Local thermal non-equilibrium in sediments: implications for temperature dynamics and the use of heat as a tracer

Understanding streambed thermal processes is of fundamental importance due to the effects of temperature dynamics on stream ecology and solute exchange processes. Local Thermal Equilibrium (LTE) between fluid and solid is usually assumed for modelling heat exchange in streambeds and for inferring pore water flow velocities from streambed temperature data. By examining well established experimental and theoretical relationships of the fluid–solid heat transfer coefficient in a numerical scheme for a range of Reynolds (Re) numbers (0.01 > Re > 0.001), we show here that, for a range of typical streambed conditions, LTE is not attained. Thus errors in velocity estimates obtained when inverting streambed temperature data assuming LTE can be considerable especially at relatively low flow rates. We show that for certain conditions were the LTE assumption is not valid, inferred pore water velocities of up to 1 m/d can be obtained with LTE assumption even if the actual velocities are much smaller or even zero. Ignoring the possibility of Local Thermal Non-Equilibrium (LTNE) will have consequences for the correct estimation of streambed pore water velocity and heat fluxes at low Re values. More laboratory studies are urgently needed to supplement the sparse existing data in this area and further test the findings of this study

Hydrologic investigations of surface water groundwater interactions in a sub-catchment in the Namoi Valley, NSW, Australia.

In catchments with multiple inputs and outputs of water it can be difficult to reconcile why various reaches of a river are gaining or losing water. If rivers and adjacent aquifers are to be managed sustainably, while balancing environmental, economic and social goals, it is important that the link between the river and the aquifer is correctly characterised. This paper demonstrates how the joint analysis of rainfall, streamflow and borehole hydrograph data can contribute to elucidating the hydrological processes occurring in the catchment and hence understanding these links. In particular, the impact of the groundwater abstractions are examined by analysing the hydrological data over large time scales (decades) which span the pre- and post-irrigation development periods as well as short time scales (weeks) during pumping and flooding events

River–aquifer interactions in a semi-arid environment stressed by groundwater abstraction

Rivers and aquifers are, in many cases, a connected resource and as such the interactions between them need to be understood and quantified for the resource to be managed appropriately. The objective of this paper is to advance the understanding of river– aquifer interactions processes in semi-arid environments stressed by groundwater abstraction. This is performed using data from a specific catchment where records of precipitation, evapotranspiration, river flow, groundwater levels and groundwater abstraction are analysed using basic statistics, hydrograph analysis and a simple mathematical model to determine the processes causing the spatial and temporal changes in river–aquifer interactions. This combined approach provides a novel but simple methodology to analyse river–aquifer interactions, which can be applied to catchments worldwide. The analysis revealed that the groundwater levels have declined (~ 3 m) since the onset of groundwater abstraction. The decline is predominantly due to the abstraction rather than climatic changes (r = 0.84 for the relationship between groundwater abstraction and groundwater levels; r = 0.92 for the relationship between decline in groundwater levels and magnitude of seasonal drawdown). It is then demonstrated that, since the onset of abstraction, the river has changed from being gaining to losing during low-flow periods, defined as periods with flow less than 0.5, 1.0 or 1.5GL/day (1 GL/day = 1106m3/day). If defined as10 years) between the onset of groundwater abstraction and the changeover from gaining to losing conditions. Finally, a relationship between the groundwater gradient towards the river and the river flow at low-flow is demonstrated. The results have important implications for water management as well as water ecology and quality

Using groundwater modelling to enhance the understanding of the Maules Creek alluvial aquifer, Upper Namoi, NSW.

A groundwater flow model was developed for the Maules Creek alluvial aquifer, in the Namoi Valley (NSW, Australia). The objectives were to provide a better understanding of the dynamics of the aquifer and to provide integrated modelling of the catchment water resources, including an assessment of groundwater abstraction. The model was developed using FEFLOW 5.4. The hydrogeological system is represented by 9 layers bounded by impermeable bedrock. The hydraulic conductivity distribution in the model was based on a 3D geologic model built with EarthVision® and Mathematica™ using bore logs from the area. Hydrologic stresses include diffuse recharge, irrigation recharge, stream-aquifer interaction, lateral groundwater inflow/outflow and groundwater pumping. The hydraulic head distribution for 1978, obtained from a steady state model, was used as the initial condition for the transient model which was run from January 1978 to April 2007. Model calibration was performed by a trial-and-error method based on matching modelled bore hydrographs with measured hydrographs. Overall the model performance is good with the model correctly capturing the recovered water levels after each irrigation season, as well as the long-term trends. However, in areas with large groundwater abstraction-induced drawdowns (up to 8-10 m) the seasonal dynamics are not captured well by the model. For assessing the effect of groundwater abstraction on stream-aquifer interactions, two scenarios were run: without groundwater pumping and with a 2-fold increase in the pumping rate. The baseflow fluxes to the river and water budgets were computed for each scenario and compared with the calibration scenario. The groundwater model has provided a better understanding of the alluvial aquifer system and its dynamics. The results show the impact of irrigation on the hydraulic head distribution and baseflow to the river. Limitations and possible methods of resolving model uncertainties and improving calibration performance were also investigated

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

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

The Influence of Syndepositional Macropores on the Hydraulic Integrity of Thick Alluvial Clay Aquitards

Clay‐rich deposits are commonly assumed to be aquitards which act as natural hydraulic barriers due to their low hydraulic connectivity. Postdepositional weathering processes are known to increase the permeability of aquitards in the near surface but not impact on deeper parts of relatively thick formations. However, syndepositional processes affecting the hydraulic properties of aquitards have previously received little attention in the literature. Here, we analyze a 31 m deep sediment core recovered from an inland clay‐rich sedimentary sequence using a combination of techniques including particle size distribution and microscopy, centrifuge dye tracer testing and micro X‐ray CT imaging. Subaerial deposition of soils within these fine grained alluvial deposits has led to the preservation of considerable macropores (root channels or animal burrows). Connected pores and macropores thus account for vertical hydraulic conductivity (K) of urn:x-wiley:00431397:media:wrcr23221:wrcr23221-math-0001 m/s (geometric mean of 13 samples) throughout the thick aquitard, compared to a matrix K that is likely < urn:x-wiley:00431397:media:wrcr23221:wrcr23221-math-0002 m/s, the minimum K value that was measured. Our testing demonstrates that such syndepositional features may compromise the hydraulic integrity of what otherwise appears to have the characteristics of a much lower permeability aquitard. Heterogeneity within a clay‐rich matrix could also enhance vertical connectivity, as indicated by digital analysis of pore morphology in CT images. We highlight that the paleo‐environment under which the sediment was deposited must be considered when aquitards are investigated as potential natural hydraulic barriers and illustrate the value of combining multiple investigation techniques for characterizing clay‐rich deposits.The work was financially supported by the National Centre for Groundwater Research and Training, supported by the Australian Research Council and the National Water Commission. Mark Cuthbert was financially supported by the European Community Seventh Framework Programme (FP7/2007–2013) under grant agreement 299091. C.H.A. acknowledges funding by the Australian Research Council through an ARC Future Fellowship (FT120100216)