Stormwater Particles Sampling Literature Review

Transcription

1 Stormwater Particles Sampling Literature Review Greg DeGroot Pete Weiss, Ph.D., P.E. St. Anthony Falls Laboratory University of Minnesota June 2008

2 Foreword This literature review is divided into two sections. Section I is a review of studies and findings with a focus on particle sizes and associated pollutants. Section II is a review of particle size distribution (PSD) sampling methods, as well as factors affecting PSD.

3 Introduction One of many possible pollutants, sediment in stormwater runoff can be detrimental to a receiving water body. Not only is the sediment itself detrimental, but nutrients such as phosphorus and nitrogen and pollutants such as heavy metals (e.g. lead, zinc, copper, etc.), polycyclic aromatic hydrocarbons (PAHs), and pathogenic bacteria are often bound to the sediment and transported to the receiving water body with the sediment. In order to protect water bodies and improve the quality of runoff, many stormwater management plans seek to remove sediment before the runoff reaches the receiving water body. Two primary mechanisms for removal are sedimentation and filtration. Both mechanisms are dependent on particle size, as well as particle shape and density. Typically the larger the particle size the more readily it can be removed. This relationship is due to that fact that larger particles settle to the bottom of a water column relatively quickly and are also more easily filtered (i.e. physically strained). As a result, runoff containing large solids often experiences a large reduction in solids after treatment by a stormwater BMP whereas runoff containing small solids will usually experience less of a reduction. The importance of suspended solid particle size and the particle size distribution in runoff has led many to investigate these properties in stormwater runoff. Equally important is the amount of pollutant sorbed to the solids. As this paper will show, the amount of pollutant sorbed to solids is highly dependent on the particle size. Thus, when attempting to remove sediment and the associated sorbed pollutants from stormwater runoff, two very important variables are the particle size distribution of the solids in the runoff and the amount of the target pollutant that is sorbed to the sediment. The large spatial scale of watersheds and the temporal and spatial variability of storm events have made it historically difficult to consistently sample stormwater runoff. This is especially true for stormwater particles, which are, at any given time, buoyant, suspended, settled or in the process of settling. The varying size, shape, density and composition of stormwater particles affect the types and relative concentrations of pollutants sorbed to or absorbed within stormwater particles. This review will examine several studies of stormwater particles, their associated pollutants and the methods used for collection and analysis.

4 Section I: Studies, Particle Sizes and Pollutants Figure 1 below shows EPA and NURP study results from 1986, as well as results of more recent studies. Of particular note is the wide range of variability in particle size distributions, oftentimes up to two orders of magnitude. Figure 1. Comparison of particle size distributions (Rinker Materials 2004) Sansalone and Buchberger (1997) sampled rain and snow events from Interstate I-75 near Cincinnati, OH. The section of highway investigated had average daily traffic of 150,000 vehicles. The PSD for two snow events are represented by the two smaller graphs at the top of Figure 2. Each of the smaller graphs represents one snow event; one line is for samples taken near the beginning of the event and the other line is data from samples taken near the end of the snow event or after the snow had sat by the side of the road for several days. The large graph at the bottom of Figure 2 shows particle size distributions from composite samples collected during five single-day rain events (8 April 1995, 30 April 1995, 15 July 1995, 8 September 1995, and 3 October 1995).

5 Figure 2. Particle size distributions from Sansalone and Buchberger (1997). Sansalone and Buchberger (1997) also investigated the concentration of sorbed pollutants as a function of sediment particle size. Their results are reproduced in Tables 1-4 below. The data show that for all the pollutants investigated, the mass of sorbed pollutants per mass of sediment increases as the particle size decreases. Also, the amount of sorbed pollutants was higher in rainfall sediment as compared to snow sediment for all pollutants except lead. Table 1. Zinc concentrations sorbed to solids (ug/g) as a function of particle size (Sansalone and Buchberger 1997).

6 Table 2. Lead concentrations sorbed to solids (μg/g) as a function of particle size (Sansalone and Buchberger 1997). Table 3. Cadmium concentrations sorbed to solids (μg/g) as a function of particle size (Sansalone and Buchberger 1997).

7 Table 4. Copper concentrations sorbed to solids (μg/g) as a function of particle size (Sansalone and Buchberger 1997). Slattery and Burt (1997) investigated the particle size characteristics of suspended solids in an agricultural watershed. They first determined an effective particle size which was done without dispersing the solids. Afterwards, the particles were dispersed and the true sediment size distribution was determined. The results are shown in Table 5 below. The Dispersed soil column is the size distribution of the surface soil, the Eroded sediment column shows the particle size distribution of the effective sediment size, and the Dispersed Sediment column gives the particle size distribution after the solids in the runoff have been dispersed. Slattery and Burt (1997) concluded that that a significant mode of sediment transport in fluvial systems is in the form of aggregates, and that the dispersed sediment size distribution is inappropriate for determining the transportability of sediment by flow. Table 5. Particle size distributions from Slattery and Burt (1997)

8 Sansalone et al. (1998) reported additional PSDs for runoff from Interstate I-75. Results are shown in Figure 3 and Table 6. This study did not investigate the amount of sorbed pollutants but the data did show a first flush with respect to sediment and increasing sediment surface area with decreasing particle size. The inverse relationship between surface area and particle size, however, deviated from what would be expected with spherical particles.

9 Figure 3. Particle size distributions from Sansalone et al. (1998)

10 Table 6. Particle size distribution data from Sansalone et al. (1998). Roger et al. (1998) studied runoff from a highway in Herault, France that had an average vehicle count of 30,000 vehicles per day. Sampling was performed for over one year and it was determined that, on average, 90% of the solid matter (by weight) was in the form of solid particles less than 100 microns. Of the particles that were smaller than 50 microns, 56% were clays, 15% quartz, 12% chalk, 9% organic matter, 5% feldspars, and 2% dolomite. For a single rainfall event it was determined that 53% of the channel sediment was between 500 and 1000 microns in diameter. Roger et al. concluded that particles greater than 100 microns in diameter are easily separated by simple decantation but particles less than 100 microns remain in suspension. For this single rainfall/runoff event the overall average (i.e. including both suspension and channel sediment) of the distribution is shown in Table 7. Size (μm) Percent Mass < Table 7. Particle size distribution for one runoff event reported by Roger et al. (1998). California State University-Sacramento (2002) conducted a study in which they trapped solid particles from rainfall and snowmelt runoff using filter bags and sheets from two monitoring sites. Each monitoring site had a double barrel sand trap (one up-gradient barrel and one down-gradient barrel) from which the samples were collected. The study lasted over a year and both monitoring sites were located along a roadway in the Lake Tahoe Basin near the border of California and Nevada. The basin was comprised of forests, meadows, small urban centers, ski resorts, and vacation housing. Their results for particle size distribution are given in Figure 4 and Table 8 below.

11 Figure 4. Particle size distribution in Lake Tahoe basin (CSUS 2002). Table 8. Particle size distribution in Lake Tahoe basin (CSUS 2002).

12 The California State University-Sacramento study also investigated the concentration of 12 parameters sorbed to the solids as a function of solids size. Results for only four of the parameters, however, were reported and those are reproduced below in Figure 5. The remaining parameters exhibited similar trends in that, with perhaps the exception of iron, there was no apparent correlation between grain size and amount of sorbed contaminant. This finding was contrary to other studies (e.g. Roger et al. 1998), however. Figure 5. Amount of sorbed phosphorous, nitrogen, copper, and iron as reported by (CSU-S 2002). Vignoles and Herremans (1995), Roger et al. (1998), and Andral et al. (1999) found that particles less than 50 microns in diameter accounted for 70-80% of the total suspended sediment load by weight and Roger et al. (1998) showed that the smallest particles had the highest concentration of metals. Furumai et al. (2002) revisited the work of Sansalone and Buchberger (1997) to generate Figure 6 below. The figure shows that particles smaller than 100 microns account for more than 50% of the lead, zinc, and copper pollutant loads but their weight

13 is just over 10% of the total suspended solids load. Furumai et al. conclude that small particles are very important when targeting removal of these metals. Figure 6. Cumulative loads of heavy metals and TSS in road runoff (Furumai et al. 2002). Furumai et al. (2002) also studied road dust and runoff from a road in an 8.4 ha watershed in Winterthur, Switzerland. The road had average daily traffic counts from 25,300 to 73,300. They found that for Zn and Cu the amount of metal sorbed to solids (mg/kg) was higher in runoff samples than road dust while for Pb it was about the same. Furumai et al. (2002) combined data for 24 samples collected during one runoff event into composite samples based on suspended sediment levels and peak runoff patterns. The results are shown in Figure 7 below. The data show that samples with higher suspended solids concentrations contained larger sediment sizes. Regression equations were also fit to the particle size distributions. These results are shown in Figure 8. Figure 7. a) Particle size distribution along cumulative suspended solids load, b) Particle size distribution at different suspended solid concentrations. (Furumai et al. 2002)

14 Figure 8. Regression equations fit to particle size distributions. (Furumai et al. 2002) In a study investigating the performance of three proprietary stormwater BMPs, Al- Hamdan et al. (2007) prepared synthetic runoff by adding sediment to a relatively clean water supply. The sediment added was obtained from streets in Orlando and Tallahassee as well as from a lake in Miami. The particle size distributions of sediment from all three sources and their combined particle size distribution is given in Figure 9. Figure 9. Grain size distribution of sediments used by Al-Hamdan et al. (2007). Bäckström (2002) investigated the performance of grassed swales with regards to retaining sediment. In this study a synthetic runoff was prepared by adding sediment collected from street sweeping in the city of Lulea, Sweden. The sediment was divided into three categories based on diameter, 0-75 microns, microns, and microns. The densities of the first two categories were experimentally determined to be 2610 kg/m3 and 2580 kg/m3, respectively. This study investigated the settling properties of the solids in a column and estimated the settling velocities of particles in the range of 8-50 microns in diameter. Particles

15 smaller than 8 microns did not settle out of the water column during the 1042 minute test. These relationships are shown in Figure 10. Bäckström (2002) found that Stoke s Law could be used to accurately estimate the settling velocity of particles larger than 20 microns in diameter. Smaller particles could not be modeled with Stoke s Law, however, and Bäckström (2002) hypothesized that the deviation from Stoke s Law at lower velocities could be attributed to lower densities, non-spherical shapes, and/or electrostatic forces. Figure 10. Particle settling velocities as a function of settling diameter (from Backstrom 2002). Cleveland and Fashokun (2006) monitored the suspended sediment in water samples collected from a ditch adjacent to a road widening construction project in Harris County, Texas. Some samples were collected after a rain storm (i.e. storm samples) and other samples were collected when there had not been a recent rain storm (i.e. non-storm samples). The ditch was monitored upstream and downstream of a rock filter dam before, during, and after the construction project. The particle size characteristics of the stormwater runoff at the two locations are shown graphically in Figure 11. The authors concluded that construction activity increases total suspended solids concentrations by a factor of about two for non-storm samples and a factor of about six for storm samples. These findings were consistent with, but lower than, other studies. It was also concluded that there was no statistical difference between total suspended solids concentrations upstream and downstream of the rock filter dam.

16 Figure 11. Particle size characteristics of water samples upstream and downstream of a rock filter dam (Cleveland and Fashokun 2007). Jacopin et al. (1999) reported on the sediment trapped in a settling basin in France. Details on the particle size distribution of the trapped sediment are reported but since they are not of the sediment contained in the runoff they are not reproduced here. The investigation found that the particle size distribution of retained solids is dependent on the number and frequency of the storm events in that the percentage of fine particles increased with a higher number and frequency of runoff events. An investigation into the clogging of infiltration systems by Siriwardene et al. (2007) used semi-synthetic stormwater with characteristics typical of urban stormwater. The particle size distributions of sediment in the semi-synthetic stormwater are reported in graphical form and the paper references Hatt et al. (2006) and Duncan (1999) as the source for typical particle size distributions. Unfortunately, between these two referenced documents the only reference to particle size distribution is one sentence in Duncan (1999) that states that the greatest mass of suspended particles in runoff is usually in the 1-50 micron size range but that larger particles can occur. In an communication with Hatt (2007) it was learned that the target mean particle size of 150 microns was desired in the work done by Siriwardene et al. (2007) and that this was based on a figure in Walker and Wong (1999) that is reproduced as Figure 12. The open circles in Figure 13 show the particle size distribution in the semi-synthetic stormwater prepared by Siriwardene et al. (2007).

17 Figure 12. Particle size distribution of suspended solids in road stormwater runoff (Walker and Wong 1999). Figure 13. Particle size distributions determined by Siriwardene et al. (2007). Vaze and Chiew (2004) investigated nitrogen and phosphorus content bound to solids on a street in Melbourne, Australia. The street had two lanes of traffic in each direction and carried about 3,000 vehicles per day. A major nearby cross street carried about 30,000 vehicles per day. Vaze and Chiew collected dry samples from the road and also collected composite grab samples from six storm-runoff events. For the dry samples, two different analyses were conducted; dry sieving and wet sieving. Dry sieving gives an estimate of the total phosphorus and total nitrogen on the solids. Wet sieving gives an estimate of the total phosphorus and total nitrogen that can be washed off the solids if there is sufficient water to dissolve the pollutants. The particle size distribution of collected dry sediment is shown in Figure 14 and the total nutrient loads as a function of particle size are shown in Figure 15.

18 Figure 14. Particle size distribution of dry street sediment (Vaze and Chiew 2004).

19 Figure 15. Total Phosphorus and total Nitrogen as a function of particle size and wet vs. dry sieving.

20 Similar information related to the sediment found in the rainfall-runoff events is given in Figure 16. Figure 16. Rainfall and total phosphorus and total nitrogen as a function of particle size (Vaze and Chiew 2004).

21 Finally, Figure 17 compares the nutrient loads as a function of particle size for the dry and runoff samples. Figure 17. Total nutrient (N & P) as a function of particle size for dry and runoff samples (Vaze and Chiew 2004). The authors concluded that more than half of the sediment is larger than 300 microns and particles of this size are responsible for less than 15% of the total phosphorus and total nitrogen loads. In this study, dissolved nitrogen accounted for 20-50% of the total nitrogen and dissolved phosphorus accounted for 20-30% of the total phosphorus. Also, the authors determined that practically all of the particulate total phosphorus and total nitrogen in their stormwater samples were attached to sediment particles between 11 and 150 microns. Vaze and Chiew concluded that to effectively remove total phosphorus and total nitrogen, stormwater BMPs must be able to remove particles down to 11 microns. According to Vaze and Chiew (2004) this is a slightly different portioning than experienced by metals but metals are also associated with the smaller particles of city dust and dirt. For example, Vaze and Chiew (2004) reference Pitt and Amy (1973), North Carolina Department of Natural Resources and Community Development (NCDNRCD 1993), Woodward-Clyde (1994), and Chiew et al. (2004) who found that approximately half of the heavy metal pollutant load are bound to particles between 60 and 200 microns and that 75% are bound to particles smaller than 500

22 microns. Also referenced was an article by Dempsey et al. (1993) that found that the highest concentrations of copper, zinc, and phosphorus were bound to sand particles between 74 and 250 microns in size. Li et al. (2005) investigated many aspects of particle size distribution in highway runoff including possible errors, development of a sampling protocol, and the first flush phenomenon. They monitored five runoff events at three different monitoring sites along a major highway in west Los Angeles. The sites ranged from 0.39 to 1.69 hectares in size and all had annual average daily traffic counts of over 260,000 vehicles per day. The study did find the occurrence of a first flush phenomenon. The particle size distributions of their samples were calculated assuming spherical particles of constant density. Results are shown in Figure 18. Figure 18. Particle size distribution contained in highway runoff samples as calculated by Li et al. (2005). Li et al. (2006) further analyzed the data presented in Li et al. (2005) and calculated particle size distributions as shown in Figure 19 in which the continuous lines correspond to the right axis and the other data corresponds to the left axis. From this data, Li et al. concluded that more than 90% of the particles in number are less than 10 microns in diameter and that the mass of this 90% of particles make up less than 10% of the total mass of solids. Li et al. (2006) also found a strong first flush occurring with respect to sediment. Li et al. used Stokes and Newton s Law for settling velocities to analyze a two-compartment settling basin for sediment and sorbed pollutant removal. In a literature review, Li et al. (2006) compared data from previous studies on sediment characteristics and sorbed pollutant concentrations. Those comparisons are reproduced below in Tables 9 and 10.

23 Figure 19. Particle number and mass fraction as found by Li et al. (2006).

24 Table 9. Particle size and corresponding specific gravity (Reproduced from Li et al. 2006). Table 10. Metal concentrations as a function of particle size (reproduced from Li et al. 2006).

25 Zanders (2005) also investigated particle size distributions and concentrations of sorbed metals in road sediment. Samples were collected every two days from an intersection in Hamilton, New Zealand that had, on average, 25,000 vehicles pass through every day. One off sample was collected and analyzed at the beginning of the investigation. The average particle size distribution of six 2-day samples and the distribution of the single off sample are shown in Figure 20. The error bars represent the standard error of the mean. Figure 20. Particle size distribution in road sediment as found by Zanders (2005). Zanders found that, for the 2-day samples, 52% of the particles were less than 250 microns in size and, of this, 36% was less than 125 microns, and 6% less than 32 microns. Zanders (2006) also investigated the mass of sorbed copper, zinc, and lead and the particle density as a function of particle size. Results are shown in Table 11 and Figure 21 below. Table 11. Metal concentrations and particle density as a function of particle size (Zanders 2005).

26 Figure 21. Contribution of particle size to the total metal loads in road sediment (Zanders 2005). Zanders (2005) goes on to state that particles greater than 125 microns are more easily retained by vegetative buffer strips but the retainment drops for particles less than 60 microns and becomes poor for particles between 6 and 32 microns. For Zanders samples, particles less than 125 microns carry 57% of the copper, 64% of the zinc, and 46% of the lead and particles less than 32 microns carry 9%, 13%, and 8%, respectively. Andral et al. (1999) studied a roadway section in the Kerault Region of France. Sediment samples from a collection channel that carried stormwater runoff from the roadway were collected after each of the eight rainfall events monitored. Suspended solids content of runoff samples collected by an automatic ISCO composite sampler were also analyzed. The sediment collected from the channel was filtered through four steel square mesh filters (1000, 500, 100, 50 microns) to determine the size distribution and particles that passed through the 50 micron mesh were analyzed by a laser to determine the particle size distribution as a function of volume of this fraction and the Andreasen pipette and sedimentation column methods were used to determine the particle size distribution as a function of settling velocity. The density of the particles as a function of size was also determined. Figure 22 shows the particle size distributions (volume basis) of the eight channel sediment samples for that portion that passed the 50 micron sieve. Particles greater than 50 microns are shown in Figure 22 because some particles agglomerated into larger particles.

27 Figure 22. Size distribution of channel sediment for particles smaller than 50 microns (Andral et al. 1999). The particle size distribution as a function of settling velocity and the density of the runoff sediment (not channel) as a function of size are shown in Figure 23 and Table 12, respectively. Readers are referred to the original manuscript (Andral et al. 1999) for additional information that was deemed too detailed for this review. a)

28 b) Figure 23. Sedimentation Velocities, a) For particles smaller than 50 microns using the Andreasen pipette method, b) For particles between 50 and 100 microns using the sedimentation column method (Andral et al. 1999). Table 12. Density of runoff sediment as a function of particle size (Andral et al. 1999). Ghani et al. (2000) reports the grain size distributions of sediment in urban runoff for five cities in Malaysia with average d50 values of 0.6, 0.9, 0.8, 0.6, and 0.7 mm. Lygren et al. (1984) studied a segment of roadway in Norway in order to better understand the transport of pollutants from the road. Runoff samples, dust deposits, dust samples, and snow samples were collected. Dust transport profiles and transfer of water and contaminants in a ditch were also examined. Samples were analyzed for compounds such as Ca, Cd, Cu, Zn, Pb, Ni, Cr-T, Hg, SO4, suspended matter, COD, TOC, PAH, etc. and sedimentation characteristics of the particulate matter was determined. Road runoff sample data related to total suspended solids and settling velocity is reproduced in Table 13.

29 Table 13. Total suspended solids data from runoff samples (Lygren et al. 1984). Snow bank sample data related to total suspended solids and settling velocity is reproduced in Table 14. Table 14. Characteristics of solids from a snow bank sample (Lygren et al. 1984). This results of this study suggested that a majority of PAHs are transported to the nearby surroundings as small dust particles ( ug PAH/km vehicle day is deposited within 100 m) and only a small portion are transported via stormwater runoff (<10 ug PAH/km vehicle day). Also, it appeared for this region that approximately equal amounts of some inorganic pollutants (e.g. lead) were carried to the surroundings at dust and in stormwater runoff. Finally, the melting period produced the highest loading of inorganic pollutants. Roberts et al. (1988) investigated the size and surface texture of sediment in an urban watershed. Samples were collected from seven different areas in the watershed, each

30 area having its own unique land use and/or characteristics. Particle size distributions for each of the seven areas are shown in Figure 24. Figure 24. Sediment particle size distributions for different land uses (Roberts et al. 1988). The influence of land use on particle sizes and surface texture is summarized in Table 15.

31 Table 15. The influence of land use on particle size and surface texture.

32 Table 15. Continued

33 Table 15. Continued

34 Runoff chemical constituents such as metals, phosphorus, and polycyclic aromatic hydrocarbons (PAHs) can partition to PM and distribute across the PSD (Sansalone and Buchberger 2007, Roger et al. 1998). Walker and Wong (1999) noted that a prior study found 70% of oil and approximately 85% of polycyclic aromatic hydrocarbon (PAH) to be associated with solids in the stormwater. This study demonstrated that over a period of dry weather conditions, increasing proportions of oil become solid associated where the highest oil content was found in sediments of 200 to 400 µm in size. Roger et al. (1998) found that the proportion of organic matter increased with decreasing particle size. Schorer (1997) reported that both polychlorinated bi-phenyls (PCBs) and PAHs show an affinity for organic material. Results indicated the presence of two primary types of organic material bimodally distributed with maximum measurements recorded for fine silt and fine sand fractions. Schorer also found that heavy metal content increases with decreasing particle size, due to the high number of inorganic exchange sites in the particle fraction. Conclusions Both large and small particles can carry a significant heavy metal load. In general, small particles (< 150 μm) carry a more significant nutrient load. PCBs and PAHs tend to associate with organic material, much of which is found in finer particle fractions, but portions of which have been found in larger (>100 μm) fractions. Little research was found which correlated pathogenic bacteria with particle size distributions. It is posited here that the process of separating particles into size fractions poses a technical problem for obtaining viable bacterial samples a technical problem which does not seem as yet to have been overcome. In brief, the relative importance of specific particle sizes is dependent upon objectives. Unless specific objectives are determined in advance, it is clear that researchers and practitioners seeking to determine particle size distribution should approach the problem with measurement range as large as feasible. It is also clear that organic content plays a significant role in particle density, settling velocity, and pollutant-binding tendencies, and as such should play a role in any particle size distribution investigation. The vast majority of studies focused on freeways and urban highways, with some focus urban residential areas and intersections. Caution must be made, as such a heavy focus on roadways may not yield results that translate well to disparate land uses and even land uses which may at first glance seem similar. Additional investigation appears to be needed for land types such as agriculture, forestry, construction, industrial development, commercial development, rail facilities, airports, urban and new urban development, as well as suburban and exurban development.

35 Section II: Stormwater PSD Sampling Sampling Methods Kayhanian and Stenstrom (2008) noted in a review of previous studies that there is no standard protocol for stormwater sample collection. Sampling techniques for the surveyed studies can generally be divided into two types: dry collection versus wet collection. Dry collection generally takes the form of vacuuming roadways or other surfaces of interest. Wet collection can take several different forms: Automatic samplers (many studies) Grab sampling (Li et al. 2005) Trap collection (Jacopin et al. 1999) Entire-event capture (Kim and Sansalone 2008) Entire-stream sieving, entire event or partial event (Kayhanian and Stenstrom 2008) Automatic samplers use a pumping mechanism attached to a tube to draw and deposit stormwater into sampler containers. All samples may be combined into one storage container (composite sampling), samplers may be taken at a certain flow-volume interval (flow-weighted sampling), or some combination of the two (flow-weighted composite sampling). Automatic samplers are also used with a manifold attachment. Automatic samplers have the major advantage of being ready to sample at any time of day, with sampling commencing based on rainfall or flow data. Preliminary results presented by researchers at St. Anthony Falls Laboratory at the University of Minnesota have called into question the accuracy of automatic samplers, especially with respect particles larger than 44 to 88 microns. It has been theorized that this is a consequence of the varying relative concentration of particles in the flow column based on the size, shape and density of the particles. Grab sampling is also used to collect samples, as in Li et al. (2005), and continued with Kayhanian and Stenstrom (2008). Kayhanian and Stenstrom noted that grab sampling allows greater flexibility in the use of different sample storage containers and preservation techniques, as well as the advantage of capturing representative samples from the water column outfall. Grab sampling has the drawback of being limited to outfalls of reasonable size and flow rate which are also safely accessible during storm events. Entire-event capture is a method utilized by Kim and Sansalone (2008). The researchers placed a storage tank downstream of a hydrodynamic separator which collected runoff from a portion of elevated highway in Baton Rougue, Louisiana. As shown in Figure 25, the hydrodynamic separator captured the majority of the larger, settleable particles, while the storage tank captured the remaining effluent. This entirestream collection method has several advantages, namely the collection of all of the

36 runoff from a storm event, the separation during the storm event of particles into two major classes (sediment vs. settleable and suspended). The latter allows more accurate analysis of the coarse fraction. Disadvantages of entire-event capture include the need to provide sufficient storage volume for expected event volumes. Except in limited situations (such as with elevated structures) providing storage at an elevation appropriate to capture large runoff quantities is often cost-prohibitive. Figure 25. Schematic profile of the experimental setup and runoff collection system (Kim and Sansalone 2008) Entire-stream sieving is a sampling method where the entire runoff flow is passed through a sieve or sieve-like device. Such a device often takes the form of a sock with mesh of a certain size. Entire-stream sieving has been used for gross pollutant studies, such the litter collection devices with maximum opening size of 6000 μm used by Kayhanian and Stenstrom (2008). CSUS-UCD-Caltrans (2002) used a sieve stack as a secondary separation method downstream of a two-chamber settling and infiltration system. However, no studies have been found which use an entire-stream sieve device with a smaller (<100 μm) opening size as a primary capture method either for part of a storm event or for the entire event. Field Techniques Kayhanian and Stenstrom (2008) recommend a sampling method for first flush characterization studies where five grab samples are collected in the first hour, with the first sample being collected as soon as an adequate runoff volume reaches the sampling point and the additional four samples being collected in 15-minute intervals. They recommend continued grab sampling once every hour for the seven hours to follow. For storm longer than 8 hours, they recommend one or two additional samples

37 are recommended. Kayhanian and Stenstrom stress the importance of measuring and recording runoff volume over the entire storm event. Kayhanian and Stenstrom (2008) also note the importance of requires timely presence at site to arrive prior to the start of runoff, as well as accurate weather forecasting to avoid mobilizing for events which do not occur. In the last few years, weather forecasting and real-time radar have become readily available in the field with the reduced cost of wireless internet access and the improved availability of real-time weather data. Factors Affecting PSD Temporal and spatial variation in rainfall can have a significant impact on the rate and distribution of particle wash-off. Sansalone et al. (1998) found that particles in the 2 to 8 micron range are rapidly washed off pavement during rainfall events. Kayhanian and Stenstrom (2008) examined the concept of first flush and characterized stormwater runoff as either mass-limited or flow-limited. In brief, mass-limited runoff is an event where rainfall duration (for a given intensity?) exceeds available particle supply, where flow-limited runoff is an event where rainfall duration (for a given intensity?) is insufficient to remove the available runoff particles. Jacopin et al. (1999) found that percentage of fine particles increases with the number and frequency of storm events. Worth noting is that this study comes from a part of Europe not normally subject to storm events of large intensity. Intensity-durationfrequency theory indicates that intensity tends to decease as event frequency increases (Bedient and Huber 1992). In short, frequent storm events tend to be less intense, meaning larger particles are not as easily mobilized as smaller particles. Related to seasonal first flush, Lygren et al. (1984) found that the melting period produced the highest loading of inorganic pollutants. Kayhanian and Stenstrom (2008) note that the existence of either a storm or seasonal first flush phenomenon may present practitioners with the ability to better reduce pollutant loading by implementing treatment methods which focus on early runoff. Zanders (2005) found that vegetative buffers were effective for retaining certain particle sizes. It was found that particles greater than 125 μm were easily retained, particles less than 60 μm exhibited reduced retainment, and particles between 6 and 32 μm exhibited poor retainment. Study Factors Affecting PSD It can be observed from Figure 26 from Kim and Sansalone (2008) that the measured particle size range often corresponds to an abrupt or semi-abrupt attainment of 100%

38 finer by mass and to a lesser extent the location of 0% finer by mass. When comparing PSDs across studies, one may be inclined to compare common PSD points such as D 15, D 50, D 85. As seen below, these values can vary over several orders of magnitude. How much of this variation can be attributed solely to the measured particle range is unclear. The abruptness of some PSDs suggests that the chosen measurement range may significantly impact the outcome of the particle size distribution. Figure 26. Particle Size Distributions from Various Studies (Kim and Sansalone 2008)

39 Conclusions Multiple sampling methods have been developed for sampling stormwater particles. There is, thus far, no standard sampling method in existence. The use of automatic samplers is most common, however recent research efforts have called into question the accuracy of automatic sampling for particles greater than 44 to 88 microns. There are multiple external factors that affect PSD. Among these are rainfall intensity, duration, and frequency. Also important is the effect of study factors on PSD. Study measurement ranges have the potential to impact particle size distribution. It is recommended that future work focus on developing improved and verifiable methods for sampling stormwater particles. Due to their preparedness and ability to reduce labor hours, automatic samplers are desirable as a means of sampling stormwater particles. It is recommended that improvements be made to improve the accuracy of automatic sampling methods and to develop procedures to verify the accuracy of samplers in the field.

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