Groundwater drain An underground pipe or tunnel through which groundwater is artificially diverted to surface water for the purpose of reducing erosion or lowering the water table. A drain is typically open to the atmosphere at the lowest elevation, in contrast to a well which is open at the highest point. Groundwater Sites Adapted from USGS Site Types. http://wdr.water.usgs.gov/nwisgmap/help/sitetype.html ODM2 SiteType Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of a data collection Site. To some extent, these types represent the ultimate feature of interest that the site was established to measure. For example, a Stream Site was established to measure properties of a Stream.