Hydrological modeling of fecal indicator bacteria in a tropical mountain catchment

Authors: KIM, MINJEONG - University Of Ulsan College Of Medicine; BOITHIAS, LAURIE - University Of Toulouse; CHO, KYUNGHWA - University Of Ulsan College Of Medicine; SILVERA, NORBERT - Bureau Of Land Management; THAMMAHACKSA, CHANTHAMOUSONE - Bureau Of Land Management; LATSACHACK, KEOOUDONE - Bureau Of Land Management; ROCHELLE-NEWALL, EMMA - Pierre And Marie Curie University; Pachepsky, Yakov; SENGTAHEUANGHOUNG, OLOTH - Bureau Of Land Management; PIERRET, ALAIN - Pierre And Marie Curie University; RIBOLZI, OLIVIER - University Of Toulouse

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2017
Publication Date: 6/15/2017
Citation: Kim, M., Boithias, L., Cho, K., Silvera, N., Thammahacksa, C., Latsachack, K., Rochelle-Newall, E., Pachepsky, Y.A., Sengtaheuanghoung, O., Pierret, A., Ribolzi, O. 2017. Hydrological modeling of fecal indicator bacteria in a tropical mountain catchment. Water Research. 119:102-113.

Interpretive Summary: Fecal indicator bacteria are found in high concentrations in many tropical surface water sources which presents a significant public health hazard. In spite of the severity of the problem, little is known about the processes that cause these high concentrations. In particular, little is known about the fate and transport of fecal indicators, such as Escherichia coli, in tropical streams. Utilizing monitoring data collected at a 60-ha catchment in Northern Laos, and the USDA-ARS watershed water quality model SWAT, we evaluated the role of three candidate fate and transport processes: regrowth, release of bacteria during the bottom sediment resuspension, and the release from the bottom sediments without resuspension. We found that release from the bottom sediments without resuspension appeared to be the essential process in conditions of tropical streams. The SWAT model was amended to include the effect of this process. Results of this work are expected to be very useful for modeling microbial water quality for policy and regulation, and risk assessment purposes in tropical streams.

Technical Abstract: The occurrence of pathogen bacteria in surface waters is a threat to public health worldwide. In particular, inadequate sanitation resulting in high contamination of surface water with pathogens of fecal origin is a serious issue in developing countries such as Lao P.D.R. Despite the health implications of the consumption of contaminated surface water, the environmental fate and transport of pathogens of fecal origin and their indicators (Fecal Indicator Bacteria or FIB) are still poorly known in tropical areas. In this study, we used measurements of flow rates, suspended sediments and of the FIB Escherichia coli (E. coli) in a 60-ha catchment in Northern Laos to explore the ability of the Soil and Water Assessment Tool (SWAT) to simulate watershed-scale FIB fate and transport. We assessed the influences of 3 in-stream processes, namely bacteria deposition and resuspension, bacterial regrowth, and hyporheic exchange (i.e. transient storage) on predicted FIB numbers. We showed that the SWAT model in its original version does not correctly simulate small E. coli numbers during the dry season. We showed that model’s performance could be improved when considering the release of E. coli together with sediment resuspension. We demonstrated that the hyporheic exchange of bacteria across the Sediment-Water Interface (SWI) should be considered when simulating FIB concentration not only during wet weather, but also during the dry season, or baseflow period. In contrast, the implementation of the regrowth process did not improve the model during the dry season without inducing an overestimation during the wet season. This work thus underlines the importance of taking into account in-stream processes, such as deposition and resuspension, regrowth and hyporheic exchange, when using SWAT to simulate FIB dynamics in surface waters.