AN INVESTIGATION OF MTBE/BTEX DEGRADATION IN A UK DUAL-POROSITY CHALK AQUIFER

Matrix and fracture water solute concentration profiles from a BTEX/ oxygenate plume in the UK chalk are presented to show the potential for natural attenuation in this regionally important dual porosity aquifer. Samples taken over a two-year period from multilevel samplers (MLS) installed along the plume flowpath show almost complete removal of BTEX components by natural attenuation within 100m of the plume source. Fuel oxygenates persist beyond the zone of BTEX degradation to form a mixed MTBE/ TAME plume extending 200m downstream of the source. Concentrations of dissolved oxygen and nitrate are close to zero throughout the BTEX plume, with alkanes and BTEX components being rapidly degraded by the bacterial reduction of sulphate (present at >100 mg/L in the chalk matrix). In the oxygenate plume, dissolved oxygen and nitrate concentrations increase towards background levels, as a result of recharge and diffusion of electron acceptors from the chalk matrix.

The effect of dual porosity on contaminant transport was investigated using profiles of matrix porewater chemistry through the plume. Good agreement was found between contaminant concentrations in the chalk matrix and adjacent fracture waters (sampled using an MLS in a cored borehole). This indicates that the residence time of solutes in the matrix blocks is significantly less than the age of the plume (3 years), a surprising conclusion given that the permeability of the chalk matrix is low (typically 0.0001 to 0.01 m/day). Core recovered from within the depth interval of the plume shows locally intense fracturing, this being associated with high hydraulic transmissivity (60-300 m²/day vs 0-2 m²/day for underlying strata). The plume is therefore localised within a highly transmissive zone where the high fracture density renders the matrix more accessible to diffusive exchange of solutes. The results imply that both biodegradation and matrix diffusion are important processes controlling the natural attenuation of hydrocarbons in this BTEX/ oxygenate plume. Matrix diffusion plays a fundamental role in regulating fracture water chemistry, biodegradation, contaminant fluxes and plume development in this dual porosity aquifer. The results are likely to be relevant to other similar aquifers.