Theme: 10. Characterization, Fate and Transport

Topic Area: 10d. Conceptual Site Models: Improvements in Development and Application

Formatted Title:

Fractured Rock Groundwater Conceptual Site Model: Diabase and Sandstone Contact in a Basin-Edge Context

Background/Objectives:

The site is an industrial area located in the countryside of Sao Paulo State, Brazil, where volatile organic compound (VOC) soil and granular aquifer groundwater contamination occurred.

After routine groundwater analysis of abstraction wells PP-01 and PP-02 in 2011, VOC concentrations (mainly chloroform) were detected. Ramboll started working in the area in 2018 and installed its first batch of deep monitoring wells into the rock formation when a sandstone body was encountered before reaching the diabase and detections of chloroform were in the same order of magnitude as the ones observed in the center of mass.

Approach/Activities:

The scope of work included a first round of geophysical survey (electrical) that was performed throughout the site trying to identify contacts between possible sandstone bodies and the diabase. Based on those results, three new deep monitoring wells were installed in the sandstone, which laterally delineated the plume, and one intercepting a shallow fracture (upper 5 meters) on the diabase. Sampling results indicated contamination also in the diabase. To refine the understanding of the area, results from the previous geophysical survey were reprocessed and reinterpreted. A deep penetration radar (DPR) mapping was also performed along the same path where the electro-resistivity survey was executed to obtain additional details on local geology. Field-scale activities are being conducted to achieve the aforementioned objectives. In the current investigation phase, Continuous Multi Tubing (CMT®) wells with three channels are being installed in arenite and diabase rocks, including altered rock material. Open hole geophysics surveys are being conducted, utilizing optical televiewer, natural gamma logger, caliper log, heat pulse flowmeter and acoustic televiewer. Packers are being used for discrete groundwater sampling in the productive fractures and for hydraulic tests. Rock core sampling is being conducted using nondestructive methods.

Results/Lessons Learned:

To improve the conceptual site model all information obtained up to this point was compiled into a single 3D geological model which was used to guide the location of new boreholes and monitoring wells. The water results obtained from the CMT wells indicated concentrations above the reference in the portion of the soft altered rock of the diabase, without showing a correlation between the fractures observed from the cores. The concentrations obtained in the deeper channels are significantly lower, indicating that there is restrict transport from the fractures. In the sandstone, geophysical tests indicated transmissive zones associated with fractures and the concentrations obtained were similar between the three levels of the CMT well. Geophysical data indicate the existence of a possible geological fault in the area and the set of information indicates the possibility of two fracture systems. It is understood in this way that there is no contamination in the entire vertical profile of the sandstone until now, with the main portion of migration being at its base, at the interface with the diabase. The structures identified in the zone of greatest sandstone flow do not contribute to vertical dispersion in this portion.

In this study it was possible to understand that this kind of investigation is still challenging in Brazil, however, it is clear that carrying out investigations using geophysical means associated with data collected directly in the field provides more reliable information when updating the conceptual model.

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Thiago Favaro