Chloride Concentration Evaluation: Villanova Porous Concrete Site CEE Undergraduate Research Submitted To: Dr. Andrea Welker Submitted By: Andrea M. Braga Submitted On: April 28, 2004
TABLE OF CONTENTS Topic Page No. Abstract 3 1.0 Introduction 4-6 2.0 Villanova Porous Concrete Site 7-14 2.1 Site Location 7 2.2 Site Layout 8 2.3 Site Operation 10 2.4 Monitoring for Contaminants 14 3.0 Contamination Concerns 15-19 3.1 Potential Contaminants 15 3.2 Chloride Contamination 17 3.3 Movement of Chloride in Soil 18 4.0 Research from VU Tests 20-27 4.1 VU Monitoring Systems 20 4.2 Chloride Results 23 4.3 Interpreting the Results 26 5.0 Conclusions 28-29 Works Cited 31 Appendix 32 2
ABSTRACT As a direct result of urbanization and the increasing development that has spread across most cities, grasslands and wooded areas are rapidly depleting. With less pervious area available for natural infiltration, the need for ground water recharge is a mounting concern. Instead of infiltrating into the ground, stormwater is being piped from parking lots, streets, and buildings directly into nearby rivers and streams, having detrimental consequences. The river systems are now being inundated with additional volumes of runoff, causing erosion, sedimentation, flooding, and the ultimate destruction of the natural aquatic life that exists in these areas. Stormwater infiltration is a new and innovative technique aimed at returning stormwater to the hydrologic cycle. By infiltrating water locally, in underground beds or basins, stormwater will be captured and infiltrated into the ground before having a chance to run off into conventional sewer systems and into the rivers further increasing these problems. However, when stormwater is artificially infiltrated a potential for groundwater contamination emerges. This report is a study of the research done at Villanova University on the Villanova Porous Concrete Site and the effects that infiltrating stormwater contaminated with chloride may have on the soil and groundwater at the site. The secondary drinking water standard published by the USEPA for chloride is 250 mg/l, therefore it is important to understand how chloride interacts with the soil once infiltrated into the ground and the potential effects its infiltration may have on the groundwater table. 3
1.0 INTRODUCTION Urbanization has a significant effect on the water quality of both the ground and surface water sources of the geological area in which it is introduced. With increasing developmental progress, there is a quantifiable decrease in area free to allow for stormwater infiltration and recharge into the water table. Instead of entering normally back into the hydrologic cycle through infiltration into the soil, stormwater is forced to bypass this critical step and flow over of parking lots, rooftops, and roadways. This can result in direct runoff into nearby surface waters. This excess water quickly turns into elevated volumes of runoff carrying with it sediments, suspended and dissolved solids, metals, and other toxins. Not only does this adversely affect the ecology and health of the local rivers and streams, but it also has a regional effect, with the potential to cause flooding and erosion and sedimentation miles downstream from the source. During rain storms and snow melting events the decrease in pervious land cover, such as grasslands and wooded areas, and consequent increase in runoff volume, sends stormwater straight to rivers at such high rates that the river systems are unable to balance the flows. This results in flood events, erosion, sedimentation, and the ultimate destruction of the natural habitat and overall water quality of the river systems. Infiltrating stormwater locally into the ground instead of discharging it through conventional pipe sewers is increasingly considered as a means of controlling urban stormwater runoff, thereby reducing runoff peaks and volumes, and returning the urban hydrological cycle to a more natural state (Mikkelsen, 827). Stormwater Best Management Practices (BMPs) include the concept of source control, which establishes a passive system that intercepts pollutants at the source and disposes of stormwater close to the point of the rainfall (Barbosa, 1014). BMPs include detention systems, 4
such as wetlands, and wet detention ponds and infiltration systems. Specifically, there are two types of infiltration systems: surface infiltration and sub-surface infiltration. Surface infiltration systems allow stormwater to permeate green surfaces, such as rain/recharge gardens, swales, and grass trenches. This type of infiltration system is a practice that comes closest to natural infiltration of rainwater, allowing runoff to slowly soak into the soil, thus removing most of the toxins present in the stormwater. However, collecting stormwater from impermeable surfaces and spreading it out onto green areas puts high restrictions on the design and land use of urban space, which is often highly limited (Mikkelsen, 828). Therefore, sub-surface infiltration is the most common type of system used in these highly urbanized areas, and is the system chosen for the Villanova Porous Concrete Site. Sub-surface infiltration systems typically consist of large cavities dug out of the ground that are then filled in with clean stone. Stormwater is stored temporarily in these cavities in the voids between the stone while it slowly percolates into the surrounding soil. Water can enter the system in a number of ways. Roof drains and inlets directly convey water from the rooftops and surrounding impervious pavement into the bed bottom through perforated piping. However, a new and innovative way to get stormwater runoff into these underground infiltration beds is to have it directly infiltrate through porous pavement. Special mixes of both asphalt and concrete are used in these systems and allow for the water to soak directly into the underlying bed without the use of inlets and pipes, thus maximizing the volume of stormwater captured and infiltrated into the ground. Porous pavement systems with underlying infiltration beds present a new way of looking at development. Instead of limiting the effects of urbanization, porous pavement systems incorporate it into the design while, simultaneously, continuing the process of recharging the 5
groundwater table and returning the water back to its natural place in the hydrological cycle. Unfortunately, by using this technique, stormwater does not have a chance to infiltrate the grass and thus bypasses the humic root zone, creating the potential for a much larger risk of groundwater contamination (Mikkelsen, 828). With groundwater being such an important resource for consumptive drinking water use, chemical contamination is an aspect of stormwater infiltration that cannot be ignored. Porous pavement systems are mainly used for parking lots, driveways, and walk ways. During the winter months snow and ice must be removed from these areas with road salts. The secondary drinking water standard published by the USEPA for chloride is 250 mg/l. Therefore, salt impacts to the groundwater through the porous pavement and infiltration beds are of high concern. 6
2.0 VILLANOVA POROUS CONCRETE SITE There are two main types of porous pavement: porous asphalt, consists of an open-graded coarse aggregate, bonded together by asphalt cement, and porous concrete, consists of a specially formulated mixture of Portland cement, uniform, open-graded coarse aggregate, and water. Both mixtures have sufficient interconnected voids to make them highly permeable to water and allow rapid percolation of water through the pavement ( Storm ). Porous concrete was chosen for the Villanova Porous Concrete Site due to its light gray color and high aesthetic appeal, making it a closer match than porous asphalt to the buildings surrounding the site. 2.1 Site Location The site is located at Villanova University in Villanova, PA, Radnor Township. As shown below in Figure 2.1.1 Villanova Main Campus-East, the site is on Villanova s Main Campus in front of Bartley Hall and between the residence halls of Sheehan Hall and Sullivan Hall. Figure 2.1.1 Villanova Main Campus-East 7
The location was found to be highly suitable for this type of project due to its low level of road traffic, resulting in a low level of potential pollutants. This area of campus is used mainly for foot traffic and recreational use during the year, with the majority of vehicle traffic occurring during the days of move-in and move-out from the Sheehan and Sullivan Hall dormitories. Geologically, the site is situated on a mix of sand and silty sand, which has a high potential for infiltration. An attempt was made to determine the elevation of the water table in this area, but it was unsuccessful. The groundwater table was not encountered within the first 12 feet below ground surface. Therefore, the attempt to determine the water table elevation was discontinued, because at these depths the site was not seen as a threat for most groundwater contamination. 2.2 Site Layout The Villanova Porous Concrete Site is composed of porous concrete, brick pavers, and conventional concrete pavement. The layout of the site is illustrated in Figure 2.2.1, below. Figure 2.2.1 Porous Concrete Site Layout 8
As diagramed, the porous concrete is used only as an outline for the site, under which three infiltration beds lay. Inside the area outlined by the porous concrete is conventional impervious concrete pavement, with brick pavers laid along the outside border and within the center of the site. An enlarged portion of the center of the site is displayed in Figure 2.2.2. This figure highlights the dimensions and grading plan for the center of the site. The site is graded at a 1% slope so that during a storm event, stormwater will runoff the brick pavers and standard pavement and permeate the porous pavement, allowing for flow into the infiltration beds, and ultimately into the underlying soil. The locations east and west of the center portion are also constructed and sloped in the same manner, excluding the center portion of brick pavers. Figure 2.2.2 Grading and Dimensions Plan for Site Center 9
2.3 Site Operation The site is divided into three separate drainage areas, one for each of the infiltration beds. Figure 2.3.1a Drainage Area Schematic, displays the different areas contributing to each of the three underlying beds, denoted with a 1, 2, or 3. Figure 2.3.1b displays the legend for the preceding figure. Figure 2.3.1a Drainage Area Schematic Figure 2.3.1b Drainage Area Legend 10
The three drainage areas shown in Figure 2.3.1a-b are divided into impervious and pervious areas contributing to each of the infiltration beds. These contributing areas were established by an evaluation of the grading and sloping of the region in and around the location of the site, and from the location of the roof drains running off of each of the dormitories. During a storm event, water will run off of the impervious walkways and, depending on the magnitude of the storm, the pervious grassland, in addition to being piped directly into the beds via the roof drains and perforated piping. In Figure 2.3.1a, the black hatch on the concrete site itself indicates that the stormwater that falls onto the pavement will soak directly into the bed that it falls upon. It is necessary to note that although there is pervious grassland around infiltration Bed 3, it does not contribute to the stormwater infiltrating the bed. Due to the grading of that region of the site, stormwater is diverted away from the porous concrete and towards the impervious pavement in front of Bartley Hall. Therefore, the only stormwater that infiltrates Bed 3, the lower infiltration bed, is the direct runoff from the pavement onsite and from the roof drains on Sheehan and Sullivan Halls. Once the stormwater enters the infiltration beds it is important to understand how the water moves through the system. When a rainstorm or snowstorm occurs, stormwater or snowmelt will begin to soak through the porous concrete and into the infiltration bed beneath. Infiltration into the soil will then begin, however, this is a slow process. During larger storms, runoff may begin to fill the beds at a rate higher than that of infiltration. The system is designed to handle the excess water of bigger storms, without allowing the beds to fill to capacity and overflow back up through the porous concrete and onto the ground surface. This is 11
accomplished through the construction of the infiltration beds and the outlet structure that discharges excess water from the site. Figure 2.3.2 Infiltration Bed Cross Section, illustrates a cross section of the beds underground and the depths at which each bed is located. Figure 2.3.2 Infiltration Bed Cross Section As is displayed in the above figure, the beds are staggered at different depths due to the natural slope of the site. Each bed is separated by an earthen berm, which prevents continuous flow from bed to bed and allows the water to remain in the bed for infiltration purposes. However, a 4-inch diameter pipe was placed through the berm connecting Beds 1 and 2, the middle and lower infiltration beds respectively, from bed bottom to bed bottom. This prevents water from remaining in Bed 2 for infiltration. Instead, any runoff that soaks into this bed will immediately be diverted via the pipe to the lower infiltration bed. Conversely, the upper bed has no overflow pipe, thus allowing all the water that is collected through this section of porous 12
concrete to remain in the bed for infiltration. Since there is no way for water to exit the upper bed, it is a closed system separate from the middle and lower beds, with no overflow ever exiting this region of the site. Beds 2 and 3 are connected to an outlet control structure, which during heavy storms allows water to flow out of infiltration Bed 3 through a pipe located 1.75 feet above bed bottom. When the stormwater accumulates from the middle and lower beds to an elevation of 437.35 ft, water will then begin to discharge through the overflow pipe and into the outlet control structure. The outlet control structure, shown in Figure 2.3.3, is connected to Bed 3 through the 12 diameter overflow pipe. The outlet structure is controlled by a V-notch weir and is ultimately discharged to a solid pipe, which is then connected to the existing campus stormwater system. The outlet structure controls the amount of water leaving the site and ensures that there is no opportunity for overflow back up through the porous concrete and onto the ground. Figure 2.3.3 Weir Outlet Control Structure 13
2.4 Monitoring for Contaminants Whenever stormwater unnaturally infiltrates the ground surface, there exists a potential for hazardous contamination of the underlying soil and groundwater. The basic idea is that polluted stormwater should pass through a humic layer at the soil surface that effectively screens off any present well absorbable or degradable pollutants (Mikkelsen, 834). It is essential that wherever stormwater is artificially infiltrated and bypasses this critical soil zone, precautions be taken to protect the quality of the site s groundwater. Contaminants present in stormwater runoff include organic compounds, pesticides, nutrients, heavy metals, and salts, which can originate at the land surface or in the atmosphere. Some constituents either volatize during storage or sorb to particulate matter and are not transported to the water table; others, however, are more persistent, and may threaten groundwater quality (Fischer, 464). In order to determine the effects of infiltrating stormwater at the Villanova Porous Concrete Site, monitoring systems have been put in place beneath the infiltration beds. Despite the fact that the elevation of the groundwater table at the site is over 12 feet below ground surface, it is still important to have instrumentation to monitor what and where contaminants are leaching into the soil beneath the infiltration beds. These devises will give us a better understanding of what pollutants are entering the bed, what pollutants are exiting the bed, and at what depths the pollutants are located below the infiltration bed bottom. 14
3.0 CONTAMINATION CONCERNS Natural infiltration through green surfaces, such as grasslands or wooded areas is safe for the environment because it allows for a slow infiltration process where microbial and chemical processes in the humic root zone have time to react and protect the groundwater from contamination (Mikkelsen, 828). In urban environments, these pervious areas are drastically reduced so alternative means of infiltration are necessary. When stormwater is directed underground and evades this humic root zone, contamination problems are drastically increased. 3.1 Potential Contaminants Contaminants in runoff vary in range and scope and can result from a number of activities. Vehicle traffic, agriculture, industry, landscaping, and urban pollution all provide increasing potential for hazardous material to collect in runoff and be transported into infiltration systems. Contaminants originating from agricultural activities such as pesticides, fertilizers, and weedicides may be transported during runoff and join the river system to cause surface water pollution and/or infiltrate into the ground to cause groundwater pollution (Yan, 347). The most commonly occurring organic compounds in urban groundwater include phthalate esters and phenolic compounds. Contamination by organics occurs more readily in areas with sandy soil and where the water table is near the land surface. Nutrients such as nitrates are one of the most frequently encountered groundwater contaminants. Nitrate is highly soluble and will stay in solution in the percolation of water. Contamination of groundwater can also result from municipal and private pesticide application and its subsequent collection into stormwater runoff. Pesticides decompose in soil and water but decomposition can range from days to years ( Potential, 129). Most heavy metals, collected in runoff through vehicle traffic, roof runoff, 15
and industry, are removed either in the infiltration bed by sedimentation or in the vadose zone. Microorganism causing viruses have also been detected in groundwater where stormwater recharge basins were located short distances above aquifers. Factors affecting the survival of enteric bacteria and viruses in the soil include ph, antagonism from micro flora, moisture content, temperature, sunlight, and organic matter ( Potential, 129). Induced infiltration of stormwater can compensate for reduced groundwater recharge caused by the sealing of the urban landscape and promotion of the retention and degradation of contaminants in the soil and vadose zone (Datry, 218). As previously stated, stormwater not only contains heavy metals, organic compounds, and pathogens, but can also be an important source of nutrients. Because most contaminants are adsorbed on sediment surfaces, stormwater infiltration basins are designed to retain suspended solids. Since particle sedimentation and sorption may remove the greatest mass of pollutants, considerable amounts of stormwater sediment contaminated with heavy metals and organic compounds can accumulate over time in the upper layers of the soil under the infiltration beds. This layer of sediment can act both as a filter and as a source of dissolved contaminants, thereby leading to major differences between the chemical composition of stormwater entering the basin and that of water recharging the underlying vadose zone or groundwater zone (Datry, 218). Additionally, this contaminated layer of soil can also cause the infiltration in these beds to be reduced due to the clogging of the pores in the soil. Salts, however, do not act in the same way as most major pollutants previously described. Stormwater sediments collected in infiltration systems previously studied in industry, did not appear to be a major source of Na and Cl because output concentrations were almost equal to input concentrations (Datry, 218). Sodium and chloride used for deicing during the winter 16
months collect in snowmelt and travel down through the vadose zone and to the groundwater with little attenuation. Salts still in the percolation water after it travels through the vadose zone will enter the groundwater. Sulfate and potassium concentrations decrease with depth, whereas sodium, calcium, bicarbonate, and chloride concentrations increase with depth, thus making these elements a higher risk of concern for the groundwater contamination ( Potential, 129). 3.2 Chloride Contamination Stormwater at the Villanova Porous Concrete Site is monitored for the following constituents: ph, conductivity, copper, nitrates, phosphorous, suspended and dissolved solids, and chloride concentrations. Due to the location of the site and the uses it receives during the year, most of the runoff is relatively clean in comparison to sites underneath parking lots or in more urban areas with high vehicle traffic and industrial activity. Additionally, no fertilizers, pesticides, or weedicides have been used in the area since initial construction of the infiltration beds. This will help to further reduce potential for pollution. The only contaminant that seems to be presenting concern is chloride. As a result of the reasonably harsh Pennsylvanian winters, deicing salts are used on roads and sidewalks to increase the safety of those who travel across them. Although this site is not used for vehicle traffic, it does receive a high volume of foot traffic from hundreds of Villanova students every day. Therefore, the use of deicing salts are essential for keeping this area safe for passersby despite the potential hazard to the infiltration beds laying beneath the surface. In order to decrease this hazard, Peladow, calcium chloride pellets, manufactured by Dow Chemical Company, are used exclusively in this vicinity of the Villanova Campus. 17
Peladow is composed of the following: Calcium Chloride: 90-97% Sodium Chloride: 1-2% Potassium Chloride: 2-3% Although Peladow is a specialized form of deicing salt used mainly for this area of campus it is still composed mainly of chloride. Therefore, it is imperative that we understand how chloride moves in the soil beneath the infiltration beds and how it reacts with the soil in this area. This will allow for better protection of the groundwater of the site. 3.3 Movement of Chloride in Soil Most salts are not abated during movement through the soil. In fact, salt concentrations typically increase with depth due to leaching of salts out of soils (Pitt, 82). A decrease in groundwater salinity may occur only when dilution with that of less saline recharging waters. Salt leachate is at the greatest concern in arid areas of the United States because the irrigation requirement for these arid areas are great and the irrigation water collects the salts that have been concentrated in the soil by increased evapotranspiration. Chloride concentration is also a concern in other less arid areas, due to the potential for groundwater contamination. Salts that are still in percolation water after traveling through the vadose zone can contaminate groundwater. The rate of contaminated water movement in a water table aquifer is highest when the water table is the closest to the surface. Studies in relation to a shallow, unconfined aquifer have shown that sulfate and potassium concentrations decrease with depth, while sodium, calcium, bicarbonate and chloride concentrations increase with depth in the soil. It was also 18
noted that the heavy metal loading was significantly reduced during passage through the soil, while chloride was not reduced significantly, which is a clear indication that the soil does not contain an effective removal process for chloride salts (Pitt, 83). Once contamination with salt begins, the movement of salts into the groundwater can be rapid. The salt concentration may not lessen until the source of the salts is removed. The secondary standard for chloride in drinking water in the United States is 250 mg/l published by the USEPA. Since this is not a primary standard it does not necessarily have to be abided by law, however, high concentrations of chloride ions in water does have an adverse affect on its taste, in addition to accelerating the corrosion of pipes and household appliances, thus the need for concern (Pitt, 84). 19
4.0 RESEARCH FROM VU TESTS The research for the Villanova Porous Concrete Site is funded by the Pennsylvania Department of Environmental Protection (PADEP) for a total of 25 storms per year. At the completion of a storm event, data must be collected from the instrumentation and water samples must be collected to evaluate the efficiency of the porous concrete system. Quality and quantity parameters are under investigation, so not only is the potential for contamination assessed, but also the amount of water that actually flows into the beds, out of the beds, and the rate of infiltration are also important factors to understand. However, in this report we will be focusing on the quality of the stormwater that infiltrates the porous concrete, and more specifically the chloride concentration thereof. 4.1 VU Monitoring Systems There are two different strategies for performing field investigating of existing stormwater infiltration systems. In the first strategy, and the approach used by the Villanova University research team, the groundwater quality below and downstream from an infiltration system is directly brought into focus. By sampling and analyzing seepage water and groundwater, it is supposed that it can be directly assessed whether infiltration leads to changes in groundwater quality. The second strategy focuses on the soil phase by sampling and analyzing the soil in or beneath an infiltration system. For many substances, in particular heavy metals, the soil between the bottom of the infiltration system and the groundwater table acts as an effective pollutant trap, however high concentrations of pollutants in the soil phase do not necessarily mean that the pollutants cannot be leached during the next decades or centuries 20
(Mikkelsen, 832). Therefore, it is extremely important to examine the effects on artificial infiltration below such infiltration beds. The monitoring instruments in use at the site include lysimeters, moisture meters, and pressure transducers. In this investigation, the quality of the infiltrating water is the only element of concern. As a result, the only instrument utilized by the research team is the lysimeter. Figure 4.1.1, illustrates where these devises are located at the site. Lysimeter Group Sullivan Hall Bartley Hall Middle Bed Lower Bed Figure 4.1.1 Location of Monitoring Devises Soil water pressure-vacuum lysimeters allow numerous water samples to be collected from the same location in the unsaturated zone through a relatively inexpensive process. Therefore, when a storm has passed the research team can collect water from different depths below the ground surface and test them for different water quality criteria. After a storm has begun the technician visits the site and applies a vacuum to the lysimeters by use of a suction pump. The next day, pressure is added and the sample of water is removed from the ground via a tube. As can be seen in Figure 4.1.1, there are two different locations for the monitoring instrumentation. 21
The first location is situated closer to Bartley Hall and on the west side of the bottom infiltration bed (when facing Bartley Hall). At this location the lysimeters are labeled B11, B12, and B13, which correspond to the location near Bartley (B), directly under the bed (1), and at depths of 1, 2, and 4 feet beneath the bottom of the third infiltration bed, respectively (1, 2, 3). In this case, the samples are from water that has directly infiltrated through the lowest of the three beds. Therefore, after these samples are tested the research team is able to quantify how much chloride is in each sample at the different depths and get a true representation of how chloride travels through the soil. The second instrument location is positioned on the opposite side of the same infiltration bed and farther away from Bartley Hall. This location has only one functional lysimeter, A21, which corresponds to the location away from Bartley Hall (A), outside the vicinity of the bed (2), and 1 foot beneath the bottom elevation of the infiltration bed (1). This lysimeter can be found in the soil under the grass in front of Sheehan Hall. The water taken from this location signifies a type of control for the other water samples, which simulates the effects of natural infiltration through untouched pervious grasslands. Figure 4.1.2 illustrates a normal cross section of the instrumentation. Figure 4.1.2 Cross Section of General Layout of Instrumentation 22
As shown in Figure 4.1.2, the lysimeters are located below the bottom of the infiltration bed at the designated depths below the bed bottom elevation. At the same depths of each lysimeter there is a moisture meter, which measures the rate of the infiltrating water at that point in the soil, and a pressure transducer is located on the bed bottom, which determines the depth of the water in the bed. In addition to these two monitoring locations, there are also two different locations were water samples are collected and tested for contaminants. There is a port located in infiltration Bed 3 from which samples of water can be taken directly from the bed. This is the water that has collected and seeped through the porous concrete, but has not yet had a chance to infiltrate into the soil. These samples tell us the quality of the water that is infiltrating through the beds. The second location is from the gutters of Sheehan Hall, which quantify what contaminants are in the water before it reaches the ground. 4.2 Chloride Results After the water samples are collected from the site they are brought into the lab and tested for different water quality criteria. The water samples are run though an I.C. or Ion Chromatograph to obtain the concentration in parts per million (PPM) or mg/l. The I.C. uses a technique that pushes the water sample through a column, and based on the conductivity of the different compounds in water sample, separates the sample into its individual components. These components are then converted into concentrations that can be analyzed. The chloride concentration data from 23 storms over a year s time are analyzed in this report from the dates of March 20, 2003 to February 9, 2004 (See Appendix for raw data). Using this information we can then plot chloride concentration vs. time for each location. In the first 23
graph in Figure 4.2.1, the relationship between the chloride concentration and depth can be seen for the 23 storms tested. The results in this figure are taken from the instrument locations B11, B12, and B13 and represent the chloride concentration for water samples taken 1, 2, and 4 feet below the bottom of Bed 3, the lowest bed. Chloride Concentration 1800 1600 B11 B12 B13 1400 1200 Cl (mg/l) 1000 800 600 400 200 0 02/23/03 04/14/03 06/03/03 07/23/03 09/11/03 10/31/03 12/20/03 02/08/04 03/29/04 Figure 4.2.1 Chloride Concentration as a Function of Depth As depicted in the Figure 4.2.1, the chloride concentration in the months between 3/20/03 and 6/03/03 are the highest for B13, with B12 second highest and B11 lowest. In the summer and fall months the concentrations of chloride in the sample water at all three locations are approximately the same. Most of these water samples contained chloride concentrations below 50 mg/l. Subsequently, when winter begins slightly before 12/20/03, the concentrations of 24
chloride begin to climb once again with B11 having the highest concentration and B12 and B13 mostly staying together at slightly lower concentrations. In the second graph, Figure 4.2.2, the relationship between the chloride concentration and location can be seen for the 23 storms tested. Chloride Concentration 1800 1600 A21 B11 Port Gutter 1400 1200 Cl (mg/l) 1000 800 600 400 200 0 02/23/03 04/14/03 06/03/03 07/23/03 09/11/03 10/31/03 12/20/03 02/08/04 03/29/04 Figure 4.2.2 Chloride Concentration as a Function of Sample Location Unlike Figure 4.2.1, Figure 4.2.2 illustrates the difference in chloride concentration of the water samples taken at the different locations around the site. Over a years time the chloride concentration of A21, the location in the grass in front of Sheehan Hall, essentially remains the same, changing in range from approximately 5 to 150 mg/l. Since there is no salt application on the soil itself, and the range in concentration is so slight, it is assumed that the soil has a normal naturally occurring concentration of chloride. Furthermore, due to the fact that this 25
concentration, at its highest value, is still 100 mg/l less than the secondary drinking water standard for chloride of 250 mg/l, it is not considered a threat to the groundwater at the site. Contrarily, the water samples taken from B11 and the port do have notably high chloride concentrations in the winter and spring months, which is due to the seasonable salt application to the concrete on site. Figure 4.2.2 also displays the chloride concentration for the gutter samples. These samples have insignificant levels of chloride, indicating that the rainfall itself has no discernable chloride concentration before it hits the ground surface. 4.3 Interpreting The Results The movement of chloride through soil is demonstrated through the tests completed by the research team at the Villanova Porous Concrete Site. The effects of deicing salts are seen immediately in the winter during application, and also throughout the spring when the rains begin to melt the accumulated snow and wash the excess salts into the beds. Figure 4.3.1 Spring Chloride Concentration presents the data for the first five storms of the year, in the late winter and spring between the months of March and May, which exemplify the trend of chloride s movement through soil. 26
2000 1800 1600 1400 Chloride Concentration B13 B12 B11 Cl (mg/l) 1200 1000 800 600 400 200 0 3/15/2003 3/22/2003 3/29/2003 4/5/2003 4/12/2003 4/19/2003 4/26/2003 5/3/2003 Figure 4.3.1 Spring Chloride Concentrations Since soil is not effective at removing most salts the chloride concentration moved through the soil with the highest concentrations at the deepest locations tested. In each storm tested in Figure 4.3.1, the water sample concentration at A21 is negligible in comparison to those of B11, B12, and B13. Also, in each case B13, the lysimeter set at the greatest depth below the ground, has the highest concentration of chloride, with B12 having the second highest concentration and B11 having the lowest. This further proves that chloride s movement through soil is not impeded, but instead the chloride concentration in the water samples intensify as depth below the bed bottom increases. 27
5.0 CONCLUSIONS Research has shown that general advantages and disadvantages exist in stormwater infiltration systems. Infiltration systems collect stormwater near the source, reducing the costs of conveying runoff through sewer systems and discharging to wastewater treatment plants and/or to receiving waters. Water treatment by pollutant removal, improved road safety because of better skid resistance, and recharge to local aquifers are all incentives to utilize infiltration systems. However, these systems also have a tendency to become clogged if improperly installed or maintained. They also present a risk of contaminating ground water depending on soil conditions and aquifer susceptibility. Porous pavement pollutant removal mechanisms include sorption, straining, and microbiological decomposition in the soil. As runoff enters the system, particles may settle as stormwater infiltrates into the soil. The dissolved and colloidal fractions of pollutants are transported by the water through the soil pores, and should be retained by the soil. An estimate of porous pavement pollutant removal efficiency is provided by two long-term monitoring studies conducted in Rockville, MD and Prince William, VA. These studies indicate removal efficiencies of between 82% and 95% for sediment, 65% for total phosphorus, and between 80% and 85% of total nitrogen. The Rockville, MD, site also indicated a high removal rate for zinc, lead, and chemical oxygen demand (Barbosa, 1014). Chloride does not act in the same manner as most pollutants, but instead flows straight through the soil and into the groundwater beneath. This does not necessarily mean that it is a concern for groundwater safety. Historical data trends show that chloride concentration levels in the infiltrating water entering the Villanova Porous Concrete Site are high during the winter and spring months. The samples with the highest concentration detected were nearly 1800 mg/l, 28
which exceeds the USEPA s secondary drinking water standard of 250 mg/l. However, once chloride filters through the water table this concentration will be diluted rapidly to below risk based levels. There is no immediate risk to the groundwater caused by infiltrating water contaminated with chloride. There may be potential for concern regarding long-term exposure of the high levels of chloride during the winter and spring months. However, due to the fact that deicing salt applications and thus the elevated chloride concentrations occurrences are seasonable, the groundwater table has a longer time to recover between exposure, minimizing the potential for even long term concern. No evidence has thus far identified infiltration systems as a considerable risk for groundwater contamination, but stormwater infiltration will eventually lead to contamination of surface soils. Stormwater infiltration systems may act as effective traps for well absorbable contaminants, thus pointing at sustainable solutions for control of stormwater contaminants (Mikkelsen, 834). As long as contaminants can be effectively retained in the infiltration systems without causing harm to human activity, infiltration may be seen as an effective means of pollution source control, which may again be seen as a step towards sustainable stormwater management. The positive effects of infiltrations are clear: the runoff peaks and volumes are reduced, and the hydrological cycle is returned to its natural state. 29
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Works Cited Barbosa, A. E.; Hvitved-Jacobsen, T. Infiltration Pond Design for Highway Runoff Treatment In Semiarid Climates. Journal of Environmental Engineering November 2001: 1014-1022. Datry, T., et al. Solute Dynamics in the Bed Sediments of a Stormwater Infiltration Basin. Journal of Hydrology 273 March 25, 2003: 217-233. Fischer, D.; Charles, E.; Baehr, A. Effects of Stormwater Infiltration on Quality of Groundwater Beneath Retention and Detention Basins. Journal of Environmental Engineering May 2003: 464-471. Pitt, Robert., et al. Potential Groundwater Contamination from Intentional and Nonintentional Stormwater Infiltration. Risk Reduction Engineering Laboratory, Office of Research and Development US EPA. May 1994. ---. Potential Groundwater Contamination from Stormwater Infiltration. Groundwater Management (1996): 127-132. Mikkelsen, P.S.; Jacobsen, P.; Fujita, S. Infiltration Practice for Control of Urban Stormwater. Journal of Hydraulic Research 34 (1996): 827-840. Storm Water Technology Fact Sheet: Porous Pavement. Environmental Protection Agency. September 1999. <http://www.epa.gov/owm/mtb/porouspa.pdf> Yan, M.; Kihawita, R. Modeling Pollutant Transport in Overland Flow with Infiltration. Environmental and Coastal Hydraulics: Protecting the Aquatic Habitat (1997): 347-351. 31
Appendix 32
Raw Chloride Data Date A21 B11 B12 B13 Port Gutter 3/20/03 0:00 22 1245 1200 1750 3/26/03 0:00 68 680 1090 1490 4/7/03 0:00 80 481 972 1591 4/9/03 0:00 96 436 880 1376 5/6/03 0:00 109 145 367 766 6/3/03 0:00 92 34 83 277 6/19/03 0:00 126 15 20 75 6/22/03 0:00 117 10 11 39 9/3/03 0:00 112 6.8 7.2 15 9/5/03 0:00 107.1 5.7 5.9 9.6 9/24/03 0:00 50 10 6.8 3.5 10/6/03 0:00 52 2.5 1.3 0.7 10/22/03 0:00 66 5 7.3 6.9 10/27/03 0:00 70 5.1 5.8 9.5 10/29/03 0:00 100 4.9 4.7 4.9 11/6/03 0:00 60 0.28 0 0.5 11/19/03 0:00 115 3.2 3 3.3 12/9/2003 116 11 2.9 5 12/11/2003 145 30 63 19.4 12/12/2003 123 184 8.9 47 2/3/2004 100 250 50 40 2/4/2004 100 280 70 70 2/9/2004 100 300 200 90 3/31/03 0:00 500 4/6/03 0:00 226 6/4/03 0:00 8 6/18/03 0:00 5.9 6/19/03 0:00 73.7 6/22/03 0:00 6.7 9/2/03 8:30 5.9 9/2/03 17:40 7.6 9/3/03 9:00 5.1 9/4/03 10:45 5.1 9/4/03 12:15 9.4 9/4/03 14:00 5.9 9/5/03 10:00 5.8 9/23/03 9:45 3.4 9/23/03 21:45 2.3 9/24/03 9:00 4.6 10/27/03 9:45 5.1 10/27/03 11:45 4.9 10/28/03 9:45 5.9 10/29/03 10:45 9.2 10/30/03 9:45 4.9 11/7/03 9:00 14.2 11/20/03 8:30 3.2 33
10/4/03 11:00 0.1 10/22/03 9:30 15.3 10/27/03 9:45 4.8 10/27/03 11:45 4.1 10/29/03 10:45 9.9 11/6/03 13:00 1 11/19/03 0:00 8.3 34