Using a colloidal borescope to define groundwater flow directions at contaminated sites

Detailed knowledge of the groundwater flow field is one of the fundamental technical prerequisites for designing effective remediation measures for contaminated aquifers.

The environmental issues under discussion concern both the implementation of emergency measures, commonly carried out by means of Pump&Treat (P&T) hydraulic containment systems, and remediation actions, developed through the adoption of in-situ technologies and treatments.

In hydrogeological practice, the analysis of groundwater flow directions is implemented on a piezometric basis, reconstructing the water table trend from point-level measurements. These point measurements are appropriately interpreted using geostatistical contouring algorithms.

However, within a single contaminated site, very specific circumstances may be found, due to both natural and operational reasons or other logistical challenges. The first group includes intrinsic geological factors, which derive e.g. from depositional mechanisms: typically, saturated alluvial systems are characterized by structural heterogeneity and anisotropy that can sometimes influence the design choices for remediation. Similarly, the geometry of the site and the presence of structures in relation to the position of the contamination source and the direction of groundwater flow can influence the location of interventions and installations, such as the position of pumping and monitoring wells, the latter placed behind the hydraulic barrier to verify its effectiveness.

In these environmental contexts, a methodological in-depth study is underway, aimed at combining traditional hydrogeological parameterization techniques with an advanced experimental measurement system with colloidal borescope.

The borescope is an optical device capable of following the movement of colloids in monitoring wells: field observation of the motion of these particles shows that the directional measurements, in addition to being generally consistent with the expected flow field, also provide complementary point-based results compared to those achievable with traditional techniques (Kearl, 1997). The instrument detects the flow of colloids from stagnant conditions up to flow rates close to 3 cm/s, allowing the prevailing azimuthal direction of transport to be established using statistical criteria.

This work describes the preliminary results of unpublished measurements, carried out in contaminated aquifers controlled by hydraulic barriers in operation. The results achieved to date suggest the opportunity for the use of the colloidal borescope to refine the conceptual hydrogeological model in contaminated sites even in dynamic regime situations.