Natural Sequence Farming (NSF) modifies streamflow by installing instream structures to decrease the velocity of the streamflow and re-connect it to the flood plain. This work aims to determine if these structures affect the stream-flood plain groundwater connectivity.
A series of instream structures (leaky weirs) were constructed in 2006 and 2007 along a flood plain reach of Mulloon Creek. Groundwater levels and electrical conductivity (EC) are being monitored through a network of 13 boreholes located at 3 cross sections on the floodplain, which were installed in 2008. Water level and EC are also monitored in the ponds which intersect each borehole transects.
Equipotential lines indicate that the groundwater flows in a north north-east direction towards the outlet of the study area and does not interact with the stream. However, during the wet winter of 2016 the groundwater flowed from the hills on the east and west of the stream towards two groundwater sumps located at the top and bottom of the reach. Each sump is located 30-50 m west of the stream. Equipotential lines do not indicate any connection with the stream.
Across all the bores the EC of the alluvial groundwater varied from 156-954 µS/cm. During the 2016 wet winter, EC of the groundwater varied from 76-635 µS/cm. The EC across and down the flood plain is variable.
During the 2016 wet winter, the EC of the stream increased marginally along the monitored reach from 61 µS/cm to 69 µS/cm at the outlet. When flows are low, the EC increases from 93 µS/cm to 140 µS/cm at the commencement and outlet respectively.
The connection between the water in Mulloon Creek and its floodplain groundwater is tenuous. This may be explained by the low hydraulic gradient between the stream and the floodplain and the complexities of the floodplain sedimentary sequences.