Federal Classifications

Class I wells inject hazardous, industrial and municipal wastes below the lowermost formation containing an underground source of drinking water with in one-quarter mile of the well. Site selection for Class I injection systems is dependent upon geological and hydro-geological conditions. Currently only certain parts of the U.S. are suitable (mid-continent, gulf coast and Great Lakes). Class I systems are continuously monitored and tested for mechanical integrity. An important part of the determination is the evaluation of geologic stability based on the history of earthquake activity. Class I injection is not suitable for use in Oregon and are not allowed in Oregon. 
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Class II injection systems are associated with oil and gas production fields. There are three types: 1) saltwater disposal; 2) enhanced oil recovery (EOR); and 3) hydrocarbon storage. As oil and gas are brought to the surface they are often mixed with salt water. Only approved geological formations can receive the produced waters that are re-injected. EOR injection systems are used to increase production and prolong the life of oil and gas fields through water flooding which results in the recovery of additional product. Hydrocarbon injection is used for underground storage of crude oil and liquid hydrocarbons. Class II systems are regularly required to be tested, monitored, and mechanical integrity testing is to occur every five years. These wells are to be located below underground sources of drinking water. Currently Oregon has only a few registered Class II systems associated with the Mist Gas field and the proposed Coos Bay gas field. Class II hydrocarbon injection systems are not allowed in Oregon. ​​

Class III injection systems are related to mineral extraction and leachate. There are two basic types: 1) solution mining of salts and sulfur and 2) in situ (in place) leaching for minerals such as uranium, gold or copper. Class III systems are not allowed in Oregon.​​

Class IV injection systems are used for hazardous waste disposal as defined under the Resource Conservation and Recovery Act (RCRA), and radioactive wastewater disposal into or above formations containing an underground source of drinking water within one-quarter mile of the well. These wells have been identified by EPA as a threat to human health and have been banned from use in all states in amendments to the 1986 Safe Drinking Water Act. In 1999 EPA revised this rule allowing Class IV injection systems to be rule authorized if used for bio-remediation at cleanup sites under state or federal oversight. Some pump and treat systems in use in Oregon qualify as Class IV systems. They like all other types of injection systems must be registered with DEQ and may operate with written approval in coordination with on-going remediation work by with Cleanup. ​​

There are 30 types of injection systems ("UICs" or "wells") recognized by EPA ranging from simple stormwater UICs to complex geothermal power generation reinjection systems. Not all Class V systems are used for injection, some are used for recharge, de-watering, remediation and recovery. Due to this they are called injection systems in Oregon. Currently in Oregon the majority of registered and approved injection systems are Class V (75%) and are predominantly used for stormwater disposal. About 50% of the cities and counties in Oregon have injection systems instead of stormwater sewers. Class V systems must meet the Safe Drinking Water Act Standards as well as the existing state Groundwater Act requirements. Pretreatment is required in most instances prior to discharge.

EPA defines Class V systems as:

1. any bored, drilled or driven shaft; or 
2. a dug hole whose depth is greater than its largest surface dimension; or 
3. an improved sinkhole; or 
4. a subsurface fluid distribution system (an assemblage of perforated pipes or drain tiles used to distribute fluids below the surface of the ground). 

EPA Region 10 is interpreting the definition to be any system, structure, or activity that is created to discharge directly into the subsurface. 

Class V systems commonly found in Oregon (by their EPA Classification) include:

1. Stormwater
• 5D2: Stormwater injection systems such as sumps, drywells and commonly receiving stormwater runoff from roof drains (single family residential roof drains are excluded from permit requirements), parking areas, and roads
• 5D3: Stormwater drill holes receiving stormwater in basaltic terrain
• 5D4: Industrial/commercial sites with stormwater discharges such as dry wells at loading docks, which are subject to spills and leaks due to the presence of toxics or petroleum products. Note this type must hookup to the municipal system and isolate potential contaminated discharges.
• 5F1: Agricultural drainage. Not allowed in Oregon.
2. Geothermal
• 5A6: direct heat
• 5A7: heat pumps/air conditioning return flows
• 5A8: aquaculture
• 5A5: electrical power generation
3. Domestic Wastewater
   Domestic wastewater disposal systems (e.g. domestic onsite systems) serving 20 or more people, or with an aggregated design capacity of 2,500 gpd) receiving only sewage (no mixed waste streams) and including multifamily domestic systems of any size.
• 5W9: Untreated sewage or sewage drill holes. Not allowed in Oregon, must be closed and replaced with an approvable onsite system.
• 5W10: Cesspools serving 20 or more people. Federally banned. Includes cesspools discharging to drywells.
• 5W11: Septic systems (undifferentiated primary treatment) from multiple dwellings or business
• 5W31: Septic systems (e.g. attached to seepage pits, drywells etc.)
• 5W32: Septic systems with drainfield, tile lines or trenches
• 5W12: Domestic wastewater treatment plant disposal well (e.g. small package plant with secondary treatment or more)
4. Industrial
   
• 5A19: Cooling water return flows (open and closed loop) permit required
• 5X28: Motor vehicle drains/automobile service station discharges. Any floor drainage system receiving fluid wastes associated with vehicle servicing or repair (e.g. petroleum products). Federally banned must be formally closed upon discovery.
5. Others
• 5R21: Aquifer recharge wells (ASR and others)
• 5S23: Subsidence control wells
• 5G30: Dewatering wells (e.g. for tunnels, foundations)
• 5X26: Aquifer remediation wells (if receiving treated water). If the injectate is over SDWA MCLs then it is a Class IV system.
• 5X29: Abandoned wells used for waste disposal upon discovery must be closed.
• 5X27: Experimental wells and others
• 5X13 - 5X16: mineral and fossil fuel recovery​
  

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Class VI wells are used for injection of carbon dioxide into subsurface rock formations for long-term storage, or geologic sequestration. Geologic sequestration refers to technologies that may be used to reduce carbon dioxide emissions to the atmosphere to help mitigate climate change.​​