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Watershed Management for Potable Water Supply: Assessing the New York City Strategy 6 Tools for Monitoring and Evaluation The previous chapters have laid the groundwork for analyzing several aspects of New York City's watershed management strategy as embodied in the Memorandum of Agreement (MOA). In conducting its analysis, the committee identified components of New York City's strategy that correspond to the necessary components of a source water protection program described in Chapter 4. In some cases, this was relatively straightforward; in others, corresponding activities were more difficult to identify. The following six chapters consider how effectively New York City is carrying out source water protection and other related activities. Because of the study's broad statement of task, it was necessary to group particular programs and issues for analysis and discussion. Whenever possible, programs were grouped to correspond with specific necessary elements of a source water protection program. The first step in any watershed management program is to set goals and objectives, both numeric and narrative. The stated goals of the New York City MOA are many. First, as with all other source water protection programs, one goal of the MOA is to comply with local, state, and federal statutes that protect drinking water quality. Thus, the City has developed an extensive water quality monitoring program, active disease surveillance, a Total Maximum Daily Load (TMDL) program, and a variety of programs for controlling and treating pollution. These programs, when carried out successfully, contribute toward the delivery of clean drinking water and the maintenance of healthy water supply reservoirs. However, the City's overall goals clearly go beyond those mandated by environmental laws. Supporting economic development within the watershed region is desired, as evidenced by the Watershed Agricultural Program, Watershed Forestry Program, and Watershed Protection and Partnership Programs.
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy Finally, a primary motivation for New York City to draft new watershed rules and regulations was to avoid filtration, which is considered to be an expensive water treatment option for this system. Although cost estimates of filtration differ greatly, all exceed the estimated costs of fully implementing the MOA. This chapter considers two fundamental activities that inform decision-makers about the quality and safety of water from the Catskill/Delaware system. The first activity is water quality monitoring of all sections of the water supply system, including groundwater, streams, reservoirs, and the delivery system. The physical, chemical, and biological parameters being measured, their importance in assessing the condition of the water supply, and their role in water quality modeling are discussed. The Geographic Information System (GIS) developed for watershed inventory and other purposes is critically evaluated. Second, the safety of the drinking water system is considered by reviewing the role of active disease surveillance in New York City and by conducting a microbial risk assessment on the source water. Current activities that are evaluated include the detection of waterborne disease outbreaks and epidemiological studies for determining the proportion of illness attributable to drinking water. WATER QUALITY MONITORING PROGRAM The quality of the drinking water in New York City depends highly on the water quality of the 19 reservoirs of the Catskill/Delaware and Croton watersheds. Reservoir water quality is directly coupled to, and dependent upon, the loadings of pollutants from the individual drainage basins and from atmospheric deposition. Each of the drainage basins of the individual reservoirs combines a unique set of physical, chemical, and biological characteristics. These characteristics—including elevation, geomorphology, rock and soil composition and distribution, soil chemistry, rates of runoff and water residence times, types and extent of plant cover, and human modifications by land use, development, and waste releases—can vary markedly from basin to basin. Because the volume of drinking water required by New York City is so large and entirely dependent upon the aggregate sources of these reservoirs, users rapidly realize changes in reservoir water quality. Therefore, any response by water managers and treatment operators requires rapid acquisition of information concerning changes to the reservoir ecosystems and their drainage basins. The regulatory steps that are taken to control various water quality parameters may be different. In addition, management steps may differ with varying seasonal, meteorological, and human influences. Thus, monitoring frequency must be flexible and respond to the rates of change that are observed for individual water quality parameters. Four types of monitoring are discussed in Chapter 4: compliance monitoring, operational monitoring, performance monitoring, and monitoring to support modeling efforts. Efforts of the New York City Department of Environmental
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy Protection (NYC DEP) in most of these areas are analyzed below, and recommendations for improvement, when necessary, are given. A full description of all monitoring efforts conducted by NYC DEP is available in the Water Quality Surveillance Monitoring report (NYC DEP, 1997a). Compliance Monitoring In order to comply with the Safe Drinking Water Act (SDWA) and the filtration avoidance determination, a variety of biological, chemical, and physical parameters are measured in the New York City source water reservoirs and the water distribution system (Table 6-1). In almost all cases, the New York City water supply has met the requirements for the physical and chemical parameters up to the present time. Compliance monitoring of fecal and total coliform measurements revealed increases in these parameters during the winters of 1991–1992 and 1992–1993 in the Kensico Reservoir. However, violations of the SDWA were avoided by bypassing the Kensico Reservoir (see Chapter 5 for details). Operational Monitoring Operational monitoring refers broadly to activities that are necessary for both short-term and long-term operation of the water supply system. This category, which encompasses much of NYC DEP's efforts, includes both routine activities and special projects (1) to follow changes in water quality and (2) to TABLE 6-1 Frequency with which Parameters are Measured during SDWA Compliance Monitoring Parameter Catskill System Delaware System CAT(LEFF)a CAT(EV)b DEL18a DEL19b Turbidity Continuous Continuous Continuous Continuous pH Daily Continuous Daily Continuous Free Chlorine Residual Not determined Continuous Not determined Continuous Total Coliform Daily Daily Daily Daily Fecal Coliform Daily Daily Daily Daily Temperature Daily Continuous Daily Continuous a CAT(LEFF) and DEL18 are the effluent locations for the Catskill and Delaware systems, respectively, at Kensico. b CAT(EV) and DEL19 are within the Catskill and Delaware aqueducts, respectively, just downstream of Kensico. Note: SDWA compliance monitoring also includes some pesticides within the distribution system, which are not listed in this table. Source: NYC DEP (1997a).
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy identify sources of pollution that affect reservoir water quality. Monitoring of physical and chemical parameters is discussed first (organized by waterbody type) followed by a review of the microbial monitoring efforts that occur throughout the watersheds. Regional Meteorology Meteorological data, including air temperature, relative humidity, rainfall, snow depth, solar irradiance, photosynthetically active radiation (selected sites), and wind speed and direction, are measured at 22 stations throughout the Catskill/Delaware and Croton watersheds at one-minute intervals. This new (1998) network of meteorological stations has been established based on a reasonable set of criteria, including precipitation patterns, elevational gradients, and modeling requirements. Each station contains instrumentation for a large and complete set of meteorological parameters, and the one-minute interval frequency is satisfactory. One could champion for greater meteorological data collection in a region such as the Catskill/Delaware watershed, where topography is quite heterogeneous. However, the new network is greatly improved over the previous system, which measured precipitation only, and it is adequate for most of the eutrophication and public health questions of concern. Groundwater and Shallow Subsurface Monitoring Regular groundwater monitoring has only occurred in the Kensico watershed, as this area has a high potential for contamination related to urbanization (NYC DEP, 1997b). Eighteen monitoring locations exist, consisting of 13 wells, some of which have multiple depths (ranging from 3.5 to 120 ft). These locations, which were selected in relation to geology, land and chemical use, and proximity to sewer lines, are reasonable for the Kensico watershed. All 13 wells were monitored for turbidity, pH, alkalinity, conductance, total and fecal coliforms, and nutrients on a monthly basis between April 1995 and April 1997, and static water levels were measured weekly. Analysis of monitoring data collected during that time period led to biannual sampling of all wells, which has continued to the present time. This frequency of sampling is sufficient for the deep subsurface but not for the shallow subsurface, which is influenced by seasonal variations. Other groundwater monitoring activities associated with special projects are discussed in a later section on performance monitoring. Stream Monitoring Monitoring of stream inputs is critical for determining reservoir water quality and managing reservoir operations. One of the first steps in understanding pollutant dynamics within short-detention-time reservoirs is to sample their tributaries
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy frequently and to construct input–output budgets of key physical and chemical parameters. There are two essential monitoring parameters for streams: (1) volume of influent water to compute loadings and for use in nutrient loading and productivity models and (2) concentrations of critical chemical parameters to evaluate the loadings of nutrients, potential toxic heavy metals, and organic compounds. The locations of the 145 sampling sites throughout the Catskill/Delaware and Croton watersheds include primary river inflows, sites above and below selected wastewater treatment plants (WWTPs) and towns, and outflows from subcatchment basins. Grab samples are generally collected at biweekly intervals and are analyzed for temperature, pH, alkalinity, conductance, dissolved oxygen, major cations (Ca2+, Mg2+, Na+, K+), turbidity, color, suspended solids (SS), nutrients, total organic carbon (TOC), silica, chloride (Cl–), trace elements, and total and fecal coliform, among others. Forty-eight (48) U.S. Geological Survey gauging stations measure water level (stage) continuously. The computerized data acquisition system being developed at present appears to be adequate. Because samples are collected at fixed time intervals rather than on the basis of discharge, it is certain that major fluctuations in the loading of chemical and biological parameters are not captured. This issue, which pertains to stream sampling, precipitation measurements, and sampling of shallow subsurface flows, is discussed below with regard to certain parameters, and it is generally addressed later under a separate section titled Flow Proportional Monitoring. Physical Parameters and Cations. Automation of stream monitoring can ease the transition from fixed frequency sampling to event-based sampling. Within the New York City watersheds, the monitoring of some stream parameters, most notably temperature, conductance, pH, and dissolved oxygen, could easily be automated. This is possible even at remote sites via telemetry of data to data acquisition centers. Although the initial expense to install automation would be high, costs could subsequently be reduced by decreasing the required personnel and by eliminating other analyses. For example, rather than being measured directly, the concentrations of Ca2+, Mg2+, Na+, and K+ could be obtained from strong correlations with conductance within 5 percent to 10 percent accuracy, which would be quite adequate for the purpose of determining hardness and reactivity (Otsuki and Wetzel, 1974). Alkalinity could also be continuously estimated with reasonable accuracy from the parameters measured automatically. Turbidity, Color, Suspended Solids, Nutrients, Total Organic Carbon, and Silica. These parameters are currently measured at stream sites on a fixed biweekly or longer interval. The usefulness and validity of these data, particularly for use in nutrient loading models, are unclear. A fixed biweekly or longer sampling interval is marginally satisfactory for the monitoring of large reservoirs with moderately long (> six months) residence times. However, these sampling
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy intervals are not satisfactory for surface water influents and nonpoint shallow subsurface inflows. Fixed interval sampling will miss significant loading events and will substantially underestimate true loadings. It would be better to thoroughly sample stream inflows at fewer stations with automated discharge-mediated samplers, particularly close to the reservoir inlet mouth, than to sample many stations at infrequent, fixed intervals. In addition to the parameters currently measured, dissolved organic carbon (DOC) should also be measured in streams on a flow proportional basis, as suggested by others (ILSI, 1998). Chloride, Trace Elements, and Toxic Compounds. Chloride and trace element analyses are adequate at present frequencies. Toxic compounds are not measured on a regular basis at stream sites. In 1997, concentrations of several pesticides and polychlorinated biphenyls (PCBs) were monitored at two stream sites in the Kensico watershed; compounds were detected at very low levels (NYC DEP, 1998a). Monthly sampling of streams for pesticides is a planned future activity of NYC DEP that is strongly supported by the committee as a way to determine presence/absence, establish ambient concentrations, and better pinpoint sources. Sampling should be timed to correspond with the application of pesticides and expected pesticide transport by stormwater from rainfall and snowmelt. This activity is needed most in the Cannonsville and Pepacton watersheds because of the density of pesticide application and in the Kensico and West Branch watersheds because of their proximity to the distribution system. Macroinvertebrates. Monitoring of stream macroinvertebrates is done annually in August and September at 14 regular stream sites throughout the entire watershed region and at additional sites that vary in location. This occasional macroinvertebrate sampling is being used as a biotic index of relative stream ''health" based on indicator species. Based on monitoring results, the stream sites are classified as nonimpaired, slightly impaired, or severely impaired. Between 1994 and 1997, 29 sites were nonimpaired, 19 sites were slightly impaired for at least one year, and no sites were severely impaired (NYC DEP, 1999a). Because the present frequency of sampling is too low and the quantitative measures are too marginal to overcome high natural variance, the usefulness of these data in relation to the information accrued and the effort expended is questionable. Aqueduct Monitoring The aqueducts (or tunnels) are sampled at ten locations west of the Hudson River and at 11 locations east of the Hudson River, generally where water enters and exits the tunnels. The sampling interval is daily or weekly, depending on the proximity of the tunnel to Kensico Reservoir. Twenty-seven (27) different parameters are measured within the aqueducts for both operational and compliance monitoring purposes and to support the Process Control-Remote Monitoring pro-
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy gram. Those measured on a daily basis include odor, color, turbidity, temperature, specific conductivity, pH, free chlorine residual, fluoride, and total and fecal coliforms. In addition to the 27 parameters, some pesticides are being monitored on an annual basis at six aqueduct (key point) locations. As part of the Process Control-Remote Monitoring Program, sampling of turbidity, pH, conductivity, temperature, free chlorine, and fluoride is automated at 13 aqueduct locations east of the Hudson River. NYC DEP plans to extend automated sampling to aqueduct key points west of the Hudson River when resources are available. The automated sampling is conducted at a significantly higher frequency than grab samples, and the results are continuously downloaded to chart recorders that display the data. The frequency and analytical techniques used for within-aqueduct analyses of water quality are adequate. Reservoir Monitoring Reservoirs receive collective loadings both from the atmosphere and from their drainage basins. The effects of these pollutant loadings on reservoir water quality are relatively low because loading volumes are significantly less than total reservoir volume. In addition, the residence time of reservoirs is increased compared to streams. Therefore, assuming that withdrawal volumes are relatively small in proportion to total reservoir volume, the frequencies of monitoring for most water quality parameters can be reduced from that of stream monitoring. This reduction, however, should not exceed the generation times of controlling processes, including biological processes. The water supply reservoirs are sampled monthly from late March to early December, with biweekly sampling occurring at some reservoirs. Temperature, pH, dissolved oxygen, and specific conductivity are measured in situ with automated samplers at 1-m depth intervals. Other measured parameters can be found in NYC DEP (1997a). Present sampling includes a depth-integrated water sample (e.g., 1–3 m) from the euphotic or light-penetrating zone, the depth of which may or may not represent an integrated sample of the epilimnion1 during periods when the reservoirs are thermally stratified. Metalimnetic and hypolimnetic samples are taken in the deeper portions of the reservoirs. For the general purposes of assessing water quality in these moderately impacted reservoirs, the spatial sampling sites for monitoring are generally adequate. 1 Reservoirs with moderate to long retention times stratify thermally, with less dense, warner water overlying cooler, more dense water in summer. The water of the upper stratum, the epilimnion, is uniformly warm, circulating, and fairly turbulent. The epilimnion essentially floats upon a cold and relatively undisturbed bottom stratum, the hypolimnion. The intervening stratum between the epilimnion and the hypolimnion is the metalimnion, characterized by a steep thermal gradient from warm to cool water (decreasing at least 1°C per meter of increasing depth).
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy Growing Season Considerations. The in-reservoir "growing season" (March to December) is assumed to be the period during which the reservoir is most susceptible to water quality changes. Thus, there is little reservoir sampling during the winter, except for sampling of the aggregated outflows within aqueducts. This strategy may be acceptable at the present with low mesotrophic to moderate eutrophic conditions in the reservoirs. However, if the productivity increases further, major chemical and biological changes will occur in winter periods that must be monitored. For example, at this latitude 25 percent of the annual primary productivity can easily occur under ice cover of reservoirs. Color, Secchi Depth, and Euphotic Depth. The euphotic depth refers to the depth of water that receives sufficient solar radiation to support net photosynthetic growth of phytoplankton. This depth is usually limited to the penetration depth of 0.1 percent to 1 percent of light reaching surface waters. Secchi depth is an estimate of water transparency and is equal to the approximate penetration depth of 15 percent of surface light. These parameters, along with color, give an indication of algal levels and other color-producing compounds. The color and clearness of the New York City reservoirs is often governed by dissolved organic matter (DOM). In the case of the Catskill/Delaware reservoirs, the DOM responsible for color and clearness originates largely from terrestrial and wetland sources of higher plant decomposition (allochthonous sources) rather than from inreservoir sources (autochthonous sources). This is because only under hypereutrophic conditions are light penetration and euphotic depth appreciably influenced by the densities of algae and cyanobacteria, and such conditions are not found in most of the Catskill/Delaware reservoirs. Much of the loading of allochthonous DOM is directly correlated with precipitation events within the drainage basin. Once in the reservoir, this recalcitrant pool of DOM is biologically degraded at a relatively slow rate. Therefore, biweekly sampling of euphotic depth, Secchi depth, and color should be adequate. Dissolved Organic Matter. Because DOM can serve as a precursor of trihalomethanes (THMs) and other chlorination byproducts, NYC DEP has spent considerable energy investigating the sources of DOM in the water supply reservoirs. These efforts have consisted of continuous reservoir monitoring as well as special activities to measure a variety of parameters, including DOC and THM formation potential (NYC DEP, 1997c; Stepczuk et al., 1998a–c). Although preliminary evidence suggests that autochthonous sources may predominate during certain times of the year (NYC DEP, 1997c), the slower degradability of allochthonous sources means they will be dominant for significant periods, especially during the winter (December through March). This potential switch in the dominance of different sources of DOM implies that monitoring must take place year round rather than just during the growing season. DOM is almost always quantitatively expressed as DOC. In order to deter-
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy mine the pool of DOC available for reaction with chlorine, quantitative assays should be conducted using modern DOC analyses (heat oxidation followed by IR or coulometry to measure the CO2). Measurements of DOC made by the Carlo Erba (EA1108) instrument for CHN analyses (as used by NYC DEP in Stepczuk et al., 1998a) are not satisfactory because the instrument was not designed for such analyses. DOC measurements should occur in depth profiles at regular intervals with a frequency of at least every 2 weeks in the reservoirs. For making such measurements in streams and shallow subsurface sources, the frequencies for such analyses should be based on discharge, not time. It should be kept in mind when conducting such analyses that DOC is, in general, a poor surrogate measure of THM precursor concentration. To complement DOC measurements, selective ultraviolet absorption spectrophotometry has been employed as a general index of DOM concentration in natural waters (American Public Health Association, 1998; Thurman, 1985; Wetzel and Otsuki, 1974). Ultraviolet absorption (UV254) permits a relative measure of the stable recalcitrant dissolved organic components derived from allochthonous sources. However, it is not particularly useful for detecting variations in autochthonous sources because DOM produced by phytoplankton is more labile by one or two orders of magnitude than allochthonous DOM. UV254 measures tend to exhibit little variation over time because the decomposition rates of allochthonous DOM of about one percent per day approximately balance allochthonous influxes. This trend has been borne out by NYC DEP studies; UV254 data collected in Kensico Reservoir effluents on an irregular basis were found to average around 0.036 OD with little variation (S. Freud, NYC DEP, personal communication, 1998). No correlations were found between UV254 and TOC or trihalomethane formation potential (THMFP) values. Total DOC and spectral data yield no information about the sources of DOM. Only a detailed structural analysis of the compounds found in water samples can reveal the relative levels of autochthonous and allochthonous DOM. Allochthonous DOM is in large part composed of yellow organic humic acids of plant origin and consists of a mixture of fulvic acids, aromatic polyhydroxy carboxylic acids, and phenolic residues and polymers of these acids. Such compounds do not originate from phytoplankton algae and cyanobacteria. The more complex organic compounds have been variously categorized on the basis of their structure and their solubility characteristics in acids and bases. Quantitative differentiation of the relative amounts of autochthonous and allochthonous DOM can be estimated by detailed organic chemical analyses using solid-state 13C-nuclear magnetic resonance and gas chromatography–mass spectrometry analyses (e.g., Wetzel et al., 1995; McKnight et al., 1997). Less demanding analyses can be used to estimate the likely proportions of aliphatic, aromatic, and "excess" carbon in a complex mixture of DOM from different sources by evaluating its elemental composition and carboxy1 content (Purdue, 1984; Wilson, et al., 1987). At best, these methods yield approximations of ratios
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy between autochthonous and allochthonous sources and should be coupled to more accurate estimates of external DOM loading to reservoirs. If the NYC DEP wants to determine the relative sources of DOM in the water supply reservoirs, these methods must be employed. In addition to determining sources of DOM, NYC DEP may want to consider regularly measuring disinfection byproduct (DBP) formation potential in the reservoirs, particularly Kensico Reservoir. DBP yields (mg DBP/mg DOC) vary from source to source. Determining the seasonal DBP yields from the outflows of each reservoir would provide information that could help focus control efforts on the most important pollution sources. Temperature, pH, Specific Conductance, and Dissolved Oxygen. In medium to large natural lakes in the temperate zone with depths greater than 10–15 m, temperature, pH, specific conductance, and dissolved oxygen can be evaluated adequately at biweekly intervals. However, caution should be used when applying these monitoring frequencies to reservoirs because they are subject to more rapid changes than natural lakes. Such rapid changes are most common when the collective reservoir volume is moderate or small in relation to inflows and outflows. In most of the New York City reservoirs, changes in inflow and outflow volume can be large in relation to total reservoir volume. Hence, two-week sampling intervals for these four parameters may not be adequate. In particular, sampling might be increased during the week of autumnal turnover,2 which is often quite predictable and is a time of major chemical redistribution. Total Suspended Solids, Volatile Suspended Solids, and Turbidity. Total suspended solids, volatile suspended solids, and turbidity are moderately useful metrics to approximate the loading of organic and inorganic particles. Total suspended solids indicate the presence of inorganic and organic particulate matter, while volatile suspended solids reflect organic particulate matter. Both provide information about particle composition that cannot be derived from turbidity measurements. In most natural lakes, levels of inorganic particulate matter are low in comparison to organic particulate matter. In reservoirs (including the New York reservoirs), however, inorganic particulate matter such as clays is sometimes found in high concentrations, reversing the ratio toward a dominance of inorganic matter. Whether this is true depends upon geomorphology, precipitation events, and other factors. Because levels of total suspended solids, volatile suspended solids, and turbidity are heavily dependent on precipitation events, the biweekly sampling of these parameters that is currently taking place is likely to be of 2 Autumnal turnover refers to loss of thermal stratification and complete water circulation.
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy marginal operational value. However, the sampling will indicate which of the reservoirs are routinely problematic. The more rigorous monitoring done at the aqueducts is more responsive to the aggregate loadings from all reservoir sources and therefore should be continued. Odor. It is assumed that odor measurements are precautionary and that odor is generally not a problem within the reservoirs because of low to moderate production of algae and cyanobacteria. Thus, biweekly sampling is adequate at this time. Nutrients. Chemical analyses, particularly of nutrients, should be performed at frequencies commensurate with changes induced by loadings, biotic utilization and recycling, and losses. In the case of phosphorus, more than 90 percent to 95 percent is found within living and dead particulate organic matter. Soluble reactive phosphorus (SRP) cycles very rapidly (minutes to a few hours), as does much of the soluble organic phosphorus. Hence, the present biweekly measures of total phosphorus and monthly measures of SRP and total dissolved phosphorus (mostly organic) are likely to be adequate for the predictive modeling purposes. Total nitrogen (inorganic and organic, particulate and dissolved) tells one relatively little in any functional sense because of the complexities of the oxidative–reductive interactions among the different chemical compounds. It is assumed (and it is likely correct, although algal bioassays have not been conducted) that nitrogen is not limiting phytoplanktonic productivity in these reservoirs except when phosphorus loading is excessive and the availability of phosphorus exceeds demand. Therefore, determinations of combined nitrogen (nitrate/nitrite and ammonium ion concentrations) are useful in relation to vertical intensities of bacterial metabolism, rates of hypolimnetic oxygen reduction, anoxia and related problems (such as sulfide production and iron reduction), and potential nitrogen limitation under eutrophic conditions caused by excessive phosphorus loading. Biweekly sampling frequency is generally adequate to follow seasonal changes in stratified reservoirs. Chemicals. Chloride is a highly conservative ion of relatively minor limnological interest in freshwater inland lakes. Concentrations nearly always exceed biological requirements, they change little either spatially or temporally, and they are not a problem. Depending on the composition of road salt used in the watershed region, however, the reservoirs may have a chloride gradient. In order to assess this, the present sampling schedules are adequate. Bromide is currently not measured in the reservoirs. However, should New York City decide to install ozonation, bromide should be measured on a regular basis in the Kensico Reservoir using the IC method with a detection limit of 10 µg/L or less. Sulfate ions can become reduced and depleted in the hypolimnia of productive stratified reservoirs, with the production of hydrogen sulfide and related
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy Results: Monte Carlo Simulation Although useful, point estimates of risk do not reveal the degree of uncertainty in the risk estimate. Monte Carlo simulations are currently the most rigorous way to take uncertainty into account during risk assessment, assuming high quality data are available. Summary statistics on 10,000 iterations of the Monte Carlo model are shown in Table 6-11. For this computation, the entire oocyst concentration database (1992–1998) was used. The main result of the analysis is that the mean individual daily risk is estimated as 3.4 × 10-5. For an exposed population of 7.5 million, this would translate into an estimated 255 infections per day. The range of the 95 percent confidence limits would translate into an estimated range of infections per day from 2.6–1,643. It should be noted that the results of the Monte Carlo analysis bracket the range of point estimates observed by considering each year's data set separately, whether maximum likelihood or "fill-in" methods are used. As part of this computation, a sensitivity analysis was conducted. The rank correlation of the individual daily risk with the various input parameters was computed (Table 6-12). The densities of pathogens in the two effluent flows TABLE 6-11 Summary of 10,000 Monte Carlo Trials on Kensico Risk Assessment: Daily Risk of Cryptosporidium Infection (× 10-5) Statistic Individual Daily Risk Daily # of Infectionsa Mean 3.4 255 Median 0.7 53 Standard Deviation 19.8 1,485 Lower 95% confidence limit 0.034 2.6 Upper 95% confidence limit 21.9 1,643 a Based on an exposed population of 7.5 million persons. TABLE 6-12 Rank Correlation of Input Parameters with Daily Risk of Infectiona Input Parameter Rank Correlation with Daily Risk DEL 18 oocyst density 0.61 CATLEFF oocyst density 0.56 Water consumption 0.24 Dose-response "k" value –0.20 a Rank correlation is the correlation between two sets of data when the individual observations in each set are replaced by the rank in that set.
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy from Kensico have the greatest correlation with the estimated daily risk. The other inputs (water consumption, dose-response parameter) contribute only a minor amount to the uncertainty and variability of the estimated risk. This suggests that to reduce the degree of uncertainty and variability in the estimated risk, attention should be paid primarily to obtaining better (more precise) estimates of the effluent oocyst concentrations. Caveats Dose-Response Relationship The above risk assessment has a number of caveats that should be taken into account in making a decision based on these results. First, the dose-response relationship was obtained from a study on healthy volunteers who were believed to have no prior history of cryptosporidiosis (Dupont et al., 1995). Prior exposure to Cryptosporidium may result in reduced susceptibility (Okhuysen et al., 1998). On the other hand, the elderly, children, and persons with lowered immunity (e.g., those on antirejection drugs after organ transplantation, recipients of cancer chemotherapy, and persons with HIV infection) are in general more susceptible to infectious diseases such as cryptosporidiosis (Gerba et al., 1996). The same populations may also suffer more severe symptoms than the general population, as was demonstrated during the cryptosporidiosis outbreaks in Milwaukee (Hoxie et al., 1997) and Las Vegas (Goldstein et al., 1996). Secondary Infection The above risk analysis does not incorporate consideration of community-level impacts, such as the formation of secondary cases. Such cases refer to individuals not directly infected by water exposure but who are exposed to other infected individuals. A consideration of secondary infections can raise the risk estimate. For example, in the 1993 Milwaukee outbreak, 4.2 percent of households with one or more ill persons also contained one or more secondary cases (MacKenzie et al., 1995). In a foodborne (apple cider) outbreak of cryptosporidiosis, for each primary case, there were one-third as many secondary cases (Millard et al., 1994). Oocyst Viability and Recovery The above analysis used total oocysts found in the Kensico Reservoir raw water sampling locations to assess exposure. Not all of the total oocysts represent viable, human, infectious forms. Although several methods are available, there is no rapid, inexpensive, and reliable method for determining oocyst viability on a routine basis. One frequently used method is to calculate viability based on the
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy ratio of "confirmed" to "presumed" oocysts, terms that are applied to oocysts during microscopic observation. However, this viability analysis is not scientifically defensible because it has never been conclusively established that oocyst collection and processing steps do not themselves result in loss of microscopic features required for confirmation. Simple shaking of oocysts with sand has been found to cause loss of internal structure of oocysts (Parker and Smith, 1993). The efficiency with which the sampling methodology can recover oocysts can be quite low (Clancy et al., 1994). Oocyst recovery in New York City currently ranges from 30 percent to 70 percent (Stern, 1998). In prior risk assessments for protozoa, it has been assumed that the two errors (errors related to viability and to recovery) roughly cancel (Regli et al., 1991; Rose et al., 1991). Endpoints The focus of this risk assessment is on infections. It should be recognized that the end-point of clinically confirmed human illness may be substantially less than this. Based on human feeding studies, only about half of all infections progress to frank illness (Dupont et al., 1995). Even if illness occurs, in normal healthy individuals symptoms may be mild and not cause medical attention to be sought. If the endpoint of illness is used, the risk assessment would predict a lower number of symptomatic cases. The endpoint for this risk assessment—infection—is different than the endpoint measured by active disease surveillance—frank illness that is diagnosed and reported. This is one of the reasons that the risk estimate predicts a higher rate of infection than is observed in the active disease surveillance program. Other factors, such as the limited sensitivity of active disease surveillance and the contribution of other vectors such as food, are also responsible for discrepancies between the risk assessment and measured surveillance rates. Time Period The exposure estimate in the risk analysis used the entire data record at the two locations. If the lower oocyst levels, which have been seen in more recent years (Figure 6-6), are assumed to be more typical of future oocyst levels, then a lower exposure and consequently a lower risk would be estimated. Strains of Cryptosporidium It is noted that the present dose-response relationship derives from a single set of studies on a single oocyst strain (the "Iowa" or "Harley Moon" strain). Information currently being analyzed suggests that other strains of oocysts may have higher and lower infectivities and different dose-response curves than the
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy ''Iowa" strain (Chappell, 1999). This represents an additional source of potential variability in the risk assessment that should be amenable to quantitation. Other Sources of Water Supply The above analyses assumed that the sole oocyst loading reaching consumers was from the Catskill/Delaware supply. Although this represents the dominant source of drinking water for New York City, some residents are primarily served by water from the Croton system. To the degree that the (current) quality of raw water from the Croton system with respect to oocysts is different than the Catskill/Delaware, the numbers in Table 6-11 may under-or overestimate the total risk to some consumers of New York City water. Impact of Treatment and Watershed Management If the watershed management programs described elsewhere in this report are successfully implemented, the level of oocysts in the Catskill/Delaware supply may decrease. Although it is not yet possible to quantitatively forecast the magnitude of this decrease, a reduction in risk to consumers is expected. A primary motivation for conducting microbial risk assessment in water supply systems that pursue watershed management should be to determine the contribution of watershed management to overall risk reduction. Quantifying the impacts of other treatment processes on the risk estimate is a more straightforward task. Ozonation and particle removal will decrease the oocyst levels in drinking water, and it is possible to determine the magnitude of this reduction based on pilot-scale testing. Given standard treatment efficiencies, a properly functioning water filtration plant is expected to achieve at least a 2-log removal of Cryptosporidium oocysts (Nieminski and Ongerth, 1995). For both treatment processes and watershed management activities, a 1-log reduction in oocyst concentration translates directly into a 1-log reduction in the risk estimate because the dose-response relationship is linear at low dose. In other words, any process that reduces oocyst levels in the Kensico Reservoir by a factor of ten will reduce the risk estimate to 0.34 × 10-5 per person per day, or 25.5 infections per day, in a population of 7.5 million persons. Conclusions and Recommendations on Risk Assessment 1. Based on the committee's risk assessment using data from 1992 to 1998, the current daily risk of Cryptosporidium infection in New York City is 3.4 × 10-5, with a 95 percent confidence interval ranging from 3.4 × 10-7 to 21.9 × 10-5. EPA has stated that less than one microbially-caused illness per year
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy per 10,000 people is a reasonable policy (EPA, 1989).5 This risk level, which corresponds to 10-4 per year or 2.7 × 10-7 per day, is smaller than the lower 95 percent confidence interval of the estimated daily risk for New York City based on the Catskill and Delaware supplies (Table 6-11). It is also below the point estimates for risk during individual years. Hence, based on the assumptions used, the calculated risk of cryptosporidiosis would appear to be in excess of the frequently propounded acceptable risk level. The calculated risk estimate must be considered in conjunction with the caveats listed above. The risk estimate does not take into account measurements of oocyst viability and recovery, secondary infection, multiple strains of Cryptosporidium, or multiple dose-response relationships. The endpoint of the risk assessment was assumed to be infection, several years of data were used, and only the Catskill/Delaware system was considered. Finally, the impacts of watershed management on the risk estimate are not quantified. 2. It is recommended that a Cryptosporidium risk assessment be performed on a periodic basis for New York City. The goal of these efforts should be to help determine the contribution of watershed management (vs. other treatment options and management strategies) to overall risk reduction. Data that are sufficient for these purposes are currently collected as part of the NYC DEP Pathogen Studies. As new methods for oocyst recovery, detection, speciation (bird vs. human vs. animal), and viability become available, the risk assessment methods used in this report should be improved upon. Depending on the frequency of monitoring, risk assessment can be calculated for varying time periods to assess potential high-risk exposure times, such as during certain seasons and during storm events. Prior to commencing this regular effort, a decision must be made as to what level of risk is deemed to be acceptable to the regulatory agencies, the City, and the affected parties. This level should be arrived at after full and open discussion with the various stakeholders. Should an annual risk level of greater than 10–4 be regarded as acceptable by NYC DEP or other relevant risk managers, then the risk estimates computed in this report can be compared to such alternate yardsticks. 3. An ongoing program of risk assessment should be used as a complement to active disease surveillance. Risk assessment allows one to ascertain the level of infection implied by a very low level of exposure that would go undetected by active surveillance, thus acting as a complementary source of information 5 It should be noted that although the 10-4 risk level was developed for giardiasis, it is the only EPA-endorsed value available with which to compare current risks of cryptosporidiosis. As suggested in the second recommendation, New York City should determine an acceptable risk level before undertaking regular risk assessments.
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Watershed Management for Potable Water Supply: Assessing the New York City Strategy about public health. In combination, risk assessment and active disease surveillance data could be used to estimate the proportion of gastrointestinal disease cases attributable to drinking water (as in Perz et al., 1998). These estimates could then be validated by comparison with epidemiological data from a cohort study. In general, periodic risk estimates should be examined for concordance with prior computed risks and observed illness rates when formulating subsequent water treatment and watershed management decisions. REFERENCES American Public Health Association (APHA). 1998. Standard Methods for the Examination of Water and Wastewater. 20th Edition. Clesceri, L. S., A. E. Greenberg, and A. D. Eaton (eds.). Washington, DC: American Public Health Association. Angulo, F. J., S. Tippen, D. J. Sharp, B. J. Payne, C. Collier, J. E. Hill, T. J. Barrett, R. M. Clark, E. E. Geldreich, H. D. Donnell, and D. L. Swerdlow. 1997. A community waterborne outbreak of salmonellosis and the effectiveness of a boil water order. Am. J. Pub. Health 87(4):580–584. Ashendorff, A. 1999. NYC DEP. E-mail memorandum to the National Research Council dated January 6, 1999. Bagley, S. T., M. T. Auer, D. A. Stern, and M. J. Babiera. 1998. Sources and fate of Giardia cysts and Cryptosporidium oocysts in surface water. Journal of Lake and Reservoir Management 14(2-3): 379–392. Boyce, T. G., A. G. Pemberton, and D. G. Adiss. 1996. Cryptosporidium testing practices among clinical laboratories in the US. Ped. Infect. Dis. J. 15:87–88. Bukowski, J., L. Korn, and D. Wartenberg. 1995. Correlated inputs in quantitative risk assessment: The effects of distributional shape. Risk Analysis 15(2):215–219. Burmaster, D. E., and P. D. Anderson. 1994. Principles of good practice for the use of Monte Carlo techniques in human health and ecological risk assessment. Risk Analysis 14(4):477–481. Cabelli, V. J. 1977. Clostridium perfringens as a Water Quality Indicator. Bacterial Indicators/Health Hazards Associated with Water. A. Hoadley and B. Dutka. Philadelphia, PA: ASTM. Carmichael, W. W., C. L. A. Jones, N. A. Mahmood, and W. C. Theiss. 1985. Algal toxins and water-based diseases. Crit. Rev. Environ. Control 15:275–313. Carmichael, W. W. 1986. Algal toxins. Adv. Bot. Res. 12: 47–101. Centers for Disease Control and Prevention (CDC). 1995. Assessing the public health threat associated with waterborne cryptosporidiosis: report of a workshop. MMWR 1995; 44(RR-6): 1–19. Chappell, C. 1999. Presentation at Heath Effects Stakeholder Meeting for the Stage 2 DBPR and LT2ESWTR. USEPA. February 1999, Washington, DC. Chute, C. G., R. P. Smith and J. A. Baron. 1987. Risk factors for endemic giardiasis. Am. J. Public Health 77(5):585–587. Clancy, J. L., W. Gollnitz, and Z. Tabib. 1994. Commercial labs: How accurate are they? Journal of the American Water Works Association 86(5):89–97. Clancy, J. L., C. R. Fricker, C. R. Fricker, and W. Telliard. 1997. New US EPA Standard Method for Cryptosporidium Analysis in Water. AWWA Water Quality Technology Conference. Denver, CO: American Water Works Association. Clarke, R. T. 1994. Statistical Modeling in Hydrology. New York, NY: John Wiley & Sons. Codd, G. A., S. G. Bell and W. P. Brooks. 1989. Cyanobacterial toxins in water. Water Science and Technology 21:1–13. Craun, G., G. Birkhead, S. Erlandsen, F. Frost, W. Jakubowski, D. Juranek, R. Soave, C. Sterling, and B. Ungar. 1994. Report of New York City's Advisory Panel on Waterborne Disease Assessment. NYC DEP.
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Representative terms from entire chapter: