Flow reversals in groundwater-lake interactions: A natural tracer study using δ18O

Lake Hampen, (Central Jutland, Denmark) is located high in the landscape near a seasonal moving groundwater divide. The lake is regarded as a flow-through lake during average to wet weather conditions with a large catchment making the lake groundwater-dominated. Monitoring of delta O-18 in mini-piezometers in the lake bed and wells at the lake shore and in the catchment was used to investigate changes in groundwater-lake interaction as a result of three years with below-average precipitation (2009-2011). It is demonstrated that the lake experienced a flow reversal, where the lake switched to being mainly a recharge lake during this period, before returning to a flow-through lake in early 2012. The observations correlate well with observed changes in hydraulic gradients. A 2D groundwater flow and transport model was used to simulate this natural tracer experiment in order to evaluate the extent and timing of the flow reversal as well as supply information during periods, where there were no delta 18O and hydraulic head data. The results show that delta 18O is a robust tracer for tracking the extent of flow reversals. Furthermore, time series of hydraulic heads (2007-2013) demonstrate that the lake experienced short-lived annual flow reversals typically during late summer.