Piloting Activities for the Design of a Large-scale Biobarrier Involving In Situ Sequential Anaerobic–aerobic Bioremediation of Organochlorides and Hydrocarbons

In situ bioremediation (ISB) is a widely accepted method for eradicating petroleum hydrocarbons (PHCs) and chlorinated aliphatic hydrocarbons (CAHs) from contaminated aquifers. To achieve full removal of all toxic compounds that originated from microbial degradation, sequential anaerobic/aerobic bioremediation systems are recommended. While several works based on laboratory analyses targeting sequential bioremediation have been documented, examples of sequential ISB are limited. The purpose of this study is to report and analyze the results obtained from the multiscale characterization activities propaedeutic to the construction of Italy’s largest (> 400 m long) sequential ISB system. The rich wealth of information produced during this study provides a useful example that can be followed for the construction of new sequential ISBs. The system was set up to remediate a solute plume containing PHCs and CAHs in an alluvial aquifer in Italy. Microcosm experiments were carried out to determine the biodegradation potential under anaerobic and aerobic conditions. In situ tests were performed by installing two 40-m-long pilot biobarriers for sequential anaerobic and aerobic degradation intercepting part of the contamination plume. These experiments pointed out the need of adding biostimulating compounds to accelerate the biodegradation process, under both aerobic and anaerobic conditions. In situ tests showed removal efficiencies of up to 95 and 99% for total CAHs and PHCs, respectively, proving the feasibility of the full-scale ISB system. Apparent discrepancies between laboratory and in situ tests can be ascribed to scale effects and aquifer heterogeneities.