 Abstract | Knowledge about the human toxicity of arsenic (As), combined with widespread naturally and anthropogenically-induced elevated As concentration in groundwaters in many parts of the world (e.g. Argentina, Bangladesh, India and Vietnam), have sparked an increasing interest in the factors controlling the distribution of As and the mechanisms that influence the fate of As in groundwater systems.
Several common naturally-occurring geochemical processes can play an important role in controlling the distribution of As. However, in natural systems, it is often difficult to discern which chemical or biochemical processes take the lead in controlling the fate of arsenic, or whether its fate might be predominantly controlled by physical transport processes. In such cases integrated flow and reactive transport modelling can provide an important and consistent quantitative framework for advancing our understanding of the complex and often non-intuitive field-scale behaviour of arsenic.
This thesis describes the development and evaluation of detailed process-based simulation capabilities for two selected managed aquifer recharge (MAR) operations in Langerak, the Netherlands and Bradenton, Florida. At both field sites, injection of potable, oxygenated water into anoxic aquifers for storage and later withdrawal resulted in the mobilisation of arsenic. Both sites were well-characterised and benefited from the controlled hydraulic flow conditions that were induced by the MAR operations and from the availability of comprehensive data describing the geochemical evolution of the aquifer.
The simulators used for the studies were the USGS flow model MODFLOW in conjunction with the reactive multi-component transport model PHT3D (Prommer et al. 2003). PHT3D couples the three-dimensional transport simulator MT3DMS (Zheng and Wang, 1999) with the geochemical model PHREEQC-2 (Parkhurst and Appelo, 1999).
The model-based data interpretation provided conceptual insight into the predominating reaction patterns, their spatial variability and their dependence on the flow regime under a variety of MAR operating conditions. The integrated flow and reactive transport modelling illustrated that arsenic was initially released/mobilised following pyrite oxidation triggered by the injection of oxygenated water into the anoxic aquifers. Dissolved concentrations were controlled by complexation to neo-formed hydrous ferric oxides during injection. Modelling suggested this to be an effective arsenic attenuation mechanism, albeit a temporary one. During recovery arsenic was remobilized as a result of both dissolution of hydrous ferric oxides and displacement from sorption sites by competing anions.
The numerical framework allowed detailed assessments of arsenic partitioning among mineral phases, surface complexes and aqueous phases during injection, storage and recovery and the evaluation of the temporal and areal extent of arsenic mobility and capture within the aquifer.
During the model development and applications it became clear that computational efficiency and accuracy consideration can play an important role for the simulation of arsenic fate at field scale. This motivated additional and more systematic investigations on the efficiency and accuracy of the numerical modelling approaches for multi-dimensional field-scale reactive transport.
Taken collectively, this thesis creates a depth of knowledge on the science and simulation capabilities of field-scale As behaviour. The work demonstrates, that a clear understanding of the fundamental geochemical processes affecting the mobility of arsenic and their interaction with physical transport can only be achieved, if flow, transport and reactive processes are considered simultaneously. A contribution to understanding the complete cycling of arsenic in complex field-scale groundwater systems as a coupled process of hydraulic and geo(bio)chemical controls is made.
The practical aspect of the work is the provision of a tool to assess the suitability of different MAR sites and techniques in relation to As mobility, to optimize operational conditions as well as to evaluate proposed engineering solutions that could mitigate the As problem at affected MAR sites.
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