Preliminary assessment of infiltration rates and effects on water quality of selected infiltration media for use in highway runoff retention basins in Washington State

Infiltration experiments were undertaken to investigate an infiltration medium that could be used in retention basins to decrease the infiltration rate to between 5 and 10 inches per hour and to also decrease the concentrations of some pollutants in highway runoff. Fourteen infiltration media formulations were tested in small-scale preliminary infiltration tests to estimate their infiltration rates and select candidates for further study. Subsequent cylinder infiltrometer tests were then conducted using two of the media to gather additional infiltration rate estimates and to investigate the potential effects of the infiltration media on the water quality of highway runoff. A formulation of 70-percent sand, 15-percent clay, and 15-percent mulch had a steady-state infiltration rate of between 1 and 3 inches per hour. Two batches of the 90-percent sand, 5-percent clay, and 5-percent mulch formulation were tested. The first batch of the formulation had a steady-state infiltration rate of between 9 and 10 inches per hour and was consistent between replicate tests. The second batch had a steady-state infiltration rate of approximately 36 inches per hour, and a subsequent test using the second batch in a second infiltrometer had a steady-state infiltration rate of approximately 50 inches per hour. Water-quality samples taken prior to and after infiltration of highway runoff through the 90-percent sand, 5-percent clay, and 5-percent mulch formulation indicate that there may be a decrease in the concentrations of dissolved copper, lead, zinc, and cadmium as well as total petroleum hydrocarbons and nitrate plus nitrite. The infiltration medium may also increase the concentrations of total and dissolved arsenic, total lead, total copper, and suspended and dissolved solids. Further testing would be needed to establish if the changes in water quality are statistically significant.