removal capacity over a range of influent concentrations, reinforcing the importance of the environmental services provided by this and other small tropical wetlands.