Optimal designs of bioretention cells in shallow groundwater

Abstract

Bioretention cells, as one representative low impact development practices, have been proved to be effective in controlling surface runoff, removing pollutants and recharging groundwater. However, they are often not recommended in shallow groundwater areas due to potential groundwater pollution, reduction in runoff control performance and groundwater drainage through the underdrain. Most design guidelines only require a minimum distance between bioretention cell bottom and seasonal high groundwater table without guiding the design of bioretention cells to mitigate the problem of shallow groundwater. This study therefore proposed some design recommendations of bioretention cells for different rainfall runoff loads, native soil types and initial water table depths. A variably saturated flow model was employed to conduct event-based simulations on one single hypothetical bioretention cell in shallow groundwater, which was calibrated using experimental and simulation data of an on-site bioretention cell. A wide range of climatic and geophysical factors (i.e. initial groundwater depths, native soils, rainfall runoff loads) and bioretention designs (i.e. media soil types and underdrain sizes) were considered. Surface runoff reduction, time before groundwater mound formation, as well as maximum height of groundwater mound were evaluated. Less-permeable media types (i.e. sandy loam) are recommended in areas with many extreme rainfall events (i.e. 40 – 70 mm/h or larger) and of shallower groundwater, which can better protect groundwater from mounding and possibly contamination although may slightly compromise the runoff control performance. For areas having seasonal high groundwater table of 0 – 1 m below bioretention bottom, underdrain is recommended to maintain good infiltration capacity without draining groundwater. However, underdrain is not recommended for areas of groundwater table always near or above the bioretention bottom, only if an impermeable sheet is added. Generally, groundwater interference is a concern only when groundwater table is above 1 – 2.5 m below bioretention bottom and runoff loads are very high. The results of this study overall could benefit the implementation of bioretention cells in shallow groundwater areas, and the establishment of relevant design guidelines.

PLAIN LANGUAGE SUMMARY

Bioretention cells are one type of low impact development practices, that are used to control rainfall runoff, and mitigate impact brough by urban development. It can infiltrate, store and delay rainfall runoff, and also intercept pollutants during the process. The infiltrated water becomes a good source of groundwater recharge. However, it also brings problems in shallow groundwater areas, which may lead to groundwater contamination due to less traveling time, reduction in runoff control performance due to higher soil moisture in surrounding areas. And groundwater can also be drained out from the underdrain (one component of bioretention cell) when groundwater table rises up above the underdrain. Dealing with these problems is a prerequisite of implementation of bioretention cells in shallow groundwater areas, but no detailed design guidelines have been provided so far. This study used computer modeling to simulate a hypothetical bioretention cell, to propose some design recommendations of bioretention cells for different rainfall runoff loads, native soil types and initial water table depths. These recommendations may be useful to the establishment of relevant design guidelines, and the implementation of bioretention cells in shallow groundwater areas in the future.