NEMLA. Feasibility study on applicability of membrane interface probe in limestone

This report presents the outcome of a feasibility study of the applicability of the MIP technology in limestone. The study was initiated as a result of a number of innovation workshops – the NEMLA workshops - hosted by Capital Region of Denmark (CRD) with the overall aim to initiate the development of new emplacement methods in limestone for measurement of contamination with chlorinated solvents.

The present study was carried out as an experimental study in collaboration between COWI/Probing.dk and GEUS, and was funded by CRD. The study was executed in two phases:

• Phase 1 with the aim to evaluate the feasibility of measuring field relevant PCE concentrations in limestone with a standard MIP probe in a controlled set-up mimicking the situation where a standard MIP probe is placed in an open borehole. In this phase of the project, also simple numerical calculations were carried out with the purpose to better understand the results obtained in Phase 1 as well as to provide input for the experiments in Phase 2.

• Phase 2 where supplementary experiments were carried out with the purpose to point at possible options for development of the MIP technology to improve the measurement of chlorinated solvents in open boreholes in limestone.

The main finding of the study is that the MIP technology is generally only capable of measuring PCE contamination in limestone up to >1,000 ppb if close contact between limestone and MIP probe is ensured. Even at small gaps (2 mm) between limestone and MIP probe, the heating from the heater block of the MIP probe will cause convection in the gap, and as a consequence the contamination is carried away from the MIP membrane. Efforts to improve the measurements by confining the gap between MIP probe and limestone showed that contamination of the limestone with 500 ppb PCE could be detected with the MIP probe at a gap distance of up to 4 mm. At higher gap distances, the MIP probe could not detect the contamination.

Combining these findings with the fact that the caliper of an open borehole in limestone often may vary by several centimeters over tens of centimeter distances, it is not likely that a simple modification of the current MIP probe design can enable the development of new emplacement methods in limestone for measurement of contamination with chlorinated solvents. More radical changes to the current design should apply if the MIP membrane technology must be utilized.

A secondary finding of the study is that porosity and grain density of Copenhagen Limestone may vary considerably over very short distances (tens of cm). Thus, measured porosities varied between c. 15-45% and grain densities varied between c. 2.66-2.71 g/cm 3. The exact reason for this variation is yet unexplained.