Understanding groundwater processes in a Southeast Asian Quaternary
      aquifer using a combined multi-tracer and modelling approach
Creators: name: Chambers, Lee
Publisher: name: Lancaster University
Year: 2018
Subjects: Hydrogeology
environmental tracers
numerical modelling

description: 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 &lt; 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 &gt; 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.