Theme: 10. Characterization, Fate and Transport

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

Formatted Title:

Control of Structural Geology in the Off-Site Migration of Organochlorine Plumes in Free Aquifers

Background/Objectives:

The off-site migration of organochlorine contamination plumes in aquifers is one of the biggest concerns in contaminated site management, especially in areas close to residential receptors and discharge zones into water bodies that supply populations. This is the scenario at an industrial unit in the interior of Brazil, a factory that began operating more than 60 years ago with the use of carbon tetrachloride-based degreasers until 2014. The plume of carbon tetrachloride contamination in groundwater has concentrations that exceed 1,000 µg/L in monitoring wells installed outside the Unit, extending for approximately 300 meters to the discharge zone into a river that is the main source of water supply for a city with 500,000 inhabitants. However, during the investigations, the geometry and distribution of the plume of contamination could not be explained by hydrogeology only, as the plume did not behave within the expected dispersion patterns when the direction of the groundwater was evaluated exclusively.

Approach/Activities:

During the updating of the site's conceptual model, which began in 2019, to understand the controls governing the dispersion of the organochlorine plume, the studies focused on understanding the regional and local geology, since the industrial unit is located exactly on a regional geological fault in a SW-NE direction marked by the contact of two Gneisses from the Neoproterozoic Metamorphic Cover of the Andrelândia Megasequence. In this phase, the detailed investigations focused on geological mapping of the region and detail the interface between the residual soil and the bedrock to detail the direction, depth, and dip of the bedrock. Eighteen multi-level pairs of groundwater monitoring wells were installed, the shallow wells were installed up to 3 meters below the water table and the deeper wells were installed on the bedrock.

Results/Lessons Learned:

The geological detailing showed that the aquifers from each geological formation had their characteristics. The aquifer from the banded Biotite gneiss is heterogeneous, showing characteristics of the original rock (saprolite) with a small thickness ranging from 3 to 7 meters. The Sillimanite-garnet-biotite gneiss residual soils form a homogeneous free aquifer with progressively deeper thickness (> 20 meters) as you further away from the contact zone. In the contact zone between the two gneisses, it can be observed the presence of soils from banded biotite gneiss interbedded with sillimanite-garnet-biotite gneiss with a high presence of quartzite, causing an extreme variation in the characteristics and thickness of the free aquifer in this region, which can vary from 0 to more than 20 meters in a few meters of horizontal distance. The results of the environmental investigations indicated that the distribution of contamination in the free aquifer is primarily controlled by the geometry of the bedrock of the gneisses, which corresponds to the direction and dip of the regional fault, i.e. at the soil/bedrock interface there is the formation of a "subterranean valley" marked by the rise of the bedrock in the subsurface which limits the dispersion and flow of the plume primarily within its limits in the subsurface. A detailed understanding of the plume's migration behavior in each hydrostratigraphic layer of the free aquifer was fundamental for the new phases of investigation and intervention in the neighborhood affected by the carbon tetrachloride plume. No carbon tetrachloride was detected in the river (discharge zone), but initial studies in the fractured aquifer showed concentrations of the compound in the pumping wells installed in the gneiss interdigitation zone, indicating a potential impact on the fractured aquifer as well.

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Marcella Lopes