Aiken et al. 2023 — Mn disparities in Central Valley drinking water: domestic wells and small CWSs bear disproportionate burden
Integrating 2011–2019 monitoring data from 667 community water systems (CWSs) and modeled groundwater quality for 69,733 domestic wells across California’s Central Valley, this study estimates that approximately 2,342 domestic well users (0.4% of the domestic well community population) rely on groundwater with predicted Mn >300 µg/L, matching the US federal health advisory limit (HAL). In community water systems, 904 users (2.4%) served by small CWSs and 3,072 users (0.4%) served by medium CWSs rely on delivered water with mean Mn >300 µg/L. Large CWSs show no users above 300 µg/L. Small CWSs report Mn measurements at far lower rates than large systems (58.2% vs 77.4% reporting), yet a higher proportion exceed regulatory standards. The study highlights equity dimensions: domestic well communities disproportionately served by disadvantaged populations face both elevated Mn exposure and financial barriers to treatment.
Key numbers
- Federal secondary maximum contaminant level (SMCL) for Mn in drinking water: 50 µg/L
- Federal health advisory limit (HAL): 300 µg/L
- California customer notification level (CWS): 500 µg/L
- WHO provisional guideline (2021, revised): 80 µg/L (protective of bottle-fed infants)
- Population served by domestic wells with predicted Mn >300 µg/L: 2,342 (0.4% of DWC population)
- Small CWS population with mean Mn >300 µg/L at point of entry: 904 (2.4% of small CWS population)
- Medium CWS population with mean Mn >300 µg/L: 3,072 (0.4%)
- Large CWS population with mean Mn >300 µg/L: 0 (0%)
- Mn concentration range in untreated US groundwater: 0.4–550 µg/L typical; up to 28,200 µg/L reported nationally
- Mn >500 µg/L in untreated groundwater found in 47 of 58 California counties (2011–2019)
- Total CWS Mn observations used: 2,323 (small), 4,736 (medium), 6,054 (large)
- Predictive model for DWCs based on groundwater Mn probability at 33 m, 50 m, 67 m, 84 m, 100 m depths
Methods (brief)
Secondary data analysis integrating state CWS monitoring data, groundwater Mn predictive model (1 km² raster), US Census population, and OEHHA disadvantaged community designations. Spatial apportionment matched CWS boundaries and PLSS grid DWC boundaries. Reported Mn concentrations are point-of-entry (last measurement before distribution). Predictive model outputs are probability of exceeding threshold values, not measured concentrations for domestic wells. Key limitation: small CWSs under-report, and domestic well data rely on model predictions rather than measured household samples.
Implications
Certification: Manganese in drinking water is relevant as a dietary intake route, particularly for formula-prepared infant foods in affected communities; supports including Mn in drinking-water-source risk profiling. Courses: Illustrates regulatory gap between SMCL (aesthetic, 50 µg/L) and health advisory (300 µg/L) vs WHO provisional guideline (80 µg/L, infant-protective); strong equity dimension for public health curriculum. App: No direct food-matrix Mn data; informs drinking-water context for formula reconstitution risk where tap water is the diluent. Microbiome: Not directly relevant.