Concern about both water resource vulnerability and the long-term sustainability of our water resources has rapidly increased across the United States over the past few decades. Broadly defined, water resource vulnerability refers to the overall vulnerability of surface water and ground water. Vulnerability includes water quality issues, such as pollution, as well as water supply issues, such as aquifer recharge or overuse. The causes, which can vary greatly, may stem from human populations or natural systems.

Long-term sustainability means that enough water is available to support natural systems and human populations over time, and that the supply of water is naturally replenished. Sustainability can be affected in many ways. For example, once a water supply is contaminated, the contamination may be difficult or impossible to fix. Or, if the natural flow of water is disrupted (for example, if stormwater is channeled directly to rivers and lakes in a series of pipes or ditches rather than being allowed to flow freely over the surface of the land and replenish underground aquifers or fill wetlands), its potential benefits are lost.

This paper explores the issue of water resource vulnerability, using the St. Marks Basin in northwest Florida as a model, and suggests how an assessment of vulnerability can be used as a planning tool in community design. The approach outlined here provides an avenue for raising important questions about the unique surface water and ground water resources of individual river basins, exploring the most crucial problems and issues from different perspectives, using our scientific knowledge as a basis for developing solutions, and building community support for resolving problems.

Vulnerability can be analyzed from two different perspectives: place and intended use. Important attributes of place include the following:

• Physical form comprises physical characteristics such as soils, geology, hydrology, weather, and climate.

• Basin function describes the valuable and varied "services" that the natural system carries out, such as flood control, water storage, and filtering of pollutants.

• Assimilative capacity refers to a natural system’s ability to assimilate wastes, both biologically and chemically, through its soils and native characteristics.

• Ecosystem integrity means the long-term diversity, stability, and sustainability of a river basin’s natural systems and biological communities.

• "Uniqueness of place," a key concept, refers to the fact that a river basin’s physical characteristics give rise to unique biological systems.

Intended use includes the following:

• Potable water supply, shellfish harvesting, recreation, and agriculture comprise four major water resource uses. They also constitute Florida’s four functional classifications, used to regulate surface waters. These classifications reflect a historically utilitarian approach to water resources in the United States that began with the European settlement in the 1500s. Almost completely centered on human needs and focused on the efficient use of water for energy production, navigation, flood control, irrigation, and drinking water, this approach ignored other important issues.

• Waste disposal, a common use of our water resources, was long taken for granted as the United States and Florida developed. The pollution that resulted from using surface waters as dumping grounds for human and industrial waste, however, had become evident by the 1950s and 1960s and led to many new federal and state laws.

• Natural system needs have only been recognized and acted on in the past fifteen years. A lesson on the needs of natural systems in Florida was learned the hard way with one well-known project—the ditching, draining, and filling of the Everglades—that was probably the largest water-control project in human history. The original Everglades system once comprised the entire lower third of Florida.

We can use the information we have about a place—including its unique geological structure, hydrology, chemical and physical characteristics, and historical, current, or future land uses—to map specific vulnerabilities for surface water and ground water resources. In Florida, these vulnerabilities are often closely linked: for example, underground aquifers and surface waters are connected in many areas, allowing polluted agricultural and urban stormwater to flow directly into drinking water supplies.

Figure 1 shows overall surface water vulnerability in the St. Marks Basin, with the darkest areas being the most vulnerable. This is the Tallahassee Hills physiographic region, where the land surface is underlain by clayey sediments that retard the downward movement of water. Although there is an occasional surface connection to ground water through sinkholes, pollution mainly flows across the surface into the area’s rivers and lakes. In contrast, the region that is least vulnerable to surface water pollution is the light area in the center, where soils are porous and water seeps rapidly into underground aquifers.

Additional vulnerability comes from both the intensity and type of local land uses. In the city of St. Marks, for example, these land uses include an industrial complex, the Purdom power plant, refineries, and a fuel-storage area—all on the banks of the St. Marks River. This kind of situation is common in the United States because, as discussed previously, water resources were long considered strictly utilitarian. Surface water vulnerability also increases because hazardous materials such as petroleum products are transported on the St. Marks River.

Figure 2 shows groundwater vulnerability in the St. Marks Basin. The darkest area, the Woodville Karst Plain, shows the greatest vulnerability. This area has a veneer of sands over limestone, and limestone outcrops often occur directly at the surface. Because the region contains many solution holes, sinkholes, and springs, many connections exist between surface water and groundwater. By contrast, the Tallahasee Hills area is least vulnerable because the near surface materials do not readily transmit water.

Source: Mark Brown, Ph.D., Center for Wetlands, Department of Environmental Engineering Sciences, University of Florida

As Figure 3 shows, over half of Florida’s aquifers are vulnerable to ground water contamination. In addition, because 90 percent of the state’s drinking water comes from aquifers, the effects of any contamination are potentially serious.

Figure 4 shows local and regional influences on groundwater vulnerability. The area closest to the withdrawal point is most vulnerable, while the area farthest away is the least vulnerable.

Source: Paul Lee, Ph.D., Florida Department of Environmental Protection

The cross-section of Leon and Wakulla counties, Florida, in Figure 5 illustrates these local and regional influences, which can also be interpreted in terms of travel times. For the most vulnerable areas in the Woodville Karst Plain and the coastal lowlands that have numerous underground connections, travel times can be brief—even as little as twenty-four hours—while for the least vulnerable areas in the Tallahassee Hills they can be years or even decades.

Source: Lyle Hatchett, Florida Department of Environmental Protection

Figure 5 also demonstrates how vulnerability can be short-circuited by interactions between surface water and ground water, ground water and surface water, surface water to surface water, and ground water to ground water. These interactions can be natural (for example, sinkholes), or human induced (for example, drainage canals). The diagram in Figure 6 illustrates how short circuits can affect local and regional influences on groundwater vulnerability.

Source: Paul Lee, Ph.D., Florida Department of Environmental Protection

Figure 7 illustrates a natural short circuit. The first picture shows Lake Iamonia in 1932, complete with canoeists paddling on the lake. Because the lake periodically drains through a sinkhole and refills, however, a picture taken the preceding year shows it completely empty.

The Lake Munson drainage basin in the St. Marks Basin is an example of a human-induced short circuit. A network of drainage ditches carries two-thirds of Tallahassee’s highly polluted stormwater to Lake Munson. The lake then drains via Munson Slough and flows underground at Munson (Ames) Sink, ten miles north and upgradient of Wakulla Springs.

Fortunately, over the years Lake Munson has filtered most of the pollution before it was carried underground. Figure 8 shows a screen capture from the Florida Department of Environmental Protection’s water quality assessment Web site, known as eBASE. The figure confirms that the water eventually making its way to Munson (Ames) Sink is of good quality.

In addition, historically high phosphorus levels dropped to a fraction of their previous levels once sewage effluent was diverted from the lake in 1985. Figure 9 shows that phosphorus loads and concentrations declined steeply once sewage effluent was diverted from Lake Munson.

Nitrate pollution remains a serious concern, however, and nitrate levels in Wakulla Springs have increased tenfold over the last twenty years. Although the process may take decades, the cross-section in Figure 5 clearly illustrates the fact that land uses in the Tallahassee area can eventually affect Wakulla Springs.

Source: Joe Hand, Florida Department of Environmental Protection

Surface water and ground water vulnerability can also be mapped jointly, as Figure 10 shows. Again, the darker areas represent the most vulnerable regions. In the St. Marks Basin, the Gulf Coastal Lowlands and the Tallahassee Hills are the least vulnerable areas, while the Woodville Karst Plain, coastal areas, and rivers are the most vulnerable.

Source: Mark Brown, Ph.D., Center for Wetlands, Department of Environmental Engineering Sciences, University of Florida

In designing new communities and retrofitting older ones, it is crucial to understand which areas within a river basin are most vulnerable if our water resources are to be protected and conserved over the long term. As Figure 11 shows, the relative vulnerability of individual basins varies significantly.

Source: Mark Brown, Ph.D., Center for Wetlands, Department of Environmental Engineering Sciences, University of Florida

Not only do we need to understand which river basins are most vulnerable, we must also understand the implications of specific land uses and human activities on those basins. For example, the extensive paving—for roads, houses, parking lots, and driveways—that accompanies development has an enormous impact on both the quality and quantity (amount, timing, and distribution) of water resources.

Generally, a natural system with no paved surfaces has about 10 percent runoff, 25 percent shallow infiltration, 25 percent deep infiltration, and 40 percent evapotranspiration. With 75 to 100 percent of the surface paved over, however, these proportions shift dramatically. Runoff increases to 55 percent, while shallow and deep infiltration fall to 10 and 5 percent, respectively, and evapotranspiration drops to 30 percent. As a result, beneficial water uses are lost—for example, aquifer storage diminishes significantly and evapotranspiration, which is essential to maintaining Florida’s cyclical summer rains, is reduced by a fourth. In addition, the five-and-a-half-fold increase in polluted runoff significantly affects water quality in receiving waters.

Vulnerability also varies depending on one’s location in a river basin. For instance, from the viewpoint of a homeowner who uses a septic tank to treat sewage, the tank fails only when sewage from the system actually backs up in the home. Otherwise the adage "out of sight, out of mind" usually applies. Yet those living downstream who are affected by septic tank pollution or whose wells become polluted with nitrate have a different perspective on system failure.

Septic tanks may be a relatively minor contributor to nitrate contamination compared with the major sources: agricultural fertilizers, dairies, and poultry farms. Yet we need to control all sources, including septic tanks, for once nitrate pollution enters ground water, it is difficult to clean up and often travels great distances to public and private wells or natural springs.

Once surface water and ground water vulnerabilities are understood, that knowledge can be used to plan the long-term development of our communities and make better land use decisions—for example, designating protection zones or buffers around lakes, rivers, and sinkholes or using other means to protect the most vulnerable areas in each river basin. The information can also be used to restore water bodies that have been injured in the course of development.

Figure 12 shows the Lake Lafayette Protection Zone that has been proposed within the St. Marks Basin, based on the lake’s vulnerability to contamination. A protection zone would help to filter polluted stormwater before it enters the lake. Significant changes will need to be made in the future land use map at the right, however, if the lake is to be protected over the long term. Currently, significant development is shown around the lake but no protective buffers have been planned.

Figure 13 shows the boundaries of Wakulla Springs State Park. Wakulla Springs, located within the park boundaries, is buffered from polluted stormwater runoff from surrounding land uses. These boundaries, however, do not limit the flow of ground water that might carry pollutants from sources as far away as Tallahassee and southern Georgia. The vulnerability of Wakulla Springs to ground water contamination demonstrates the need to connect local and regional land use decisions with natural resource protection needs.

As illustrated by the example of the St. Marks Basin, water resource vulnerability can be an important tool in planning and designing future communities and retrofitting older ones. Analyzing vulnerability from different perspectives such as place and intended use allows us to understand and explore a basin’s crucial problems and issues, develop science-based solutions, and build community support for those solutions.

In this and many similar situations, we can choose to acknowledge the valuable services that natural systems provide—such as flood protection and filtering of pollutants—and protect our water resources at the outset. By far the cheapest way to protect these resources is to preserve natural buffers along lakes and rivers rather than pave them over. If we allow our water resources to become contaminated, the cleanup will be costly (or impossible, given the limits of current technologies). Engineering for services such as water storage and filtration is also expensive.

If we fail to understand the basic vulnerabilities of our water resources and the link between land uses and pollution, our natural systems will eventually be obliterated and the quality of life for residents and visitors will diminish. The effects would be devastating for us, for Florida’s economy, and for the other species who share this unique and valuable place.