EFFECTIVENESS OF PERMEABLE PAVEMENT INSTALLED IN LAKE HITCHCOCK SEDIMENTS

Permeable pavement has been proposed as an alternative to standard pavement in parking lots as a strategy to reduce storm water runoff and increase groundwater recharge. The success of this strategy is dependent on the ability to infiltrate clean water into the underlying geologic materials over the long-term.

A small permeable pavement parking lot “laboratory” has been constructed on the Smith College campus. The 12-space lot is underlain by fine grained bottom sediments of Glacial Lake Hitchcock. One half of the lot is constructed with standard pavement and the other half with permeable asphalt. Pan lysimeters were installed under the permeable side to collect the infiltrated water for analysis. In addition, two groundwater wells were installed, one within the storage reservoir directly under the pavement and the second in the adjacent Lake Hitchcock sediments. Both wells were instrumented with Leveloggers that measure water levels at 15 minute intervals.

During construction an average infiltration rate of 25 mm/hr was measured in the lake sediments. Since construction in April 2016, water was detected in the 75cm thick subpavement gravel filled reservoir 5 times, each associated with a significant rain or snowmelt event. At no time was the reservoir filled to capacity and it was observed to completely drain within a few days of each event. Lysimeter volumes were compared to rainfall and collection efficiencies were calculated for each event. During periods when rainfall was low, efficiency dropped to about 50 percent, but during periods of frequent high rainfall, efficiency increased to over 150%. The low efficiency during dry periods is likely due to evaporation of water lowering the moisture content within the permeable pavement system below field capacity. Efficiencies over 100% likely reflect water running off the standard pavement side onto the permeable pavement side during extreme rain or snowmelt events.

The major water quality issue appears to be infiltration of road salt contaminated water. During most of the year lysimeter chloride values were less than 1mg/L but during winter they occasionally rose above 1500 mg/L. During these same periods runoff from the standard pavement side of the parking lot exceeded 4000 mg/L. The ambient groundwater had Cl levels that varied from 180mg/L in summer to 800 mg/L in winter.