Infiltration

        A key to managing stormwater runoff involves "infiltration".  As defined by the U. S. Geological Survey, infiltration is the downward movement of water from the land surface into soil or porous rocks.  

        Best management practices that hold runoff onsite, depend in part on infiltration and evaporation.  Infiltration should never be the sole solution to stormwater management.  Infiltrative techniques include:  Detention ponds, swales, shallow drywells or catch basins, and bioretention areas. 

        The speed and capacity of infiltration as a management technique is determined by several factors:

• Soil characteristics--Sands and gravels have a higher capacity for infiltration of surface waters, whereas clays that are finer, more densely packed soils have a lower capacity.  Porosity of the soil--large grain vs. small grain will determine the speed and capacity of water moving through the soil, off the surface.
  

• Soil moisture--Soils already saturated, whether due to seasonal high watertables or close proximity to a surface waterbody, will have reduced capacity to accept surface water through infiltration.  Just as a sponge soaking in water will have no further capacity to accept water poured into it, soils naturally at capacity for moisture, will have no room for further water funneled into it.
  

• Precipitation--How much rain historically falls in an area--frequency, duration, and intensity--all play a role in how much infiltrative capacity soils will have.  As frequent storms, with high amounts of rain occur, the capacity of the soil to infiltrate additional water funneled to a specific location decreases.
  

• Slope--Water moving over a steep slope has less "residence time" on the surface, and less time to infiltrate into the surface.
  

• Landcover--The amount and type of vegetation on the ground surface affects the infiltrative capacity of the soil.  Vegetation may slow down surface runoff, but dense vegetation may serve as a barrier to the water/soil interface necessary for infiltration.  Soil surface compaction reduces the amount of infiltrative capacity of even porous soils.

Cautions associated with infiltration practices and devices:

• Infiltration is not the solution to every stormwater management incident.
  

• Each technique has a different factor for maintenance and longevity.
  

• Maintenance and effectiveness for removal of pollutants is dependent on surrounding landuses.  Examples:  Landuses that contribute oils and greases will require periodic removal of surface layers to remove sealed surface.  Practices that serve a dual function--sediment trap as well as infiltration basin--require regular maintenance to remove sediment in order to maintain holding capacity of the basin.