One of the major success stories in the use of natural ecosystems to deliver vital ecosystem services is the use of a series of river-reservoir ecosystems located in the Catskill Mountains to provide water for New York City's nearly nine million people (Ashendorff et al. 1997). Three large reservoir systems (Croton, Catskill, and Delaware) containing 19 reservoirs, 3 controlled lakes, and numerous tributaries cover an area of 5,000 km2 with a reservoir capacity of 2.2 X109 m3. The US Environmental Protection Agency issued a "filtration avoidance status" in 1997 for five years in response to the city's request to upgrade their watershed management and enhance the capacity of natural ecosystems to maintain clean water. To avoid the potential expense of US$2—8 billion over 10 years to build new, larger filtration plants to meet drinking water standards, the city invested US$1 — 1.5 billion to restore natural ecosystem processes in the watershed (Ashendorff et al. 1997; Foran et al. 2000). The city agreed to construct a filtration plant if natural processes failed to meet EPA standards. Filtration is viewed as essential because chlorination is not completely effective in killing pathogens, particularly when there are high levels of suspended materials (Schoenen 2002).
New York City increased the capacity for natural nutrient retention and lower erosion by protecting riparian buffer zones along rivers and around reservoirs. Road construction within 30 m of a perennial stream and 15 m for an intermittent stream was prohibited. Non-point sources of nutrients and pesticides from stormwater runoff, septic tanks, and agricultural sources were also regulated. Water managers continued to monitor for protozoans, such as Crytosporidium parvum and Giardia lamblia, that cause cryptosporidiosis and giardiasis.
The city is now expected to save some US$300 million annually that would be necessary to run new filtration plants. The investment in natural capital reduced risks of contaminants, and the city can now focus on minimizing disinfectants at the final treatment stages. Although chlorination of drinking water is widely used, it can produce carcinogenic byproducts (e.g., chloroform, trihalomethane, and 260 other known chemicals) in drinking water, especially in ecosystems with high levels of organic matter (Zhang & Minear 2002).
Increasing effectiveness of natural ecosystem processes by watershed protection, restoration, and riparian management provides an example of how planners can cope with highly variable inputs that characterize this catchment (e.g., Frei et al. 2002). Boston, Seattle, San Francisco, and Greenville, South Carolina, are other examples where natural ecosystem services are used in conjunction with water treatment plants to ensure high-quality drinking water (O'Melia et al. 2000). This final case study illustrates the potential value of maintaining and enhancing natural ecosystem functioning in order to provide our vital ecosystem services.
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