A snapshot comparison of surface water and groundwater isotopes in the thur catchment
2 Eawag - Swiss Federal Institute of Aquatic Science and Technology
14.1 in Forum Junge Hydrogeologen
25.03.2020, 14:00-14:15, Telemann-Saal
Often field-scale studies observe small catchments, where a high monitoring resolution is possible. This study was conducted in the 1700 km² catchment of the Thur River in northeastern Switzerland. The δ18O and δ2H isotope signatures of water were used as an environmental tracer. A spatial snapshot monitoring approach, involving water from groundwater and surface water sites, was applied. Event-sampling campaigns of surface waters were conducted in spring and summer in the Thur catchment to capture the high-flow and low-flow conditions of the river. In addition, groundwater samples were collected bi-annually from aquifers in the Thur valley, which are recharged by the Thur River or its tributaries. Longterm data of isotopes in the local precipitation was available from a monitoring station in St. Gallen.
Seasonal and spatial variations of the isotopic signatures were identified. Surface water from the upper reaches of the river was found to be depleted in heavy isotopes during the high-flow events in spring. However, the isotopic variations are not correlated to discharge. Altitude and temperature effects and the influence of meltwater could be observed. The surface water samples from higher altitudes had a deuterium excess which could not be observed in the local precipitation data. Surface water samples from the lower reaches showed signs of evaporation. The isotope signature of groundwater from aquifers in the upper Thur catchment indicated a comparable depletion in 18O and 2H isotopes, while the groundwater isotopes sampled from the lower region had a wide seasonal variation. Temperature signals and chloride concentrations propagated from surface water to groundwater within one month, irrespectively of the distance between river and well, which varied between 30 m and 1380 m. Similar observations could be made regarding the isotope signatures.
Based on the regional altitude gradient, which lies between 0.4 ‰ and 0.6 ‰ for δ18O, the average groundwater recharge altitude was calculated. It averages at 790 m above sea level for the aquifer of the lower Thur valley. However, the calculated catchment altitude shows monthly variations, which reflect the seasonal changes of isotopic signatures. Using a two-component mixing equation, it was determined that groundwater recharge occurs predominantly in spring and the source of recharge is 73 % river water and 27 % direct infiltration of precipitation. The resulting, isotopically lighter than average recharge contributes to a biased, exaggerated, calculated groundwater catchment altitude.
Sampling catchment-wide water isotopes can provide insight into surface water – groundwater interactions and aid in the efficient management of aquifers, with respect to potential changes in precipitation, flow-regime and recharge. However, separate aquifers within the hydrologic catchment should be addressed individually, especially if a direct connection between the aquifers is unlikely.
Chittoor Viswanathan, V. et al (2016): An integrated spatial snap-shot monitoring method for identifying seasonal changes and spatial changes in surface water quality. – In: Journal of Hydrology.
Vitvar, T. & Balderer, W. (1997): Estimation of mean water residence times and runoff generation by 18O measurements in a Pre-Alpine catchment (Rietholzbach, Eastern Switzerland). – In: Journal of Hydrology.
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