Theme: 01. Remediation Technology Innovations

Topic Area: 01c. Permeable Reactive Barriers: Best Practices and Lessons Learned

Formatted Title:

Passive Mass Flow Reducer with Funnel-and-Gate Technique

Background/Objectives:

An industrial plant producing refrigeration systems operating since 1973 in the state of São Paulo generated an environmental liability composed mainly of VOCs, especially TCE, with contamination reaching the external area and a river located more than 700 m away from the industrial unit. Several attempts to contain and remediate contamination were carried out over the years until 2020 when intervention plan in progress underwent a new review, with the aim of promoting the protection of receptors and reducing the area's rehabilitation time. A series of integrated measures were proposed for the case, one of which is a passive barrier to reduce mass flow, designed with the aim of protecting the river from the input of contaminants from groundwater. For this barrier, the concept of funnel-and-gate was used, aiming to reduce hydraulic conductivity of soil creating walls and funneling groundwater into permeable zones with activated carbon (in situ reactors), for the adsorption of VOCs. The methodology for building the barrier had to be changed from steel sheet piling to cement injection in high permeability zones and drilling for creation of cement and carbon columns.

Approach/Activities:

The environmental history was evaluated taking into account the geological complexity of the region, composed of an alluvial sequence that intersperses clay packages with packages of coarse sand and gravel of different thicknesses and lateral extensions, where the hydraulic conductivity varies several orders of magnitude at nearby locations. As well as a large extent of the VOC plume in groundwater.

Variography studies of groundwater concentrations obtained from monitoring wells indicated high variability in the distribution of contaminants spatially and temporally, indicating that transport via groundwater occurs in very specific locations and moments, which suggests the existence of natural underground “channels” of sand/gravel. This characteristic may explain why a hydraulic barrier installed in the past was not successful, since its wells did not intercept the correct high permeability layers. Although there is no longer an active source in the region, data indicates that back diffusion of contaminants from secondary sources (which can be low permeable zones) may take a role in the groundwater contamination.

Although concentrations in groundwater does not indicate a potential risk to human health in the downstream open area, an intervention was planned to reduce possible future inputs of upstream residual concentrations into the river via groundwater.

A 3D mathematical model was created to visualize the area of occurrence of the dissolved phase plume in order to choose the location and extension of the barrier. The mass of contaminants and the amount of carbon required to adsorb this mass were calculated. Surveys were also carried out to acquire geological and geotechnical properties of soil and to identify permeable and impermeable layers in the barrier installation area.

The initial project for the barrier included the installation of steel sheet piles as walls, but this became economically unfeasible in the years 2020-2021, during the Covid pandemic. Therefore, cement injection techniques were tested to reduce permeability of sandy portions of the soil, combined with drilling holes filled with cement and coal.

Results/Lessons Learned:

Some difficulties appeared while Injecting cement and trying to completely fill sand layers, despite all geotechnical data acquired, therefore adjustments were made. On the other hand, drill holes filled with cement and carbon showed efficiency, implementation is in process. The barrier is planned to remain permanently in the area guaranteeing VOC adsorption for a minimum period of 100 years.

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João Moreno