Theme: 10. Characterization, Fate and Transport

Topic Area: 10e. Groundwater Modeling: Advancements and Applications

Formatted Title:

Calculation of Contaminant Mass Balance for a Cr(VI) Plume in Groundwater through 3-D Numerical Modelling

Background/Objectives:

This work presents a case study of the application of 3-D numerical groundwater contaminant transport modelling to calculate the mass balance of a contamination by hexavalent chromium, or Cr(VI), to assess the responsibilities of the different subjects that operated in a National Interest contaminated site in Italy. The site, currently a brownfield, is a former industrial area which was involved, since the beginning of the Twentieth century, in the production of a wide range of chemical products by several different industrial entities. Due to the large contamination found in soil, sediments and groundwater, the former owner of the area was identified by the Authorities as the responsible party, resulting in the implementation of a hydraulic barrier and related water treatment. To manage the barrier, since 2018 the Public Authority, in charge of the technical evaluation of the environmental site conditions, developed a 3-D, numerical, groundwater flow and Cr(VI) transport model for the area using the public domain codes MODFLOW/MT3D-MS, and made it available for public consultation and use.

Approach/Activities:

ERM used this numerical model and, through a specific computer program created for the elaboration of the MODFLOW and MT3D results files, ERM calculated a series of Cr(VI) mass balances to understand the correlation of different sources in the contamination of groundwater. In detail, the model has been used to 1) quantify the mass of Cr(VI) released to groundwater by different sources within the area under investigation, 2) evaluate the quantity of Cr(VI) originated in the area by sources outside the area under investigation and 3) evaluate the quantity of Cr(VI) that reached the hydraulic barrier in year 2020 and that, potentially, has been originated outside the site.

Results/Lessons Learned:

Using the model, it has been possible to demonstrate that the existing contamination for Cr(VI) in groundwater is associated to the operations of the current site owner with very limited or no contribution from others. In fact, as of 2020, more than 96% of the Cr(VI) present in groundwater was originated by the current owner, while the Cr(VI) originated by former site owners was already abstracted by the hydraulic barrier and treated. Moreover, it has been possible to estimate the amount of total Cr(VI) mass released to the aquifer by the current site owner. Finally, it has been demonstrated that, as of 2020, 7% of the Cr(VI) that was present in the aquifer was originated by industrial facilities located outside from the site. The overall work done with the model allowed to quantify the responsibilities of the different operators that succeed in the production activity, demonstrating that the current site owner provided the largest contribution to the Cr(VI) contamination present in groundwater in 2020. The adopted approach allowed ERM to provide a new and more valid site conceptual model using hydrogeological information from the numerical model.

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Gennaro Stefania