Alabama estuaries cover an area of 610 square miles, including Mobile Bay, the fourth largest estuary on the North American continent. Mobile Bay drains a watershed of approximately 43,662 square miles, receiving an average of 460,000 gallons per second of freshwater.
The U.S. EPA's National Coastal Assessment (NCA) is a multi-year partnership with EPA's Office of Research and Development (ORD), EPA's Regional office, all coastal states, and selected territories. Alabama entered the program in 2000 and sampled through 2004. Samples were collected to determine water quality, sediment quality, and biota at fifty sampling locations, each to be revisited on a yearly basis. The NCA program is based on EPA's Environmental Monitoring and Assessment Program (EMAP), using a compatible probabilistic program and a common set of environmental indicators to survey each state's estuaries and assess their condition. These estimates can then be aggregated to assess conditions at EPA's regional, biogeographical, and national levels.
The goal of NCA is to make statistically unbiased estimates of ecological condition with known confidence. To approach this goal, a probabilistic sampling framework was established among the overall estuarine areas along the Alabama coast. Under this design, each sampling point is a statistically valid probability sample. Thus, percentages of estuarine area with values of selected indicators differing from established environmental guidelines can be estimated based on the conditions observed at the individual sampling locations. Statistical confidence intervals around these estimates also can be calculated. Moreover, these estimates can be combined with estimates from other states or regions that were sampled in a consistent manner to yield national estimates of estuarine condition.
Alabama's sampling design for the base sites consisted of partitioning the estuaries and rivers of the coastal area into hexagonal quadrates. Each hexagon covered 55.2 square kilometers and the grid was placed randomly over a map of the estuaries of the State of Alabama. Computer iterations were then performed to randomly select at least one site in each quadrate. This was repeated until all sampling locations were at a water location (some quadrates were only partially over the water).
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