Groundwater in Streams: Understanding the dynamics of travel times, nutrients and temperature

Surface waters worldwide are threatened by anthropogenic influences. Many of these waters gain part of their flow from groundwater and it has been noted that surface water and groundwater are effectively a single resource. However, the importance of this connection between surface- and groundwaters in multi-stressed catchments is not yet well understood. The research presented in this thesis therefore aimed to quantify the impact and importance of the groundwater contribution to streams and aimed to improve understanding of how the dynamics of the groundwater affect the propagation of stressors to streams. A conceptual framework is presented for the analysis of linked multiply-stressed GW-SW systems. This framework is illustrated by applying it on four European catchments to analyse the propagation and buffering of multi-stressors through groundwater to surface waters. The importance of the groundwater system varies between the catchments due to differences in catchment characteristics and stressor combinations. Results from these catchments show that the effect of stressors can propagate through the groundwater system towards surface waters, and groundwater thus functions as a connection between a catchment and its stream(s). In addition to this, the flow of groundwater over potentially long distances and over longer time scales creates a buffer capacity to the effect of stressors by adding time lags and attenuating stressor signals, although this strongly dependents on groundwater flow paths and the shape and dynamics of groundwater travel time distributions. Travel time distributions are studied by calculating them dynamically for three Dutch lowland catchments using a high-resolution spatially-distributed 3D groundwater flow model in a novel way. These detailed distributions showed that the streams consist of younger water in the wet periods and older water in dry periods due to the interplay between the activation of shallow short flow paths contributing young water towards for instance shallow drains and the intensification of fluxes through all flow paths when groundwater levels rise. The calculated TTDs were combined with chemical input curves to reconstruct concentrations in the stream water. We were able to explain the short-term variability of nitrate and chloride in the stream with the variable contribution of different groundwater flow paths and the time-variable part of the catchment that is contributing to streamflow. We used the model conceptually to advance our understanding of time lags in the breakthrough of agricultural nitrate. Parameters such as the thickness of the unsaturated zone, mean travel time, input patterns and the distance of agricultural fields determine time lags between peak inputs and maximum concentrations in the stream. The importance of groundwater for stream temperature is investigated by locating seepage in two of the study streams by measuring temperatures using long fibre-optic cables in combination with measurements of the groundwater tracer Radon-222. Using the field measurements, a stream temperature model was made to separate the different energy fluxes which showed that the cooling effect of groundwater. Scenario modelling showed that groundwater in groundwater-fed catchments may play an important role in the mitigation of climate warming because of its capacity of reducing summer maximum temperatures.