Groundwater is water that originates from rain or from melting snow and ice. Absorbed by the top soil layers, it collects or flows within the geological layers of earth. The speed and depth of water flow is determined by the types of soil and geological layers found in a specific area. The upper surface of groundwater, called the water table, eventually flows to the surface and forms springs, seeps, and wetlands.
The Colonial Parkway travels the York-James Divide, an upland ridge from which rainwater flows northward into the York River or southward into the James River. Water falling on park lands forms the numerous streams, creeks and ponds that flow through the park, feeding directly into one of these two rivers. As the hardwoods, shrubs, and grasses in the park absorb the impact of raindrops, water seeps first into the soil and then down into slow-moving underground streams called aquifers.
The importance of this groundwater cannot be overemphasized. During much of the year, particularly during the growing season, groundwater can be the only source of water available to streams and vegetation. In Virginia's coastal plain, 75% of the annual flow in streams is generated by groundwater. Springs and seeps emerge where these aquifers are near the surface. They create small ecosystems that provide critical habitat for animal, insect, and plant species.
Groundwater quality is impacted by nearby human activities, both past and present, such as agricultural runoff or runoff from adjacent residential and commercial development. The amount of water supporting water-dependent ecosystems is also affected by the amount of water that is extracted from the aquifers for human consumption.