Cumulative Frequency Analysis of Fractured Bedrock in Conceptual Site Models and Remedial Design

Cumulative Frequency Analysis (CFA) of geological structures is a technique commonly deployed on borehole core and in outcrop by the hydrocarbon industry to elucidate geofluid flow through fractured bedrock. Typically, CFA forms the primary input data for complex quantitative fracture network models used by exploration, reservoir engineering and production geologists. The CFA technique is not commonly deployed in the environmental sector, where structural data analysis is mainly qualitative or, more rarely, in discrete feature network models.

The CFA technique was applied as part of a thermal remediation project in the UK, where trichloroethene impacts were identified at depths up to 18 m in fractured Carboniferous mudstones and sandstones. Careful logging of bedrock structures and the degree of bedrock weathering was undertaken. Graphical field sketch logs were utilised to capture structural data, with care taken to avoid the inclusion of mechanically-induced fractures.

A CFA methodology was developed using industry-standard tools to process structural data and provide critical information on contaminant fate and transport. This showed that, at this particular site, there was no coincidence of logged fractures with the distribution of contaminant plumes. Rather, the contaminant plume was seen to be migrating within a finely anastomosing mesh observed in the highly weathered mudstones, with no penetration into structures within the fresh bedrock. This may be explained by the frequent observation of a clay fill within the structural features at the interface zone of fresh and weathered bedrock. The revised conceptual site model was used to optimise the thermal remedial design to focus on the identified contaminant source zones and migration pathways, greatly reducing the depth and lateral extent of rock to be treated. This resulted in a lower cost, more rapid and more sustainable remedial outcome that ultimately recovered 1.6t trichloroethene. The applicability of CFA to environmental projects is discussed.