Contamination of soil and groundwater by stormwater

Môi trường ngày càng ô nhiễm nặng, việc chung tay bảo vệ là việc của tất cả mọi người trên trái đất này. Sau đây Dịch thuật Hồng Linh dịch thuật tiếng anh giá rẻ xin giới thiệu một số thuật ngữ tiếng anh ngành môi trường. > English Việt Nam absorptionabsorbent (sự, quá trình) hấp thụchất hấp thụ absorption field mương hấp thụ xử lý nước từ bể tự hoại acid deposition mưa axit acid rain mưa axit

Introduction

Non-point source pollution from stormwater runoff is a major contributor to the degradation of freshwater lakes, rivers, and estuaries The replacement of permeable surfaces with impervious materials, like roads and rooftops, leads to increased runoff that carries pollutants into storm and sanitary sewers, ultimately affecting water bodies Effective stormwater management is crucial for public health and environmental protection, prompting municipalities to explore innovative solutions One such approach is Low Impact Development (LID), which is gaining traction for its ability to promote sustainable water resource management while supporting local economic growth LID not only enhances water quality but can also improve air quality and overall quality of life Research indicates that, when implemented correctly, LID can restore hydrological conditions to near predevelopment levels.

Historically, stormwater management focused primarily on reducing peak runoff flows from developed areas, often neglecting water quality Traditional methods, such as detention ponds, have proven insufficient in addressing ecological stream degradation However, with an increased emphasis on water quality, alternative stormwater management approaches, particularly Low Impact Development (LID), are being adopted LID aims to decrease runoff volume while improving both water quantity and quality through decentralized infiltration structures that replicate predevelopment hydrology Techniques such as greenroofs, infiltration trenches, constructed wetlands, and rain gardens are now being implemented Greenroofs help slow runoff and promote evapo-transpiration, while infiltration trenches store and allow runoff to infiltrate soil Constructed wetlands, initially designed for municipal wastewater treatment, are now also effective for stormwater management Rain gardens serve as shallow, vegetated areas for stormwater recharge Collectively, these methods are recognized as stormwater Best Management Practices (BMPs).

Management Practices (BMPs) in protecting water quality (Clary et al., 2002)

Alternative stormwater management techniques often focus on the infiltration of stormwater into the soil, which aids in reducing runoff quantity and enhancing groundwater recharge while promoting pollutant removal through various physical, chemical, and biological processes Common infiltration practices include rain gardens, bioretention systems, and infiltration basins and trenches Additionally, some stormwater management methods, such as standard detention ponds, primarily utilize sedimentation for contaminant removal, yet still facilitate infiltration through their bottom and sides.

Contamination due to Stormwater Infiltration 2 Weiss, LeFevre and Gulliver

As stormwater management techniques that promote the infiltration of polluted runoff gain popularity, concerns about groundwater contamination have emerged This article summarizes common stormwater infiltration practices and the pollutants typically present in urban runoff Additionally, it reviews existing literature on the fate of these stormwater pollutants after infiltration and the associated risks of groundwater contamination.

For more detailed information on groundwater contamination resulting from stormwater infiltration, please see Pitt et al., (1996) which is referenced several times in the following sections

Stormwater management practices aim to minimize runoff volumes by promoting the infiltration of stormwater into the soil, thereby enhancing groundwater recharge Techniques such as bioretention systems, rain gardens, and infiltration trenches are specifically designed for effective infiltration In contrast, detention ponds are typically focused on reducing peak runoff flow rates and are often assumed to lack infiltration capacity However, it is crucial to consider pond infiltration when evaluating pollutant fate and groundwater contamination risks This paper reviews various stormwater management techniques that prioritize infiltration, as well as those, like detention ponds, that, while not primarily designed for infiltration, still possess the ability to manage stormwater effectively.

Infiltration basins are designed to effectively store and infiltrate stormwater runoff, aiming to achieve a specific design volume According to the "Assessment of Stormwater Best Management Practices" manual by the University of Minnesota (2007), these structures play a crucial role in managing stormwater.

An infiltration basin is a designed or natural structure that temporarily holds and allows stormwater to seep into the ground within a specified timeframe These basins feature a flat, densely vegetated surface over permeable soil, facilitating the removal of nutrients and pollutants from the infiltrated water through various chemical, biological, and physical processes.

Infiltration basins are ideal for managing drainage in areas ranging from 5 to 50 acres (2.03–20.25 hectares) and are most effective on land with slopes under 20 percent These basins typically have depths between 2 to 12 feet (0.61–3.66 meters).

Infiltration basins often require relatively large land areas and, with well chosen vegetation, are often aesthetically pleasing

Infiltration trenches are designed to effectively collect stormwater and minimize runoff by facilitating the infiltration of water into the surrounding soil According to the "Assessment of Stormwater Best Management Practices" manual, these structures play a crucial role in managing stormwater sustainably.

“An infiltration trench is a shallow excavated trench, typically 3 to 12 feet deep

Infiltration trenches, ranging from 0.91 to 3.66 meters in depth, are backfilled with coarse stone aggregate to temporarily store runoff in their voids This stored runoff is discharged through infiltration into the surrounding permeable soil, making infiltration trenches ideal for drainage areas of 5 acres (2.03 hectares) or less.

Porous pavements are designed to minimize runoff by allowing stormwater to permeate through the pavement and infiltrate the soil beneath While porous asphalt and concrete are the most common types, there are nine categories of porous pavements, including porous aggregate, porous turf, plastic geocells, open-jointed paving blocks, open-celled paving grids, soft porous surfacing, and decks, as identified by Ferguson (2005) In systems using porous asphalt or concrete, stormwater infiltrates through the upper layer and into a stone reservoir below, where it either percolates into the soil or is collected by a perforated pipe underdrain for surface discharge.

Porous pavements are gaining in popularity; however, their use is sometimes met with (not necessarily valid) concerns of increased maintenance costs and decreased durability

Rain gardens are specially designed low-lying areas that capture stormwater runoff from impervious surfaces through features like curb cuts These gardens promote water infiltration, which helps reduce runoff volume and can recharge groundwater supplies In cases where soil infiltration is poor, rain gardens may include underdrains, constructed by excavating soil, installing a perforated pipe system, and backfilling with permeable material before planting vegetation This design allows for efficient water management while minimizing concerns about groundwater contamination.

Swales are vegetated channels designed to manage stormwater by filtering solids and promoting infiltration of runoff Also known as ditches, grassed channels, or bioswales, these structures can include permeable features like check dams to slow down water flow and enhance infiltration capacity.

Filter strips, also known as buffer strips, are specially designed vegetated areas that manage overland sheet flow of stormwater runoff These strips effectively filter out particulate pollutants and decrease runoff velocities, enhancing water infiltration For optimal performance, filter strips require sheet flow to treat stormwater runoff efficiently.

Detention ponds are engineered depressions that capture and store stormwater runoff, with dry detention ponds specifically designed to discharge all collected water within 48 hours after a rainfall event Traditionally, the primary focus for designing these ponds was peak flow reduction; however, recent approaches also prioritize water quality, allowing for the settlement of solids Notably, significant infiltration can occur in these ponds unless they are lined with an impermeable membrane or clay barrier.

Stormwater Pollutants and their Fate in Infiltration Systems

Urban stormwater runoff arises from various human-made structures such as roads, buildings, and lawns, leading to significant environmental concerns Numerous studies have identified common contaminants in urban runoff, including nutrients, heavy metals, suspended solids, petroleum hydrocarbons, pathogens, and salts The concentration of these pollutants can vary greatly depending on factors such as season, location, traffic volumes, and rainfall intensity When these contaminants infiltrate the soil, they pose a risk to soil and groundwater quality, highlighting the need for effective management and mitigation strategies.

Nutrient pollution in stormwater is primarily driven by phosphorus and nitrogen, which can lead to harmful algal blooms and eutrophication in water bodies While nitrate is a common groundwater contaminant, phosphorus contamination is less frequent and severe Nutrient contamination can also arise from sources other than stormwater, such as ammonium oxidation in certain sedimentary soils Key contributors to nutrient pollution include animal waste, septic system leaks, fertilizers, and atmospheric deposition Although stormwater nutrient loadings are generally lower than those from treated wastewater, they can exceed those levels during wet weather events Seasonal variations and land use significantly influence event mean concentrations of nutrients, but the relationship between these factors and nutrient flux remains weak due to the complexities of non-point source pollution.

Phosphorus is a significant environmental concern due to its role in promoting algal blooms and eutrophication in water bodies where it serves as the limiting nutrient Key sources of phosphorus include motor oil, animal waste, plant debris, and fertilizers The most bioavailable form of phosphorus, orthophosphate, is commonly found in stormwater Research indicates that phosphorus loading is closely linked to urban land use intensity, with levels increasing logarithmically alongside the area of impervious surfaces in traditional curb-and-gutter stormwater management systems, a trend not observed in comparable low-impact development (LID) projects.

Clausen, 2008) In this study, traditional development produced a phosphorous export rate of 2 kg/ha/yr, while 0.4 kg/ha/yr was observed in the LID development

Orthophosphate (PO4 -3) can be effectively removed from stormwater once it infiltrates the soil, primarily through precipitation or chemical adsorption to soil particles, facilitated by reactions with iron, calcium, or aluminum The precipitation processes are influenced by pH levels, resulting in the formation of iron and aluminum phosphates in low pH environments and calcium phosphates in high pH conditions Under neutral pH conditions, these reactions are limited by their rate, leading to a higher apparent solubility compared to acidic or basic conditions (Pitt et al., 1999).

Wu et al (1996) conducted a study of stormwater pollutants in urban wet-detention ponds, and found removal rates ranging from -55% (phosphorus was released) to 100% Dietz and Claussen

A study conducted in Connecticut revealed that a rain garden designed to manage roof runoff was unexpectedly a source of phosphorus rather than a remover Researchers attributed this to the partial breakdown of new plant materials, leading to short-term phosphorus release Over time, however, they observed a decrease in the influent-effluent differential, indicating a potential steady state equilibrium Notably, the highest phosphorus retention was found in the rain garden's mulch, while the soil media retained significantly lower nutrient levels, with plants assimilating only 3% of the total phosphorus.

P entering the test cell Overall, due to leaching of phosphorus, it was concluded that long term phosphorous retention would be minimal

A study by Davis et al (2001) demonstrated a significant reduction of approximately 70% in phosphorus levels when synthetic stormwater was processed through a rain garden-like media However, later research by Hsieh and Davis (2005) indicated much lower phosphorus retention rates, ranging from 41% to 48% Although they hypothesized that adding mulch would enhance phosphorus removal by retaining phosphorus complexes, their findings largely contradicted this assumption Additionally, tests on various media compositions aimed at increasing phosphorus sorption through silt/clay complexes revealed no correlation between media composition and phosphorus removal It was also observed that sorbed phosphorus was leaching out from the columns' bottom.

In contrast, studies have found media composition to be influential to P retention (Arias et al.,

A study on a subsurface flow constructed wetland revealed that the chemical properties of granular media significantly affect phosphorus removal efficiencies Higher calcium content improved phosphorus removal through precipitation under neutral to slightly basic conditions, while iron and aluminum in the sand played a crucial role under more acidic conditions Additionally, phosphorus was found to be incorporated into biofilms and plants, as well as being sorbed Research by Erickson et al (2007) demonstrated that incorporating chopped granular steel wool into soil media enhances dissolved phosphorus removal without compromising effluent quality.

Nitrogen in stormwater can come from both natural and human-made sources Atmospheric nitrogen exists as nitrate from combustion processes or as ammonium from ammonia volatilization in soils and animal waste In urban settings, road runoff is a primary contributor to groundwater nitrogen contamination.

Nitrogen in stormwater exists in various forms, with ammonia (NH4+) being the most toxic to aquatic life Common inorganic forms include nitrate (NO3-) and nitrite (NO2-), although nitrite is rarely found in urban stormwater High concentrations of nitrate have been observed in heavily industrialized areas, leading to groundwater contamination from stormwater infiltration Total Kjeldahl nitrogen (TKN) accounts for ammonia and organic nitrogen The U.S EPA sets the drinking water standard for nitrate at 10 mg/L due to its adverse health effects on fetuses and infants Nitrate is often the primary focus in studies of groundwater pollution due to its solubility and health implications While typical nitrate concentrations suggest low to moderate groundwater contamination potential, elevated levels indicate a higher risk of contamination.

Nitrogen in stormwater can be utilized by plants; however, if it is not absorbed, it may leach into groundwater The extent of nitrate movement into groundwater is influenced by several factors, including the rate and volume of water infiltration, the flow dynamics of groundwater both horizontally and vertically, the depth to the water table, and the existence of preferential flow paths in the soil.

In regions characterized by traditional development without Low Impact Development (LID) practices, nitrate export increases logarithmically with the rise in impervious surfaces Conversely, in LID areas, there is no correlation between nitrate export and impervious surface area (Dietz and Clausen, 2008) Additionally, ammonia-N concentrations are lower in LID developments than in their non-LID counterparts Overall, total nitrogen export from traditional developments averages 10 kg/ha/year, while LID developments show a significantly lower average of 2 kg/ha/year.

A study conducted by Davis et al (2001) demonstrated that stormwater passing through soil media resulted in a significant reduction of total Kjeldahl nitrogen (TKN) by 65% to 75% and ammonia by 60% to 80% However, it was noted that nitrate concentrations in both the column media and effluent increased, likely due to biological activity Subsequent research by Kim et al focused on enhancing nitrate removal through the addition of organic and inorganic electron donors, which created saturated layers and an anaerobic environment conducive to denitrification.

In a study conducted in 2003, newspaper clippings emerged as the most effective material for enhancing denitrification among various tested substances, which included sawdust, elemental sulfur, alfalfa, straw, and woodchips Pilot scale experiments demonstrated an impressive nitrate-nitrite retention rate of 70-80%.

A study by Hsieh and Davis (2005) investigated the impact of media on nitrate removal, revealing that nitrate removal rates in columns varied between 1% and 43%, while ammonia removal ranged from 2% to 26% The research indicated that mulch was the most effective medium for removing nitrates and ammonia, followed by native soil, with sand being the least effective Notably, the removal efficiency did not correlate with the clay or silt content Additionally, a field study involving six rain gardens showed that all had nitrogen retention rates of less than 10%.

Research indicates that a field rain garden designed for pollutant removal achieved less than 36% retention of total nitrogen, TKN, organic nitrogen, and nitrate, while ammonia retention was notably higher at 85% (Dietz and Clausen, 2005) Additionally, a subsequent study explored the effectiveness of a saturated mulch layer to enhance denitrification, revealing that the incorporation of this layer significantly reduced nitrate and ammonia concentrations in the rain garden (Dietz and Clausen, 2006).

Groundwater and Soil Contamination

The previous section examined common contaminants in urban stormwater runoff and the effectiveness of infiltration systems in retaining them Many studies measured influent and effluent contaminant concentrations to determine the percentage of removal Additionally, research has explored the potential for stormwater contaminants to migrate to aquifers and compromise groundwater quality Other investigations have assessed the contaminant levels in soil resulting from the infiltration of polluted stormwater Protecting groundwater and soil from stormwater contamination is crucial for safeguarding human and environmental health, warranting further research (Lind and Karro, 1995).

This section provides a literature review of the latter two types of studies: groundwater contamination and soil contamination that results from infiltrating stormwater The section ends

Stormwater infiltration can lead to contamination, as highlighted by Weiss, LeFevre, and Gulliver in their review of existing literature Their work summarizes findings on pollutant transport associated with infiltration systems and includes model simulations to illustrate these effects.

A study by Stephenson et al (1999) investigated highway runoff entering a sinkhole in Knoxville, Tennessee, and a nearby spring, monitoring various contaminants during a single runoff event The research found that while most contaminants peaked at the spring approximately one hour after the sinkhole, total dissolved solids (TDS) peaked only 20 minutes later Notably, the peak loading rates for TDS and dissolved zinc were higher at the spring compared to the sinkhole, whereas other contaminants showed lower levels at the spring The study concluded that karst aquifers are particularly vulnerable to contamination from highway runoff due to their ability to transport pollutants with minimal attenuation or filtration.

Fisher et al (2003) conducted a study comparing groundwater beneath 16 detention basins in developed areas with groundwater from monitoring wells in undeveloped areas The findings indicated that pesticide concentrations varied, with some being higher in undeveloped areas while others were elevated in developed regions The authors proposed that the presence of detention basin water may dilute certain pesticide levels in monitoring wells A significant observation was the lower dissolved oxygen (DO) levels in the water beneath detention basins compared to the higher DO levels in undeveloped areas, which correlated with increased concentrations of nitrite and nitrate due to enhanced nitrification in aerobic conditions Conversely, ammonia and organic nitrogen concentrations were elevated in the low DO water beneath detention basins Chloride levels remained consistent across both environments, although the study occurred during a year with minimal snowfall, resulting in reduced salt application on roadways.

Datry et al (2004) examined the impact of infiltration on groundwater quality beneath an infiltration basin, revealing that at a depth of 1 meter below the water table, the groundwater was predominantly composed of stormwater, which did not extend beyond 3 meters They found higher concentrations of dissolved phosphate in the groundwater compared to the stormwater influent, suggesting that mineralization of organic sediments within the infiltration bed was a significant contributor Additionally, the study indicated that organic sediments enhanced the dissolved organic content of the water.

In the water samples from the infiltration bed, ammonium was the primary nitrogen species observed during dry conditions; however, it was absent in the groundwater, suggesting its oxidation to nitrate within the infiltration bed Additionally, heavy metals and hydrocarbons were not detected in the groundwater, indicating a clean water quality.

Contamination due to Stormwater Infiltration 16 Weiss, LeFevre and Gulliver infiltration during cold rains raised the groundwater DO slightly and decreased the DO during warm rains

Research indicates that hydrocarbons are primarily concentrated in the top few centimeters of soil within infiltration basins The type of hydrocarbon influences its behavior, with mineral oil-type hydrocarbons (MOTHs) being more effectively captured and degraded in soil compared to polycyclic aromatic hydrocarbons (PAHs) Studies show that PAHs tend to adhere to soil particles, but they pose minimal risk to groundwater contamination Furthermore, investigations into pretreatment facilities before infiltration basins revealed that simple sand filters are not effective in capturing hydrocarbons.

A study by Hong et al (2006) is the only one that explicitly investigated the fate and biodegradation potential in infiltration systems Building on the findings of Hsieh and Davis (2005), which indicated that oils and greases were primarily trapped in a top mulch layer without degradation, Hong et al designed a reactor featuring a 3 cm deep mulch layer fed with synthetic stormwater containing contaminants like naphthalene, toluene, and motor oil The high lignin content of mulch allows it to retain hydrocarbons effectively, as it has a strong affinity for nonionic organic compounds Additionally, filtration plays a crucial role in removing hydrocarbons, with 74% to 90% of these compounds in urban runoff being linked to particulate matter For effective biodegradation to occur, the mulch layer must support a sufficient population of hydrocarbon-degrading microbes and provide adequate contact time.

Naphthalene was effectively removed from stormwater at a rate of approximately 90%, while toluene and motor oil were reduced by 80%, and particulate-associated naphthalene was decreased by 97% The observed reduction in contaminant concentrations within the mulch layer, alongside an increase in the microbe population, indicates that biodegradation processes were actively taking place in the mulch.

Mikkelsen et al (1997) conducted a study on pollutant levels in soil media within an infiltration system in Switzerland, analyzing soil characteristics and contaminant concentrations at two distinct sites dominated by calcareous gravel deposits Site A, influenced by runoff from agricultural, residential, and light industrial areas, experienced an average traffic density of 37,000 vehicles per day In contrast, Site B was situated near a city with heavy traffic and a waste incineration plant The findings, which detail the soil properties and contaminant concentrations at both sites, are illustrated in Figure 1.

Figure 1 Vertical profiles of soil properties and pollutant concentrations (Mikkelsen et al 1997)

The study concluded that runoff solids serve as both a source of contaminants and a medium for contaminant sorption High pollutant levels were detected in the topsoil, with concentrations decreasing significantly at greater depths, suggesting that groundwater contamination is not an immediate concern However, non-sorbing contaminants, such as salts, may still reach groundwater It's important to note that these findings are influenced by the local geology and may not be applicable to other infiltration systems.

Bucheli et al (1998) examined the presence of pesticides in rainwater, roof runoff, and artificially infiltrated runoff, finding that pesticide concentrations in percolating groundwater were comparable to those in the runoff The results indicate rapid infiltration of runoff and a lack of pesticide retention in the soil media, as illustrated in Figure 2, where black dots represent roof runoff samples and lines depict data from three lysimeters This lack of retention raises concerns about the potential contamination of groundwater supplies by these harmful substances.

Figure 2 Atrazine concentrations in roof runoff and infiltrated water (upper graph) and runoff flow rate (lower graph) as a function of time (Bucheli et al 1998)

Legret et al (1999) conducted a study on the infiltration of stormwater containing metals such as copper, cadmium, lead, and zinc into porous pavement and the underlying soil, utilizing laboratory experiments and mathematical modeling, alongside field verification in France The concentrations of these metals in rainwater were found to be within specific ranges: lead (28-50 µg/l), copper (17-24 µg/l), cadmium (0.4-0.8 µg/l), and zinc (250-370 µg/l) The findings indicated that after 50 years, the increase in lead, copper, and zinc levels in soil serving as filter media for stormwater was minimal and remained below regulatory thresholds, while cadmium exhibited notable migration to depths of 30 cm.

Dierkes and Geiger (1999) conducted a study on pollutant concentrations, including zinc, copper, cadmium, and hydrocarbons, along five major highways near Essen, Germany Soil samples were collected at depths of 0-5 cm, 5-10 cm, and 10-30 cm, revealing that pollutant levels were influenced by traffic density and the duration of runoff infiltration The highest concentrations were found within the top 5 cm of soil, particularly within 2 meters of the road, while concentrations decreased significantly with depth In the 10-30 cm range, only 7% to 25% of the metal concentrations detected in the top layer were found, with the most significant decrease observed for lead and copper, and the least for cadmium.

At a distance of 10 meters from the road, copper concentrations dropped to 7% of the levels found near the road, while lead and zinc decreased by 30% each, and cadmium saw a reduction of 45% Although the study indicated minimal impact on groundwater, an increase in heavy metal concentrations was observed These findings are summarized in Table 1.

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