| Water Conditioning | Virtually any form of water treatment designed to improve the aesthetic quality of water by the neutralization, inhibition or removal of undesirable substances. (Not health related.) | |
| Water Cycle | See hydrologic cycle. | |
| Water Hammer | A shock wave or series of waves produced by the abrupt acceleration or deceleration of water flow, due to inertia. Water hammer may produce instantaneous pressures many times the normal pressure. | |
| Water Softening | The removal of calcium and magnesium, the ions which are the principle cause of hardness, from water. | |
| Water Table | The level of the top of the zone of saturation, in which free water exists in the pores and crevices of rocks and other earth strata. | |
| Watershed | The land area from which water drains into a stream, river, or reservoir. | |
| Weak Base Load Faction X | Is referred to as the "Theoretical Mineral Acidity (TMA). It is the sum of the chloride, sulfate, and nitrate. | |
| Wellhead Protection Area | The area surrounding a drinking water well or well field which is protected to prevent contamination of the well(s). |