Infiltration and Potential Groundwater Recharge Performance of Stormwater Bioretention Designed for Semiarid Climates

Bioretention is a structure that captures runoff from small catchments and stores it in porous vegetated areas with the intent of infiltrating all or a large fraction of the annual runoff volume. In practice, the effects of bioretention on potential groundwater recharge are oftentimes unknown because of varying storage capacities and infiltration rates. This study examined the infiltration performance of a field facility on the University of Utah campus in Salt Lake City, Utah. Data were collected over the course of nine months between March, 2012 and November, 2012. Soil moisture sensors were installed at six, nine, and twelve foot depths within the bioretention cell to measure the volumetric soil water content as the infiltrating wetting front advanced. Six pressure transducers and three lysimeters provided complementary infiltration data. Overall, the site demonstrated substantial improvement in volume retention and infiltration over the prior conditions. The average storm event produced approximately 5.6 mm (0.22 in) of precipitation. For all storm events examined, nearly all of the inflow volume was retained and either infiltrated, lost through evapotranspiration, or utilized by plants. Average vertical and horizontal infiltration rates ranged between 0.5 cm/hr and 20 cm/hr for the sandy loam subsoils. The wetting front beneath the bioretention took 1 to 2 days (24 to 48 hrs) to reach the 1.8 m (6 ft) depth and 7 to 14 days to reach the 3.7 m (12 ft) depth depending on the spatial location. At depths of 1.8 m (6 ft), 3.7 m (12 ft) and 4.6 m (15 ft) outside the basin, the wetting front was shown to progress at least 3 m (10 ft) laterally in three days (72 hrs) time, but without additional sensors located at larger lateral distances from the basin, it remains unclear exactly where the lateral extent of the wetting front ceases. Without additional engineering to protect infrastructure such as building foundations and retaining walls, it is recommended that bioretention cells constructed in semiarid climates and with similar subsoils be located at least 6.1 m (20 ft) from the infrastructure to prevent unintentional damage. Overall, this research indicates that bioretention is a viable stormwater best management practice in Utah. It was shown that with proper design and sizing, nearly all annual runoff volume can be controlled on site and either infiltrated or utilized by native plant species. As measured infiltration data were limited to the vadose zone, the infiltrated volume was considered potential recharge; future work may include modeling and installation of deeper sensors as a means of approximating actual recharge