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Phosphoric fertiliser has enormously contributed to agriculture; however, it generates five tonnes of phosphogypsum per ton of phosphoric acid synthesised. Phosphogypsum houses heavy metals and long-lived radioactive elements that represents an environmental issue requiring remediation. This paper presents a methodology for characterising phosphogypsum deposits using geophysical, geochemical, and statistical tools. Gamma-ray probes determined the abnormal radioactive zones within the phosphogypsum deposits while electrical resistivity tomography provided the geometry and distribution of the phosphogypsum deposits. Chemical results confirmed the high presence of heavy metals in the waste determining chromium as the most concentrated metal. Radiological measures indicate that the effective ambient dose equivalent average in the study area surface is approximately 8.5 times higher than the average for Europe. While at 1.0 m depth, in the phosphogypsum layer, the ambient dose equivalent average surpasses approximately 27 times the European average. Statistical correlation analysis supports that the radiation increases due to the uranium presence. This methodology might reduce time and cost avoiding the use of expensive traditional methods, and it is exportable to any deposit.

This study presents the results of a physical-chemical characterisation of phosphogypsum deposits generated with hydrochloric and sulphuric acid during the wet acid process. The paper aims to establish an efficient methodology based on electrical resistivity tomography (ERT), chemical analysis and multivariate analysis identifying the areas most contaminated by heavy metals in an abandoned factory where fertiliser was derived from phosphoric rock. This fertiliser has provided many benefits to agriculture; however, it generates a vast amount of waste (5 tonnes phosphoric rock/1 tonne fertiliser). The chemical composition of this by-product varies according to the industrial process performed. Hydrochloric acid (HCl) recovers more than 90% of phosphorus, while sulphuric acid (H2SO4) recovers around 30%. Therefore, a chemical assessment of the remaining waste is a necessary step prior to initiating any remediation process. ERT provided the geometry of the deposits and the distribution of the phosphogypsum. The chemical analyses consistently validated the electrical contrast found within the deposits. We employed a correlation analysis combined with multivariate analysis to identify the relationships among the metal concentrations and resistivity. Principal component analysis (PCA) reduced the information contained in all the variables to a few principal components. The first three principal components accounted for 74% of the variability of all the studied variables. Partial least squares-discriminant analysis (PLS-DA) for classification allowed the discrimination of the two populations. Electrical resistivity was the most influential variable for separating HCl waste from that of H2SO4. The use of ERT saves time and reduces costs yielding a methodology which facilitates the environmental assessment of large areas.

This communication reports an improvement of the quality of the electrical data obtained from the application of electrical resistivity tomography method on archaeological studies. The electrical contact between ground and electrode enhances significantly by using carbomer-based gel during the electrical resistivity tomography measurements. Not only does the gel promote the conservation of the building surface under investigation, but it also virtually eliminates the necessity of conventional spike electrodes, which in many archaeological studies are inadequate or not permitted. Results evidenced an enhancement in the quality of the electrical data obtained in the order of thousands of units compared with those without using the carbomer-based gel. The potential and capabilities of this affordable gel make it appropriate to be applied to other geoelectrical studies beyond archaeological investigations. Moreover, it might solve corrosion issues on conventional spike electrodes, and electrical multicore cables usually provoked for added saltwater attempting to improve the electrical contact.

[SPA] Esta tesis doctoral se presenta bajo la modalidad de compendio de publicaciones. Los fertilizantes fosfóricos han tenido un impacto positivo en la industria agrícola. Sin embargo, este tipo de fertilizante, además del fertilizante, genera una cantidad significativa de fosfoyeso como subproducto. Dependiendo del ácido utilizado en el proceso industrial del fertilizante se pueden producir hasta cinco toneladas de fosfoyeso. En la zona de estudio se empleó el proceso de ácido húmedo que consiste en atacar la roca fosfórica con un ácido fuerte, generalmente ácido sulfúrico. Las investigaciones previas de fosfoyeso lo han estudiado con la ayuda de análisis químicos y radiológicos, pero no con tomografía eléctrica. Por lo tanto, la tesis pretende caracterizar una antigua zona de fosfoyeso ubicada en la Región de Murcia, en el sureste de España, utilizando la tomografía eléctrica como principal herramienta para la caracterización y así, establecer una metodología eficiente para evaluar áreas de residuos. La presente tesis doctoral utiliza la modalidad de compendio de publicaciones. Tres publicaciones científicas componen este documento. Inicialmente, se caracteriza físicamente el área de estudio; calculando el volumen existente en los depósitos de residuos. En segundo lugar, se realiza una caracterización geoquímica; determinando que el cromo es el metal más presente en los residuos. Finalmente, se establece una metodología eficiente para identificar y clasificar el depósito más contaminado por metales pesados. La primera publicación utiliza LiDAR y tomografía de eléctrica para determinar con alta precisión el volumen de los depósitos en el área de estudio. Esta combinación no solo mejora la precisión del volumen, sino que también establece una metodología para calcular el volumen en este tipo de depósitos. El estudio presentado mejoró los resultados de estudios previos realizados algunos años antes por otra empresa. La segunda publicación determina la distribución e, [ENG] This doctoral dissertation has been presented in the form of thesis by publication. Phosphoric fertilisers have impacted positively on the agricultural industry. Nevertheless, this type of fertiliser, apart from the fertiliser itself, also generates a significant quantity of phosphogypsum as a by-product. Per one ton of fertiliser, five tonnes of phosphogypsum are generated. This occurs due to the industrial process followed in the study area. The factory employed the wet acid process which consists in attacking the phosphoric rock with a strong acid, usually, sulphuric acid. Previously, phosphogypsum has not been studied with electrical resistivity tomography but rather radiological and chemical analysis. Therefore, the thesis intends to characterise an ancient phosphogypsum area located in the Murcia Region in the Southeast of Spain with ERT as the principal tool to set an efficient methodology to assess waste areas. The present doctoral thesis uses the compendium of publications mode. Three scientific publications compose this document. Initially, it assesses the whole area volumetrically with significant accuracy. Next, it evaluates geochemically one specific pond determining that chromium is the most present metal in the waste. Finally, an efficient methodology is established to identify and classify the deposit most contaminated by heavy metals. The first publication uses LiDAR and electrical resistivity tomography for determining the volume of the deposits in the study area accurately. Not only does this combination enhances the precision of the results, but it also sets a methodology for calculating the volume of the deposits. The presented study improved the results of previous studies carried out some years before by another company. The second publication determines the spatial distribution of the heavy metals presents within deposits of phosphogypsum. Electrical resistivity tomography and geochemical data were merged by using non-linear regression