| Groundwater | (Environmental Engineering) Water which is contained in geologic strata. Also properly written as two words, ground water. |
| HAPs | (Environmental Engineering) Hazardous air pollutants. |
| Heterotrophic | (Environmental Engineering) A group of organisms which obtain carbon for synthesis from other organic matter or proteins. |
| Hindered (Zone) settling | (Environmental Engineering) Settling in which particle concentrations are sufficient that particles interfere with the settling of other particles. Particles settle together as a body or structure with the water required to traverse the particle interstices. |
| Hydrocarbon | (Environmental Engineering) Any organic compound composed entirely of carbon and hydrogen. Two examples are methane gas and octane. |
| Hypolimnion | (Environmental Engineering) The lower layer of a lake. |
| In situ treatment | (Environmental Engineering) Treatment of a waste in place, as opposed to pumping or digging the waste up and then treating it. |
| Infectious disease | (Environmental Engineering) A disease caused by pathogenic organisms. |
| Influent | (Environmental Engineering) The fluid entering a system, process, tank, etc. An effluent from one process can be an influent to another process. See effluent. |
| Ion exchange | (Environmental Engineering) An adsorption process in which one ion is exchanged for another ion of like charge. There is an equivalence of exchanged charge. |