PCL COMMON

Bioretention systems are emerging as a preferred BMP (Davis et al., 2009). Bioretention systems were originally designed for reducing runoff volumes by enhancing infiltration

(Morzaria-Luna et al., 2004). However, a number of studies have reported additional benefits of bioretention through surface water attenuation and pollutant removal (Morzaria-Luna et al., 2004). Many bioretention studies have confirmed the removal of suspended solids, phosphorus, heavy metals, oil and grease, chlorides and fecal indicator bacteria (Davis et al., 2009).

The majority of bioretention studies have focused on conventional systems.

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soil, and sand layers, as shown in Figure 2.1. Davis et al. (2006) performed laboratory studies on conventional bioretention systems with sampling ports constructed at varying depths. One of the experiments in the study used a 4.1 cm/hr synthetic storm event lasting for six hours. Overall, TKN removal efficiencies between 74 and 83% were observed, with 42 to 63% of the TKN removal occurring in the mulch layer. In addition, TN removal efficiencies between 66 and 83% were measured. However, effluent NO₃⁻ concentrations from all sampling ports were greater than the influent. When the flow rate was reduced to 2 cm/hr, 19 to 79% NO₃⁻ removal was observed in the lower port. The decrease in the flow rate most likely created anoxic conditions in the lower section; thereby, creating a mechanism for denitrification.

Various types of bioretention configurations are shown in Figure 2.2. Conventional bioretention systems are best for infiltration and/or if the surrounding soil characteristics are sandy, as shown in Figure 2.2a. Bioretention systems sometimes include a gravel layer or geotextile fabric encompassing the discharge pipe to prevent clogging, as shown in Figures 2.2b and 2.2c (Davis et al., 2009). Also, overflow weirs are sometimes used to ensure that the water surface elevation does not exceed the depth of the ponding area to prevent on-site flooding. Conventional bioretention systems with under-drains can incorporate impermeable liners and are good for reducing on-site flooding, reducing groundwater contamination and/or if soil

characteristics are poorly-drained, as shown in Figure 2.2b (PGC, 2007). Modified bioretention systems can incorporate impermeable liners and are desired for attenuation, reducing

groundwater contamination, NO₃⁻ removal and/or if soil characteristics are poorly-drained, as shown in Figure 2.2c (PGC, 2007).

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Nitrogen removal efficiencies from conventional and modified bioretention system studies can be found in Collins et al. (2010). Both systems (conventional and modified) have similar sedimentation, filtration and nitrification performance. TKN is removed through all of these processes, which can explain why median TKN removal efficiencies of conventional (44.4%) and modified (54.1%) bioretention systems are comparable. However, the main

advantage of modified bioretention systems is NO₃⁻ removal performance. Unlike TKN, median NO₃⁻ removal efficiencies for modified systems (65%) are greater than conventional systems (8%). Denitrification can occur in conventional systems, but the lack of a carbon source and anoxic conditions greatly inhibits NO3⁻ removal. The numerous reports of negative NO3⁻

removal efficiencies for conventional systems (Collins et al., 2010) provide insight to this phenomenon.

Many water quality treatment processes occur in bioretention systems. Bioretention design guidelines provide information on how physical, chemical and biological processes can be incorporated into the system; however, little research has focused on providing design guidance for the IWSZ. Brown et al. (2011) observed longer IWSZ retention times (greater depths) increase total nitrogen and phosphorus removal; however, this study compared two field bioretention systems with different media layers, vegetative covers, IWSZ depths and runoff volumes when comparing two bioretention cells. Laboratory studies by Kim et al. (2003) and Zinger et al. (2007a) were more controlled, which provides greater insight.

Kim et al. (2003) developed the modified bioretention system by incorporating an IWSZ with carbon-containing media under the sand layer. The NO₃⁻ removal performance of various

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sand-mixed electron donor media types can be found elsewhere (Kim et al., 2003; Gibert et al., 2010). A decrease in NO₃⁻ removal efficiency was observed with higher flow rates and/or

influent NO₃⁻ concentrations. A lag period before NO₃⁻ removal was observed when the columns were drained and then operated after 30 and 84-day dormant periods. However, nearly complete nitrate removal was observed when the columns were left submerged and then operated after 7 and 37-day dormant periods (or ADCs). The authors concluded that newspaper was the best electron-donor and that near complete NO₃⁻ removal efficiency could be achieved if stormwater remained in the IWSZ for more than seven days.

The study by Kim et al. (2003) has some drawbacks. The water source used in the study was dechlorinated tap water, with additional inputs of total dissolved solids, NO₃⁻ and

phosphorous, and no added organic carbon. Nitrate removal rates in the control columns could have been greater if organic carbon was included in the source water because a carbon source could be used for denitrification. Also, this study considered the NO₃⁻ mass loading rate

(mg/day-N) as the prime independent variable of interest; however, designers also need to know NO3⁻ removal efficiency with respect to IWSZ depth (or volume). For example, two bioretention

systems could have the same NO3⁻ mass loading rate, but have different IWSZ depths. In this

case, the NO₃⁻ removal efficiency of each system will be different, as indicated by Zinger et al. (2007).

Zinger et al. (2007a) performed mesocosm studies on modified bioretention units with different sand-mixed carbon sources (no carbon, pea straw and red-gum) and IWSZ depths (0, 15, 45 and 60 cm). An IWSZ depth of 45 cm was observed to be the optimum depth with TN

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and NO₃⁻ removal efficiencies of 74 and 99 percent, respectively. However, TN removal

efficiencies for all units were between 70 and 74%. In addition, declining removals of ammonia and org-N were observed with increasing IWSZ depth. The authors reasoned that the decreases in ammonia and organic nitrogen occurred because mineralization was inhibited by the anoxic conditions present in the IWSZ.

Little detail was provided in the study by Zinger et al. (2007a). The study did show how the IWSZ depth affects different nitrogen species, but did not consider varying nitrogen loading rates or detention times. In addition, the study investigated one storm event type, which was a slug load of the average runoff volume from a storm event. This is a concern because it is impossible to know the difference in removal performance between the discharged portion retained in the IWSZ from the previous storm event and the runoff portion entering and leaving the system on the same day. In addition, nitrogen species removal performance is likely to change with different storm event types, making the usefulness of the data limited.

Though much research has been performed on bioretention systems, peer-reviewed studies have yet to focus on bioretention system performance in high water table environments. In addition, poor NO₃⁻ removal performance has been observed when a permanently saturated IWSZ is not incorporated into the system (Davis et al., 2001; Hsieh and Davis, 2005a; Hsieh and Davis, 2005b; Davis et al., 2006; Dietz and Clausen, 2006; Hunt et al., 2006; Hsieh et al., 2007; Lucas and Greenway, 2011b). Both of these issues can be solved by introducing an impermeable liner around the IWSZ. Such inclusion would prevent bioretention systems from draining

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