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The average depth to the drainage trenches, he said, must be increased to 30 inches if parking is planned above them. With the regulations requiring at least another foot between the bottom of the trenches and bedrock or the water table, “that really limits you when you are looking for a four-and-a-half foot depth over a one- to two-acre site.”
He explained that the regulation for parking is based on the inherent contradiction between the purposes of the porous parking surface and the subsurface drainfield. The Water Control Board has studied a project of just such a grass-surface parking lot off U.S. 29 north of Warrenton, Mr. Shepherd said.
“It is specifically designed to drain rainwater,” rather than allowing it to run off, creating erosion problems. However, that quick absorption into the subsurface places the water directly where the Health Department is counting on a specific percolation rate to filter the sewerage effluent. “The ground acts as a natural filter, but its efficiency is compromised by fast absorption.”
If the water runs through the subsurface to the acquifer too quickly, the risk of contamination increases because it is not adequately filtered. And, by adding rainwater
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