Pruning stormwater biofilter vegetation influences water quality improvement differently in Carex appressa and Ficinia nodosa

Abstract

The maintenance of stormwater biofilter vegetation is conducted under local guidelines, which often include seasonal pruning. However, the effects that pruning has on water quality improvement remain unknown. This study used experimental columns to investigate the effects of pruning on effluent concentrations of nitrogen, phosphorus, and metals when planted with two common biofilter plant species, Carex appressa and Ficinia nodosa. Effluent was monitored in pruned, non-pruned, and unplanted control columns during a 70-day regrowth period, with monthly composite water sampling encompassing the flushed saturated zone water and effluent of each column to best represent a biofilter during a storm event. Differences between pruning treatments and the control were often species-specific and varied with nutrient type. No significant differences between treatments were found for total phosphorus, but pruning treatments affected nitrogen oxide removal in later sampling dates for F. nodosa, but not C. appressa. Total N and P removal ranged from 77 to 88% and 66–93 %, respectively, by both pruned and non-pruned plants. The overall amount of N and P removed in the pruned biomass was 2.1–3.5 times more than the estimated amount removed from the influent by the regrowth of the pruned columns alone during the regrowth period. Consequently, the amount of nutrients removed via pruning may significantly impact long-term removal. Cadmium, copper, lead, and zinc effluent concentrations were similar between treatments with removal efficiencies over 95 %. Overall, pruning appears to affect water quality improvement, but optimal pruning practices that may enhance long-term removal should be investigated further.

Introduction

Stormwater runoff becomes increasingly contaminated with both dissolved and particulate contaminants as it traverses the urban landscape (Phillips et al., 2018). These contaminants, without removal, are eventually released into downstream water courses. Excess nutrients are likely to cause algal blooms and eutrophication and additional heavy metals can critically affect living organisms within the system (Phillips et al., 2018; Gunawardena et al., 2015).

Water Sensitive Urban Design (WSUD) systems, also known as Low Impact Development systems and Sustainable Urban Drainage systems (Bratieres et al., 2008), seek to make the way water moves through the urban landscape more closely mimic natural hydrology. One system commonly employed is the stormwater biofilter. Biofilters help remove contaminants from urban stormwater runoff by allowing captured water to percolate through vegetated filter media beds to enhance the sorption of contaminants and to promote biological uptake in both plants and in their rhizosphere (Bratieres et al., 2008).

Biofiltration systems are being increasingly used to combat water contamination in urban areas, but their maintenance requirements, and how to ensure systems are most effective, requires further investigation (Erickson et al., 2018; Delgrosso et al., 2019; de Macedo et al., 2017). Considered designs are also essential to reduce pollutant loading, with not all media- and plant-types being equal. Vegetated biofiltration systems typically remove nutrients more than non-vegetated systems (Henderson et al., 2007; Lucas and Greenway, 2008), leading many researchers to investigate the relative abilities of plant species to further increase removal efficiency (Bratieres et al., 2008; Read et al., 2008; Payne et al., 2014b). Biofilter vegetation may also offer several other benefits, such as wildlife habitat, pollinator services, and biodiversity support (Le et al., 2021). The various roles of biofilter plants have been summarised in Dagenais et al. (2018). However, it is also important to ensure that biofilters do not become long-term sources of pollutants. Hatt et al. (2007) noted that non-vegetated soil-based columns were potential sources of pollutants, especially nitrogen, while sand-based columns likely converted particulate nitrogen compounds into dissolved forms. Studies have also shown that vegetated columns can become nutrient sources, with vegetation selection critical to biofilter performance (Bratieres et al., 2008; Payne et al., 2014a).

Most of the roles that plants have on biofilter function are likely impacted by the common maintenance practice of pruning (Payne et al., 2015; Erickson et al., 2018). This practice is intended to increase visibility for pedestrians and motorists, and to enhance aesthetics, but it may also offer an additional opportunity to increase pollutant removal efficiency in biofilters (Davis et al., 2006). Plant biomass pruning has been investigated previously for nutrient removal in constructed wetlands (Fogli et al., 2014; Vymazal, 2007; Graber and Junge-Berberovic, 2008) and for specific nutrients and elements (Kim and Geary, 2001; Vymazal et al., 2010), but has still been identified as a key area that is needed in biofilter research (Muerdter et al., 2018; Davis et al., 2006; Roy-Poirier, 2009), and is important for the implications on urban biofilter maintenance and efficiency.

This study seeks to build upon previous research on the capabilities of pruned plants to increase pollutant removal in constructed wetlands and extend it to urban stormwater biofilters using two well-known biofilter plant species. The aim of the current study is thus to determine the impact that pruning vegetation has on water quality improvements in stormwater biofilters, and whether this impact is species dependent. These findings will help landscape and WSUD asset managers determine appropriate maintenance practices for stormwater biofilter vegetation.