U.S. drinking water challenges in the twenty-first century.

The access of almost all 270 million U.S. residents to reliable, safe drinking water distinguishes the United States in the twentieth century from that of the nineteenth century. The United States is a relatively water-abundant country with moderate population growth; nonetheless, current trends are sufficient to strain water resources over time, especially on a regional basis. We have examined the areas of public water infrastructure, global climate effects, waterborne disease (including emerging and resurging pathogens), land use, groundwater, surface water, and the U.S. regulatory history and its horizon. These issues are integrally interrelated and cross all levels of public and private jurisdictions. We conclude that U.S. public drinking water supplies will face challenges in these areas in the next century and that solutions to at least some of them will require institutional changes.

The access of almost all 270 million U.S. residents to reliable safe drinking water distinguishes the United States in the twentieth century from that of the nineteenth century, and the United States from much of the rest of the world even for this century (1). Circa 1900, annual typhoid rates in large U.S. cities were about 40/100,000; by 1920, they averaged about 2/100,000 per year, reflecting new water intakes, filtration, and chemical treatment (2). By 2000, there were no typhoid cases in the United States attributable to public drinking water.
In this article we address the state of U.S. public drinking water systems at the turn of the millennium. Enormous improvements have occurred during the past century. About 54,000 public water systems now serve over 250 million people (Table 1). [Under the Safe Drinking Water Act (SDWA) (3), "public water systems" are defined as those regularly serving at least 25 people or 15 service connections.] More than 80 specific contaminants are regulated, and hundreds of water quality parameters are monitored. Total annual expenditures for public drinking water in the United States are about $36 billion (4). Furthermore, the United States is a relatively water-abundant country with moderate population growth (5).
Nonetheless, the availability of fresh water is finite, and current trends are sufficient to strain water resources over time, especially on a regional basis (6). Thus, many challenges face public water suppliers in the United States at the opening of the twenty-first century. Some are systemic, such as deteriorating infrastructure, whereas others are quite local and specific. Some represent newly emerging circumstances; others have dogged us for decades. Prudent water professionals are addressing many of them already (7)(8)(9)(10).
These challenges cross all levels of public and private jurisdictions, from local to international. Some are generally tractable; some are intractably political; a few components present purely technological barriers; most are a combination. Most are also shared by other industrialized nations, and attention by developing countries may enable them to avoid some of the pitfalls we have encountered.
Finally, these challenges are integrally interrelated. An integrated or at least systematic approach is necessary to facilitate efficient, effective, and sustainable solutions.

The State of U.S. Public Water Infrastructure
Investment by the United States in maintenance and repair of public water infrastructure has generally been inadequate over the past half century (11)(12)(13).  amendments to the Safe Drinking Water Act (14) required the U.S. Environmental Protection Agency (U.S. EPA) to regularly conduct a survey of the infrastructure needs of public water supplies. In its recent survey on these needs, the U.S. EPA estimated that the nation's water utilities must increase investments at least $151 billion over the next two decades to maintain our public water infrastructure and to ensure safe and healthful community water supplies (4). Of this total, about $38 billion is for water treatment, $83 billion to repair and/or replace components of the distribution system, and $28 billion to protect watersheds and maintain storage reservoirs. Only a small part of the total-20.7%-is for investments required by the SDWA.
Two other studies support these estimates. The Water Information Network (WIN), a coalition of engineering and construction firms, an environmental group, and water utilities, recently estimated that total annual spending for capital investments and operations by U.S. community water supply systems, currently about $36 billion, must increase by $15 billion (15). The estimated needs for wastewater infrastructure are even larger: an increase of $19 billion over the current annual expenditure of $25 billion. [In contrast to drinking water, a large portion of the wastewater expenditures are attributable to requirements of the Clean Water Act (CWA) (16). ] The American Society of Civil Engineers (ASCE) just released its 2001 Report Card for America's Infrastructure (13), which included analyses not only for drinking and wastewater but also for bridges, schools, roads, and so forth. The ASCE estimated an annual shortfall of $11 billion for drinking water and $12 billion for wastewater, due to the need to replace aging facilities and to comply with existing and upcoming federal regulations. Both ASCE and WIN advocate enormous subsidies of local water supplies by the federal government, amounts that would dwarf existing federal programs that provide grants or lowinterest loans to local governments for water supply and treatment.
Federal subsidies alone, however, are unlikely to address the real causes of the inadequate maintenance: the institutional arrangements that govern local public water providers and their managerial practices. Federal subsidies alone cannot foster the changes necessary to ensure sustainable investment and maintenance practices. Unsustainable practices include the pricing of the product, the disposition of the revenues, the consolidation of the industry, and ownership.

Pricing
In many water supply systems and for many years, water rates have been insufficient to cover long-run costs (12,13,17). In addition to adequately financing the maintenance of public infrastructure, water pricing should include the costs of watershed or aquifer management (5,18).
Water system revenues are also sometimes used for other purposes not related to water supply or wastewater disposal. In some cities, for example, water system revenues are not separated from other public funds, an arrangement that allows water supply to subsidize-or be subsidized byother municipal activities.
In addition to cost recovery, a welldesigned pricing system can also encourage water conservation, which will not only reduce pressure on water resources but also reduce future infrastructure requirements. A recent survey of studies found the long-run demand for water to be inelastic but not totally unresponsive to price, with elasticities ranging from -0.2 to -0.4 for residential users and from -0.5 to -0.8 for industrial users (19). Table 2 compares the average water use per capita and price per cubic meter among selected Organisation for Economic Cooperation and Development (OECD) countries. The lowest per capita use rates shown in Table 2, as well as the highest prices, are found in the countries of northern Europe. Highest average use rates are found in North America, and the prices there are among the lowest. Table 2 shows that while prices exert a strong effect on consumption, they are not the only factor. Furthermore, comparison between North America and northern Europe, regions of comparable wealth, suggests that water prices considerably higher than those in the United States are both conceivable and precedented, and that substantial opportunities for water conservation likely exist in the United States (20).
Conservation opportunities exist on both the supply and the demand sides. Among utilities, leakage is a major issue: drinking water systems lose 6-25% of their finished water through leaks and breaks (21). Leaks are also a potential health risk, a source of contamination in systems subject to occasional negative pressure episodes (22,23). Water conservation among users can reduce requirements not only for water supply facilities but also for wastewater facilities.
Water utilities are increasingly adopting pricing structures that encourage conservation and are moving from flat-fee pricing (a fixed amount per month) to rate structures that charge consumers according to the amount of water consumed. Utilities are also slowly moving to an "increasing block" rate design, where per-unit rates are higher for greater rates of consumption. Compared with a uniform rate structure, increasing block rates reduce the economic hardship associated with high prices while retaining most of their incentive effects.
Surveys from 1987 and 1998 show that the fraction of U.S. utilities using increasing block rates increased from 17 to 37% (24) during this 11-year period, but the penetration appears to have stagnated for the past few years (25). The reluctance of utilities to adopt conservation-oriented rate structures may arise from the potential conflict between the conservation objective and the revenue objective. That is, higher rates encourage a demand response, but that very response makes it more difficult to predict revenues. If conservation is too successful, revenue shortfalls are possible (26).

Consolidation
In 1997, there were about 54,000 permanent community water supplies in the United States (Table 1), only 1,500 more than 10 years earlier (27,28). Over 90% of these systems serve fewer than 10,000 customers and together account for less than 20% of the U.S. population. By comparison, the United Kingdom has fewer than 30 public water systems. The greater area and relatively low population density of the United States only partially explains the very large number of water systems; the U.S. water supply industry has remained quite decentralized even while other local public services such as schools and police have consolidated substantially.
Despite the obvious forces favoring decentralization, this situation may change in the coming decades. First, greater population densities, especially on the coasts, will necessitate coordinated water supply decisions for both surface and groundwater systems. In addition, utility consolidation will be driven by the search for operational efficiencies in such areas as billing, customer service, and water testing, as have other industries from banks to airlines to local trash collection. With over 50,000 supply systems, there must be much duplication of effort. Finding efficiencies will become especially important given the increasingly stringent drinking water regulations of the SDWA, which will require ever greater expertise by utility operators. Monitoring and testing requirements are already a burden on small systems, many of which do not even have full-time operators. The pressures to coordinate demands on water resources and to find operating efficiencies will, we believe, inevitably lead to some consolidation of the industry, which in turn will make it somewhat easier to meet infrastructure (and watershed or aquifer) needs.

Ownership
Community water systems (as defined by the SDWA, i.e., systems regularly serving at least 25 people or 15 connections) can be public entities, as part of the municipal government, an independent agency, or a special district, or they can be privately owned. Over half the community water supplies in the United States are privately owned; however, together they serve only 16% of the population ( Table  1). The private companies are operated much like public utilities and are subject to rate-ofreturn regulations administered by state public utility commissions. Although situations vary among publicly owned water utilities, generally those not subject to public utility regulation must answer to the voters. At present, both theory and evidence are inconclusive on the superiority of private versus public ownership. Because both drinking water and wastewater services are natural monopolies, they will almost certainly be provided by a single enterprise. Without the threat of competition, the social benefits of private ownership are not likely to be realized.
However, we think it is likely that private systems have more success in raising investment funds. It is harder for public systems to set rates that maintain their systems properly, for many reasons. That the rate payers are also voters provides a constant downward pressure on rates (17). The limited terms of most politicians adds to the myopia, as the benefits of system maintenance will only appear after they have left office. Finally, after World War II many large cities suffered from stagnating local economies and an exodus of the middle class; these difficult economic conditions also inhibited investment in infrastructure.
Private water utilities, on the other hand, are somewhat insulated from public anger. They are usually subject to public utility regulation, which is a much easier environment in which to raise rates for investment funds. The United Kingdom, which in 1989 opted for 100% privatization of drinking water supply, provides an example: within 2 years, investment expenditure increased by more than 80% (29). After comparing public with private water companies, the Congressional Budget Office reported that rates of private water supply companies in the United States exceed by 30-80% those of public suppliers, whose rates often do not permit them to cover the cost of depreciation (30). Higher utility rates do not guarantee that the needed investments will be made, but they are a necessary condition for it.
The institutional factors determining the management behaviors of local water providers have not produced adequate expenditure levels to maintain public infrastructure, appropriate investments to develop new drinking water technologies (17), or successful strategies to protect watersheds and aquifers (18). Therefore, employing these alternatives in a serious way will likely require institutional changes.

Global Climate Effects on U.S. Drinking Water Quality and Quantity
During the 1990s, data on the gradual warming of the earth's atmosphere have shown a dramatic acceleration at the end of the twentieth century. These data include the "fingerprint" studies that show a) the warming pattern in the mid-troposphere in the Southern hemisphere (31), b) the disproportionate rise in nighttime and winter temperatures (32), and c) the statistical increase in extreme weather events occurring globally (33,34). Even more recently, multiple "paleothermometers" have shown that the twentieth century is the warmest in over 1,000 years (35). Other data demonstrate a significant increase in the rate at which warming is occurring, increasing from about 1°C per century through the 1980s to 3°C per 100 years during 1997 and 1998 (36). Figure 1 shows (simulated) annual global mean temperatures over the period 1850-2000, indicating that models incorporating anthropogenic loadings better approximate actual temperatures than models using only data on natural temperature variability and volcanic activity (37). Table 3 shows examples of impacts resulting from projected changes in extreme climate events (38).
If these trends continue, the resulting global warming may adversely affect water distribution, availability, and quality in the United States. Various studies are showing changes in hydrologic cycles, shifting the timing, intensity, seasonality, and spatial distribution of precipitation throughout the world (39). These changes are likely to further perturb already stressed ecosystems (40)(41)(42).
Global warming may affect both surface water and groundwater. A warmer atmosphere holds more water vapor (about 6-8% per 1°C), so evaporation rates will be higher. Droughts followed by severe weather events can result in more polluted runoff to surface waters and less infiltration to replenish aquifers. Conversely, where overall precipitation increases, depleted aquifers may be recharged. Warming of the oceans results in sea level rise from thermal expansion and the melting of glaciers and ice sheets (43). A rise in ocean levels may then result in increased salt water infiltration of coastal aquifers (44).
Warmer temperatures of surface water sources may also contribute to increased harmful algal blooms, which already appear to be occurring worldwide (45)(46)(47)(48). Algal blooms in inland aquatic systems can degrade drinking water odor and taste and have caused fatalities in farm animals that drank directly from the water (49). The risk to humans from drinking contaminated water is less definitive; certainly, deaths of children and other vulnerable populations are documented from consumption of cyanobacteria toxin (49). Several algal bloom biotoxins, associated primarily with consumption of contaminated shellfish, are also known neurotoxins (50). Some aerosolized red tide toxins produce asthmalike and other respiratory effects (51). Some algal blooms also cause dermatitis (52) or are cytotoxic (53). Year

Year Year
Theoretically, warmer temperatures and especially warmer winters may result in higher microbial and nutrient loadings in drinking water supplies, promoting biofilm growth within the distribution system and, in turn, supporting survival of some pathogens and their indicators. Warmer temperatures will also mean increased water use (e.g., drinking water consumption, bathing, watering lawns, irrigating crops, swimming, etc.), increasing demands on drinking water systems.
Snowpack, especially in mountains, holds water until late spring or even summer and then melts over several months, generating stream flow seasonally when water typi-cally is much needed but less available from rainfall. Warmer winters, particularly if precipitation decreases, may produce less snowpack and earlier snowmelt, which would then provide less water during the drier growing season and hence strain other freshwater supplies (54).
In addition, sequential extremesdroughts punctuated by heavy rains-can destabilize natural biological controls of pests and pathogens (55). Evidence since the 1980s suggests that the geographic range and virulence of some established diseases (e.g., malaria) are expanding (56). Heavy rain events and flooding are associated with waterborne disease outbreaks (57) and algal blooms, often resulting in "dead zones" (46,58,59).
Because of the global nature of the changes, even well-protected watersheds will not be immune to these conditions. And the effects are likely to be distributed widely across the country.

Waterborne Disease
Improvements in public drinking water during the twentieth century, including more protected water intakes, filtration, and chemical treatment, virtually eliminated the most deadly waterborne diseases such as typhoid and cholera from the United States (1).
Nonetheless, numerous surveys have shown widespread contamination of U.S. surface waters by multiple pathogens, even in pristine waters (60)(61)(62)(63)(64)(65). In addition, while surface water supplies are the major risk for waterborne infectious disease (WBID), myriad data show that wells, especially relatively shallow wells, are also vulnerable to microbial contamination. In data published by the U.S. Centers for Disease Control and Prevention (CDC), about half the documented waterborne disease outbreaks have a groundwater source (66)(67)(68)(69)(70). A recent study found that 5-50% of wells and springs tested were contaminated with Giardia or Cryptosporidium, suggesting that groundwater systems also need to be monitored for microbial contamination (71). Furthermore, studies have shown that many microorganisms can exist for prolonged periods under harsh conditions in a viable but nonculturable form (65,72).
The full extent of WBID in the United States presently is not known. Data from the CDC (collected through a passive, voluntary, self-reported system) are widely thought to underrepresent actual incidence (63)(64)(65)(66)(67)73). Many WBID outbreaks are never detected (73)(74)(75)(76). Current incidence estimates are three to four orders of magnitude higher than the CDC data (77)(78)(79). Empirical evidence from a variety of water systems meeting federal drinking water standards suggests that 6-40% of gastrointestinal illness in the United States may be water related (74)(75)(76)(80)(81)(82); recent data from Canada present a similar picture there (83).
In the United States, disinfection is required of all public water supplies served by surface water or by groundwater "under the influence of surface water" (84). By far, the most common disinfection approach in the United States is the use of chlorine species.
A complication of drinking water disinfection is the emerging evidence of carcinogenic and possibly other health effects associated with disinfection by-products (DBPs) such as trihalomethanes (THMs)   (78,(85)(86)(87)(88). Common sense as well as several cost-benefit analyses (17,89,90) suggest that present disinfection rates cannot be compromised. Furthermore, numerous investigations have shown that low-level WBID continues in the United States even at current disinfection levels (74,76,(80)(81)(82). Nonetheless, we believe that the new evidence of carcinogenic and other health effects from exposure to disinfected water cannot be ignored and will likely challenge the public health and water utility communities in the twenty-first century.
In addition, as the limits of disinfectants to control some of the most common waterborne pathogens (e.g., Giardia, Cryptosporidium, and probably some viruses) become clearer, the advantages of a multiple barrier approach to WBID are again highlighted (91)(92)(93)(94). Filtration and disinfection efforts will need to be strengthened along with enhanced watershed protection.
Our ability to detect waterborne diseases is constrained by the limits of current methods for each specific pathogen (95), and one third or more of documented WBID outbreaks in the United States have an unidentified etiology (66)(67)(68)(69)(70)73). Current methods for bacteria focus only on those that can be cultured in the laboratory, although there is evidence that many microorganisms can survive in a viable but nonculturable form (65,72). Current methods for protozoa and viruses are expensive and require concentration of large volumes of finished water through adsorption, filtration, centrifugation, and coagulation/ precipitation or a combination of these techniques, followed by separation and quantification. Each of these multistage procedures can contribute to significant losses in recovery. As a result, the lower limit acceptance criteria for ongoing precision and recovery is, for instance, 19% for Cryptosporidium and 16% for Giardia, using the accepted methodology (96). Molecular methods based on polymerase chain reactions show promise for improving our ability to detect specific pathogens (97) and, in certain cases, their viability (98). In combination with cell-culture techniques, infectivity of certain organisms may also be assessed (99). These techniques, however, are still primarily applicable for research studies and are beyond the scope of all but the most highly trained staff and most affluent utility laboratories.

Emerging and Resurging Pathogens
Levels of the most deadly WBIDs in the United States, such as cholera and typhoid, are currently extremely low-indeed, virtually nonexistent. However, open and easy worldwide commerce compromises the invulnerability of individual nations (or even continents) to dangers from elsewhere.
There are unsettling trends throughout the world in the emergence and resurgence of diseases, including waterborne diseases, and in the expanding geographic range and virulence of some established diseases. There has also been a resurgence of older diseases in certain parts of the world, for example, cholera in South America (100). It is more difficult, however, to define what is really the emergence of a new disease (101,102).
Altered or new routes of exposure to previously uncharacterized pathogens may result in the emergence of disease. An increase in the number of susceptible individuals (the very young, pregnant women, the immunocompromised population and-in the United States especially-the elderly) (103) will extend the human reservoir for opportunistic pathogens and change virulence patterns, even in developed countries. Indeed, the U.S. elderly population is likely to triple between 1985 and 2015. In addition, increased adaptation to the human host by pathogens could increase infection rates in populations with no underlying susceptibilities.
Many infectious agents have been categorized as emerging diseases and have not been recognized until recently, or at least not in association with water, including Legionella pneumophila, Cryptosporidium parvum, Escherichia coli O157, Vibrio cholerae O139, hepatitis E, and Helicobacter pylori. The dangers of emerging diseases have been discussed widely in the popular and scientific press (104,105).
We should possibly add to this list every waterborne pathogen that has developed resistance to antibiotics, or changed apparent virulence, as they emerge as a higher mortality risk (75). Resistance to multiple antibiotics has been well documented in waterborne pathogens (106)(107)(108)(109) and represents a major public health threat. There are many pathways for antibiotics and antibiotic-resistant organisms to enter the drinking water supply (110,111). Source water can become contaminated through antibiotic use in the human population (pharmaceuticals and biologics) as well as the use of a diverse group of bioactive chemicals, including active ingredients in personal care products such as diagnostic agents, "nutriceuticals," fragrances, sun screen agents, and so forth (112), with subsequent discharge in sewage. However, massive and unregulated use of antibiotics in agriculture and aquaculture may present the greatest risk to the aquatic environment (113).
The water distribution system itself may provide an opportunity for exchange of both antibiotic resistance and virulence factors among microbes. The biofilms or "slimes" that form on the inner surfaces of pipes are potential sites for gene exchange (114,115).

Land Use Issues
The twentieth century provided engineering solutions to lower-quality sources of drinking water, principally filtration and disinfection with chlorine. This afforded cities the alternative to continue using more contaminated source waters, as did Philadelphia, Pennsylvania, or to seek more pristine and higher-quality sources far upstream, as did New York City and Boston, Massachusetts (116). This financial decision still faces water utilities (and the U.S. EPA) at the beginning of the twenty-first century.
Land use pressures will also challenge U.S. drinking water systems in the next century (117). Multiple economic and social indicators suggest that the population will continue to grow, the size of households is likely to continue to fall, per capita water use will at least remain constant but may also continue to increase accompanying a rising standard of living, and even leisure activities that require water are likely to increase as travel costs decrease. In addition, technological advances in the provision of water (and an infusion of significant federal resources) have facilitated large population shifts to arid areas, increasing the pressure on their limited water resources. These factors may result in water scarcity in certain locales.
Control of unregulated and other "nonpoint" sources of discharge to surface waters, such as agricultural activities and much street runoff, will become more important to water quality as "point" sources are controlled (117,118). Systems sharing watersheds or aquifers will need to work together to protect their common resource. As population densities increase, especially along the coasts, coordinated land use strategies will be necessary. For instance, water diverted from infiltration into the soil and occurring as runoff to surface water will result in more rapidly depleting aquifers and more contaminated surface waters. A recent survey found a variety of pesticides in both surface water and groundwater in all basins with appreciable agricultural activities or urbanized development (119). In these circumstances, competition among sources (drinking water, agriculture, fish and wildlife habitats, residential development, energy production, leisure, etc.) is likely to increase (117).
An interesting case study of land use issues, including competing needs and externalized costs, is New York City's recent decision to invest in upgraded protection of its Catskills watershed to avoid the high costs of building a filtration plant (120,121). The city estimated that building the filtration plant would cost $6 billion to $8 billion in capital, with annual operating expenses of about $300 million. Costs to repair the degraded watershed to ensure a higher-quality water source were estimated at $1 billion to $1.5 billion, for land acquisition, new watershed rules and regulations, and financial assistance to watershed communities to promote environmental quality and their local economies. New York City chose to invest in the protection of its drinking water source, a decision supported, at least in the short run, by a scientific review by the National Research Council (121).
Alternatively, given finite water and land resources, a growing population, and a vast array of technological developments, the United States can anticipate new interpretations of the maxim that drinking water should be obtained from the best-quality source available. Reclaimed water and water reuse, for instance, can enhance both groundwater and surface water supplies. Current potable reuse projects and studies have demonstrated the capability to produce reclaimed water of excellent measurable quality and to ensure system reliability but only with frequent, careful, and thorough monitoring (122). Water reuse plans are already being investigated in several states, including Arizona, California, Florida, Texas, Utah, and Virginia.
These alternative solutions, however, come with high price tags. Because U.S. water pricing over the past century has not been adequate to maintain the public water infrastructure and to cover the costs of watershed and aquifer management, inefficient and ultimately more expensive (and unsustainable) water consumption behaviors have developed (5,18,26,120,121). In addition, substantial costs have been deferred, such as the cost of preserving the quality of watersheds, that are now coming due. We believe that, in the next century, economic and related political forces will force the rest of the United States to evaluate local and national policies on, for instance, agriculture, development (especially in naturally arid areas), and other high-water-demand activities, as New York City did recently. Inevitably, consumers and polluters must pay to fully protect our shared resources.

Groundwater Issues
Groundwater currently is the drinking water source for almost 80% of the public water systems in the United States, although only about one third of the U.S. population is served by those systems. The average groundwater system serves under 500 people (Table 1). Figure 2 shows the general distribution of systems served by groundwater versus surface water in the United States.
Some U.S. aquifers are refilled (recharged) regularly by rainfall, from surface water bodies, or both. Other aquifers, however, contain water that is thousands of years old ("fossil water") and that cannot be replenished. Once the fossil water in the latter aquifers has been exhausted, the areas dependent on them will have to procure alternative drinking water sources.
Even in aquifers that can be recharged, however, at present the rate of groundwater extraction exceeds long-term rates of recharge from precipitation and other sources in many basins (117). This is exacerbated by a reduction in natural recharge rates due to an increase in nonpermeable areas associated with land development. Depletion of groundwater in storage increases the costs of extraction and may induce water quality degradation (such as seawater intrusion), land subsidence, and eventually loss of the resource. Use of simplified concepts such as "safe yield" to determine allowable groundwater withdrawals ignores the dynamics and the interconnection of all components of groundwater budgets (123). Contamination of groundwater supplies will continue to be an important issue in the United States in the twenty-first century. Much contamination results from local human activities, including industrial activities, hazardous waste sites, residential development, and transportation. There also are more pervasive contaminants to groundwater supplies, including nonpoint sources associated with agriculture and animal husbandry (124,125), and naturally occurring contaminants such as arsenic and other trace metals (126). Finally, recharge of contaminated water into groundwater systems can introduce harmful chemical and microbial contaminants into drinking water supplies (127).
Groundwater and surface water function as linked resources, so contamination of surface water bodies can contaminate groundwater (128). Because residence times in groundwater tend to be much longer than in surface water, "short-term" contamination events in surface water can result in longterm contamination of groundwater, with consequent negative impacts on drinking water supplies.
The link between groundwater and surface waters permits the design of strategies that maximize the advantages of each resource (129). The National Research Council defines conjunctive use as "a plan that capitalizes on the combination of surface and groundwater resources to achieve a greater beneficial use than if the interaction were ignored" (129). For example, this could involve artificially recharging aquifers (via percolation ponds or injection wells) with surface water, thereby using the groundwater system as a storage and conveyance facility, and also exploiting the natural filtering ability of aquifers to clean polluted surface water. It seems likely to us that these actions can have significant benefits for providing drinking water, including better reliability of supply, reduced costs, and potentially improved water quality.
Closely linked to conjunctive use projects is the increasing use of reclaimed sewage effluent as a source of groundwater recharge and, ultimately, a source of drinking water. This is particularly the case in the western United States, where reclaimed water is considered the main "new" water supply. Faced with increasing demand, limited increases in traditional water supply sources, and potentially large costs of building/enlarging sewage outfalls, water managers are likely to accelerate their use of reclaimed water for groundwater recharge (and other uses, such as direct irrigation). Research is beginning to address public health concerns about the microbial and chemical fate and transport of contaminants in reclaimed water that is recharged into aquifers (122,130,131).
Climate change and climate variability may have complex impacts on U.S. groundwater resources. Net impacts on groundwater will depend on the local, relative changes in recharge and pumping demand that occur in a given basin (132,133). Potential rises in sea level may result in increased rates of seawater intrusion, depending on the change in gradients and onshore groundwater levels (44). Global warming may also result in reduced recharge (e.g., due to increased evaporation) and/or increased pumpage (e.g., due to increased consumption).
Extensive and effective monitoring is required to ensure that groundwater supplies remain available for drinking water. Monitoring of groundwater is fundamentally different from that for surface water; the threedimensional physical, chemical, and microbial characteristics of the resource must be estimated from point measurements at individual wells. Therefore, assessing the quality of a groundwater basin is more complex and inherently uncertain. We suggest that the millions of U.S. residents who use private wells for drinking water and are not covered by the SDWA are particularly vulnerable; they have no systematic monitoring and thus the potential for a large undetected population exposure to chemical or microbial contaminants exists.

Surface Water Supplies
In June 1969, the Cuyahoga River caught fire, because of the wide array of flammables in the water and on its surface; it burned for 4 days. Shortly thereafter, the CWA was passed to address the widely and heavily polluted surface waters in the United States. Since then, controls on point sources and extensive sewage treatment have reduced U.S. water pollution significantly (134). A major unresolved issue is nonpoint source contamination, that is, unregulated discharges such as runoff from agriculture and animal husbandry, roads and other developed areas, and other sources (118,124,134).
About 167 million Americans use surface water as their public drinking water source (Table 2, Figure 2). Virtually none of the surface water in the United States is drinkable without treatment. Indeed, federal rules require disinfection of surface water used for drinking.
The 1996 amendments to the SDWA (14) placed a new focus on identifying and protecting drinking water sources. In coordination with water pollution control programs implemented under the CWA and other water quality protection laws, the 1996 SDWA amendments required that all states assess the problems that impair the designated uses of water, with a priority for use as drinking water. These assessments, done on a watershed basis, were required under both the SDWA ( §1453) and CWA ( §305[b]) and strengthened U.S. drinking water protection efforts, especially watershed protection.
The reports submitted by all 50 states listed siltation, nutrients, pathogens, oxygendepleting substances, metals, habitat alteration, pesticides, and organic toxic chemicals as the most common causes of surface water quality impairment (118,135,136). This pollution results primarily from runoff related to human activities (Table 4). Nationwide, agriculture is the most extensive source of water pollution, affecting 70% of impaired rivers and streams and 49% of impaired lake acres. Other national or regional causes of water quality degradation include municipal point sources, hydrologic and habitat modification, urban runoff and storm water (especially sanitary sewer overflows during rain events), resource extraction, and removal of streamside vegetation. Decreasing water quality and increasing eutrophication of many freshwater bodies are also resulting in increased algal blooms, especially cyanobacteria (45)(46)(47)(48)137,138).
As with groundwater, technological advances are occurring rapidly in the treatment of surface waters. For instance, over the next century, we believe that technological improvements may reduce the costs of desalination to the point where seawater becomes an economically viable source of drinking water supply in some U.S. coastal areas.

Regulatory History and Horizon
In the first half of the twentieth century, the U.S. Treasury Department and the U.S. Public Health Service (U.S. PHS) adopted drinking water guidelines for a few contaminants, including coliform bacteria (first issued in 1914) and some chemicals, such as arsenic (set in 1942) (139,140). These standards were binding on "interstate carrier conveyances" such as trains and aircraft, but states could adopt or reject federal standards for stationary public water systems. In fact, most states had not adopted or enforced these federal standards before 1974 (141). In 1974, Congress passed the SDWA (3). Over the next 12 years, the U.S. EPA ratified most of the 30 or so older U.S. PHS guidelines as "interim standards" and adopted one new standard (for THMs in 1979) (142). These standards then became enforceable nationally on all public water systems. In 1986, the U.S. Congress, frustrated by the slow pace of drinking water regulation, revised the SDWA significantly and mandated the U.S. EPA to establish new or revised standards for 83 specific contaminants; Congress also ordered the U.S. EPA to adopt 25 new contaminant standards every 3 years thereafter. The U.S. EPA issued over 80 new drinking water rules in the 10 years that followed.
A backlash ensued against the issuance of these new standards, resulting in an effort joined by many water utilities and state and local government officials to relax the 1986 requirements of the SDWA. However, this effort clashed with increasing public concern about drinking water safety. Events such as the 1993 cryptosporidiosis outbreak in Milwaukee, Wisconsin, that sickened over 400,000 people (143), and mounting evidence that some common contaminants posed significant health risks, created a countercurrent favoring more stringent protection of drinking water.
Congress overhauled the SDWA again in 1996. Important revisions included a new emphasis on public involvement and public "right to know" about drinking water quality through annual reports, which community water systems are now required to provide to their customers, as well as the "Boxer Amendment" requiring consideration of vulnerable populations such as pregnant women, children, and the chronically ill in setting standards. The law also established new provisions that allow consideration of costs and benefits and risk-risk trade-offs in setting tap water standards. The 1986 law's requirement for 25 new contaminants standards every 3 years was changed to require no fewer than 5 new contaminant standards to be considered every 5 years. The U.S. EPA is charged with reviewing and, if feasible, strengthening all tap water standards every 6 years. In addition, the 1996 law specifically requires the U.S. EPA to issue or update standards for arsenic (144), radionuclides, surface water treatment, filter cleaning and backwash practices, groundwater disinfection, and DBPs. Finally, the U.S. EPA is required to conduct a survey of the infrastructure needs of public water systems every 4 years.
An examination of the current status of individual state drinking water regulations revealed significant gaps between federal and state authority (145). There were also discrepancies among states.
We anticipate additional U.S. EPA standards in the coming decades. Certainly, microbial contaminants are likely to be the subject of upcoming standards (78). Rules related to disinfection of both groundwater and surface water supplies will likely be strengthened, for two reasons. First, filtration and disinfection practices that were believed to be adequate to control microbial risks have in some cases been shown to be inadequate (78,90). Second, there is ample evidence that groundwater systems are also vulnerable to microbial contamination (71).
Moreover, the U.S. EPA has agreed in 1992, as part of a regulatory negotiation, to adopt a rule that will address risks posed by distribution systems, such as cross connections, backflow, and other significant health risks from pipes that deliver treated water to customers (146,147). Existing standards for many other chemical contaminants are also likely to be strengthened, due either to legal requirements or to new evidence of health risks.
Given the long and ever-growing list of contaminants that the U.S. EPA is anticipated to regulate on a chemical-by-chemical and microbe-by-microbe basis, however, we note that a fundamental shift in regulatory approach may be preferable. Instead of continuing to rely upon the case-by-case approach with constantly changing standards as our knowledge of contaminants increases, water utilities (and public health) may be better served by adopting broadspectrum risk reduction, treatment, and prevention approaches that ameliorate many contaminants simultaneously. For example, a vigorous source water protection program may be able to reduce multiple chemical and microbial risks. In addition, membrane filters, advanced water treatment trains such as granular activated carbon, advanced filtration, and ultraviolet radiation disinfection can reduce or eliminate a wide array of chemical and microbial risks at once. Other possible approaches include risk-based strategies (as in New Zealand), allowing utilities a flexible method to sum across risks.
However, a continuing challenge has been persistent noncompliance with the U.S. EPA current regulations. Although 80% of U.S. public water systems have no reported violations, about 30 million Americans drink water each year from systems that report violations of health-based standards (148). The U.S. EPA reports that in 1998, over 10,000 systems violated health-based drinking water standards (149). In addition, there were 86,000 violations of federal requirements to monitor water or to report results. Overall, the data likely underestimate noncompliance because data audits show that states reported to the U.S. EPA only 55% of major violations and 10% of monitoring and reporting violations (147).

Conclusions
Among the major public health achievements of the United States in the twentieth century was the access of virtually all U.S. residents to a safe, reliable water source. Nonetheless, the nation faces numerous challenges to the continued provision of safe drinking water in the coming century: • The state of the public water infrastructure is inadequate to meet even our current needs. A major increase of resources is necessary. Changes in pricing, consolidation, and ownership may help to address some of these issues. • Global warming may have significant impacts on drinking water quality and quantity, affecting both groundwater and surface waters in the United States. • The risks of WBIDs, which have become more evident recently, are underestimated and underappreciated. Resurging and emerging diseases, along with a significant growth in the size of the U.S. population sensitive to infectious disease, may also necessitate additional public health attention. • A particular complication relates to drinking water disinfection. Growing evidence suggests that U.S. drinking water currently is associated with mild to moderate levels of WBID in the United States, even in systems meeting federal standards. At the same time, several studies are finding carcinogenic and other health effects associated with exposure to DBPs. • Land use pressures will also challenge U.S. drinking water systems in the next century and will require the coordinated actions of all those sharing watersheds or aquifers. • Groundwater aquifers are being depleted and contaminated; remediation will be expensive at best. Also, those using private wells may require regulatory or public health attention. • Surface waters have been cleaned up somewhat since the low point in 1969 when the Cuyahoga River caught fire. Still, virtually none of the surface water in the United States is drinkable without treatment. • We need better, more efficient and sensitive monitoring tools and strategies, especially to assess microbial risks and groundwater contamination. * U.S. EPA will need to update many regulations to address legal requirements and new health data; the current case-by-case approach may need to be reevaluated. Compliance with current regulations, especially in small systems, remains a challenge. It is likely that solutions to at least some of these will require institutional changes.