3D time and space analysis of groundwater head change for mapping river and aquifer interactions

This study demonstrates how multidimensional spatial analysis of hydrograph data enables the 3D mapping of hydraulic pathways through complex sedimentary aquifer systems in the Namoi Catchment (New South Wales, Australia). Historical groundwater head records over the past 40 years capture the influence of irrigation extractions. Analysing the head change throughout the aquifer in 3D and with respect to time clearly shows both the yearly and long term impacts of groundwater extractions on river-aquifer interactions. The 3D analysis also maps the recharge pathways, delineating the primary zones of recharge. The hydraulic data were analysed and cross validated with lithological logs, groundwater temperature and pH values. Data analysis was undertaken using spatial analysis techniques available in ArcGIS and EarthVision facilitated by extensive use of Python scripting. Some hydrographs show that aquifer heads respond to variations in extraction differently at different depths, indicating that there are impervious or leaky semi-impervious layers. Other hydrographs show heads from different depths all responding in the same way to extraction and subsequent recovery, indicating that locally the system is vertically hydraulically connected. The head data were analysed over one year periods with no flood events. During low rainfall years, groundwater usage is at its highest level, resulting in maximum pumping related head change throughout the aquifer. Positioning the head values in 3D space at the slotted depth of the boreholes highlights hydraulic connectivity between boreholes and through the alluvial sequence. Correlating lithological logs in sedimentary environments containing numerous sand and clay units is difficult. Mapping the horizontal continuity in head change aided the borehole log correlation, showing which sedimentary units are hydraulically connected. The 3D mapping of head change due to pumping stress enabled the 3D mapping of palaeochannels, clearly delineating the meandering path of pre-existing water courses and the link to the present day stream channels. Spatial data analysis techniques adopted for this research have successfully enhanced the understanding of river and aquifer interactions and our knowledge of the 3D geometry of the aquifers, showing that shallow and deep aquifers are more complex than previously conceptualised for the region. The resulting 3D conceptual models have provided an improved framework for the construction of 3D groundwater flow models. Presenting the data in 3D has also proved to be a powerful communication tool that can be used in public meetings, improving the conceptual understanding of water dynamics for all stakeholders. People who are not specialists in hydrology, but who are either users or managers of the water can obtain a better visual understanding of the impact of groundwater extractions.