Subterranean transport of microplastics as evidenced in karst springs and their characterization using Raman spectroscopy

The increasing use of plastic materials has led to accumulation of large amounts of plastic waste in environment and a global challenge to be tackled with. The natural process of macro-plastics aging generates a multitude of secondary microplastic fragments accumulating in all areas of the planet. The pollution with microplastics of large water bodies, such as rivers, seas and oceans was already proven, but the presence of microplastics even in karst spring water was not reported yet. In this study, Raman micro-spectroscopy was used to confirm the presence of microplastics in the spring water samples collected from two rural karst springs in the Apuseni Mountains (Țarina and Josani), North-Western Romania. Two sets of water samples of 1000 L collected in spring time 2021 and one in autumn 2021 were filtered and analyzed. Using the Python programming language and combining two separate Raman databases, one for plastics and the other for pigments, we established a customized database to unambiguously identify the type of plastic and pigment present in the discovered micro-fragments. The generated reference pigment-plastic spectra were compared to those of potential microplastics found on filters and Pearson's coefficient was used to measure the level of similarity. The presence of microplastics in karst spring waters was confirmed and a quantitative estimation expressed as number of fragments or fibers per liter was 0.034 in Josani and 0.06 in Țarina karst spring. Five months later sampling (autumn 2021) revealed 0.05 microplastics per liter. The spectral results revealed that most microplastics found were dominated by polyethylene terephthalate (PET), followed by polypropylene and interestingly, abundant blue micro-fragments were identified according to their copper phthalocyanine pigments (pigment Blue 15) or indigo carmine (pigment Blue 63) characteristic spectral fingerprints, which surpassed the inherent spectral background level characteristic for the Raman spectra of naturally contaminated waste micro-samples. Their origin in mountain karst spring waters and potential decrease in time is discussed.