Assessing the transport and fate of MTBE amended petroleum hydrocarbons in the UK Chalk aquifer

Conclusions

The methodology used to characterise contaminant fate and transport at this site provides an integrated approach to the assessment of NA in dual-porosity aquifers. In summary:

• Fractures form preferential pathways for the migration of LNAPL and dissolved phase contaminants in the Chalk aquifer
• The main controls on the subsurface geometry of the LNAPL source term are transverse spreading of the LNAPL, penetration to below the water table, and redistribution within the unsaturated zone due to water table fluctuations (smearing)
• High concentrations of dissolved phase contaminants at 40 m depth close to the source and negligible vertical hydraulic gradient imply penetration of LNAPL below the water table along vertical fractures
• The distribution of dissolved phase contaminants in the aquifer is controlled by site geology, aquifer hydrogeology, contaminant properties and groundwater chemistry. Plume migration depth appears to be controlled by the regional structural dip of ca. 1-3°, based on the structural analysis and contaminant distribution
• Contaminant fluxes from the site and the vertical distribution of contaminants in the aquifer are strongly controlled by water table fluctuations and fracture transmissivity. Contaminant concentrations and fluxes are higher in fractures with higher transmissivity
• Episodes of higher contaminant flux from the site are associated with recharge events that increase the water table elevation and allow dissolution of fresh product in source zone fractures that are otherwise not accessible
• A mass balance suggests that over 90% of the contaminant mass within the saturated zone could be present, dissolved or sorbed within the aquifer matrix, under worst case conditions and that diffusion into the aquifer matrix is an important natural attenuation mechanism limiting the migration of dissolved phase contaminants

The authors gratefully acknowledge the sponsorship of this research by TotalFinaElf UK Ltd. We thank QDS environmental consultants for assistance with field activities at the research site. We also acknowledge the support of CL:AIRE and the Environment Agency in the completion of this work.