Experimental and Modelling Analyses of Saltwater Upconing

Author: Danica Jakovovic

Jakovovic, Danica, 2014 Experimental and Modelling Analyses of Saltwater Upconing, Flinders University, School of the Environment

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Abstract

Saltwater upconing is a process that occurs when salty groundwater that underlies fresh groundwater rises towards a pumping well. It is an important problem in many coastal aquifers around the world, leading to the deterioration in water quality of freshwater wells. Once a well is intruded by saltwater, it may require the well to be decommissioned, and hence, controlling bore salinisation through upconing is essential. In this study, the mechanisms of saltwater upconing are assessed to improve the current body of knowledge of the associated density-dependent flow and transport processes. Saltwater upconing is particularly difficult to measure under field situations, and there are no previous examples of well characterised field-scale saltwater upconing plumes. Prior to the current research, there were also no published observations of upconing under controlled laboratory experimental conditions, notwithstanding previous studies of lateral saltwater intrusion in which incidental vertical movements in saltwater plumes were observed. Laboratory experiments and numerical modelling analyses of saltwater upconing processes were undertaken to provide insight and understanding of the mechanisms responsible for the salinisation of freshwater wells. The research focuses mainly on laboratory-scale upconing, through which saltwater rise under a pumping well and the related impacts in terms of well salinity and plume rise and extent are examined. Firstly, saltwater upconing observations from four controlled sand-tank experiments were quantified and compared to an existing analytical solution of transient upconing. These results were subsequently extended using a numerical modelling analysis of the laboratory experiments to better understand the flow and transport processes occurring in the sand tank. An important outcome of this work is the numerical reproducibility of the experimentally observed temporal development of saltwater plumes under a pumping bore, albeit for three of the four experiments. The 'double peak' upconing observed in one of the laboratory experiments was not reproduced by this model. Numerical modelling results were compared with an existing sharp-interface analytical solution, which corresponded well with the numerical modelling results for early stages of the four upconing experiments. Secondly, additional laboratory experimentation and numerical modelling were undertaken to investigate double-peaked upconing that remained unresolved. Laboratory experiments successfully reproduced the double-peaked plume demonstrating that this phenomenon was not an experimental nuance in previous experiments. The modelling undertaken in this analysis demonstrated that sorption is an important consideration when using Rhodmaine WT as a visual aid in sand-tank experiments, especially under slow flow, density-dependent conditions. The final component of the study extended the laboratory-scale investigation to scales that apply to real-world settings. The aim was to define and characterize the 'saltwater upconing zone of influence', which is the extent of saltwater upconing impact, in terms of saltwater rise attributed to pumping, in a largely hypothetical, three-dimensional coastal setting involving a sloping regional freshwater-seawater interface. Both radial and three-dimensional numerical modelling of saltwater upconing at the field scale were undertaken. The results indicate that the sharp-interface approximations of SUZI, for both radial and three-dimensional cases, are larger compared to the numerical model predictions. It was also found that the lateral flow towards the coast significantly influences both the SUZI and the salinity of the extracted groundwater. This part of the study demonstrated that the three-dimensional modelling that includes inclined interfaces and lateral flow towards the coast is essential in studying SUZI in typical coastal areas. That is, radial modelling, which does not capture the lateral flow effects, over-estimates the SUZI extent as well as the pumped water salinity.

Keywords: saltwater upconing,coastal aquifers,numerical modelling
Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2014
School: School of the Environment
Supervisor: Adrian Werner