Groundwater in the Watershed


What is a watershed?
All land is part of one watershed or another. When rain falls, much of the water runs across the surface of the land toward a stream, river, pond, or lake as surface runoff. The land area that drains runoff to the stream, or other surface water body, is called a watershed, or drainage basin. Watershed boundaries, also called drainage divides, are the high lands that divide one watershed from another. Theoretically, a drop of rain that falls squarely on a drainage divide is split into halves; one half flows into one watershed and the other into the adjacent watershed.

Groundwater is found beneath the surface of the ground within drainage basins. It does not move in underground rivers from distant watersheds. The source of all groundwater in each watershed is the precipitation that falls there. Groundwater divides usually occur approximately beneath surface water divides. Occasionally, however, the groundwater divide may not coincide with the watershed boundary. In that case, a recharge area for an aquifer in one watershed may extend partially into the adjacent watershed, but these conditions are relatively rare, and, in any case, quite local.

Since groundwater occurs within watersheds, and groundwater divides are usually approximately beneath surface water divides, watersheds are often used as the basic hydrologic unit for both surface water and groundwater planning purposes. Massachusetts has been divided into 27 major drainage basins for water resources planning. Each one includes a number of tributary watersheds, or sub-basins, that are drained by smaller streams or rivers. The Assabet River watershed is a subbasin of the SuAsCo (Sudbury, Assabet & Concord), which is, in turn, a subbasin of the Merrimack River watershed.

How groundwater moves through the watershed
Groundwater moves slowly from recharge areas, where precipitation is absorbed, down to discharge areas, where it flows or seeps out of the ground and becomes part of the surface water. When groundwater discharges into surface water, they flow together. Streams and rivers flow down the valley of the watershed until they join larger rivers and, eventually, reach the ocean. Thus, groundwater typically flows toward a stream, while the stream flows toward the ocean.

The fact that groundwater becomes surface water when it reaches discharge areas can't be overemphasized. Groundwater and surface water are interconnected and can only be fully understood and intelligently managed when that fact is acknowledged. For example, if pumping wells remove too much groundwater, there will not be enough groundwater discharge to maintain stream flow and aquatic habitats such as wetlands and ponds. (Contrary to popular belief, wetlands in Massachusetts are not usually important groundwater recharge areas. Recharge through wetland soils occurs very slowly and introduces only minor amounts of surface water into the groundwater system.)