Evaluating hydrologic performance of bioretention cells in shallow groundwater

Abstract

Bioretention cells, which are generally effective in controlling surface runoff and recharging groundwater, have been widely adopted as low impact development practices. However, shallow groundwater has limited their implementation in some locations due to the potential problems of a reduction in surface runoff control, groundwater pollution, and continuous groundwater drainage through the underdrain. Many guidelines have established minimum requirements for the groundwater depth below bioretention cells, but they may not be optimized for certain environmental conditions and bioretention cell designs. This study made use of a variably saturated flow model to examine the hydrologic performance of a single bioretention cell in shallow groundwater with event‐based simulations, considering a wide range of initial groundwater depths, media and in situ soil types, surface runoff loads, and underdrain sizes. Performance indicators (e.g., runoff reduction, time for infiltrated water to reach the bioretention cell bottom and the groundwater table, and height and dissipation time of groundwater mound) were evaluated to examine the processes of runoff generation, the formation and dissipation of groundwater mounds, and the bioretention cell's performance in a shallow groundwater environment. The most influential factors were the initial groundwater depth, the hydraulic conductivity of the media soil, and the rainfall runoff load. With a deeper initial groundwater table, infiltrated water took longer to reach the bioretention cell bottom and groundwater table. Groundwater mounds, however, took longer to dissipate even though they were smaller. The groundwater quality can be better protected if relatively less‐permeable soil types (e.g., sandy loam) are used as the media, although it may compromise the performance in runoff quantity control. However, only very high surface runoff loads would cause concerns regarding a reduction in runoff quantity control and possible groundwater contamination due to the shallow groundwater. A distance of 1.5–3 m between the bioretention cell bottom and the groundwater table is generally sufficient. The results of this study could help to guide the planning and design of bioretention cells in areas of shallow groundwater.

Is there a limit to bioretention effectiveness? Evaluation of stormwater bioretention treatment using a lumped urban ecohydrologic model and ecologically based design criteria

“…A bioretention cell is conceptualized with multiple layers (e.g., ponding, filter media, and underdrain) with the water balance and (in some models) pollutant balance are solved vertically between layers. Single cell models, validated against experimental observations, show skill in predicting outflow discharge, filter‐layer soil moisture, and effluent concentrations for design and planning purposes of bioretention (e.g., Brown, Skaggs, & Hunt, ; Daly, Deletic, Hatt, & Fletcher, ; Gülbaz & Kazezyılmaz‐Alhan, ; Kabir Md, Daly, & Maggi, ; Palhegyi, ; Randelovic, Zhang, Jacimovic, McCarthy, & Deletic, ; Stewart et al, ; Zhang et al, ).…”

Modeling Connectivity of Non‐floodplain Wetlands: Insights, Approaches, and Recommendations

“…VS2DI is a two-dimensional, groundwater model developed to examine flow and transport in variably saturated porous media along a specified vertical transect (Rossi and Nimmo 1994;Hsieh et al 2000). Developed and distributed by the U.S. Geological Survey (USGS), VS2DI has been widely used for a variety of purposes, ranging from aquifer recharge simulations (e.g., Heilweil et al 2015), to evaluating performance of bioretention cells (e.g., Zhang and Chui 2017) and investigating connectivity of NFWs to downstream waters (e.g., Neff and Rosenberry 2018). VS2DI utilizes a finite-difference approach in conjunction with Richard's equation to simulate unsaturated groundwater flow.…”

Evapotranspiration from Green Infrastructure: Benefit, Measurement, and Simulation

“…Overcompensating groundwater recharge can lead to deleterious effects on downstream waters and ecosystem like in arid regions with intermittent and ephemeral streams [24]. Moreover, excessive recharge from GI may cause groundwater mounds, which, taking a long time to dissipate [69], endanger the foundations of other infrastructures and compromise drought resilience by promoting shallow-rooted plant systems that do not extract water from deep soil [70]. Therefore, determining the appropriate ET amount for an urban watershed is complicated and requires an overview of the complete water budget.…”