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How to Treat Parking Lot Run Off

Parking lots are impervious surfaces, meaning that water cannot penetrate them. As a result, rainwater has to go somewhere else. Often, it runs over the parking lot surface and usually down a storm drain, taking pollutants with it. This is called ̶0;parking lot runoff." Water from storm drains usually end up in main water systems, polluting streams, lakes, and other bodies of water. A better way to deal with parking lot runoff is the installment of rain gardens or stormwater retention ponds, which are strategically placed lowered terrain that can retain runoff, allowing it to sink into the ground over time. A properly designed rain garden will filter and retain many pollutants, while allowing filtered, cleaner water to sink deeper.

Things You'll Need

Shovel
Measuring Tape
Selection of native plants
Sand
Topsoil
Compost
Mulch
Flat stones

Instructions

Building a Rain Garden
- 1
  Measure out the area of parking lot that your rain garden will treat (or drain). Calculate 10 to 30 percent of the measured parking lot area to determine the area of your rain garden -- 10 percent for sandy soils, 15 to 25 percent for silty soils, 30 percent for soils with high clay content. Add 2 to 3 feet on all sides to allow for sloping sides when dug. Find a place downhill from the parking lot to situate your rain garden. Pick a rain garden shape that is aesthetically pleasing.
- 2
  Mark out the shape for your rain garden. Dig, within the area marked out, to a depth of 18 to 30 inches; slope the sides gradually inward to the desired depth. Leave the downstream side of the dug out area slightly lower than the upstream side. Pick a spot on the downstream side of the dug out area, and carve or dig out a little divot that is pointed at a storm water conveyance system like a drain to direct water overflow during a large storm.
- 3
  Mix equal parts compost, topsoil and sand together in quantities large enough to fill your rain garden until the surface is 6 to 12 inches below ground level. Fill the basin to this level with your mixture. Place rocks around the edge of the rain garden, particularly the inflow and outflow edges, to prevent erosion.
- 4
  Consult a native plant guide for your region, and select a mix of drought tolerant, semi-wetland and wetland plants that will thrive in the rain garden. Plant the wetland plants in the bottom of the hole, the semi-wetland plants on the sloping edge of the hole, and the drought tolerant plants around the upper edge. Layer mulch over the the depression, and water your rain garden thoroughly.

Nature