A novel framework for measuring natural attenuation in the unsaturated zone below MSW dumpsites

Natural attenuation is a promising alternative to commonly employed active remediation strategies for municipal solid waste (MSW) dumpsites, such as capping, excavation, or pump-and-treat systems. While several studies and guidelines exist for monitoring natural attenuation of organics and inorganics in groundwater, limited research addresses its measurement in the unsaturated zone for inorganics. This study proposes a novel framework to evaluate natural attenuation for inorganic contaminants in the unsaturated zone, demonstrated through a case study of the Bhalswa dumpsite in Delhi, India. The framework integrates high-resolution site characterization using cone penetrometer, injection logger, and electrical conductivity (EC) probe, with the continuous soil sampling and pore water extraction for detailed contaminant profiling. The soil and pore-water samples were then analyzed for inorganic contaminants using standard analytical methods. The unsaturated zone of 2-4 m depth at Bhalswa dumpsite revealed significant attenuation of NH₄⁺, Ni, Cr and As in regions where a 1-2 m natural clay layer was present. Attenuation reduced pore-water contaminant concentrations by 73 %-92 % and bulk soil EC by 40 %-50 %. In regions without clay, no attenuation was observed, as indicated by constant EC, total concentration in soil, and pore water concentration. Speciation modelling of the pore water was also conducted to get some insights into the attenuation mechanism for the contaminants. The proposed framework offers a structured, multi-step approach for assessing natural attenuation of inorganic contaminants in the unsaturated zone, particularly at MSW dumpsites.