Water and solute transport revealed by travel time distribution and concentration-discharge relations in groundwater and lake systems

Abstract

Time is a fundamental quantity determining transport and reaction progresses in hydrologic and hydrogeological systems. Travel time distribution (TTD) is a lumped representation of discharge and solute export responding to input events in a system. It encapsulates storage dynamics and transport process into a single mathematic function, and resolves mixing process by differentiating water in discharge into a series of water parcels with different ages or from different historical input events. The TTD theory provides a powerful tool to study transport and reaction processes in a time-integrated Lagrangian perspective. This thesis investigates the properties of TTD and the theoretic relation between TTD and transport. The transport timescale, mixing of surface water with different ages in Lake Taihu and mixing of the groundwater with different ages in Po Shan hillslope influenced by tunnel drainage process are studied using the TTD theory. Reactive transport in Po Shan hillslope is investigated by concentration-discharge relations.

To link TTD with both conservative and reactive solute transport in aquifers, a general framework based on Green’s function is proposed. The Green’s function which represents the influence of water or solute source at one point on storage volume or solute concentration at another point after a certain elapsed time is a fundamental function in the TTD theory. The relation between TTD and reactive transport can be derived from the relation of Green’s functions between water or conservative solute and reactive solute. To explore the influence the internal variability of groundwater flow system on time-variant TTD, a multi-fidelity model is developed to solve the equation of groundwater age distribution under the scenarios with and without considerations of molecular diffusion and hydrodynamic dispersion. The absence of molecular diffusion and hydrodynamic dispersion can lead to biased TTD.

Lake Taihu is a large shallow lake suffering from a severe eutrophication problem. Travel time is an important quantity determining biogeochemical and ecological processes in the lake. Long-term deuterium isotope data in the lake and rivers around the lake are used to estimate the time-variant TTD of lake water. The travel time ranges from 4-8 months in dry season and 2-5 months in wet season. Lake Taihu shows an inverse storage effect, i.e., more young water is released when lake volume is large, and vice versa. Temporal variation of spatial distribution of deuterium isotope content indicates storage selection preference of lake water with different ages is controlled by the variation of flow paths and velocities.

The long-term monitored flow rate data and water chemistry data in Po Shan hillslope show that the water and solute transport are highly influenced by the tunnel drainage process. The temporal variation of TTD and storage selection preference are different from those in natural hillslopes. In wet season, some sub-vertical drains may drain additional old water to the tunnels, which may accelerate the chemical weathering process in the hillslope.