Using high-resolution in situ radon measurements to determine groundwater discharge at a remote location: Tonle Sap Lake, Cambodia
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Tonle Sap Lake (Cambodia) is the largest freshwater lake in SE Asia, and is reported to have one of the highest freshwater fish productions anywhere. During the dry season (November–April) the lake drains through a tributary to the Mekong River. The flow in the connecting tributary completely reverses during the wet monsoon (May–October), adding huge volumes of water back to the lake, increasing its area about fourfold. We hypothesize that nutrients are at least partially delivered via groundwater discharge, especially during the draining portion of the annual flood cycle. We surveyed over 200 km in the northern section of the lake using a customized system that measures natural 222Rn (radon), temperature, conductivity, GPS coordinates and water depth while underway. Results showed that there were portions of the lake with significant enrichments in radon, indicating likely groundwater inputs. These same areas were generally characterized by lower electrical conductivities. Samples collected from nearby wells also showed a general inverse relationship between radon and conductivity. Our data suggest that groundwater pathways are important, accounting for roughly 10–20 % of the freshwater flow of the Tonle Sap tributary (connection to the Mekong River), the largest single source of fresh water to the lake. Nutrient inputs from these inputs, because of higher concentrations in groundwater, will be correspondingly higher.
KeywordsRadon Tonle Sap Lake Groundwater discharge Remote locations Nutrients
The authors thank Ms. Rawiwan Kritsananuwat and Mr. Sompop Rungsupa of Chulalongkorn University for their assistance in the field and in the laboratory. We also acknowledge several Cambodian colleagues who were instrumental in helping us complete our field research. These included Mr. Bunseang Suy and Mr. Veasna Chhan (Tonle Sap Rural Water Supply and Sanitation Sector Project), Ms. Leakhena Phuong (Pure Water Association), and Mr. Sothea Kok, Ms. Eliyan Chea, Ms. Sodaneath Hong and Ms. Thavin So, all of the Royal University of Phnom Penh. Financial support for this research was received from a grant from the National Geographic Society to Florida State University.
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