Probing sources of strontium exposure in pregnant individuals living near unconventional oil and gas wells using urinary 87Sr/86Sr isotope ratios.

Abstract

Background: In the Exposures in the Peace River Valley (EXPERIVA) study, pregnant individuals living in a region of natural gas exploitation had higher biological concentrations of certain trace elements, including strontium (Sr), than the general population. However, sources remained unidentified.

Objectives: To measure urinary ⁸⁷Sr/⁸⁶Sr isotope ratio in EXPERIVA participants, assess its reliability, and explore how its variance fluctuates based on Sr concentrations in biological (urine, hair, nails) and environmental (tap water) samples, as well as the density/proximity of unconventional oil and gas wells around participants' residence.

Methods: Participants provided urine daily over seven consecutive days. We measured ⁸⁷Sr/⁸⁶Sr in each urine sample from 7 participants and in pooled daily samples for all 75 participants. We used serial measurements to determine the intraclass correlation coefficient (ICC). We calculated the density/proximity of unconventional oil and gas wells around participants' homes using inverse distance weighting (IDW). We assessed the variance of urinary ⁸⁷Sr/⁸⁶Sr based on Sr concentrations in biological/environmental samples and IDW through visual inspection and Levene's test. We also performed unsupervised clustering to explore whether certain characteristics of the participants may be associated with a specific ⁸⁷Sr/⁸⁶Sr signature.

Results: Urinary ⁸⁷Sr/⁸⁶Sr ranged from 0.70798 to 0.71437. The ICC was 0.797 (95% CI: 0.574-0.953), indicating moderate to excellent reliability. Increasing Sr concentrations in hair were marginally associated with a decrease in urinary ⁸⁷Sr/⁸⁶Sr variance (p = 0.066). A similar but less consistent association was observed with increasing IDW. We observed no association between Sr concentrations in water and variance in urinary ⁸⁷Sr/⁸⁶Sr. No clear pattern was found using unsupervised clustering.

Impact: To our knowledge, this study is the first to explore the use of urinary ⁸⁷Sr/⁸⁶Sr isotope ratios to investigate sources of Sr exposure. Results are consistent with the hypothesis that a predominant source contributes to Sr exposure in most exposed EXPERIVA participants, but the contribution of unconventional oil and gas wells around participants' residences remains unclear. Findings should be considered as exploratory given the many limitations of this study. Our effort will hopefully benefit future studies aimed at identifying the sources of exposure in human populations.