{"schema_version":"1.2","pwsid":"WI6320327","system_name":"ONALASKA WATERWORKS","city":"Onalaska","state":"WI","population_served":18000,"source_type":"GW","summary":"ONALASKA WATERWORKS serves approximately 18,000 people in Onalaska, Wisconsin. Based on contaminant analyses from DNR, WQP, EPA ECHO, EPA UCMR5, 0 contaminants have been measured above federal EPA standards (MCL) and 3 exceed health-based guidelines. The most significant finding is Arsenic at 0.002 mg/L, which is 500x the health guideline. Data is drawn from source-level compliance monitoring and covers 1997 through 2026. Results reflect conditions at the point of collection (wells, treatment plants), not necessarily at the tap.","data_period":{"start":"1997-06-03","end":"2026-04-06"},"contaminants_tested":136,"contaminants_detected":25,"exceedances":0,"guideline_exceedances":3,"contaminants":[{"name":"Chloramines","code":"0600","cas_number":null,"group":"disinfectants","section":"other_detected","measured_value":0.35,"unit":"mg/L","sample_date":"2026-04-06","source":"DNR","source_scope":"source_monitoring","health_guideline_value":4,"health_guideline_source":"EPA MCLG","epa_mcl_value":4,"status":"below_limits","detection_limit":null},{"name":"Chlorine (free)","code":"0700","cas_number":null,"group":"disinfectants","section":"other_detected","measured_value":0.81,"unit":"mg/L","sample_date":"2026-04-06","source":"DNR","source_scope":"source_monitoring","health_guideline_value":4,"health_guideline_source":"EPA MCLG","epa_mcl_value":4,"status":"below_limits","detection_limit":null},{"name":"Chlorine 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UCMR5","url":"https://www.epa.gov/dwucmr/fifth-unregulated-contaminant-monitoring-rule"}],"pfas_combined_display":{"value":5.5,"unit":"ng/L","sample_date":"2026-02-11","note":"Legacy 2016 EPA Health Advisory combined PFOA+PFOS metric. Not a current MCL."},"well_counts":{"active":4,"retired":3,"facilities_synced_at":"2026-04-11T13:48:07.246Z"},"historical_wells":[],"has_only_historical_data":false,"pfas_hazard_index":{"value":0.682,"compounds_used":["PFHXS","PFBS"],"is_partial":true,"threshold":1,"components":[{"code":"PFHXS","value_ng_l":6.8,"hbwc_ng_l":10,"ratio":0.68},{"code":"PFBS","value_ng_l":4.1,"hbwc_ng_l":2000,"ratio":0.002}]},"tthm_lraa":[{"monitoring_point":"system_wide","value":18.55,"unit":"ug/L","quarters_used":["2022Q3","2023Q3","2024Q3","2025Q3"],"sample_count":10,"mcl":80,"exceeds_mcl":false}],"haa5_lraa":[{"monitoring_point":"system_wide","value":21.99,"unit":"ug/L","quarters_used":["2022Q3","2023Q3","2024Q3","2025Q3"],"sample_count":10,"mcl":60,"exceeds_mcl":false}]}