Using isotopes (strontium and radon) and microbial communities to quantify groundwater mixing influenced by anthropogenic factors at riverside area

Abstract

Investigating mixing processes between groundwater and surface water is important to manage groundwater system. Especially, because the study site is located around a groundwater heat pump (GWHP) system at a riverside area, external effects such as an operation of GWHP system and/or river level fluctuations can disturb the water system. It can result in artificial mixing of two water bodies, thereby changing original hydrogeochemical and biological characteristics. For this research, a multi–tracer approach combining strontium isotopes, 222Rn, microbial communities, and hydrochemical composition was undertaken for nineteen water samples to quantify how and to what extent mixing may occur due to external processes. Before this approach was applied, the physical experiment using flowmeter was also conducted for defining the effects of dam discharge rates to the flow system. Estimated mixing ratios based on radon tracer showed spatio–temporal variations, influenced by the dam discharge rate, seasonal effects, and GWHP. The average mixing ratio values by strontium tracer were in accordance with the results of radon tracer, suggesting that a dual-tracer approach would be a more reliable means. Mass–transfer calculation indicated that spatio–temporal variations in strontium isotopes were affected by hydrological events. Results from the microbial community using a heat map and principal coordinate analyses also supported the mixing process characteristics of the study area. Based on the results, the interactive and dynamic mixing occurred in the riverside area in relation to external factors causing hydraulic disturbances, and the mixing could be estimated from the combined application of isotopic tracers with microbial community.