| #D001 | | Class I wells | Class I wells are used to inject hazardous and non-hazardous wastes into deep, isolated rock formations. | medium |
| #D002 | | Class II wells | Class II wells are used exclusively to inject fluids associated with oil and natural gas production. | medium |
| #D003 | | Class III wells | Class III wells are used to inject fluids to dissolve and extract minerals. | medium |
| #D004 | | Class IV wells | Class IV wells are shallow wells used to inject hazardous or radioactive wastes into or above a geologic formation that contains a USDW. | medium |
| #D005 | | Class V wells | Class V wells are used to inject non-hazardous fluids underground. Most Class V wells are used to dispose of wastes into or above underground sources of drinking water. | medium |
| #D006 | | Class VI wells | Class VI wells are wells used for injection of carbon dioxide (CO2) into underground subsurface rock formations for long-term storage, or geologic sequestration. | medium |
| #D007 | | Endangerment | No owner or operator shall construct, operate, maintain, convert, plug, abandon, or conduct any other injection activity in a manner that allows the movement of fluid containing any contaminant into underground sources of drinking water, if the presence of that contaminant may cause a violation of any primary drinking water regulation under 40 CFR part 142 or may otherwise adversely affect the health of persons. | high |
| #D008 | | UIC | Underground Injection Control | high |
| #D009 | | USDW | underground sources of drinking water | high |
| #D010 | | CO2 | carbon dioxide | high |