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  • 2018chambersphd

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Understanding groundwater processes in a Southeast Asian Quaternary aquifer using a combined multi-tracer and modelling approach

Research output: ThesisDoctoral Thesis

Publication date2018
Number of pages480
Awarding Institution
  • Lancaster University
<mark>Original language</mark>English


To advance understanding of the hydrological controls on the distribution of contaminants within aquifers of Southeast Asia, a minimally exploited arsenic (As) rich Quaternary aquifer system in Cambodia was investigated using a combined multi-tracer and modelling approach. A hydrogeological conceptual model was first developed using traditional physical approaches (e.g., water levels, lithology and geophysics). Two-dimensional resistivity tomography data were used to constrain the aquifer and aquitard structure at the local and regional scale. The model was then further developed using an independent multi-tracer approach, incorporating groundwater recharge, flow velocity (age) and transport processes through direct observation of mass transport by measuring environmental tracers (e.g., noble gases, tritium, CFCs and SF6). Initial results demonstrated that a significant component of groundwater recharge occurs over timescales < 55 years, even in the absence of significant groundwater abstraction, contradicting previously proposed hydraulic models of the system. This is due to dramatic seasonal changes in hydraulic gradients driven by the Southeast Asian monsoon cycle, which reverses hydraulic gradients annually, presenting the ideal conditions for rapid groundwater recharge and mixing. However, it was difficult to accurately resolve groundwater age and mixing processes due to the complex nature of the system (e.g., heterogeneity) and microbial degradation of the CFCs under highly reducing conditions. In addition, the inversion of tracer concentrations to derive age rests on considerable simplifying assumptions. Therefore, a numerical groundwater flow and contaminant transport simulation was developed to advance conceptual understanding. Simulations revealed that groundwater reaching a well typically consisted of wide and multimodal distributions of age (often spanning > 100 years), even over short well screens (1 m), as a result of the extreme changes in hydraulic gradients and dispersion processes. This study avoided the issues associated with groundwater age as a scalar value through simulating distributions of age and tracer concentrations directly. This is typically rare, particularly in transient systems. In doing so, it was possible to expose some of the potential ambiguities of standard fluid dating techniques. It is suggested that the term ‘groundwater age’ should be used with caution in such systems. Further, dispersion of groundwater age and reversal of hydraulic gradients (monsoon cycle), implies that the processes controlling groundwater quality trends in response to point or non-point source contamination (anthropogenic or natural) are best evaluated through long-term monitoring, which is rare. Ultimately, the effects of groundwater contamination may not be realised until decades or centuries into the future, following a gradual and continual decline in groundwater quality.