To control pollutants found in surface runoff, permeable paving surfaces capture the stormwater in the soil or aggregate base below the road or pathway, and subsequently treat the runoff via percolation, which allows water to infiltrate, supporting groundwater recharge or contain the stormwater to be released back into municipal stormwater management systems after a storm. Permeable paving systems have shown effective in reducing suspended solids, Biochemical Oxygen Demand (BOD), chemical oxygen demand, and ammonium concentrations within groundwater.  In areas where infiltration is not possible due to unsuitable soil conditions, permeable pavements are used in the attenuation mode where water is retained in the pavement and slowly released to surface water systems between storm events.