Transient Infiltration/Exfiltration from Low Impact Development Facilities in Urban Areas with High Groundwater: Impact on Hydrologic Efficiency and Contribution to Nonstationary Hydrological Response

Urbanization in coastal plain areas with high groundwater poses challenges for stormwater management strategies. Low Impact Development (LID) such as raingardens, bioretention basins, enhances infiltration to offset increasing runoff. Decreases in LID hydrologic efficiency over time is commonly attributed to reduction of infiltration due to pore clogging and lack of maintenance. Recent LID implementation in high groundwater areas in Mediterranean climates, has revealed the complexity introduced by artificially enhanced infiltration on the urban subsurface. Examples include transient infiltration/exfiltration rates, water table mounding and groundwater discharge via stormwater infrastructure. How these processes impact on the nonstationary response of surface water remain unknown, yet is crucial for assessing urbanization impacts.
This work presents findings of the role of artificially enhanced infiltration on surface-groundwater (SW-GW) interaction, in high groundwater urban areas of the Perth Coastal Plain (Australia). Raingardens and bioretention basins have been implemented, and drains and compensating basins have been rehabilitated into living streams. A multi-technique approach (hydrometric, passive tracers and chemical data) was used to undertake water and solute mass balances to quantify exfiltration rates (across event, seasonal and interannual scales) and their effects on groundwater mounding and SW-GW interaction.
For raingardens, results showed differences in exfiltration rates leading to localized mounding. The bioretention basin displayed changes in hydrologic functioning as groundwater mounding and SW-GW interaction reduced exfiltration rate. Interaction timing and duration were correlated with annual rainfall and water table height. Living streams showed complex SW-GW interactions, with large volumes of infiltrated water discharged for a few weeks following rainfall events; this was attributed to underground pipe conveyance of infiltrated water. All cases showed an increase in groundwater contribution to surface water. A framework based on LID facility size, inflow volumes and groundwater contribution was established to address both hydrologic and nutrient attenuation performance and possible sources of nonstationary in the hydrological response.