Characterisation and quantification of groundwater-surface water interactions along an alluvial stream using geophysical, hydraulic and tracer methods

Abstract:

Qualifying and quantifying water, nutrient and contaminant exchange at the groundwater-surface water interface are becoming increasingly important for water resources management.

The objectives of this study are to characterise an alluvial stream using geophysics in addition to traditional geological and geomorphological data and quantify the groundwater seepage to the stream on point-to-reach scale using both hydraulic and tracer methods.

We mapped the very shallow subsurface along an alluvial stream using a GCM system (DUALEM421S, an electromagnetic system that can be operated behind a boat or towed behind a motorized vehicle) as well as using geological logs from a large number of old wells. Furthermore we made geomorphological observations through digital maps (old topographical maps and aerial photos) and field observations.

We measured stream discharge (quasi-) simultaneously at several positions along the stream using both an Ott-C31 propeller instrument and an Acoustic Doppler Current Profiler instrument. The measurements were made during dry summer periods when baseflow is expected to be the dominating contribution to streamflow.

Preliminary findings show that the GCM system reveals small-scale structures not seen with other data types. Furthermore, based on the GCM results and stream discharge results we have identified gaining, losing and zero exchange sections of the stream.

During late summer 2015 we will collect additional hydrological data in order to support or modify our preliminary findings. To further investigate the spatial and temporal variations of the groundwater-surface water interactions along the stream we will measure groundwater seepage to the stream using:

• seepage meter (point-scale)
• DTS (reach-scale)
• temperature stick measurements (point-in-space-and-time-scale)
• temperature loggers installed in the streambed (month-scale)

The measurement sites are chosen based on our geophysical, geological, and geomorphological mapping as well as our stream discharge measurements.