Small Community Water Systems Have the Highest Prevalence of Mn in Drinking Water in California, USA

Abstract

Manganese (Mn) is currently regulated as a secondary contaminant in California, USA; however, recent revisions of the World Health Organization drinking water guidelines have increased regulatory attention of Mn in drinking water due to increasing reports of neurotoxic effects in infants and children. In this study, Mn concentrations reported to California's Safe Drinking Water Information System were used to estimate the potentially exposed population within California based on system size. We estimate that between 2011 and 2021, over 525,000 users in areas with reported Mn data are potentially exposed to Mn concentrations exceeding the WHO health-based guideline (80 μg L^-1), and over 34,000 users are potentially exposed to Mn concentrations exceeding the U.S. Environmental Protection Agency health-advisory limit (300 μg L^-1). Water treatment significantly decreased Mn concentrations compared to intake concentrations for all system sizes. However, smaller water systems have a wider range and a higher skew of Mn concentrations in finished water than larger systems. Additionally, higher Mn concentrations were found in systems above the maximum contaminant levels for chromium and arsenic. The treatment of these primary contaminants appears to also remove Mn. Lastly, data missingness remains a barrier to accurately assess public exposure to Mn in very small, small, and medium community water system-delivered water.

Figure 1

A ten-year mean Mn concentration measured at points leading directly into distribution systems calculated per year from 2011 to 2021. Boxes represent the 25th, 50th, and 75th percentile of concentrations and whiskers represent 5th and 95th percentiles. Outliers are represented as black points. A red dashed line is the USEPA Health-Advisory Limit for Mn (300 μg L^–1) and WHO Provisional Guideline (120 μg L^–1). No significant differences were observed (p < 0.01). Very small = <500 users (n = 823 CWS) , small = 501–3300 users (n = 170 CWS), medium = 3301–10,000 users (n = 86 CWS), large = 10,001–100,000 users (n = 151 CWS), very large = >100,000 users (n = 56 CWS).

Figure 2

The impact of non-specific treatment on Mn concentration within CWS stratified by size. Pre-treatment is the 10 year mean of the reported values occurring first in the flow path and post-treatment is the 10 year mean of reported value at point-of-entry between 2011 and 2021. All data that did not have reported values for pre- or post-treatment are excluded from the analysis. The boxes represent the 25th, 50th, and 75th percentile of concentrations, and whiskers represent the 5th and 95th percentiles. Outliers are represented as black points. Very small = <500 users (n = 134 CWS) , small = 501–3300 users (n = 69 CWS), medium = 3301–10,000 users (n = 48 CWS), large = 10,001–100,000 users (n = 96 CWS), very large = >100,000 users (n = 50 CWS). Asterisks (*) indicate a significant difference in median pre- and post-treatment between each system size classification (p < 0.01, Table S6).

Figure 3

Raw groundwater Mn concentration in relation to primary groundwater contaminants As and Cr, and water pH from samples collected on the same day and sampling location. Boxes represent the 25th, 50th, and 75th percentile of concentrations, and whiskers represent the 5th and 95th percentiles. Outliers are represented as points. Asterisks (*) indicate significantly different median Mn values between concurrently measured co-contaminant or water quality values listed as categories at the bottom of the plot (p < 0.000, Table S7). The count of all samples analyzed is listed in Table S7.

Figure 4

Raw groundwater Mn concentration in relation to other redox-sensitive groundwater constituents sampled on the same day and sampling location. Boxes represent the 25th, 50th, and 75th percentile of concentrations, and whiskers represent the 5th and 95th percentiles. Outliers are represented as points. * = significantly different median Mn values between concurrently measured co-contaminant or water quality values listed as categories at the bottom of the plot (p < 0.000, Table S7). The count of all samples analyzed is listed in Table S7.