The development of a multidisciplinary investigation ensures the success of the remediation strategy.

The Litoclean team has implemented different soil investigation techniques at a site contaminated by metals.

Natàlia BlÁzquez-Pallí, Carlos Herrarte, Jenny Nilsson Vidal de Llobatera, Lucía de la Ossa, Marçal Bosch, David Garriga.

In any soil and groundwater remediation project, the investigation phase determines the ultimate success, as obtaining thorough and accurate site data will allow the best treatment options to be chosen.

The subsurface investigation techniques are diverse and, depending on the characteristics of each case, some will be more advisable than others, but often a combination of several will optimize the results.

Litoclean is committed to multidisciplinary research to characterize the impact of contamination and has done so in different projects, such as that of an industrial site contaminated by metals. This is a 60-hectare area that was used from 1960 to 2005 to obtain metallic zinc (Zn) and, due to mining activity, the site’s soils became heavily contaminated with zinc and other metals, posing a risk to human health and environmental ecosystems.

Litoclean’s team of specialists opted for the implementation of different soil contamination investigation techniques, integrating field work, laboratory tests, geostatistical data analysis and in situ applications, together with desk work.

Figure 1. Equipo de LITOCLEAN durante los trabajos de investigación.

Research by phases

To design the first phase of the characterization, all potentially available technologies were studied to define the work plan for the subsoil characterization of the site and a historical investigation of the site was carried out to determine all past industrial and agricultural activities and relevant environmental studies performed. In this first stage, a sectorization of the site was carried out according to the main activities developed and, after a first field visit, a subdivision was identified according to the observations and results obtained through the implementation of different in situ investigation tools (boreholes and monitoring wells, in situ XRF metals analysis, laboratory analysis of soils and leachates). All this information was integrated into a first geostatistical model.

Figure 2. Sectorización de la zona de estudio.

After studying the findings of the first phase, the second, very detailed phase was designed, in which the number of soil samples and borings was multiplied, scheduled technical stops were carried out to analyze the data collected, and a geophysical survey was conducted in the area of the sludge ponds.

In total, during the on-site investigation, 200 test pits were performed, 16 groundwater monitoring devices were installed, 715 soil samples were analyzed by XRF, 60 leachate samples, 18 groundwater samples and 12 water samples from the ponds. Subsequently, 136 interpolation maps were made to identify soil contamination and 17 electrical tomography profiles were made to estimate the depth of the sludge ponds.

Figure 3. Técnicas in situ de investigación de suelos.

Based on the detailed geochemical characterization of the site, four soil types were sampled from different subdivisions of the study area for the preparation of laboratory treatability tests to investigate the immobilization and stabilization of metals by different amendments. These were applied at different concentrations (% by weight) according to the experimental setup and for each soil type. In total, 9 treatability studies were conducted for each soil type and the efficiency of each treatment was evaluated through the concentrations of metals in soils and leachates.

Figure 4. Ensayos de laboratorio.

Comprehensive results

The integrated approach used during the investigation is necessary to identify the key factors affecting the remediation strategy to ensure successful implementation, minimize uncertainties and prevent potential setbacks.

The results of the site characterization revealed that contamination was mainly concentrated in the upper soil layers (0-2 m) and that its content decreased significantly with depth. The main metals detected were Zn, cadmium (Cd), lead (Pb) and copper (Cu), however, antimony (Sb) and mercury (Hg) were also recorded in some sectors of the site. In terms of concentrations, the maximum values detected corresponded to Zn (above 200000 mg/kg in some cases) and, depending on the area of the site, sometimes all the metals analyzed exceeded the soil background values used as a reference. In addition, groundwater samples confirmed that metal contamination had also reached the saturated zone, also exceeding the reference values, and that the plume was not delimited within the geographical limits of the site under study.

Figure 5. Resultados de la investigación.

In the laboratory, treatability tests applied to four different types of soils showed that the application of amendments, which promoted a change in soil pH, were able to affect the content of metals in the leachates, compared to the controls. However, some of the amendments increased the pH so much that they allowed the stabilization of some metals, but also the re-solubilization of others. Thus, the best results were obtained with those that acted as a buffer and were able to maintain a pH between 8-10. In these cases, the metal content in the leachates was minimized for all the compounds analyzed and the concentrations did not exceed the landfill acceptance limit for inert wastes, with the exception of Cd.

The results obtained through this multidisciplinary soil investigation were integrated to design the most cost-effective and sustainable remediation plan.