Abstract. Braided rivers, while uncommon internationally, are significant in terms of their unique ecosystems and as vital freshwater resources at locations where they occur. With an increasing awareness of the connected nature of surface water and groundwater, there have been many studies examining groundwater–surface water exchange in various types of waterbodies, but significantly less research has been conducted in braided rivers. Thus, there is currently limited understanding of how characteristics unique to braided rivers, such as channel shifting, expanding and narrowing margins, and a high degree of heterogeneity affect groundwater–surface water flow paths. This article provides an overview of characteristics specific to braided rivers, including a map showing the regions where braided rivers are mainly found at the global scale: Alaska, Canada, the Japanese and European Alps, the Himalayas, Russia, and New Zealand. To the authors' knowledge, this is the first map of its kind. This is followed by a review of prior studies that have investigated groundwater–surface water interactions in braided rivers and their associated aquifers. The various methods used to characterise these processes are discussed with emphasis on their effectiveness in achieving the studies' objectives and their applicability in braided rivers. We also discuss additional methods that appear promising to apply in braided river settings. The aim is to provide guidance on methodologies most suitable for future work in braided rivers. In many cases, previous studies found a multi-method approach useful to produce more robust results and compare data collected at various scales. Given the challenges of working directly in braided rivers, there is considerable scope for the increased use of remote sensing techniques. There is also opportunity for new approaches to modelling braided rivers using integrated techniques that incorporate the complex river bed terrain and geomorphology of braided rivers explicitly. We also identify a critical need to improve the conceptual understanding of hyporheic exchange in braided rivers, rates of recharge to and from braided rivers, and historical patterns of dry and low-flow periods in these rivers.
Fresh groundwater is thought to occur off the coast of Perth, Western Australia, in the confined Leederville and Yarragadee aquifers. Onshore hydraulic heads suggest that offshore groundwater may be augmenting onshore groundwater extraction, which is a critical component of Perth's water supply. To assess offshore freshwater conditions, we apply variable-density flow and transport modelling to a simplified cross-sectional representation of the Perth Basin offshore aquifers, developed using available hydrogeological information. Simulations suggest Perth's offshore fresh groundwater was emplaced during glacial periods (when sea levels were up to 120 m lower than today), and the interface between seawater and freshwater is likely still moving landward in response to paleo-conditions, albeit slowly (i.e., a maximum rate of 0.74 m/y was predicted). Onshore groundwater extraction is predicted to have increased the rate of inland interface movement by up to 75%, compared to the rate under paleo-conditions alone. Simulations including the offshore Badaminna Fault suggest that this feature truncates the offshore extent of fresh groundwater and reduces the rate of inland interface movement. The results of this investigation demonstrate that paleo-stresses may impose stronger controls than modern, human-induced factors on offshore freshwater extent in the Perth Basin, and that offshore faults may play a critical role in controlling offshore freshwater extent.
Abstract Sea-level rise (SLR) causes groundwater salinisation and water-table rise. The impacts these processes can have on water security, agricultural production and infrastructure are becoming widely recognised. However, some misconceptions relating to SLR impacts on groundwater have been observed among students, which may interfere with further learning and the application of science principles to everyday life. These misconceptions include the following: (1) water-table rise will equal SLR; (2) inland movement of the interface causes the rise in the water table under SLR; (3) seawater intrusion (SI) caused by SLR is small compared to SI caused by pumping. These misconceptions are explored with the aid of simple analytic solutions and a Jupyter Notebook. It is shown that: (1) water-table rise is only equal to SLR above the interface under flux-controlled inland boundary conditions; (2) water-table rise under SLR is not caused by SI, but rather is caused by the change in levels at the coastal boundary; (3) SI caused by SLR is a considerable risk, especially under the head-controlled conditions, which will become more common when land is drained to counter the effects of groundwater shoaling.
Sea water intrusion into aquifers is problematic in many coastal areas. The physics and chemistry of this issue are complex, and sea water intrusion remains challenging to quantify. Simple assessment tools like analytical models offer advantages of rapid application, but their applicability to field situations is unclear. This study examines the reliability of a popular sharp-interface analytical approach for estimating the extent of sea water in a homogeneous coastal aquifer subjected to pumping and regional flow effects and under steady-state conditions. The analytical model is tested against observations from Canada, the United States, and Australia to assess its utility as an initial approximation of sea water extent for the purposes of rapid groundwater management decision making. The occurrence of sea water intrusion resulting in increased salinity at pumping wells was correctly predicted in approximately 60% of cases. Application of a correction to account for dispersion did not markedly improve the results. Failure of the analytical model to provide correct predictions can be attributed to mismatches between its simplifying assumptions and more complex field settings. The best results occurred where the toe of the salt water wedge is expected to be the closest to the coast under predevelopment conditions. Predictions were the poorest for aquifers where the salt water wedge was expected to extend further inland under predevelopment conditions and was therefore more dispersive prior to pumping. Sharp-interface solutions remain useful tools to screen for the vulnerability of coastal aquifers to sea water intrusion, although the significant sources of uncertainty identified in this study require careful consideration to avoid misinterpreting sharp-interface results.
This paper explores post-pumping seawater intrusion (PP-SWI), which is the phenomenon of seawater intruding further inland than the location of a well, after pumping has ceased. Despite numerous papers on the topic of seawater intrusion and pumping, this is the first time that PP-SWI has been described in the literature, to our knowledge. This paper describes a laboratory-scale investigation of the phenomenon and we demonstrate that PP-SWI can be reproduced within physical experiments. We also show, using numerical modelling, that PP-SWI is caused by disequilibrium in the flow field following the cessation of pumping. Specifically, in our simulations, the cone of depression persisted after the cessation of pumping (first moving inland and then retreating toward the coastal boundary) which caused a lag in the reestablishment of fresh water flow toward the coast, after pumping had stopped. It was during this period of flow-field disequilibrium that PP-SWI occurred. We expect systems with larger postextraction disequilibrium to be most susceptible to PP-SWI and recommend future research to improve understanding of the relationship between hydrogeological parameters, extraction rates, well location, and incidence of PP-SWI. In those systems where PP-SWI is most likely, quantitative analysis of groundwater extraction and SWI will need to employ transient approaches to ensure that SWI is not underestimated.
Twenty-eight coastal aquifer case study areas across Australia. Seawater intrusion causes degradation of groundwater resources in coastal areas. The characterization of seawater intrusion is difficult and expensive, and there is therefore a need to develop methods for rapid assessment of seawater intrusion as part of large-scale screening studies in order to guide future investment. We use a steady-state analytic approach to quantify seawater extent and propensity for change in seawater extent under different stresses, in combination with findings from a previous qualitative investigation, which relies on a data-based assessment of regional trends. The combination of methods identified areas of highest risk to SWI including unconfined aquifers at Derby (WA) and Esperance (WA), and confined aquifers at Esperance (WA) and Adelaide (SA). The combination of analytic and qualitative approaches offers a more comprehensive and less subjective seawater intrusion characterization than arises from applying the methods in isolation, thereby imparting enhanced confidence in the outcomes. Importantly, active seawater intrusion conditions occur in many of Australia's confined coastal aquifers, obviating the use of the analytical solution, and suggesting that offshore groundwater resources provide significant contributions to these systems.
The changes in seawater volumes caused by seawater intrusion are often neglected in coastal aquifer management studies. The conditions under which this can result in significant water balance errors are not well understood. Interface movements also influence temporal trends in coastal aquifer water levels, but there is little guidance on this effect. In this study, we use steady-state, sharp-interface, analytic modelling to generate idealised relationships between seawater volume, freshwater volume and water levels. The approach assumes quasi-equilibrium conditions, which are evaluated using a selection of transient, dispersive simulations. The results demonstrate that seawater volume changes can influence significantly coastal aquifer water level trends, relative to the corresponding non-coastal aquifer situation, particularly within deep aquifers with high hydraulic conductivity and low net recharge. It is also shown that seawater volume changes can be a significant component of coastal aquifer water balances, e.g., relative to freshwater discharge to the sea, especially within deep aquifers characterised by low hydraulic conductivity and low freshwater discharge. Transient simulations show that steady-state conditions are a reasonable approximation for a range of transient seawater intrusion situations, including two of the three cases considered in this analysis. We conclude that changes in seawater volumes should be included routinely in coastal aquifer water balances. Also, temporal trends in coastal aquifer water levels may not provide an adequate measure of freshwater storage trends. It follows that the assessment of coastal aquifer condition should consider groundwater levels relative to the hydraulic head imposed by the ocean, accounting for density effects.
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