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Water and Environment Journal. Print ISSN 1747-6585

The USEPA’s distribution system water quality modelling program: a historical perspective Robert M. Clark Environmental Engineering and Public Health Consultant, 9627 Lansford Drive, Cincinnati, Ohio, USA

Keywords drinking water; hydraulics; modelling; network modelling—clean water; regulation; simulation modelling; water; water quality. Correspondence R. M. Clark, Environmental Engineering and Public Health Consultant, 9627 Lansford Drive, Cincinnati, Ohio, USA. Email: [email protected] doi:10.1111/wej.12132

Abstract Hydraulic and water quality models have become widely used to understand both the hydraulic behaviour, and the fate and transport of contaminants in drinking water distribution systems. Research conducted by the United States (US) Environmental Protection Agency (EPA) played a major role in the development and application of hydraulic/water quality modelling in the United States and throughout the world. Eventually this research led to the development of EPANET, an integrated hydraulic/water quality model, and had a major influence on the implementation of the United States Safe Drinking Water Act (SDWA). The modelling research conducted by the US EPA has helped many drinking water utilities throughout the world alleviate public health threats due to the deterioration of water quality in drinking water networks. The US EPA has provided over 100 000 downloads of the EPANET software over the last 2 years.

Introduction Hydraulic and water quality models have become widely used to enhance water supply professional’s understanding of both the hydraulic behaviour, and the fate and transport of contaminants in drinking water distribution systems. Even though the availability of these tools for wide application is recent, they have become adopted by both large and small utilities and even imbedded in the United States (US) Safe Drinking Water Act (SDWA) (Clark 2012, USEPA 2006). Understanding the fate and transport of contaminants in drinking water systems can have major public health implications. For example, from a public health perspective it is extremely important for water system managers to understand the factors that result in the loss of disinfection residuals in networks. Research conducted by the U S Environmental Protection Agency (US EPA) has helped provide a basic understanding as to how water quality can deteriorate due to the construction, design and operating philosophies associated with drinking water distribution systems. It has played a major role in the development and application of hydraulic/water quality modelling in the United States and throughout the world. Prior to the passage of the SDWA, most drinking water utilities in the United States concentrated on meeting drinking water standards at the treatment plant, even though it had long been recognized that water quality could deteriorate in the distribution system. After its passage, the SDWA was interpreted by the US EPA as meaning that federal water 320

quality standards should be met at the ‘consumers tap’ rather than at the entry to the distribution system. Consequently, water quality in the distribution system became a focus of regulatory action and a major interest to drinking water utilities in the United States. This article will discuss the research and studies conducted by the US EPA leading to the development and validation of hydraulic and water quality models designed to track the fate and effect of regulated contaminants and the fate of other water quality parameters such as disinfectant residuals in drinking water networks. It will also discuss the intended application of this research to help understand public health problems associated with contaminated drinking water and the many extensions and modifications that have been applied to this research.

Creation of the USEPA and passage of the SDWA The US EPA which was created on 9 July 1970 incorporated a number of functions that had previously been located in different organizations within the US Government: (http:// www2.epa.gov/aboutepa/reorganization-plan-no-3-1970\) including: • Functions carried out by the Federal Water Quality Administration (from the US Department of the Interior). • Functions carried out by the National Air Pollution Control Administration [from the US Department of Health, Education and Welfare (DHEW)].

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• Functions carried out by the Bureau of Water Hygiene, (from the US DHEW). The reorganization created a Federal agency with the ability to respond to environmental problems in a comprehensive manner. It was tasked to develop regulations and compliance strategies as well as to develop the capacity to do research on important pollutants irrespective of the media in which they appeared, and on the impact of these pollutants on the total environment. As part of the reorganization, four independent National Environmental Research Centres (NERCs) were created in Cincinnati, Ohio, Research Triangle Park in North Carolina, Corvallis, Oregon and Los Vegas, Nevada. All of the US EPA drinking water research activities were assigned to the Water Supply Research Laboratory (WSRL) which was part of NERC-Cincinnati. While the US EPA was being created, the US Congress was also considering the passage of a comprehensive SDWA. The SDWA which was eventually promulgated in 1974 became the primary federal law ensuring the quality of American’s drinking water and established the first set of national standards for drinking water. The SDWA was unusual because it emphasized that cost should be taken into consideration when setting standards and also incorporated the idea of a comprehensive, regional or system wide approach to setting drinking water standards (http://water. epa.gov/lawsregs/rulesregs/sdwa/). After passage of the SDWA the mandate to consider both cost and quality and to incorporate a system wide approach when setting drinking water standards, posed a unique research challenge. Prior to the passage of the SDWA only limited research had been conducted that focused on the deterioration of water quality between the treatment plant and the consumer. There was, however, growing awareness that water quality could deteriorate significantly between the treatment plant and the consumer (Geldreich 1996). After passage of the SDWA, NERC-Cincinnati initiated studies attempting to characterize costs and the relationship of costs to quality fate, effect and transport of water in a network. A major issue was the rudimentary state of hydraulic modelling, which is a key factor in understanding water quality fate, effect and transport. Early studies were conducted in collaboration with the Greater Cincinnati Water Works (USA) to document the cost characteristics associated with transporting water in the network and to develop a spatially based cost accounting system (Clark et al. 1979, 1982). In these early stages of the US EPA research, it was discovered that hydraulic models provided critical flow information that is essential for modelling distribution system water quality. In these early research efforts stand-alone hydraulic model were used to generate a file containing hydraulic flow conditions that were then used in a stand-alone water quality model. Eventually research conducted by the Cincinnati EPA Laboratory led to the development of EPANET and had a major influence on the Initial


The USEPA’s distribution system research program

Distribution System Evaluation provisions of the SDWA (Clark 2012).

US hydraulic modelling research In the United States, mathematical methods for analyzing the flow in networks had been in use for over half a century (Cross 1936). These early hydraulic models were developed to simulate flow and pressure under steady-state conditions. Steady-state modelling represents external forces as constant in time and determines solutions that would occur if the system were allowed to reach equilibrium (Wood 1980a). Cross’ model was based on manual techniques. Computer-based models for performing hydraulic analysis were first developed in the 1950s and 1960s and were greatly expanded and made more widely available in the 1970s and 1980s (Wood 1980b; Sarikelle et al. 1989). These models were expanded to include time varying demand and operational conditions, and are generally referred to as extended period simulation (EPS) models. At the time of the US EPA studies, there were several models available in the public sector for steady-state modelling (Jeppson 1976; Gessler & Walski 1985). Hydraulic models may also incorporate optimization components to aid the user in selecting system parameters that result in the best match between observed system performance and model results (Gessler & Walski 1985). The theory and application of hydraulic models is discussed in many widely available references (Larock et al. 2000; Walski et al. 2003; American Water Works Association 2004).

US water qualty modelling research Research into the use of water quality models developed quickly. Most of these early models were based on steadystate formulations (Clark et al. 1986). Wood (1980b) suggested the use of models to study the spatial patterns of slurry flow in a pipe network. Males et al. (1985) used simultaneous equations to calculate the spatial distribution of variables that could be associated with links and nodes such as concentration, travel times, costs and other variables in drinking water distribution systems. This model, called SOLVER, was a component of the Water Supply Simulation Model (WSSM), an integrated data base management, modelling and display system that was used to model steady-state water quality in networks (Clark & Males 1985). A similar formulation was later used in a 166-link representation of the Alameda County, California, Water District with three sources of water of differing hardness (Chun & Selznick 1985; Metzger 1985). Males et al. (1988) and Clark et al. (1988b) introduced techniques for calculating spatial patterns of concentrations, travel times, and the percentage of flow from sources. In this 321


approach, links are hydraulically ordered starting with source nodes and progressing through the network until all nodes and links are addressed. Alternative methodologies for predicting water quality and determining the source of delivered flow under steadystate conditions were investigated by Wood & Ormsbee (1989). They found that an iterative cyclic procedure was both an effective and efficient method. This procedure was similar to the incremental solutions described previously for networks that are termed as source dependent (i.e. networks in which the nodes can be hydraulically sequenced starting from sources). However, for nonsource-dependent networks, which are rare, their algorithm iterated until a unique solution was found. Although, steady-state water quality models proved to be useful tools for investigating the movement of a contaminant under constant conditions, the need for models that represented the dynamics of contaminant movement was recognized. In the mid-1980s, several models that simulate the movement and transformation of contaminants in a distribution system under time varying conditions were developed and applied (Kroon & Hunt 1989; Kroon 1990). Three such models were initially introduced at the American Water Works Association (AWWA) Distribution Systems Symposium in 1986 (Clark et al. 1986; Hart et al. 1986; Liou & Kroon 1986). Grayman et al. (1988a,b) developed and applied a water quality simulation model that used flow results previously generated by a hydraulic model and a numerical scheme to route conservative (i.e. concentration does not degrade with time) and non-conservative (i.e. concentration changes with time) contaminants through a network. Kroon & Hunt (1988) developed a similar numerical model originally implemented on a mini-computer and was originally used on a PC-based workstation. This model was directly tied to a hydraulic model and generated both tabular and graphical output displaying the spread of contaminants through a network (Liou & Kroon 1986). Hart et al. (1986) developed a model using the GASP IV simulation language. The usability of these models was greatly improved in the 1990s with the introduction of the public domain EPANET model (Rossman 2000). Males et al. (1985) demonstrated that flow, cost and quality could be fully determined in a network by a set of closedform of equations under steady-state conditions. The system was represented by a link–node configuration (i.e. pipes are represented as links, and junctions of pipes, wells, tanks or starts of pipes as nodes) and an existing hydraulic model was used to determine flows and velocities in links. This concept was then applied to a study conducted by the US EPA in collaboration with New Vienna, Ohio, a village of 1000 people (Clark & Males 1985, 1986). The model was used to characterize both the cost of transporting water in the New Vienna system and the quality variations in the network. 322

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US EPA water quality modelling studies As a next step in validating the WSSM model it was applied to the North Penn Water Authority (NPWA), in Lansdale PA, USA. The project was a comprehensive attempt to investigate the feasibility of water quality modelling development and application, to study the mixing of water from multiple sources.

North Penn study At the time of the study, the NPWA served 14 500 customers in 10 municipalities. Average demand was 5 mgd (19 mL/d) (Clark et al. 1988a). Water sources included 1 mgd (3.7 mL/d) of treated surface water and 4 mgd (15 mL/d) from 40 wells operated by NPWA. The NPWA distribution system was modeled in a network representation consisting of 528 links and 456 nodes. Water demands for modelling represented conditions from May–July 1984. The network hydraulic model used was the WADISO model, which contained provisions for both steady-state and quasi-dynamic hydraulic modelling or EPS (Gessler & Walski 1985). The model was employed to study the overall sensitivity of the system to well pumpage, customer demand and other factors resulting in the development of a number of typical flow scenarios. Fig. 1 depicts the results of an intensive sampling program using total trihalomethanes (TTHMs) as a tracer (solid line). Laboratory and field evaluations demonstrated that the trihalomethanes (THMs) in the Keystone water had reached their formation potential and were relatively stable. Any THMs formed from well sources was relatively minor. Fig. 1 also shows the variation in hardness of these same points [hardness (dotted line) was primarily associated with flow from the wells]. At the Mainland sampling point, a flushing back and forth of water between the surface source and the well sources could be seen. The peaks of the TTHMs at Mainland were approximately 12 h out of phase with the peaks from the wells. This indicated that water flow at this point was affected by the surface and groundwater sources. The results pointed out the problems in attempting to predict a dynamic situation using a steady-state approach and the dynamic nature of water movement in the distribution system. The project was originally oriented towards investigating the feasibility of developing and applying a steady-state water quality model. As the study progressed, it became obvious that the dynamic nature of both demand patterns and water quality variations required the development of a dynamic water quality model (DWQM) (Clark et al. 1988c; Grayman et al. 1988a).


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Fig. 1. Results from sampling program at North Penn at selected sampling sites.

Results from the North Penn study It became obvious form the North Penn study that both the flows and the water quality in the system were very dynamic. Adapting steady-state models to study dynamic water quality behaviour proved to be very difficult, therefore, work was initiated on developing a dynamic water quality model called the Dynamic Water Quality Model (DWQM) (Grayman et al. 1988a). It was also clear that the studies needed to be moved to a more complex system (Clark et al. 1988c). In order to extend the modelling application, USEPA initiated a cooperative agreement with the University of Michigan. Under the agreement, the university began a program with the South Central Connecticut Regional Water Authority (SCCRWA) based in Hartford Connecticut, to test the concepts developed in the North Penn case study including field studies to verify and calibrate the model (Clark & Goodrich 1993).

South central Connecticut regional water authority At the time of the study, the SCCRWA supplied water to approximately 95 000 customers (380 000 individuals) in 12 municipalities in the New Haven, Connecticut, area. The SCCRWA service area was divided into 16 separate pressure/ distribution zones. Average production was 50 mgd (189 mL/d). Surface water sources included Lake Gaillard, Lake Saltonstall, Lake Whitney and the West River system. There were five well fields serving as sources (North Cheshire, South Cheshire, Mt. Carmel, North Sleeping Giant and South Sleeping Giant). Approximately 80% of the water in use in the system came from surface sources, with the remaining 20% from wells. All water was treated using chlorination, filtration and a phosphate corrosion inhibitor. The system included 22 pumping


stations, 23 storage tanks and 1300 mi (2080 km) of water mains. From 13 to 15 August 1991, a sampling program at the Cherry Hill/Brushy Plains service area was initiated to gather information to characterize the variation of water quality in the service area and to study the impact of tank operation on water quality.

Verification study The Cherry Hill/Brushy Plains service area covered 2 mi2 (5 km2) in the town of Branford in the eastern portion of the SCCRWA area (Clark et al. 1993). The service area was almost entirely residential, containing both single-family homes and apartment and condominium units. Average water use during the sampling period was 0.46 mgd (1700 m3/d). The water distribution system consisted of 8inch (200 mm) and 12 inch (305 mm) mains as shown in the schematic in Fig. 2. The terrain in the Cherry Hill/Brushy Plains service area was generally moderately sloping with elevations varying from 50 ft (15.2 m) mean sea level (msl) to 230 ft (70.1 m) msl. Cherry Hill/Brushy Plains received its water from the Saltonstall system. Water was pumped from the Saltonstall system into Brushy Plains by the Cherry Hill pump station. Within the service area, storage was provided by the Brushy Plains tank. The pump station contained two 4 inch (100 mm) centrifugal pumps with a total capacity of 1.4 mgd (5300 m3/d). The operation of the pumps was controlled by water elevation in the tank. Built in 1957, the tank had a capacity of 1.0 mgd (3800 m3/d). It had a diameter of 50 ft (15.2 m), a bottom elevation of 193 ft (58.8 m) msl, and a height (to the overflow) of 263 ft (80.2 m) msl. During normal operation, the pumps were set to go on when the water level in the tank dropped to 56 ft (15.2 m) and to turn off when the water level reaches 65 ft (19.8 m). 323


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the distribution system, as shown in Fig. 2 included all 12 inch (305 mm) mains, major 8 inch (200 mm) mains and loops, and pipes that connected to the sampling sites. Pipe lengths were scaled from maps, actual pipe diameters used, and, in the absence of any other information, a Hazen–Williams roughness coefficient of 100 was assumed for all pipes. Fig. 3 shows the results of the fluoride sampling study and the modelling efforts at each of the sampling nodes. From these results it is clear that the modelling effort matched the sampling efforts quite well with the exception of the dead ends.

Results from the study

Fig. 2. Link–node representation of the Cherry Hill/Brushy Plains network with ‘pumps-on’ scenario.

Results from the study included: • A demonstration of the Dynamic Water Quality Modelling Algorithm. • A demonstration of the use of Fluoride as a conservative tracer. • Provision of a test-bed for future model development. • Demonstrating the effect of pipe wall demand on loss of chlorine residual. • Documenting the effect of storage tanks on loss of disinfectant residual.

Presampling procedures

Development of EPANET

Prior to initiating the SCCRWA study, the WADISO hydraulic model (discussed earlier) and the DWQM were applied to establish flow patterns within the service area. Additionally, during the periods of 21–22 May 1991, 1–3 July 1991, 8–10 July 1991 and 30 July 1991–1 August 1991, chlorine residuals were monitored (using a portable free chlorine analyzer and chart recorder) at the tank and operational patterns (pump records and tank water-level variations) were studied. Based on these model runs and sampling data, a sampling strategy was adopted. This strategy involved turning the fluoride off at the Saltonstall treatment facility and sampling for both fluoride and chlorine in the Cherry Hill/Brushy Plains service area. Defluorided water was used as a conservative tracer for the movement of flow through the system and to calibrate the DWQM. The DWQM and a chlorine decay model based on hydrodynamic principles were used to model the dynamics of this substance. Seven sampling sites in the distribution system, in addition to sampling sites at the pump station and tank, were identified.

Based on the two studies discussed earlier it had become very clear that there was a strong need for development of a model that integrated both hydraulic analysis and water quality modelling into one ‘package’. The US EPA, therefore, began an effort to develop a model that integrated both hydraulic analysis and water quality fate and effect modelling. The SCCRWA study provided a test-bed for development of the model which was called EPANET (Rossman et al. 1994). EPANET was developed with the following characteristics: • It had the ability to characterize a network which can consist of pipes, nodes (pipe junctions), pumps, valves and storage tanks or reservoirs as a link and node network. • It calculated the flow of water in each pipe, the pressure at each node, the height of water in each tank, and the concentration of a chemical species throughout the network during a simulation period comprised of multiple time steps. • In addition to chemical species, it simulated water age and source tracing. • It provided the basis for most of the current proprietary models and non-proprietary models (PIpeLine Net). As part of the development of EPANET a decision was made to make EPANET open-source software. An opportunity to validate EPANET occurred when the North Marin Water District, serving Novato, California, USA, requested that the US EPA provide assistance in meeting

Analysis of sampling results The WADISO hydraulic model and DWQM water quality model were used to simulate the Cherry Hill/Brushy Plains service area for a 53-h period from 9:00 a.m. on 13 August 1991 to 3:00 p.m. on 15 August 1991. This skeletonization of 324


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Fig. 3. Results from fluoride sampling study at nodes 3, 6, 10 and 11.

TTHMP regulations under the SDWA. At the time of the request the NMWD served a suburban population of 53 000 in or near Novato.

Validation of EPANET in the North Marin water district The district used two sources of water-Stafford Lake and the North Marin Aqueduct. The North Marin Aqueduct is a yearround source, but Stafford Lake was used only during the warm summer months when precipitation is virtually nonexistent and when demand is high. Novato, the largest population centre in the NMWD service area, was located in a warm inland coastal valley with a mean annual rainfall of 27 inch (685 mm). There was virtually no precipitation from May through September. Eighty-five percent of total water use was residential and the service area contained 13 200 singlefamily detached homes, which accounted for 65% of all water use (Clark et al. 1994). The Novato service area of the NMWD is shown in Fig. 4. The water quality of the two sources differed greatly. Stafford Lake water had a high humic content. Total trihalomethane formation potential (TTHMFP) levels in the Stafford Lake water were very high. The North Marin Aqueduct water


was technically groundwater and was very low in precursor material with a correspondingly low TTHMFP. Fig. 4 shows the distribution network for zone 1, (Novato) which was the major focus of the study. It shows both sources, a schematic of the major pipes in the service area distribution system, the major tanks and pumps, and the sampling points used in the study. EPANET was applied for the first time to the NMWD system to model the system hydraulics, including the relative

Fig. 4. Network schematic of North Marin of the Novato service area.

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flow from each source, TTHMs and chlorine residual propagation (Rossman et al. 1994). The model was based on an earlier network representation made by Montgomery Watson, for NMWD and was calibrated based on a comparison of simulated vs. actual tank levels for the 27–29 May 1992, period of operation. Fig. 5 shows the predicted values for TTHM against sample results at selected sampling points. Results from the study were used by the NMWD to help meet the requirements of the US SDWA.

Joint studies with the US water industry In addition to the field studies and the development of EPANET, the US EPA organized a major seminar with the AWWA Research Foundation now the Water Research Foundation (AWWARF/US EPA 1991). The seminar was held in Cincinnati, Ohio (USA) from 4–5 February 1991 and attempted to bring together of all the investigators both internationally and domestically working in this area. The symposium created an agenda that became the template for research in the field of distribution system water quality modelling as followed by AWWARF and the USEPA. As a complement to the North Marin study, USEPA and AWWARF (Vasconcelos et al. 1996) initiated a project that

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involved several case study utilities and was intended to gain a better understanding of the kinetic relationships describing chlorine decay and TTHM formation in water distribution systems (Rossman et al. 1996). Several kinetic models were evaluated, tested and validated using data collected in these field-sampling studies based on the EPANET distribution network model as the framework for analysis. Another goal was to provide information and guidelines for conducting water quality sampling and modelling studies by water utilities. Table 1 summarizes some of the major milestones leading to the development of EPANET.

Regulatory implications and implications for public health Water quality models can provide important insight into the factors that cause the deterioration of water quality in drinking water distribution systems. For example, water quality models have been applied to, and have proven effective is solving waterborne outbreaks in Cabool and Gideon Missouri, in the United States and an outbreak in Walkerton, Canada (Clark et al. 1996; Geldreich et al. 1992: Grayman et al. 2004). Cabool, a small town of 2100 people, located in the Southeastern corner of Missouri, USA experienced a large waterborne disease outbreak of Escherichia coli O157:H7 during the winter

Fig. 5. Comparison of modeled values for TTHM against the sampled values for four sampling stations in the network.

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Table 1 Milestones Leading to the Development of EPANETa Key event

Activates and consequences

Timeframe

Passage of safe drinking water act.

Established national drinking water standards. SDWA required standards to be met at the ‘tap’. Conducted studies on the cost of transporting water in distribution system networks. WSSM modeled cost and quality interactions in networks under steady-state conditions. Applied model to New Vienna OH water systems. Developed DWQM based on studies in North Penn Water Authority (NPWA) and applied it in the South Central Connecticut Regional Water Authority (SCCRWA). Utilized results from SCCRWA and applied model to North Marin Water District system to develop EPANET. Made decision that EPANET should be a public sector model. Conducted major seminar with AWWARF to establish an agenda for water distribution studies. Collaborated with AWWARF to apply EPANET to six participating water utilities. Conducted joint studies with the Greater Cincinnati Water Works Developed EPANET-MSX (Multispecies eXtension) and other security extensions.

1974

Ongoing studies to apply EPANET in a real time mode. Developing EPANET-RTX.

2009–present

Water system characterization studies. Developed water supply simulation model (WSSM). Developed dynamic water quality model (DWQM). Developed EPANET

Enhanced EPANET for use in the water industry. Extensions of EPANET including security applications. Application of EPANET to water system real time management. a

1976–1980 1980–1984 1984–1987 1988–1992

1992–2002

2002–2009

The US EPA recorded over 100 000 requests for EPANET software over the last 2 years (Simon 2014).

of 1989–1990, in which 243 cases were reported, with 32 hospitalizations and four deaths. In 1993, the town of Gideon, Missouri, USA (pop 1100) suffered from an outbreak of salmonellosis that affected more than 650 people and caused seven deaths (Hrudey & Hrudey 2004). The first documented outbreak of E. coli 0157:H7/Campylobacter spp. gastroenteritis associated with a municipal water supply in Canada occurred in the small rural town of Walkerton, Ontario (population 5000) in May 2000 (Grayman et al. 2004). Approximately 2500 people became ill and 7 people died. Water quality models have been used to locate water quality monitoring stations and to estimate various types of exposures from drinking water (Lee et al. 1991: Clark et al. 1992. Murphy 1991). Water quality modelling was used as the basis for reconstruction of a historical contamination event in Dover Township, New Jersey and an ongoing epidemiological study at the Marine Corps located at Camp LeJune, North Carolina, USA. In the Dover Township case study the New Jersey Department of Health determined that the childhood cancer incidence rate in Dover Township was higher than expected. A hypotheses was developed that the higher cancer incident rate was related to the higher exposure to public water supplies with documented contamination. The United States Agency for Toxic Substance Disease Registry (ATSDR) developed a water distribution model for the study area using the EPANET software to support this hypothesis. In another study conducted by ATSDR, it was found that from 1957 through 1987, US Marines and their families at Camp Lejeune drank and bathed in water contaminated with toxins. At least 850 former residents have filed claims for nearly $4 billion from the military. No official studies have definitively connected the contamination with illness, however, former residents of


Camp Lejeune suffer from a high rate of cancer and other diseases. Children from the Marines have also shown signs of brain cancer and leukemia. Data collected as part of field studies was used to calibrate a hydraulic and water quality model, based on EPANET, of the Camp Lejeune distribution system (Maslia et al. 2001; Aral 2004a, b) to assess this problem. As a reflection of the importance of water quality modelling in the United States, the SDWA was amended to require that nearly all water utilities that have a disinfectant residual in the distribution system perform an individual distribution system evaluation (IDSE). These provisions require that each drinking water utility provide: • An EPS model that has been recently calibrated using generally accepted methods. • An all-pipes model or skeletonized model of the distribution system. Clearly, water quality models have played and continue to play a major role in protecting public health throughout the world.

Summary and conclusions US EPA research into water quality modelling has had many positive regulatory and operational benefits including the development of EPANET, a public-sector hydraulic and water quality model. The critical decision to make EPANET a publicsector model (open-source model) dramatically increased general interest in network water quality and the US EPA has recorded over 100 000 requests for the software in the last two years (Simon 2014). The research underlying EPANET has played a major role in the development of private sector and quasi-public sector models such as PipeLine Net (Clark 327


2012) and played a role in the determination by US EPA’s Office of Water to determine that the SDWA maximum contaminant levels (MCLs) must be met at the consumer’s tap. US EPA’s water quality modelling research provided the basis for IDSE guidance (USEPA 2006). A number of important conclusions resulted from the US EPA studies as follows: (1) Chlorine decay in distribution systems can occur due to reactions in the bulk phase and at the pipe wall. (2) Pipe-wall reactions related to corrosion of ferrous pipe materials can consume significantly more chlorine than those related to biofilm. (3) Reaction of chlorine at the pipe wall is inversely related to pipe diameter and can be limited by the rate of mass transfer of chlorine to the pipe-wall. (4) A well-calibrated hydraulic model, one preferably based on a tracer study, such as fluoride, is an absolute requirement for a distribution system water quality study. (5) Storage tanks residence-time can significantly increase the loss of disinfectant residuals. (6) Modelling can be used to provide the basis for estimating exposure impacts from contaminants including water borne disease outbreaks and exposure to toxic chemicals. (7) Provided the basis for water security studies. (8) Provided the foundation for development of vendor models (9) Provided the basis for much of US EPA’s in-house work The development of EPANET substantially decreased the computational time required for analyzing complex networks. During the original studies conducted by the US EPA (NPWA, SCCRWA and NMWD), before the development of EPANET analyzing specific scenarios frequently required days of computation. With the advent of EPANET these same scenarios were completed in minutes. There have been numerous extensions to the original development of EPANET. For example, US EPA’s National Homeland Security Research Centre has developed an extension called EPANET-MSX (Multispecies eXtension) that allows for the consideration of multiple interacting species in the bulk flow and on the pipe walls. EPANET-MSX models complex reaction schemes between multiple chemical and biological species in both the bulk flow and at the pipe wall. This capability has been included into both a stand-alone executable program as well as a toolkit library of functions that programmers can use to build customized applications (Janke et al. 2013). EPANET-MSX requires a new input file in which the user specifies the mathematical expressions that govern the reaction dynamics of the system being studied. This allows users the flexibility to model a wide range of chemical reactions of interest. Examples include: • The auto-decomposition of chloramines to ammonia, • The formation of disinfection by-products, • Biological regrowth including nitrification dynamics, 328

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• Combined reaction rate constants in multisource systems and • Mass transfer limited oxidation-pipe wall adsorption reactions. EPANET-MSX is distributed in a compressed zip file that contains a command line executable, several libraries of functions, and a User’s Manual. The executable file can be used to run water quality analyses without any additional programming effort. The function library can be used in conjunction with the EPANET Programmer’s Toolkit to develop customized applications. At this point in time, the software has not been integrated into a Windows interface. This capability has been incorporated into both a stand-alone executable program as well as a toolkit library of functions that programmers can use to build customized applications. This research is being devoted to the development of an extension to EPANET called EPANET-RTX which is an opensource platform for real-time hydraulic and water quality modelling (Janke et al. 2013).

Acknowledgements The research and engineering studies described in this article took place over a span of more than 20 years, and could not have been completed without the advice, and encouragement of many colleagues and associates. I would like to acknowledge Dr. Walter Grayman, Dr. Richard Males, Mr. Benjamin Lykins, Dr. James Goodrich, and Dr. Rolf Deininger, who were involved in all phases of our early research. Dr. Lewis Rossman, developed and validated EPANET; Mr. Harry Borchers, Ms. Judy Coyle, and Mr. David Milan, from the NPWA were instrumental in implementing the first field studies conducted by the US EPA to demonstrate the potential hydraulic/water quality modelling; Mr. Alan Hess, Mr. Ken Skov, and Mr Darrell Smith of the South Central Connecticut Regional Water Authority, provided many useful ideas and were deeply involved in validating many concepts embodied in the research; Ms. Gayle Smalley, of the North Marin Water District, assisted in the first application of EPANET to study total trihalomethane propagation; Dr. John Vasconcelos, of MWH Global, field tested, validated and applied EPANET to a broad cross section of water utilities in the United States; Mr. John Hill of the Missouri Department of Natural Resources and Dr. Fred Angulo, from the Centre for Disease Control and Prevention, helped apply EPANET, for the first time, to a waterborne outbreak. Dr. Paul Boulos, President and COO, MWH Soft was an early participant in our studies, and provided many useful comments related to the development of EPANET. Other colleagues who have contributed substantially to this article are Mr. Robert Janke and Drs. Michael Trybe and Michelle Simon of the US Environmental Protection Agency in Cincinnati.


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