This research was first established during the ARC/NWC funded National Centre for Groundwater Research and Training (NCGRT) from 2009-2014, and is now a CWI Flagship Project.
The interconnectivity of surface water and groundwater is often not fully recognised or accounted for by water managers, and this can lead to double accounting and double allocation of resources. This problem is compounded by a lack of widely available infrastructure for monitoring surface water-groundwater connections. Scientifically, because they are at the interface between surface water and groundwater hydrology, surface water-groundwater interactions still pose major research challenges. Understanding the spatial and temporal dynamics of surface water-groundwater interaction processes is a major research area (Sophocleous 2002).
This research program will quantify fluxes and constrain biogeochemical processes and will provide important links to important groundwater dependent ecosystems; these are key gaps in current understanding and matters of critical international scientific interest. Attempts will be made to develop integrated concepts for the ecological assessment of connected surface water and groundwater. Importantly, through ecological and biogeochemical characterisation, this program will aim to determine whether ecological communities can be used to measure or indicate groundwater-surface water interactions.
Key investigators: PI Cook, PI Lamontagne CI Lockington, CI Simmons, CI Cartwright, CI Gasparon, with assistance from RI Acworth and M Andersen.
There are no accepted field methods for distinguishing between connected and disconnected streams, and currently-used theoretical criteria for assessment are now recognised to be in error (Brunner et al., 2009). Developing robust theoretical criteria to predict river connectivity remains a challenge. Field-based assessments of disconnected streams, and their transient nature, are absent in current literature since they lack a robust theoretical basis. An ability to make this distinction is vital for groundwater management, as groundwater pumping will only deplete streamflow if streams are hydraulically connected to the groundwater.
This Sub-program will develop new theoretical criteria and urgently needed field methodologies (hydraulic, heat/thermal and hydrochemical) for assessing disconnection states between groundwater and losing streams. The rate of surface water loss will be measured using river flow gauging, and relationships between loss rate and surface water and groundwater levels will be explored using both modelling and hydraulic, chemical and tracer approaches. Improved methods for estimating connectivity and river losses are particularly needed in arid and semi-arid regions where ephemeral losing streams form a major recharge mechanism. The transient nature of these processes is a major challenge that will be overcome by sampling for both hydraulics and chemistry at high spatial and temporal resolution.
Chemical baseflow separation methods typically suggest that 30-70% of the streamflow peak that follows a rainfall event is attributable to groundwater baseflow. This contradicts hydraulic analyses, which suggest a large timelag between an increase in groundwater recharge resulting from the rainfall event and the corresponding increase in discharge to the stream (Kirchner, 2003). This paradox remains an important but unresolved scientific problem in the area of surface water-groundwater interaction.
This Sub-program will investigate this paradox by conducting baseflow separation during storm events with a number of different tracers. These will include a combination of chloride and 2H and 18O, which are traditionally used for chemical baseflow separation, and novel tracers such as 222Rn, He, and H/3He, which allow groundwater residence times to be determined. This combination of tracers will allow for the critically needed differentiation between native groundwater and bank return flow, and allow mixing processes within the near-stream environment to be assessed.
Groundwater levels during the stormflow event will also be measured in transects of nested piezometers, allowing directions of flow at different levels in the groundwater system to be monitored throughout the event. This will lead to a better understanding of the hydraulic and chemical exchange processes that occur during stormflow events, and also the ability of chemical methods to accurately partition storm hydrographs. These issues also apply to lake and wetland systems, which have received relatively little study, and where hydraulic and tracer approaches yield different fluxes (Hunt et al., 1996).
Research Contributors from UNSW:
Interactions between an ephemeral stream, Maules Creek and groundwater of the underlying aquifer are being studied using a combination of geological data, hydraulic data, stream water levels, fluid EC, temperature and resistivity imaging. Zones of groundwater discharge have been pinpointed largely based on temperature anomalies and EC variations. Zones where the stream appears to be recharging the aquifer were identified based on the geology and gradients in hydraulic head. Geological heterogeneity was found to be an important factor in controlling the occurrence of surface water flow and the exchange of water between stream and aquifer. However, anthropogenic effects in the form of groundwater extraction probably enhance the aquifer recharge from the creek.
Based on EC and temperature data, zones of groundwater discharge were detected in the upper part of the stream reach that was studied in August and October, 2006. Once water discharged into the creek it appears to be flowing for several kilometers between pools in a relatively thin (2-10 m) layer of sand and coarse gravel on top of more massive clayey layers as indicated by the resistivity imaging. However, the resistivity images also suggest that the clay is not laterally continuous with possible hydraulic connections to the aquifer below. Variations in the electrical conductivity downstream indicate a possible influx of groundwater with varying EC. It cannot be discounted that evapotranspirative concentration down the reach may also have some influence on the EC data. This needs to be quantified in order to use EC data to derive quantitative estimates of the exchange of water between the stream and aquifer. Further downstream, the stream is potentially recharging the regional aquifer as were inferred by downward hydraulic gradients and by inspecting the geological data. It is possible that this stream fed recharge is enhanced by drawdowns in the regional aquifer caused by extraction of groundwater.
So far, the results of this study are of a qualitative nature and actual fluxes needs to be established. Furthermore the results are obtained during low flow conditions and the dynamics of surface water groundwater interactions in relation to major precipitation and flooding events need to be studied in greater detail. The results of this study have implications for understanding the stream fed aquifer recharge and in turn for estimating the sustainable extraction of groundwater from the regional aquifer.
