Sand is typically silica based which is not good for retention of some contaminants especially phosphorus. Limestone gravel may be better. Subsurface flow wetlands have a severe problem with clogging so that they may need a system by which flow can be reversed or surged. Hydrology - Water budgets refer to the amount of water going into, flowing out of, and remaining in the wetland. Infiltration, evaporation, transpiration, and outflow are ways the wetland loses water.
Residence time is the time it takes the water to leave the wetland.
Rule of thumb is 5-14 days. Handbook of Constructed Wetlands, Vol. 1 suggests a minimum of 12 days residence time to remove the necessary contaminants. The calculations for residence time can be very complex if accurate results are desired . Wetland water levels must remain relatively constant to provide adequate vegetation for the system to facilitate the removal of contaminates.