This paper, and the associated keynote presentation at ICWMM 2025, review several topics that show how stormwater monitoring has been critical in the development of stormwater quality models. The first example is a timeline of how street cleaning was originally conceived as a stormwater quality control and how subsequent focused monitoring enabled a more accurate representation of its benefits. The next example illustrates scaling issues in monitoring and how data have been used to verify appropriate extrapolations of the information. Finally, a short summary of emerging contaminants of current interest illustrates the need for expanded monitoring to understand these little understood stormwater constituents and how they can be modeled to predict their sources, transport, controls, and fates. Many other topics relating to monitoring and modeling exist obviously, but these examples, mostly from the author’s publications, illustrate the range of some of these issues needing further monitoring and model development. The examples shown are only a small portion of the data collected during the referenced studies, and the full reports should be consulted for further information.

Soil infiltration rate is a critical parameter in the design and evaluation of stormwater control facilities. The aim of this research was to evaluate selected infiltration models (Horton's, Kostiakov's, and Green-Ampt's) used to estimate final soil infiltration rate. Parameters of the models were estimated. The goodness of fit of the three equations for infiltration was tested using the root mean squared error (RMSE). Comparison of the field and predicted infiltration rate evaluated at each time interval indicated that the infiltration rate predicted by the Green-Ampt's and Horton's models were much closer to the measured data.

This study evaluates the performance of various conventional stormwater control measures (SCMs) in mitigating 26 per- and polyfluoroalkyl substances (PFAS), including perfluorooctanesulfonic acid (PFOS), and perfluorooctanoic acid (PFOA). PFAS with carbon chain lengths ≥ 9 exhibited greater particulate affinity, ranging from 35% to 73% of their concentration associated with suspended solids. In contrast, only 17-35% of PFAS with C ≤ 8, which dominated the detected compounds, were associated with particulate phases. As most SCMs studied (biofilters, media filters, hydrocyclones, and a retention pond) heavily rely on particle removal, the treatment of filtered-water PFAS (< 0.7 μm) was poor, with essentially no attenuation. Particulate-phase PFAS removal varied, with some systems, such as the retention pond, effectively reducing concentrations. In contrast, others, including the biofilters and one treatment train (hydrocyclone plus cartridge filters), exhibited unexpected increases, suggesting potential remobilization of particulate-associated PFAS within the system. The retention pond SCM, influenced by evaporation, infiltration, and used for local irrigation, was monitored over a one-year period to assess PFAS persistence, precursor transformation, and the effects of water loss on contaminant concentrations. Analysis of precursor compounds in the pond, including fluorotelomer sulfonates (FTS) and perfluoroalkyl sulfonamides (FASAs), showed decreased concentrations relative to runoff. When coupled with increased concentrations of stable products such as perfluorohexanesulfonic acid (PFHxS), PFOS, and PFOA, these changes suggested in situ transformation due to the extended retention time in the system. Additionally, the concentration of PFAS associated with the filtered water (e.g., PFBA, PFOA) increased over time due to evaporation of water during periods of minimal stormwater inputs or discharges. Perfluoroalkane sulfonic acid (PFSA) concentrations (primarily PFOS) in sediments were higher than those in the incoming particulate phase, suggesting sorption and accumulation of PFOS in sediments. The substantial filtered water fraction of PFAS and the limited removal of PFAS by any of the SCMs underscore the limitations of conventional SCMs in treating PFAS.

This study evaluates the effectiveness of various stormwater control measures (SCMs) in removing polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), both in dissolved forms and associated with different-sized solids. The SCMs evaluated include biofilters, a hybrid biofilter + media filter, a retention pond, and treatment trains with hydrodynamic separators and cartridge filters. The targeted particle size fractions were clay (0.7-2.7 mu m), fine silt (2.7-20 mu m), coarse silt (20-63 mu m), and sand (>63 mu m), along with their associated PAHs and PCBs. Samples were collected from multiple storm events at the inlets and outlets of these SCMs on current and former military bases in the Southwestern and Northwestern US. The study found that coarse particles (>20 mu m) contained significantly higher fractions of organic carbon (f(oc)), which correlated with higher concentrations of contaminants in these particulates. All SCMs effectively reduced particulate-bound contaminants, especially within the coarse particle fractions, but the removal of aqueous phase contaminants was generally minimal. Despite the overall effectiveness of the SCMs, maintenance challenges-such as biofilter erosion and insufficient cleanout of cartridge filters-can hinder their performance. The study highlights the importance of considering particle size and its relationship to contaminant distribution to comprehensively assess the performance of stormwater control measures and the potential for sediment recontamination.

This study focused on understanding the influence of stormwater on sediment recontamination and biota bioaccumulation. Stormwater and sediment sampling was conducted at Paleta Creek San Diego, CA, a mixed-use urban watershed. Stormwater samples were size fractioned into clay, silt, sand, and filtered fractions to better understand the association of PAHs and PCBs with different particle sizes. Settling traps were deployed during storm seasons to capture solid particles deposited by stormwater. Bioaccumulation studies of the receiving water sediments were conducted on bent-nosed clams (Macoma nasuta) both in-situ and ex-situ. The results indicated that the majority of PAHs and PCBs were discharged during the early stages of the storm and were primarily associated with coarser particles (>20 μm). Sediment near the discharge exhibited higher levels of contaminants, consistent with the findings from the settling traps and stormwater discharges. Ex-situ bioaccumulation studies demonstrated that sediment contamination did not correlate with bioaccumulation in Macoma nasuta, whereas porewater accurately reflected bioaccumulation trends. This study highlights the importance of considering stormwater discharge patterns and particle sizes when assessing sediment recontamination but also that the resulting bulk sediment contamination may not reflect bioavailability as measured by organism bioaccumulation. These insights contribute to a better understanding of the impacts of stormwater runoff on sediment and biota in southern California coastal watersheds, aiding in the development of effective management strategies.

Although biofilters and bioretention controls have been extensively studied and are encouraged as effective stormwater controls, there is remaining uncertainty concerning their long-term performance and required maintenance. Most full-scale monitoring efforts have been of short duration, with no clogging or breakthrough observed. This paper describes the long-term monitoring results of media-based stormwater controls located at the Santa Susana Field Laboratory (SSFL) in Ventura County, California, an industrial site with historic aerospace and energy research. The 10 stormwater controls examined in this paper were grouped into four types (referred to as culvert modifications, which are media filters installed at drainage road crossings, detention bioswales having large subsurface storage, a large sedimentation pond/biofilter treatment train, and a sedimentation tank/media filter treatment train). Data were available for about 6-9 years of monitoring. This paper examines the trends in performance and usage until major maintenance is required. In almost all cases, the effluent concentrations tracked the influent concentrations, with no significant performance or effluent concentration differences with time. Silt clogging at one facility occurred as predicted based on initial laboratory tests, and the media in that control was replaced on schedule. (C) 2021 American Society of Civil Engineers.

This paper focuses on specific information needed to model various aspects of the pollutant retention processes in stormwater biofilters. Updates currently being incorporated in WinSLAMM (Source Loading and Management Model) are building on expanded data from laboratory and field research mostly conducted by Pitt’s research group at the University of Alabama, Tuscaloosa, by Dr. Shirley Clark’s research group at Penn State–Harrisburg, and the Wisconsin Department of Natural Resources and the U.S. Geological Survey (DNR/USGS). These processes and data can be used in manual calculations or other models. Extensive summaries of these data sources and associated statistical analyses, plus additional references, are included in an online white paper available at the PV & Assoc. website. These tests were conducted to provide the details needed for modeling the performance of biofilters, specifically focusing on: methods to predict treatment flow rates through the media; particulate and associated particulate bound pollutant retention for several particle sizes; maintenance requirements due to sediment clogging and pollutant breakthrough; and retention of filterable pollutants. This multiyear research program also examined issues not commonly described in the biofilter performance literature, such as failure due to excessive salt loadings on media having large amounts of fines, problems associated with compaction of the media, and leaching of previously captured material from the media.

Stormwater biofilters and bioretention controls have been extensively studied and are commonly encouraged as effective stormwater controls. Most of these controls incorporate media to enhance pollutant removal. There is much published research describing laboratory tests on the performance and selection of different media, and many field tests on media treatment of stormwater, but complementary long-term, full-scale field tests compared to prior laboratory tests are not as common. The purpose of this paper is to compare the laboratory evaluations of treatment media targeting a broad range of constituents of concern, to the results of long-term monitoring of full-scale stormwater controls using the selected media. The laboratory evaluations identified which media blend would reduce concentrations of constituents of concern to help meet National Pollutant Discharge Elimination System (NPDES) permit discharge limits and benchmarks, given site specific influent concentrations, when deployed in the field. Besides pollutant reductions, treatment flow rates and maintenance requirements were also of a consideration when selecting the media blend, especially determining the useful life of the media before clogging and/or pollutant breakthrough. Statistical comparison tests of the laboratory results with the full-scale field results resulted in similar effluent quality for most measured constituents (especially for lead and the critical form of dioxin), at all field treatment locations. Therefore, the laboratory observations were confirmed by the long-term, full-scale, stormwater control monitoring activities. The selected media has resulted in reductions in concentrations of the constituents of concern, over long periods of use. (C) 2021 American Society of Civil Engineers.

Recently, the role of urban trees in stormwater management has received increasing interest. The interception of rainfall by urban trees has been proposed to p…

Recontamination of sediments by stormwater is a major concern when evaluating the potential effectiveness of sediment remediation. Stormwater and sediment sampling were conducted in a mixed-use watershed at Paleta Creek in San Diego, CA to evaluate methods for assessing sediment recontamination by metals. Size-segregated stormwater contaminant loads with simultaneous receiving water and sediment measurements were used to identify dominant sources and contaminants with respect to their impact on sediment recontamination. Most of the stormwater contaminant loads of Cd, Cu, Pb, and Zn were associated with residential and highway sources from the upstream portions of the watershed and As, Ni and Hg were more significantly influenced by the down-stream area of the watershed. Cd was strongly associated with large particles (>63 mu m) and observed to settle in near shore areas with some attenuation due to mixing and dilution. Cu, in contrast, was associated more with the filtered fraction (<0.45 mu m) and clay fraction (0.45-5 mu m), resulting in less near shore sediment recontamination. Depositing sediment and other metals, particularly Cu and Hg, exhibited greater accumulation in settling traps than could be attributed to stormwater loads indicating the importance of other sources or resuspension of bay sediments on surficial sediment concentrations. Pb, Zn, Ni, and As showed influences of both stormwater and other sources. The study showed that measurement of size-segregated stormwater contaminant mass and concentrations combined with simultaneous measurements of deposition in sediment traps could differentiate between recontamination by stormwater and that of other sources. (C) 2020 Elsevier B.V. All rights reserved.

Early management of combined sewer overflows (CSOs) incorporated sewer separation, large-scale storage, and rapid, but partial, treatment. There is increasing interest focusing on green infrastructure (GI) stormwater controls to alleviate CSO magnitudes and frequencies. GI encompasses many elements of urban infrastructure and usually stresses water and wastewater systems. Mostly, GI components for CSO management incorporate infiltration of source stormwater flows, along with possible retention and beneficial use components. There have been many monitoring projects examining the performance of small individual GI controls (such as biofilters, green roofs, and porous pavement for the treatment of stormwater flows). Many modeling efforts have demonstrated the potential of large-scale use of these controls for CSO control, but there have been few monitoring efforts examining complex implementations of these controls in large drainage systems. This chapter reviews two such large-scale projects, located in Cincinnati, Ohio, and Kansas City, Missouri, USA, which monitored GI performance in combined sewer systems, indicating expected performance and potential shortcomings. Recommendations are also provided to reduce some of the monitoring problems observed during these projects. GI infiltration controls were retrofitted in seven different test locations, ranging in area from 8 to 40 ha and included porous pavement sidewalks and parking lots, curb cut biofilters, grass swales, rain gardens, bioretention systems, and stormwater harvesting systems. These projects not only demonstrated significant flow reductions but also highlighted the need for widespread implementation of stormwater runoff controls affecting most of the site runoff to achieve the large flow reductions necessary to meet typical CSO reduction objectives. The use of GI controls in new developments would be much more efficient as their use and placement can be better incorporated in the site design.

The Santa Susana Field Laboratory (SSFL) occupies about 2,850 acres and is located in Ventura County, California. The site is jointly owned by the Boeing Company and the federal government (the National Aeronautics and Space Administration administers the federal portion of the property). Much of the site was historically used as a rocket engine testing and energy research facility from 1949 to 1998. The site stormwater discharges are permitted by the Los Angeles Regional Water Quality Control Board through an individual industrial NPDES permit that includes numeric effluent limits for a wide range of constituents, including dioxins and metals. A large portion of the site uses distributed source stormwater controls with natural treatment systems utilizing chemically active media. As part of this approach, extensive research was conducted to develop a robust media for use in these controls to meet the discharge objectives. This paper describes the development of the media and its characteristics.

The design and performance of stormwater controls is affected by the treatment flow rates of bioretention media. This article presents the results of a large number of laboratory column tests conducted to examine the treatment flow rates for various mixtures of stormwater bioretention media. Statistical analyses were conducted to identify the treatment media having targeted treatment flow rates. It was found that the bioretention media treatment flow rates were most affected by the median particle size (D50 ) and uniformity coefficient (Cu ) of the media, and the amount of organic matter. Statistical models were developed to evaluate and compare the treatment flow rates for various bioretention media mixtures. The findings of previous research (Sileshi, 2013) using two level, four factors (24 , with varying texture [T], uniformity [U], organic content [OC], and compaction [C]) full-factorial experiment study indicated that T and U of the media mixture have the greatest effect on the measured final infiltration rates of the media, followed by interactions of T and U; C; interactions of T and OC of the material; and interactions of U and OC of the material. As expected, media containing primarily larger particles (higher sand percentage) and, that is, uniformly graded (small uniformity coefficients) had the largest treatment flow rates. Compaction had minor effects if the organic matter content was low, but had significant effects on the flow rates for high organic matter content. PRACTITIONER POINTS: Bioretention media treatment flow rates were most affected by the median particle size and uniformity coefficient of the media, and the amount of organic matter. Statistical models were developed to evaluate and compare the treatment flow rates for various bioretention media mixtures. The media containing primarily larger particles and, that is, uniformly graded (small uniformity coefficients) had the largest treatment flow rate.

Underdrains are commonly used in biofilter stormwater treatment devices in areas having low soil infiltration rates. These usually are required to reduce the duration of standing water to less than 1-3days to minimize nuisance conditions and reduce mosquito breeding. The drainage rates in biofilter underdrains can be controlled using a restricted orifice or other flow-moderating component in order to maximize contact time for improved water quality while still meeting the standing water criterion. Restricted orifices are of concern due to clogging potential, because they usually are very small in order to provide sufficient contact time with the treatment media for water quality benefits, or to detain the water for a significant time when used as part of green infrastructure components in areas that have combined sewers. If the orifices are large to minimize clogging, they are less effective for flow rate reductions or for maximizing water quality improvements. This study examines the flow capacity and clogging potential of an alternative belt drain underdrain device. The sediment-carrying capacity of the slowly flowing water entering a belt drain is very low and not capable of transporting particulates into the underdrain material due to its very low Reynolds numbers during normal operating conditions. The corresponding average Reynolds numbers based on the mean-flow velocities and the drain belt microchannel diameter range from 7 to 700. The results of these tests indicate that the drain belt provides an alternative option for biofilter underdrains, showing minimal clogging while providing very low discharge rates. (C) 2018 American Society of Civil Engineers.

Stormwater bioinfiltration systems can be effective options for the treatment and disposal of stormwater runoff from urban areas. However, the performance of these systems and other infiltration devices can be affected by factors such as texture, structure and degree of compaction of the treatment media. This study provides insights on media characteristics of a poorly operating biofilter facility located in Tuscaloosa, AL, along with supporting laboratory investigations. Double ring infiltrometer tests and soil compaction measurements were conducted along a large biofilter to determine the in-situ infiltration and compaction characteristics of the media. Infiltration measurements were also made during actual rain events by observing falling water levels in ponded areas. The effects of different compaction levels on the infiltration rates through the soil media were also examined during controlled laboratory column tests for comparison to the field observations. Similar tests were also conducted examining compaction effects of the media after mixing with varying amounts of filter sand to investigate restoration options. These results indicate that soil compaction results in increased bulk densities, decreased moisture capacities and has dramatic effects on the infiltration rates.