damages will require a time scale of decades to centuries. For example, groundwater in many parts of the nation is highly enriched in nitrogen, leaving a legacy of nitrogen pollution that, even without new inputs, will take many decades to be diluted by groundwater recharge. Thus, the current quality of any water resource reflects the past as well as ongoing contamination, and water, particularly groundwater, may not become clean even if future sources of contamination are eliminated.

As the global human population grows through this century the demand for clean water will increase. Humans already appropriate more than half of the annual renewable freshwater supply globally, and there are few untapped sources of clean freshwater in the places on Earth where most people live (Postel et al., 1996). Supplying clean water to the growing human population will require much greater water reuse and water treatment than in the past. As societies attempt to increase their standard of living, economic growth will likely exacerbate problems related to water quality and availability, because cleaning water requires energy and energy generation consumes and may contaminate water. The agricultural intensification necessary to feed the growing population and an increasingly urban global population are trends that are likely to further concentrate human and livestock wastes and place additional stress on water treatment systems. The decisions societies make about how to acquire, clean, and dispose of water have enormous impacts on the aquatic ecosystems that are the source of water and the recipient of wastes. The degradation of aquatic ecosystems and the loss of sensitive aquatic taxa can lead to a reduced capacity for natural wetlands, streams, and lakes to trap, store, and transform contaminants. This provides a positive feedback that can further exacerbate water quality problems. Finally, numerous opportunities exist for climate change to impact water quality.

Access to clean water is a political and social problem, but decision makers who are tasked with resolving the problem should be informed by the results of hydrologic research. Science will ensure that the knowledge base necessary to address the challenges of maintaining good water quality where it exists and restoring it when it has been degraded will be available in the future. Research opportunities related to water quality stem largely from a need to know the processes that control the evolution of water quality in both relatively pristine and in heavily impacted environments. The requisite research also spans spatial scales from local to global, and time scales from minutes to decades. Understanding will come through research on the transport and fate of the constituents that dictate water quality of surface and groundwater. Similar to the surge in analytical techniques enabling detection of minute amounts of contaminants in water, the committee anticipates a surge of discoveries from the hydrologic sciences

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