{"hq_id":"hq-p-out-000096","name":"Geothermal Heat Pump — Propylene Glycol Loop Fluid Leakage and Soil/Groundwater Impact (Antifreeze Loop, BOD Loading, Aquifer Contamination)","category":{"primary":"renewable_energy","secondary":"geothermal_heat_pump","tags":["geothermal","heat pump","propylene glycol","ground loop","antifreeze","groundwater","BOD","aquifer","soil contamination","HVAC"]},"product_tier":"OUT","overall_risk_level":"low","description":"Ground-source (geothermal) heat pump systems circulate a water-antifreeze solution — typically 20-30% propylene glycol — through hundreds of meters of buried polyethylene loop piping to exchange thermal energy with subsurface soil or groundwater. While propylene glycol is classified as GRAS (Generally Recognized As Safe) by the FDA for food use, its environmental behavior when released to soil and groundwater through loop system leaks, pipe failures, or improper decommissioning presents distinct concerns. Propylene glycol exerts significant biological oxygen demand (BOD) during aerobic biodegradation — 1 kilogram of propylene glycol consumes approximately 1.5 kg of dissolved oxygen, potentially creating localized anoxic zones in shallow groundwater that mobilize naturally occurring iron, manganese, and arsenic from aquifer sediments. A typical residential geothermal loop contains 200-400 liters of glycol solution; commercial systems may contain thousands of liters. Loop breaches from excavation damage, fitting failures, or corrosion release glycol directly into the subsurface at depths where natural attenuation is slow due to limited oxygen availability. Several US states now regulate geothermal loop fluids under groundwater protection statutes, with some jurisdictions requiring only water-based loops (no antifreeze) within wellhead protection areas. Decommissioning of abandoned geothermal loops requires proper flushing and fluid disposal — grouting of boreholes is mandated in most jurisdictions to prevent vertical aquifer cross-contamination.","synthesis":{"derived_risk_level":"moderate","synthesis_confidence":0.5,"synthesis_method":"compound_composition","context_used":"human_infant","context_source":"available_priority","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":"households on private wells near geothermal systems (no municipal treatment), communities in arsenic-prone aquifer areas where oxygen depletion mobilizes arsenic","overall_risk":"low","primary_concerns":["Propylene glycol BOD loading can create anoxic groundwater zones that mobilize arsenic and manganese","Loop failures release 200-400 L of glycol solution directly into subsurface at depth","Abandoned ungrouted boreholes create permanent vertical aquifer cross-contamination pathways","Slow leaks may go undetected for months in buried closed-loop systems"],"exposure_routes":"Ingestion (groundwater contamination pathway — glycol and mobilized metals reaching private drinking water wells). Environmental (subsurface oxygen depletion and secondary contaminant mobilization)."},"exposure":{"routes":["ingestion"],"contact_types":["ingestion_groundwater"],"users":["community_resident","homeowner"],"duration":"chronic","frequency":"continuous_if_leaking","scenarios":["Buried loop pipe is punctured by excavation equipment — glycol solution leaches into shallow aquifer over weeks before detection","Loop fitting failure at manifold connection — slow glycol leak into basement or surrounding soil","Decommissioned geothermal system with ungrouted boreholes creates vertical pathway for surface contaminants to reach deep aquifer","Large commercial ground loop leak causes localized groundwater anoxia — arsenic and manganese mobilization in downgradient drinking water wells"],"notes":"Propylene glycol: CAS 57-55-6. FDA GRAS. Low acute toxicity (oral LD50 rat ~20 g/kg). Environmental concern: BOD of propylene glycol solution — aerobic biodegradation depletes dissolved oxygen in groundwater. Anoxic conditions mobilize geogenic contaminants: Fe2+ (aesthetic), Mn2+ (neurological at chronic exposure), As3+ (carcinogenic). USGS studies document arsenic mobilization in anoxic groundwater plumes. Loop systems: closed-loop HDPE pipe, 150-400 ft depth for vertical, 6-8 ft for horizontal. Leak detection: pressure drop monitoring. State regulations: Minnesota (Minn. R. 4725), New York (6 NYCRR 750), Massachusetts (310 CMR 46.00) regulate geothermal loop chemistry and decommissioning. Some states ban antifreeze in loops within wellhead protection zones."},"consumer_guidance":{"usage_warning":"If you have a geothermal heat pump with glycol loop, monitor system pressure regularly — any unexplained pressure drop indicates a potential leak. When decommissioning, ensure the installer properly flushes loop fluid and grouts all boreholes per state requirements. If you rely on a private well and a geothermal system is installed nearby, test well water for propylene glycol, iron, manganese, and arsenic annually. In wellhead protection areas, use water-only loops without antifreeze where climate permits.","safer_alternatives":["Water-only ground loops (no antifreeze) in climates where loop temperatures stay above freezing","Food-grade propylene glycol with corrosion inhibitors (lower environmental risk than ethylene glycol)","Standing-column well systems with reduced fluid volume","Air-source heat pumps (no ground loop, no subsurface contamination risk)"]},"regulatory":{"applicable_regulations":[{"jurisdiction":"USA","regulation":"State Geothermal Heat Pump Well Regulations and Groundwater Protection","citation":"Varies by state: Minnesota R. 4725 (Water Well Code), New York 6 NYCRR 750, Massachusetts 310 CMR 46.00","requirements":"Most states regulate geothermal loop installation as a form of well construction. Requirements include: licensed installer, loop fluid chemistry restrictions (some states ban antifreeze in wellhead protection areas), borehole grouting standards, decommissioning grouting requirements, and setback distances from water supply wells. No uniform federal standard — state environmental and health agencies regulate individually.","compliance_status":null,"effective_date":null,"enforcing_agency":"State environmental / health 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":true,"disposal_guidance":"Used loop fluid should be collected during decommissioning and disposed per local regulations — typically sewage treatment plant acceptance for propylene glycol solutions. Grouting of boreholes with bentonite-cement mixture is required in most states to seal the vertical pathway.","hazardous_waste":false,"expected_lifespan":"50+ year loop pipe life (HDPE); glycol fluid replaced every 5-10 years; system decommissioning requires professional grouting"},"formulation":{"form":"varies","key_ingredients":[],"certifications":[]},"materials":{"common":[],"concerning":[],"preferred":[]},"compound_composition":[{"hq_id":"hq-c-org-000094","compound_name":null,"role":"heat_transfer_fluid","typical_concentration":"20-30% propylene glycol in water; 200-400 L per residential system; BOD ~1.5 kg O2 per kg PG; GRAS for food but environmentally significant in groundwater"}],"identifiers":{"common_names":["geothermal heat pump — propylene glycol loop fluid leakage and soil/groundwater impact (antifreeze loop, bod loading, aquifer contamination)"],"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":"2026-05-14T01:25:43.760Z"}}