WP4 - Aquifer Interactions

Objectives
1. To investigate stream – aquifer interactions at several scales using a multi-tracer approach.

2. To use Distributed Temperature Sensing (DTS) for mapping spatio-temporal changes in discharge zones.
 
3. To analyze atmosphere-stream–aquifer dynamics using groundwater-stream numerical flow and heat transport models and energy budget models.

4. To investigate hydrograph uncertainty in small and moderately sized streams using continuous stage and Doppler-velocity data.

5. To investigate spatial and temporal differences in how groundwater discharges to a small headwater stream using Doppler velocity data and stable isotope measurements.

Work content

Task 1: Field experimentation
Two different wetland-stream sites have been monitored; site I in the period 2007-2011 and site II from 2009-2012. A new site III is under development.

2007-2011: Data has been collected from a wetland-stream site I and from ~ 15 stream stations. The hydrogeology of the wetland-stream site has been mapped with different geophysical techniques (MEP, georadar) and the flow dynamics in the wetland from regular monitoring of water level changes in 23 piezometers. Forty temperature probes have recorded seasonal changes in temperature in the stream bed and used to quantify the seepage to the stream. Stream discharge, CFC apparent ages, and water temperature at two levels have been measured at the stream stations.

2010-2012: A fiber-optic cable (DTS) has been installed at site II (partly shaded) for longer periods (up to several months) to record spatio-temporal changes in temperature at the stream bed. Two DTS columns have been installed to record the temperature at the atmosphere-stream-aquifer interfaces. Furthermore, a climate station was installed. The long-term placement of the DTS cable allows us to investigate the importance of sedimentation/scouring.

2012-2013: Site II has now been abandoned and moved to site III (with less shading).

Two different streams within the Skjern catchment have been monitored with pressure transducers and acoustic Doppler instruments during the period 2010-2012. Acoustic Doppler Velocity Meters (AVMs) have been installed in two/three different levels where they record the average flow velocity. Monthly discharge measurements using an Acoustic Doppler Current Profiler (ADCP) have also been performed at the two sites.

Influences from seasonal and short duration changes in weed distribution and abrupt changes in stage have been recognized in the velocity gradients in the streams. It has been shown that the joint use of use of stage data and multi-level acoustic Doppler velocity data can provide estimates of the hydrographs estimation with lower uncertainty than traditional stage-discharge methods.

An experimental program with sampling of tracer hydrographs and discharge hydrographs are currently carried out in one of the small headwater streams in the Skjern River catchment. Four locations along a 3 km stretch of the stream have been instrumented with water-samplers, which collect time series of the stable isotopes in the stream water during event flows. Two samplings have been carried out in the spring 2012 and one additional will be carried out during fall 2012. At two of the four locations continuous average velocities are being recorded with AVM sensors, and stage is recorded with pressure transducers in addition to near monthly ADCP-discharge measurements. Finally the concentrations of the end-members or sources of the water to the stream (rainfall, shallow or deep groundwater, etc.) have been recorded.

Task 2: Numerical analysis and regionalization
Data are now available for simulating flow and heat transport using different approaches; Numerical flow and heat transport models at the (i) stream-bed scale (1D), (ii) wetland-stream scale (2D), and (iii) catchment scale (3D) and using stream energy budget models to simulate the energy exchanges between the river and the atmosphere/groundwater under different degrees of shading.

The collected data will be used in a study on quantifying/modeling how the different components/end-members contribute with water to the stream.