1. Introduction
In settlement areas, urbanization has converted natural permeable land into impervious cover. The amount of impervious land in urban areas ranges from 20% in residential areas to as much as 85% in commercial areas [
1]. These areas, such as parking lots, concrete roads, and cemented surfaces, prevent infiltration of water through the soil. This has a significant impact on numerous biotic and abiotic components in the environment, including watershed hydrology and the quality of water resources [
2]. In streams, these impacts include higher peak stream flows, which increases channel incision, bank erosion, and increased transport of sediment and other nonpoint pollutants [
3]. Urban stormwater runoff in the United States of America is the leading concern in terms of water quality impairment and it is also the third largest source of water impairment overall [
4]. Urbanization’s extensive transformation of natural landscapes into impervious surfaces underscores the urgent need to address the environmental challenges associated with urban stormwater runoff, particularly its profound effects on hydrology and water quality.
Eastern Texas’s unique geographic location, influenced by movements of seasonal air masses, including subtropical west winds, tropical storms, and a subtropical high-pressure system, makes it prone to hurricanes [
5]. It has a humid subtropical climate with an average annual precipitation of approximately 1100 to 1300 mm, but is frequently impacted by short-duration, high-intensity storm events, particularly in the late spring and summer months. Storms associated with tropical cyclones, mesoscale convective systems, and frontal activity can produce over 50 mm of rainfall in less than an hour, with 2- to 10-year recurrence intervals for 1-hour rainfall intensities exceeding 65 mm [
6]. During recent decades, such extreme precipitation events have increased in frequency and intensity in eastern Texas [
7], leading to heavy storms and dramatically increased flooding risks [
8]. These climatic trends exacerbate the impacts of urbanization on watershed hydrology and water resources, as increased impervious cover amplifies stormwater runoff and diminishes natural infiltration processes [
9]. Elevated runoff volumes overwhelm the stormwater infrastructure, increasing the risk of flash flooding and pollutant transport into receiving water bodies. This has profound implications for water quality, including the introduction of heavy metals, nutrients, and sediments, which degrade aquatic habitats and threaten biodiversity. Furthermore, the altered hydrologic regime contributes to stream channel instability, erosion, and sedimentation, further impacting watershed health.
Thus, the main purpose of the study was to evaluate the efficacy of raingardens as structural BMPs by measuring the surface runoff water quality in eastern Texas, USA. We hypothesized that (1) Rain gardens effectively reduce pollutant concentrations in urban stormwater runoff; and (2) Pollutant removal efficiency varies across sites due to local factors such as soil composition and subsurface geology. To test these hypotheses, we conducted a field study monitoring stormwater runoff from impervious surfaces (parking lots and rooftops) entering and exiting three rain gardens in a subtropical urban setting. We evaluated the concentrations of multiple water quality parameters, including nutrients, trace metals, salts, and major ions, across inflow and outflow locations to assess pollutant removal efficiency.
2. Literature Review
Stormwater runoff mainly consists of suspended solids, heavy metals and chlorides, while it may also contain oil, grease and other related hydrocarbons [
10]. Heavy metals in stormwater are products of vehicular emission and industrial activities. They can be found in one to two order magnitudes greater than sewage effluent [
11]. The sources of heavy metals in vehicular emissions include lead deposits from lead oxides, zinc from tire wear; copper, chromium and nickel from the wear of the plating, bearings, brake linings and other moving parts of the vehicle [
12]. Runoff from non-point sources is known to carry a substantial pollutant load. Parking lots and rooftops in particular are associated with elevated concentrations of nutrients and trace metals [
13,
14]. The impacts of urban storm runoff on the receiving water bodies depend on the ambient quality of the receiving water and the quantity of the water pollutants entering that water body [
15]. Lead typically accumulates in stream sediments and can affect fish survival, growth, and reproduction [
16]. Zinc and copper at certain concentrations are toxic to fish and macroinvertebrates as well [
17]. Cadmium and chromium are mutagenic and carcinogenic to aquatic life [
18]. Excess nutrients like nitrogen and phosphorus causes algal blooms, which can increase algal turbidity thus blocking sunlight and decay of excess biomass can consume stream dissolved oxygen [
19]. Oil and grease can affect fish reproduction. Sediment particles deposited by storm runoff or resulting from channel erosion can endanger fish survival and reproduction [
20], it can cover the stream bottom reducing available spawning habitat [
21]. Therefore, the diverse pollutants in stormwater runoff, including heavy metals, hydrocarbons, nutrients, and sediments, pose significant ecological threats to aquatic ecosystems, underscoring the critical need for effective mitigation strategies to protect water quality and aquatic life.
Rain gardens, a best management practice (BMP), mitigate urban stormwater impacts by enhancing infiltration, reducing runoff, and improving water quality [
22,
23,
24]. These shallow, vegetated depressions capture and retain stormwater for infiltration, aquifer recharge, pollutant removal, and peak flow reduction [
25]. Their design includes runoff conveyance, pretreatment, treatment, and maintenance [
26] while mimicking natural hydrologic processes such as infiltration, filtration, adsorption, and decomposition [
22]. Additionally, their vegetation provides both functional hydrological benefits and aesthetic value [
25].
Over the past two decades, numerous studies have demonstrated the multifunctional benefits of rain gardens. Comparing the environmental impacts of rain gardens with rainwater harvesting systems, rain gardens offered more substantial benefits in water quality improvement and carbon footprint reduction under certain urban conditions [
27]. Rain garden also improved microclimate, enhanced urban biodiversity by supporting plants and insects, and contributed to air quality improvement [
28]. Recent research has focused on optimizing rain garden designs to enhance pollutant removal efficiency, such as implementing two-stage tandem rain gardens for improved retention [
29], integrating polyculture plantings to increase pollutant removal rate [
30], and modifying soil fauna to enhance microbial activity and pollutant breakdown [
31]. Planting mixtures significantly affected removal efficiency of heavy metals and nitrogen compounds. A comprehensive analysis of plant species was used in rain gardens in China and reported that plant configuration significantly affects decontamination efficiency, particularly for nutrients and suspended solids [
32]. Another study also showed that native polycultures generally outperform monocultures in terms of pollutant retention and resilience across seasonal variations [
33].
Recent reviews have further highlighted their importance in climate adaptation strategies. A review emphasized the benefits and limitations of rain garden applications under climate change scenarios, advocating for integrated planning to maximize ecological and hydrological benefits [
34]. The study that assessed rain garden performance in urban Japan, demonstrated their effectiveness in reducing peak runoff and improving infiltration during high-intensity rainfall events [
35]. Structural modifications such as permeable pavement layering in rain gardens enhanced their purification capacity, especially for organics and hydrocarbons [
36]. These advancements highlight the ongoing efforts to refine rain garden functionality, making them more effective and adaptable for sustainable urban stormwater management. Despite extensive research on BMPs, rain garden studies in recent five years are more focus on temperate regions [
37], relatively few have examined their effectiveness under the hydrologically volatile conditions of humid subtropical climates. In regions like eastern Texas, stormwater BMPs must contend with high-intensity rainfall, short antecedent dry periods, and high runoff volumes due to impervious surface cover. These factors can compromise bioretention performance by reducing contact time, overloading infiltration capacity, and triggering bypass or overflow events.
3. Materials and Methods
3.1. Study Area
This study was conducted at the Pineywoods Native Plant Center (PNPC), located on the northern edge of Stephen F. Austin State University (SFASU) in Nacogdoches, Texas, USA (31°36′11″ N; 94°39′19″ W). The PNPC spans 17 hectares and consists of a mix of uplands, mesic mid-slopes, and wet creek bottomlands. The area has a humid subtropical climate, characterized by hot summers and cool winters. Annual rainfall is relatively evenly distributed throughout the year, with April and May receiving the highest precipitation.
The primary contributing areas to the three rain gardens are upgradient parking lots and building rooftops (
Figure 1). Each rain garden was designed to accommodate runoffs from different infrastructure types and varied in size. Rain Garden 1 primarily receives runoff from the Raguet Elementary School parking lot, while Rain Garden 2 is supplied with runoff from the main office parking lot of the PNPC. Rain Garden 3 receives runoff from the Music Preparatory building parking lot at SFASU.
Rain Garden 1 is located on the soil of the Attoyac-Urban Land Complex (Soil Unit 8) with 0–4% slopes [
38]. The major soil components include 40% urban land, 40% Attoyac and similar soils, and 20% minor components. The parent material consists of loamy alluvium with a moderately high to high saturated hydraulic conductivity (Ksat) of 1.45–5.03 cm/h Rain Garden 2 is situated on Trawick-Urban Land Complex soils (Soil Unit 65) with 8–20% slopes. The major soil components include 55% Trawick and similar soils, 30% urban land, and 15% minor components. The parent material is clayey residuum weathered from glauconite and sandstone. The Ksat is moderately high (0.51–1.45 cm/h), and the available water capacity is low (14.22 cm). Rain Garden 3 is located on a combination of Nacogdoches-Urban Land Complex soils (Soil Unit 47) and Trawick-Urban Land Complex soils (Soil Unit 65). For Soil Unit 47, the composition consists of 45% Nacogdoches and similar soils, 35% urban land, and 20% minor components. The parent material is clayey residuum weathered from glauconite sandstone. The Ksat is moderately high (0.51–1.45 cm/h), and the available water capacity is 22.61 cm. Rain garden contributing areas had no indications of point sources of water pollution and neither were there any point-source discharge permits issued for these areas by the state environmental agency.
3.2. Rain Garden Design
The contributing area for each rain garden was determined through site evaluation and geospatial hydrological analysis. Rain gardens were constructed in accordance with the Knox County, Tennessee Stormwater Management Manual [
23]. Based on the guidelines outlined in this manual, the contributing areas for each rain garden were calculated to ensure optimal stormwater retention and treatment capacity (
Table 1).
3.3. Structural Design
The flow of stormwater through a rain garden treatment system, beginning at the parking lot, serves as the primary source of runoff. The runoff water passed through multiple structures within the rain gardens before reaching the outflow, where treated water was collected for quality analysis. The first component of the system was an instrument box equipped with a Thermo Scientific® 1000 mL HDPE Nalgene® stormwater sampler (Thermo Scientific, Mansfield, TX, USA) and a Hobo® U24-001 temperature and conductivity data logger (Hobo, Bourne, MA, USA). An untreated runoff from the parking lot was first collected in the water sampler inside the instrument box for analysis before entering the rain garden for treatment.
A 0.15 m drainage pipe transported water from the pre-treatment instrument box into the rain garden, where it flowed onto the surface and temporarily pooled within a designated ponding zone, enclosed by a 0.25 m high berm. This area was planted with native eastern Texas vegetation; however, nitrogen-fixing plants were intentionally excluded from both the rain garden and the surrounding berm. The stormwater then infiltrated through the mulch and sand layers, where natural treatment processes such as sedimentation, adsorption, and microbial activity occurred.
Water percolating through these layers accumulated at the base of the rain garden and was directed through a series of 0.1 m drainage pipes before reaching the outflow instrument box. The outflow structure was equipped with the same sampling and monitoring equipment as the inflow, ensuring consistency in data collection for water quality analysis.
3.4. Cross Sectional Structure
The cross-sectional area of the rain gardens is divided into two major zones: the rooting zone and the drainage zone, comprising five distinct structural elements (
Figure 2). The rooting zone consists of a mulch layer and sand layer. The mulch layer, approximately 0.15 m thick, is composed of dry leaves and finely shredded hardwood chips applied uniformly. This layer serves multiple functions, including filtering pollutants, preventing soil erosion, and supporting microbial activity for pollutant degradation. Beneath the mulch layer, the 0.9 m sand layer acts as an infiltration medium, slowing runoff and promoting water percolation.
The drainage zone, located below the rooting zone, consists of three key layers: filter fabric, gravel, and perforated pipes. The filter fabric separates the rooting and drainage zones, providing additional filtration while acting as a root barrier and preventing organic matter from clogging the drainage system. Below this, a 0.2 m pea gravel bed (
Figure 3 and
Figure 4) enhances stormwater storage capacity and prevents waterlogging in the upper layers. At the bottom, 0.1 m perforated drainage pipes, spaced 0.9 m apart, facilitate passive gravity drainage. These pipes are connected to a main discharge pipe that directs treated stormwater downgradient.
A selection of native plants, including Echinacea purpurea, Tradescantia humilis, Scutellaria ovata, Salvia lyrate, Phlox divaricate, and Chasmanthium latifolium, were planted in the rain gardens due to their low maintenance requirements, pollutant uptake capacity, and adaptability to local growing conditions. These species also provide habitat and food sources for native fauna while enhancing the visual appeal of the rain gardens. No nitrogen-fixing plants were included, ensuring that nitrogen levels in the system were influenced solely by the soil and incoming stormwater.
3.5. Water Sample Collection
During the study period (September 2013 to September 2014), a total of 25 storm runoff events were recorded. For each event, two water samples—one from the inflow and one from the outflow—were collected from each of the three rain gardens. However, in some instances, insufficient outflow prevented sample collection, resulting in a total of 135 samples obtained from the 25 storm events.
Water samples were collected using Thermo Scientific® 1000 mL HDPE Nalgene® stormwater samplers (Thermo Scientific, Mansfield, TX, USA) installed in the inflow and outflow collection boxes. The samplers were equipped with a float ball shut-off valve to prevent dilution from late-stage runoff. Water collected in each bottle was transferred into acid-washed 500 mL HDPE sample bottles and transported immediately to the Soil, Plant, and Water Analysis Laboratory at Stephen F. Austin State University (Nacogdoches, TX, USA).
Each sample was split for the separate analysis of total and water-soluble metal concentrations. For water-soluble metals, a subsample was passed through a 0.45 µm cellulose acetate membrane filter (VWR, Radnor, PA, USA) and analyzed without digestion. For total metals, unfiltered subsamples were digested with nitric acid according to U.S. EPA Method 200.7. Both fractions were analyzed using inductively coupled plasma–optical emission spectrometry (ICP-OES; Optima 8000, PerkinElmer Inc., Waltham, MA, USA).
Water quality parameters analyzed in this study included pH, electrical conductivity (EC), temperature (T), fluoride (F−), chloride (Cl−), nitrate (NO3−), nitrite (NO2−), phosphate (PO43−), sulfate (SO42−), salts, carbonates (CO32−), bicarbonates (HCO3−), sodium (Na+), potassium (K+), aluminum (Al), boron (B), calcium (Ca2+), magnesium (Mg2+), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), arsenic (As), lead (Pb), and mercury (Hg).
Major anions and cations—including F−, Cl−, NO3−, NO2−, PO43−, SO42−, Na+, K+, Ca2+, and Mg2+—were analyzed using ion chromatography (IC) on a Thermo Dionex ICS-2100 system (Thermo Fisher Scientific, Sunnyvale, CA, USA), in accordance with EPA Method 300.0. Carbonates and bicarbonates were assessed via acid titration and interpreted using Standard Method 2320 B, based on pH and total alkalinity.
Trace and heavy metals—including Al, B, Fe, Mn, Cu, Zn, As, Pb, and Hg—were quantified using inductively coupled plasma–optical emission spectrometry (ICP-OES) with an Optima 8000 system (PerkinElmer Inc., Waltham, MA, USA). Total concentrations were determined by digesting unfiltered samples with nitric acid, while water-soluble fractions were obtained from 0.45 µm filtered samples and analyzed without digestion, following EPA Method 200.7. Mercury (Hg) concentrations were determined under specific cold vapor mode settings to enhance detection sensitivity.
All samples were preserved with ultrapure nitric acid (HNO3) to pH < 2 and stored at 4 °C until analysis. Quality assurance measures included the use of blanks, matrix spikes, and certified reference materials.
3.6. Conductivity and Temperature Measurement
In situ electrical conductivity (EC) and temperature (T) were continuously monitored using HOBO® U24-001 conductivity data loggers (Onset Computer Corporation, Bourne, MA, USA). The loggers record EC standardized to 25 °C with an accuracy of ±3% of reading or ±5 µS/cm (whichever is greater), and a temperature accuracy of ±0.1 °C over 0 °C to 50 °C. Devices were installed inside instrument boxes at each sampling location, and data were retrieved using a HOBO® U-DTW-1 waterproof shuttle.
3.7. Rain Garden Soil Sampling
During the construction process, all three rain gardens were filled with material from the same source. However, an evaluation was necessary to ensure that the difference in performance among rain gardens was not attributable to variations in soil materials. Therefore, soil samples were randomly collected from the mulch layer and the top 30 cm of the sand layer. A total tissue digestion method was used to analyze the samples for nutrients and various metals. To analyze the difference in the soil of the rain gardens, an ANOVA test was performed. All normally distributed soil data were analyzed using an F-test (α = 0.05) and non-normally distributed data were analyzed using a Kruskal–Wallis test (α = 0.05).
3.8. Sample Period Analysis
Two distinct sample periods were evaluated. The first period ranged from September 2012 to August 2013. This period signified the time immediately following construction. During this interval, it was anticipated that the performance of the rain garden would be non-representative as the vegetation was not fully established [
22]. The second period extended from September 2013 through September 2014. This represented the first year after vegetation establishment. The results presented herein focus on the second period, although comparisons are made with the first period to evaluate the overall performance of the rain gardens [
39].
3.9. Statistical Analysis
Descriptive statistics were calculated for water quality parameters by rain garden. The water quality parameters before and after stormwater enters the rain garden were compared. Parameter concentrations were compared with the water quality standards established by the World Health Organization (WHO), the Texas Commission on Environmental Quality (TCEQ) and USEPA (
Table 2). The Shapiro–Wilks test for normality was conducted prior to performing any other statistical test. If the water sample data were normally distributed, then a parametric test (paired
t-test) was conducted between contrasting rain gardens. For all non-normally distributed water sample data, the non-parametric Kruskal–Wallis test was performed (α = 0.05). For concentrations below the method detection limit for a given chemical analysis, one-half of the method detection limit was utilized for calculation and statistical analysis. Pollutant removal efficiency was also evaluated and calculated for the rain gardens using the Formula (1):
5. Discussion
This study evaluated the effectiveness of rain gardens in managing non-point source pollution in eastern Texas by analyzing water quality parameters at inflow and outflow points. The inflow stormwater samples revealed pollutant concentrations that, in several cases, exceeded regulatory thresholds, indicating moderate to high pollution risk. For instance, arsenic concentrations in unfiltered inflow samples reached 0.05 mg/L, which is five times higher than the USEPA MCL of 0.01 mg/L for drinking water. Aluminum levels routinely exceeded the TCEQ aquatic life protection criterion (0.087 mg/L), with inflow concentrations as high as 0.28 mg/L. Copper also exceeded site-specific TCEQ hardness-adjusted standards during multiple events. While nitrate levels (maximum 2.7 mg/L) remained below the USEPA MCL of 10 mg/L, they reflect significant nutrient loading from impervious surfaces, contributing to eutrophication risk in downstream systems. These findings suggest that untreated stormwater from rooftops and parking lots poses substantial ecological and regulatory concern, particularly with respect to trace metal toxicity.
After treatment through the rain gardens, the concentrations of aluminum, copper, iron, potassium, zinc, boron, manganese, phosphorus, fluoride, and phosphate at the outflow were significantly reduced, achieving a pollutant removal efficiency exceeding 50%. Specifically, median inflow concentrations of nitrate were reduced by 52%, from 2.7 mg/L to 1.3 mg/L at the outflow, with statistically significant reductions (
p < 0.05) in over 70% of events. Copper, a key toxicant in urban runoff, showed a 44% average reduction, from 90 µg/L to 50 µg/L, with complete compliance with the USEPA action level after treatment. Phosphate removal was more variable, with reductions ranging from 10–40% depending on site and antecedent conditions. Metals such as lead, zinc, and iron also showed consistent downward trends, with Pb reduced by 38% and Zn by 56%. However, iron concentrations remained above the USEPA Secondary MCL in all cases, suggesting leaching from the rain garden media. Aluminum exceeded TCEQ criteria at both inflow and outflow, indicating potential mobilization or insufficient retention. These findings contrast with those from temperate climates, where rain garden effectiveness may be enhanced by more moderate and predictable rainfall patterns. It also underscores the need for climate-adaptive BMP designs in subtropical and tropical urban watersheds, where storm intensities can exceed conventional design thresholds and performance expectations [
37]. Similar removal efficiencies have been reported in rain garden systems, including one in Kurukshetra, India, which incorporated 75% topsoil and 25% compost and achieved a 49% pollutant removal efficiency [
33]. A hybrid rain garden system in Korea, tested with synthetic runoff, demonstrated a 56% removal efficiency for heavy metals [
42]. The mean concentrations of most water quality parameters were significantly lower in the outflow than in the inflow, confirming the effectiveness of rain gardens in treating urban stormwater runoff. Additionally, runoff estimates were calculated using the United States Department of Agriculture (USDA) Curve Number (CN) method to assess potential relationships between runoff volume and outflow concentrations, but no significant correlation was identified. Most water quality parameters in the outflow met the surface runoff standards established by the USEPA and TCEQ, reinforcing the viability of rain gardens as a sustainable stormwater management practice.
Total and water-soluble concentrations of most metals were lower in outflow water than in the inflow, demonstrating the rain gardens’ effectiveness in metal pollutant removal. Significant reductions in copper, zinc, and iron align with documented decreases in metal concentrations observed in bioretention systems designed for stormwater treatment [
43]. Similarly, sandy substrates combined with organic amendments have been associated with reductions in heavy metal concentrations exceeding 50% [
44]. Pollutant removal in rain gardens occurs through multiple mechanisms, including filtration, adsorption, precipitation, and biological treatment [
45], all of which likely contributed to the reductions observed in this study. Bioretention systems remain among the most widely implemented best management practices for urban stormwater control, effectively reducing runoff volume, peak flows, and pollutant loads, including heavy metals.
In contrast, lead and arsenic showed no significant removal, which is consistent with Trowsdale and Simcock [
46], who reported limited lead and arsenic reduction in bioretention systems due to their low mobility under typical pH conditions. The lack of significant removal may be attributed to the relatively low initial concentrations of these metals in the inflow, as well as their strong affinity for particulate matter rather than dissolved phases, which reduces their susceptibility to filtration and adsorption processes in the rain garden substrates [
47].
However, aluminum and magnesium concentrations were higher in the outflow than in the inflow, likely due to underlying geological conditions or differences in fill material composition across the three study sites. Ponded runoff in the rain gardens was exposed to Weches Formation rock, known to contain elevated aluminum and magnesium levels due to its glauconitic and clay-rich composition [
48]. The leaching of aluminum and magnesium from the Weches Formation could have contributed to the increased outflow concentrations, similar to observations of elevated dissolved metal concentrations in outflow water resulting from interactions with underlying geological substrates [
49].
Laboratory analysis of the rain garden soil revealed significant differences in manganese concentrations among the three sites. The mulch layer in Rain Gardens 2 and 3 contained substantially higher manganese concentrations (1762.96 mg/L and 1775.46 mg/L, respectively) than in Rain Garden 1 (710.42 mg/L), with statistically significant differences (
p < 0.0001). In the sand layer, manganese concentrations also varied significantly, with Rain Garden 2 having the highest (243.20 mg/L), followed by Rain Garden 3 (106.25 mg/L) and Rain Garden 1 (54.39 mg/L). These variations may have contributed to the manganese levels detected in the outflow, as bioretention media composition strongly influences manganese and other metal leaching [
50].
Elevated calcium concentrations in the outflow were likely due to the dissolution of calcium from the underlying soil. The parent material in Rain Gardens 2 and 3 consists of sandstone containing silica and calcium carbonate as cementing agents, which undergoes weathering and releases calcium into percolating water. In regions where the parent material comprises sandstone cemented with CaCO
3, chemical weathering processes can release calcium into percolating water. The dissolution of calcite, a common form of CaCO
3, in sandstone has been studied to understand its impact on reservoir properties. Research indicates that calcite dissolution rates in reservoir sandstones can be significant, leading to increased calcium levels in the surrounding water [
51]. The elevated concentrations of calcium, sodium, and manganese can also alter pH and electrical conductivity, influencing the solubility and mobility of other metals. Higher pH values, such as those observed in the outflow in this study (
Table 5 and
Table 7), can enhance metal precipitation and adsorption processes, ultimately improving the removal efficiency of certain heavy metals [
52].
The performance of the rain gardens improved from Year 1 to Year 2 of the study. During the first year, vegetation was still establishing, but by the second year, woody perennials had matured, increasing pollutant uptake and enhancing overall stormwater treatment efficiency. Metal and nutrient removal efficiency in rain gardens has been shown to significantly improve as plant root systems develop and microbial communities become more active, reinforcing the role of vegetation maturity in optimizing bioretention performance over time [
53].
Variability in removal rates among the rain gardens was influenced by differences in initial inflow water conditions and site-specific factors, particularly for aluminum, iron, manganese, sodium, boron, nitrate, and nitrite. This variation aligns with research indicating that rain garden performance is site-dependent and affected by hydrologic conditions, substrate composition, and pollutant loading rates [
54]. Future studies should consider long-term monitoring and controlled experiments to further isolate the effects of substrate composition, vegetation type, and hydrologic variability on pollutant removal efficiency.
6. Conclusions
This study assessed the pollutant removal performance of three rain gardens in eastern Texas over 25 storm events under a subtropical climate regime. Despite the challenges posed by high-intensity precipitation and short residence times, the rain gardens demonstrated substantial pollutant mitigation capacity. Median nitrate concentrations were reduced by 52%, phosphate by up to 40%, and copper by 44%, with consistent reductions also observed for lead (38%) and zinc (56%). However, some pollutants—notably aluminum and iron—remained above ecological or aesthetic thresholds, likely due to local soil composition and media characteristics. These findings confirm the effectiveness of bioretention systems in managing urban stormwater in hydrologically variable environments but also point to important limitations. This study did not quantify flow rates or total mass removal, and monitoring was limited to a one-year period. Additionally, the rain gardens varied in design and substrate characteristics, making it difficult to isolate performance drivers.
Future research should incorporate multi-year monitoring, quantify pollutant loads, and evaluate the impact of different media types, vegetation assemblages, and maintenance regimes on pollutant retention. Further work is also needed to assess how rain gardens function during overflow events and under repeated high-intensity rainfall. Given the growing intensity of storm events under climate change, adaptive BMP designs—tailored to subtropical conditions—are essential for resilient urban water management.