Does It Soak Right In? Background
Surface runoff plays a key role in determining water quality in the Chesapeake Bay watershed. Runoff carries with it sediment and other potential pollutants. Excessive runoff, especially when it flows at high rates of speed, causes erosion and flooding of waterways. The amount of runoff is a result of the physical characteristics of the land and the amount of water that soaks into the surface. A surface's ability to hold water is affected by such factors as the percentage of rock in the soil, the proximity to the surface of rock and groundwater, and the degree to which the soil is compacted.
A percolation test measures the rate at which water seeps into soil. The rate of percolation is determined by how porous a surface is. If a land surface is not porous (e.g., pavement), water will not soak in but rather run off it rapidly. If a surface is very porous (e.g., areas of thick grass), it can soak up large volumes of water. When water can soak into a surface and travel through the ground slowly, much of the pollutants are filtered out. Water that runs off the land quickly, on the other hand, carries pollutants directly to the waterways.