Theme: 05. Fractured Rock and Complex Geology

Topic Area: 5d. Process-Based Conceptual Site Models (CSMs) for Informing Remediation

Formatted Title:

High Resolution Characterization of a Complex Chlorinated Compound-Impacted Fractured Bedrock Site in South Africa

Background/Objectives:

It is now widely accepted that more accurate source area definition and contaminat distribution obtained through high resolution site characterization (HRSC) techniques enabled the development of better conceptual site models (CSMs) and identification of targeted treatment areas, which results in more effective, expedient, and cost-effective corrective actions.

In Africa the uptake has been very slow and this has mainly been associated with perceived higher upfront cost of HRSC particularly for direct sensing technologies. Moreover there is a misconception that application of HRSC is only limited to unconsolidated media and yet the bulk of impacted sites have complex geology including fractured bedrock.

In this paper we discuss the application of combined HRSC techniques including direct sensing in a 115 feet thick sandy overburden, and bedrock-specific HRSC in the underlying fractured tillite at a 100-year old agrochemicals industrial site impacted with chlorinated volatile organic compounds, with over three decades of historical conventional monitoring data.

We demonstrate the value of using the historical low resolution data together with HRSC data in the development of more rigorous CSMs and remedial design characterization.

Approach/Activities:

A systematic methodology for building an improved CSM is implemented by first building a better understanding of the geological and hydrogeological model of the site. This involves analyzing existing historical boring logs, applying environmental sequence stratigraphy (ESS) principles, and literature review. This is refined by application of direct sensing tools particularly membrane interface - hydraulic profiling tool (MIHPT) in targeted areas of concern across the site.

An analysis of the hydrogeologic and contaminant distribution model of the overburden enables us to scope HRSC work in the fractured bedrock.

The HRSC approach for the fractured bedrock is presented. Using rotary cored boreholes, the approach combines borehole logging techniques (caliper, gamma, resistivity, digital televiewer logs) and discrete interval hydraulic testing using straddle-packers, single borehole dilution testing, fracture and contaminant distribution in rock core and discrete interval groundwater sampling. This suite of data are integrated to interpret the fracture network properties, aquifer hydraulic properties and contaminant distribution in vertical profile, which identifies hydraulically significant fractures, vertical flow components and preferential transport pathways.

Results/Lessons Learned:

The results are critical in (1) improving the design of existing groundwater monitoring wells many of which are long screen wells straddling across the overburden and fractured bedrock; (2) improving design of existing permeable reactive barrier which was designed with screens missing the contaminant pathways; and (3) informing potential in situ source zone treatment design.

The use of HRSC at such sites with complex geology including fractured bedrock is a cost-effective and improvement on conventional traditional site characterisation practice for fractured aquifers using numerous long screen monitoring wells. It provides more reliable assessment of contaminant transport and biodegradation processes, peak contaminant concentrations, plume geometry and behaviour for the development and validation of robust conceptual site models. The return on investment will ultimately be seen in reduced long-term site management costs owing to less uncertainty in risk assessment and greater confidence in the targeting of more effective remediation.

Menu

Naledi Chere