New York City depends on an unfiltered surface supply to provide 9 million consumers with approximately 1.5 billon gallons of water per day. The supply consists of nineteen cascading reservoirs and three controlled lakes located in the Catskill-Delaware and Croton catchments encompassing 5100 square kilometers (i.e., 1972 square miles) Figure 1. The only forms of treatment are chlorination, fluoridation, and corrosion control. In order to meet filtration avoidance requirements, coliform bacteria, turbidity, temperature, pH, oxygen content, and other water quality indicators must be monitored at least daily. Climate change has the potential to affect such water quality characteristics, and New York City’s Department of Environmental Protection has identified the deterioration of water quality as a potential vulnerability to climate change.
Figure 1. New York City’s Water Supply System. Source: Courtesy of New York City Department of Environmental Protection.
The effect of climate change on turbidity is one of the most significant concerns for New York’s water supply, as federal regulations limit it to 5 NTU (nephelometric turbidity units) at the intakes. Currently, typical values range between 0.5 and 1.5 NTU. Extreme weather events, such as hurricanes and nor’easters, can dramatically increase the turbidity levels at upstream reservoirs. Heavy rains that erode streambeds or sedimentary deposits from the last ice age transport glacial clays into the water supply, resulting in high turbidity. The flooding caused by Hurricane Floyd (September 18, 1999; see Figure 2), Hurricane Ivan (September 17, 2004), and heavy spring rain on April 2, 2005 resulted in turbidity values in the Ashokan Reservoir between 300 and 500 NTU. If heavy rains occur when water levels are low, as they are during droughts, exposed shorelines are vulnerable to severe erosion that can also result in high turbidity. When peak turbidity levels occur, NYCDEP may treat up to approximately 600 MGD in the Catskill Aqueduct with alum and sodium hydroxide to precipitate the clay particles and remove them from water traveling to the intakes. Water quality monitoring is greatly intensified and reported to the state regulatory agencies on a daily basis. Both the physical endurance of the staff and structural fitness of the system are tested at these times. An increased frequency of such events in the future would most likely necessitate increased staffing and intensified maintenance of equipment.
Figure 2. Schoharie Reservoir after Hurricane Floyd. Source: Courtesy of New York City Department of Environmental Protection.
In order to reduce chemical treatment needs caused by extreme conditions, other turbidity reduction programs are in progress. NYCDEP is conducting a study of structural (e.g., intake design, turbidity curtains, etc.) and non-structural (e.g., operational) alternatives to control turbidity leaving Schoharie Reservoir. Other turbidity sources, such as suburban developments, are controlled by stormwater Best Management Practices (e.g., detention basins) to reduce turbidity in storm runoff at key locations near intakes. In addition, the City is currently implementing a Stream Management Program in the Catskill Mountains to reduce streambed and streambank erosion during stream baseflow using a geomorphic approach developed by Rosgen (1996). The Stream Management Program is most likely to be effective in controlling turbidity at low flows.
In addition to turbidity events, temperature change will have some important impacts on the operation of the NYC water supply. The reservoir system is a network of interconnected waterbodies linked by natural streams and aqueducts. Warm water results in more rapid settling of turbidity with the consequence that the best water (i.e., lowest turbidity) to send towards distribution is generally somewhat warmer than the high turbidity water that should be detained upstream until settling occurs. In an effort to use the best quality water, it may become increasingly difficult to balance the need to maintain low temperatures in the releases to streams (which are regulated to maintain cold-water fisheries habitats) with the quantity and quality requirements for the drinking water supply.
Another possible consequence of increases in rainfall may be increases in nutrient loadings to reservoirs and subsequent eutrophication. High phosphorous levels occur in reservoirs close to farmland. Elevated phosphorus concentrations can cause extensive blue-green algal blooms (see Figure 3) that contribute high levels of organic compounds to the drinking water. These organic compounds are pre-cursors of disinfection by-products (DBPs). On the distribution side of this problem, the Department of Environmental Protection (DEP) has conducted research to determine the optimum chlorine dose under various conditions to minimize DBP formation, while meeting the contact time requirements for disinfection.
Figure 3. Algae are a major source of disinfection byproducts (DBPs) that are regulated in drinking water. Source: Courtesy of New York City Department of Environmental Protection.
Increased water temperature affects not only concentrations of suspended sediments, but also biological agents, such as coliform bacteria and water-borne pathogens. One possible explanation is the influence of temperature on sinking rates. DEP has observed that particles and pathogen cysts appear to diminish in reservoirs as water temperature increases, but more extensive analysis is needed to define this relationship.
Temperature can also affect survival and distribution of many microorganisms, their hosts, and their predators. As an example, it may change the behavioral patterns of migrating waterfowl, such as Canada geese, that can have a major influence on fecal coliform levels in reservoirs. There is a strong and well documented positive correlation between the number of waterbirds roosting on Kensico Reservoir (that reach a peak during seasonal migration periods) and the percentage of fecal coliform samples above the Surface Water Treatment Rule (SWTR) limit. DEP currently conducts a Waterfowl Management Program to keep geese, ducks and gulls away from intake areas. This maintains fecal coliform bacterial concentrations at low levels and within regulatory limits.
Although Cryptosporidium and Giardia have not historically been a significant problem for NYC, the utility is studying the sources and behavior of these water-borne pathogens. Knowledge of pathogen sources and behavior will allow DEP to develop effective management programs to mitigate the indirect effects that temperature change or intensification of the hydrological cycle may have on water quality. For example, a DEP study used genotyping of Cryptosporidium oocysts from a stream draining a residential area to demonstrate that nearly 90% of the cysts were of non-human origin. Microbiological “fingerprinting” studies such as this can help identify pathogen sources, indicate their importance for human consumers, and guide effective control measures. These studies are excellent examples of how utilities can reduce future risks by assessing potential vulnerabilities to climate-related impacts and acting to reduce these vulnerabilities.
In 2002, NYCDEP became one of 17 partner organizations in the CLIME Project (Climate Impacts on Lakes in Europe) sponsored by the European Commission. This three-year project is investigating the impacts of future climate change on water supply. An overview of the project is given at the website www.water.hut.fi/clime. The CLIME project was designed to analyze the impacts of climate change on freshwater resources. If present trends continue, limnologists believe that weather will have a major effect on the dynamics of lakes and reservoirs, including climate-related problems such as increased productivity, increased color, and increased frequency and severity of algal blooms. The benefits to New York City include an exchange of scientific expertise that broadens NYCDEP’s current capabilities, particularly in the realm of prediction of future regional weather that will determine hydrologic conditions. It also allows first-hand involvement in the development of CLIME models and a decision support system that will lead to effective watershed management and planning for the future.
Most recently, in 2004, the NYCDEP instituted an agency-wide Climate Change Task Force (CCTF). The mission of the Task Force is to understand how climate change may affect the water supply and its infrastructure, and to provide a basis for long-term planning. The potential effects include sea level rise, temperature rise, an increase in extreme events, and changing precipitation patterns, all of which will have significant impacts on the City’s existing water supply and wastewater treatment systems. Future infrastructure will also have to take these changes into account. An interesting approach taken by the CCTF has been to use extreme events (hurricanes and droughts) to begin to quantify future needs. Infrastructure changes may take decades to implement and therefore, advance planning is essential. Policy changes may also be required to prevent degradation of water quality. Therefore, the role of the CCTF is to insure that NYCDEP’s strategic and capital planning efficiently take into account the potential effects of climate change. In addition, has looked more broadly at the vulnerability of its water supply system to climate change. Major and Goldberg (2001), for example, provide a detailed review of the impacts of global climate change on the New York City Water Supply System, and examine types of adaptation that might be undertaken to cope with climate change.