Comparing the strontium isotope signatures of human urinary stones, drinking waters and environmental matrices: A first case study from Italy

Urolithiasis is a rather common pathology among the adult population and the biominerals it produces, i.e., urinary stones, may represent a potential proxy to characterize the environmental matrices that surrounded patients before being diagnosed. The objective of the present investigation (recently published in Izzo et al., 2024) was to use 87Sr/86Sr, a peculiar geochemical tracer routinely used for interpreting geological processes, to correlate the characteristics of patients’ urolith and their lifestyle habits, trying to identify correlations with direct or indirect contacts with their geological and environmental surroundings (water, soil, rock, etc.). Analyzed samples consisted of 21 kidney and bladder stones that were collected at the Department of Urology of the San Pio Hospital (Benevento, Italy) from patients living in Campania Region admitted between 2018 and 2020. Investigation was also extended to a vital food for humans such as water. Local tap waters and bottle waters (38 samples) from totally different Italian areas were here analyzed in order to highlight if and how different geological and hydrogeological settings could influence their Sr isotope ratio characterizing the connections existing between humans and their surrounding environment. The 87Sr/86Sr ratios of uroliths ranged from 0.70761 for an uricite sample to 0.70997 for a weddellite one and seem to be partly discriminated based on the mineralogy. The comparison with the isotope characteristics of Italian drinking waters shows a general overlap in 87Sr/86Sr with the biominerals. However, on a smaller geographic area (Campania Region), we observe small 87 Sr/86Sr differences between the biominerals and local waters. This may be explained by external Sr inputs for example from agriculture practices, inhaled aerosols (i.e., particulate matter), animal manure and sewage, non-regional foods. Nevertheless, biominerals of patients that stated to drink and eat local water/wines and foods every day exhibited a narrower 87Sr/86Sr range roughly matching the typical isotope ratios of local geological materials and waters, as well as those of archaeological biominerals from the same area. This preliminary study evidences how the strontium isotope ratio of urinary stones records that of the patient's surrounding environmental matrices, although further investigations will be necessary to confirm this hypothesis.