Using the pollution load index to evaluate rooftop harvested rainwater metal(loid) contamination in environmental justice communities

Water scarcity poses a significant public health crisis exacerbated by climate change-induced disruptions to freshwater sources. Rainwater harvesting offers a sustainable solution by harnessing rooftop runoff for domestic use. This study analyzed 577 rooftop-harvested rainwater (RHRW) samples from four Arizona, USA environmental justice communities and 162 control samples from five National Atmospheric Deposition Program wet-only deposition collection sites across Arizona. The samples were tested for metal(loid)s, and the pollution load index (PLI) and Nemerow Integrated Pollution Index (NIPI) were used to assess contamination. The PLI was calculated for 11 known contaminants (As, Pb, Cd, Mn, Al, Cr, Cu, Zn, Ni, Ba, and Be), with the highest contamination factor observed for Ni (1340). PLI levels ranged from 0.118 to 65.8, with the active mining community Globe-Miami showing the highest range (0.244–65.8). The PLI was significantly greater during the monsoon season than during the winter season for all the communities (p < 0.05). Compared with urban communities (0.118–13.1), active mining communities (0.169–65.8) had higher PLI values. pH was positively correlated with PLI in Tucson (β = ln 0.27). In non-urban/rural mining communities, locations closer to potential contamination sources had higher PLI values (β = ln −0.33 to −0.38). However, in Tucson, the proximity relationship was less defined because of multiple potential contamination sources in urban areas. This study highlights the importance of using indices like PLI and NIPI to assess water quality; PLI reflects cumulative contamination burden, while NIPI contextualizes this burden within potential water uses. The strong positive correlation observed between PLI and NIPI across all use categories supports the validity of both indices and affirms utility. Together, they provide a nuanced understanding of pollution dynamics in RHRW and strengthen the case for public health interventions and ensuring the safety and sustainability of RHRW.