{"hq_id":"hq-p-fod-000116","name":"Arsenic in Private Well Water — Natural Geological Contamination, EPA 10 ug/L MCL, Chronic Low-Dose Exposure Cancer Risk (Bladder, Lung, Skin)","category":{"primary":"water_quality","secondary":"well_water_contaminant","tags":["arsenic","well water","private well","MCL","bladder cancer","lung cancer","skin cancer","geological","groundwater"]},"product_tier":"FOD","overall_risk_level":"high","description":"Approximately 43 million Americans rely on private wells not regulated by the Safe Drinking Water Act, and arsenic is among the most prevalent and dangerous contaminants in private well water. Arsenic occurs naturally in geological formations — arsenopyrite, scorodite, and arsenic-bearing iron oxides dissolve under certain pH and redox conditions, particularly in the western US, New England, and upper Midwest. The EPA maximum contaminant level (MCL) for arsenic in public water systems is 10 ug/L (reduced from 50 ug/L in 2006), but private wells are exempt from this standard. USGS surveys indicate that approximately 2.1 million domestic well users in the US are exposed to arsenic above 10 ug/L. Chronic low-dose inorganic arsenic exposure (>10 ug/L over decades) is definitively linked to bladder cancer (OR 2.7 at >50 ug/L), lung cancer (OR 3.1), and skin cancer (IARC Group 1 carcinogen), as well as cardiovascular disease, diabetes, and developmental neurotoxicity. The arsenic-cancer association has no established threshold — even exposures below 10 ug/L may carry incremental risk.","synthesis":{"derived_risk_level":"extreme","synthesis_confidence":0.744,"synthesis_method":"compound_composition","context_used":"human_infant","context_source":"product_users","exposure_modifier":1,"vulnerability_escalated":false,"escalation_reason":null,"compounds_resolved":1,"compounds_total":1,"synthesis_date":"2026-05-09","synthesis_version":"1.2.0","methodology_note":"exposure_modifier and adjusted_magnitude are computed from ALETHEIA-calibrated heuristics (route × duration × frequency multipliers, clamped to [0.5, 1.4]). Multipliers are directionally informed by EPA Exposure Factors Handbook (2011) and CalEPA OEHHA but are not regulatory consensus. See /api/methodology for full disclosure."},"hazard_summary":{"sensitive_populations":"private well users in arsenic-prone regions (western US, New England, upper Midwest), infants (formula preparation), poor arsenic methylators (genetic polymorphism)","overall_risk":"high","primary_concerns":["IARC Group 1 carcinogen — definitive cause of bladder, lung, and skin cancer with chronic exposure","2.1 million US domestic well users exceed 10 ug/L arsenic (USGS)","Private wells are exempt from Safe Drinking Water Act — no mandatory testing or treatment","No established safe threshold for cancer risk — even low-level exposure carries incremental risk"],"exposure_routes":"Ingestion (primary — drinking water and food prepared with contaminated water). Dermal (bathing — minimal absorption for inorganic arsenic). Dietary (arsenic concentrates in rice cooked with contaminated water)."},"exposure":{"routes":["ingestion","dermal"],"contact_types":["ingestion_chronic","dermal_bathing"],"users":["adult","child","infant","pregnant_woman"],"duration":"chronic","frequency":"daily_lifetime","scenarios":["Rural household: daily ingestion of 2L well water at 20-50 ug/L arsenic over decades","Infant formula: reconstituted with arsenic-contaminated well water — higher dose per body weight","Cooking: arsenic concentrates in boiled rice and pasta prepared with contaminated water","Gardening: irrigating vegetable gardens with arsenic-laden well water → soil and crop accumulation"],"notes":"Inorganic arsenic species: arsenite (As(III)) and arsenate (As(V)); As(III) is more toxic and mobile in reducing aquifers. USGS 2017: 2.1 million domestic well users exceed 10 ug/L arsenic. Cancer risk: IARC Group 1 (sufficient evidence for bladder, lung, skin cancer). Dose-response: no established threshold for cancer; EPA cancer slope factor: 1.5 per mg/kg-day. Bangladesh/Taiwan epidemiology: chronic exposure >50 ug/L → bladder cancer OR 2.7, lung cancer OR 3.1. Non-cancer effects: peripheral neuropathy, cardiovascular disease (blackfoot disease), diabetes mellitus, keratoses. Metabolism: arsenic is methylated by AS3MT enzyme; poor methylators (genetic polymorphism) have higher cancer risk."},"consumer_guidance":{"usage_warning":"If you use a private well, test for arsenic at least once (preferably annually) through a state-certified laboratory. Test results above 10 ug/L require treatment. Do NOT boil water to remove arsenic — boiling concentrates it. If your well exceeds 10 ug/L, install a point-of-use treatment system or use an alternative water source immediately while arranging treatment. Priority: protect infant formula preparation and children's drinking water first.","safer_alternatives":["Point-of-use reverse osmosis (removes 85-95% of arsenic, NSF 58 certified)","Activated alumina adsorption media (effective for arsenic, NSF 53 certified)","Iron-based adsorptive media (Bayoxide E33, GFH — specifically designed for arsenic)","Whole-house anion exchange (for arsenate removal; less effective for arsenite without pre-oxidation)"]},"regulatory":{"applicable_regulations":[{"jurisdiction":"USA","regulation":"EPA Arsenic MCL — 10 ug/L (Public Water Systems Only; Private Wells Exempt)","citation":"40 CFR 141.62; 66 FR 6976 (January 22, 2001); effective January 23, 2006","requirements":"MCL for arsenic: 10 ug/L (reduced from 50 ug/L). MCLG: 0 (zero — no safe level for Group 1 carcinogen). Applies to community water systems and non-transient non-community systems. Private domestic wells are NOT subject to SDWA regulation. EPA recommends private well owners test for arsenic. Some states (NJ: 5 ug/L, NH: 5 ug/L) have adopted stricter standards.","compliance_status":null,"effective_date":"2006-01-23","enforcing_agency":"EPA Office of Water / State primacy agencies","penalties":null,"source_ref":null}],"certifications":[],"labeling":{"required_disclosures":[],"prop65_warning":{"required":null,"chemicals":[],"endpoint":null,"notes":null},"ghs_labeling":{"required":null,"signal_word":null,"pictograms":[],"hazard_statements":[],"notes":null},"hidden_ingredients":{"trade_secret_protected":null,"categories_hidden":[],"estimated_count":null,"known_concerns":null,"notes":null},"notes":null},"recalls":[],"regulatory_gap":null,"notes":null},"lifecycle":{"recyclable":false,"disposal_guidance":"Arsenic treatment residuals (spent media, RO reject water) may contain hazardous arsenic concentrations. Dispose per state guidance — some spent media requires hazardous waste disposal (TCLP arsenic >5 mg/L).","hazardous_waste":true,"expected_lifespan":"RO membranes: 2-5 years; adsorptive media: 6-24 months depending on arsenic loading and flow rate"},"formulation":{"form":"varies","key_ingredients":[],"certifications":[]},"materials":{"common":[],"concerning":[],"preferred":[]},"compound_composition":[{"hq_id":"hq-c-ino-000002","compound_name":null,"role":"natural_contaminant","typical_concentration":"private well arsenic varies <1 to >100 ug/L by geology; EPA MCL 10 ug/L (public systems only); no private well regulation"}],"identifiers":{"common_names":["arsenic in private well water — natural geological contamination, epa 10 ug/l mcl, chronic low-dose exposure cancer risk (bladder, lung, skin)"],"aliases":[],"manufacturer":null,"brands":[]},"brand_examples":[],"brand_examples_disclaimer":null,"sources":[{"type":"expert_curation","name":"ALETHEIA Safety Database","date":"2026-03-26"}],"meta":{"schema_version":"4.0.0","last_updated":"2026-03-26","timestamp":"2026-06-02T21:28:32.122Z"}}