the coast every year (Cunningham and Walker, 1996). This increase in recreational usage, together with the impact of larger year-round populations, is stressing available resources and is making the safe and prudent management of these areas increasingly challenging. Coastal ecosystems face a variety of major environmental problems, including habitat modification, degraded water resources, toxic contamination, introduction of non-indigenous species, and shoreline erosion and vulnerability to storms and tsunamis.

Although recent improvements in hurricane, El Niño, and severe coastal storm forecasting have sharply reduced loss of life, the ongoing shift in U.S. population to the coastlines has resulted in an increase in risk to property and human life caused by storms and other geologic processes characteristic of coastal settings, such as earthquakes, landslides, and coastal erosion. Estimates of the present total value of insured property at risk range from $2 to $3.15 trillion (Lewis and Murdock, in press). In addition, millions of individuals are now at risk from rapid-onset events, such as tsunamis, for which present forecasting and early warning capabilities may be less effective. The rapidly increasing expenditures associated with relief and recovery from coastal disasters are of growing concern to both the federal government and the nation as a whole. These coastal disasters result when human behavior and natural processes combine to place homes, businesses, and the public infrastructure at risk.

In many coastal communities, declining groundwater levels have led to saltwater intrusion in previously pristine aquifers. Freshwater and saltwater flows are known to form a dynamic system on the continental margin. For example, freshwater springs have long been known off the southeast U.S. coast, and brine seeps have enabled unusual chemosynthetic biological communities to develop in deep water on the continental margin off Florida, California, and Alaska. Saltwater intrusion into freshwater aquifers in coastal areas illustrates that human activities can alter the flow unfavorably. Along the East Coast, onshore and offshore coastal aquifers form essentially contiguous regional units between Rhode Island and Florida. Little is known, however, about the details of the distribution, hydrology, and volume of freshwater in the coastal and offshore region or the extent of its connection with onshore aquifers. Understanding the controls that subsurface geology places on aquifer characteristics is critical to wise use of groundwater. How water (and other fluids, such as waste) flows through continental margin deposits, its interaction with the host sediments, and the relative role that changing sea level plays are often poorly known.

The effects of coastal problems ripple far beyond the coastal lands and beaches out on to the continental shelf and slope and even into the deep ocean. Poorly planned development in these areas can result in destruction of the salt marshes and wetlands that act as nurseries for many fish stocks. The damming of rivers can reduce the supply of sediment to beaches, accelerating coastal erosion. Pollutants can be transported either dissolved in the seawater or on particles that can carry these materials far offshore where they can affect the marine ecosys-



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