National Water Quality Assessment Program (NAWQA): Mobile River Basin Study
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Schedule of Study Activities
The Mobile River Basin study is one of several NAWQA studies that began in Federal fiscal year 1997 (October 1996). Study planning and design, and analysis of existing data will be done during the first 2 years, which is consistent with all NAWQA studies. After the 2-year planning period, surface- and ground-water and biological data will be collected intensively for 3 years during a high-intensity phase. A low-intensity phase follows for 6 years, during which water quality will be monitored at a selected number of sites and areas assessed during the high-intensity phase. This combination of high- and low-intensity-monitoring phases allows the NAWQA Program to examine trends in water quality over time. During the planning period, existing data and results from previous studies will be reviewed to understand the primary physical, chemical, and biological factors that affect water quality in the study unit and to identify gaps in the current data. Information obtained from reviews of previous studies, along with field checks of existing monitoring stations and candidate sampling sites and field reconnaissance data, will be used to design a sampling program for the study unit.
During the high-intensity phase, new chemical, physical, and biological data will be collected from selected areas on local and regional scales to describe the quality of water throughout the study unit. Measurements will be made to determine water chemistry in streams and aquifers; the quantity of suspended sediment and the quality of bottom sediments in streams; the variety and number of fish, benthic invertebrates, and algae in streams; and the presence of contaminants in fish tissues. Individual streams and aquifers, chemical constituents, and biological species will be selected for sampling to represent the important water resources and water-quality concerns in the study unit and the Nation. A series of technical and nontechnical reports describing results of high- and low-intensity-phase data collection and analysis are planned.
The NAWQA Program is designed to assess the status of and trends in the quality of the Nation's ground- and surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water. Consistent data-collection and assessment methods in all NAWQA studies make this possible and are critical for providing uniform and comparable information on water quality for the nation. Surface-water, ecological, and ground-water studies are done on local (a few square miles to hundreds of square miles) and regional (thousands of square miles) scales to understand the water-quality conditions and issues within a study unit. NAWQA study-unit data collected using this multiscale, interdisciplinary approach will be aggregated to provide national-scale water-quality assessments. Partnerships and cooperative studies between local, State, and Federal agencies can be developed to help meet specific needs. The basic design for the Mobile River Basin study unit described in the following sections is similar among NAWQA study units nationwide.
Surface-water quality is monitored at two types of sites, basic-fixed sites and intensive-fixed sites, which are determined by the frequency of the sampling. Most NAWQA study units have about eight basic-fixed and four intensive-fixed sites. Basic-fixed sites are sampled on a regular basis, usually monthly, for 2 years during the 3-year high-intensity phase. Intensive-fixed sites are monitored more frequently for at least 1 year to characterize short-term variations of water quality. Both types of sites are used to monitor water-quality constituents, such as basic field properties, major ions, nutrients, and pesticides. Volatile organic compounds (VOC's) and trace elements also may be monitored at selected sites. Monitoring sites are selected to determine representative water-quality conditions in relation to important environmental settings in the study unit.
Basic-fixed or intensive-fixed sites are further classified as either indicator or integrator sites. Indicator sites represent relatively homogeneous and usually small basins associated with environmental settings, such as a specific land use that is considered to be important for understanding water-quality in the study unit. Integrator sites are established at downstream points in large drainage basins that incorporate complex combinations of land-use settings. Water quality at integrator sites reflects the effects of multiple land uses and transport in the basin.
Synoptic surface-water sampling can be used to address a selected issue in one river basin or to provide greater spatial coverage and allow investigators to assess relations of fixed sites to streams throughout the study unit. Synoptic surface-water sampling involves short-term investigations of specific water-quality conditions at numerous sites during selected hydrologic periods, such as periods of low streamflow.
Ecological studies in conjunction with surface-water sampling activities are conducted to provide insight into ecological variability over time, relations between water quality and community structure and stability, and ecological differences with respect to various environmental settings. Aquatic biological communities are surveyed at basic- and intensive-fixed sites during the 3-year high-intensity phase. These investigations are conducted along delineated stream reaches and include aquatic and riparian habitat assessments and annual surveys of fish, algal, and benthic invertebrate communities. Trace elements and synthetic organic compounds are analyzed in bed sediment and fish tissue at selected sites to determine their occurrence and distribution and relation to land use and environmental setting. Ecological synoptic studies are conducted to evaluate spatial variability of biological communities or address issues of special concern within the study unit.
Ground-water studies in the NAWQA Program are typically composed of three components: (1)study-unit survey, (2) land-use studies, and (3) an optional flow-path study. The study-unit survey is intended to characterize water quality in the major aquifers of the study unit without targeting specific land uses. About 30 wells are randomly selected for sampling in each major aquifer subunit in the study unit. Ground-water samples are analyzed for major ions, nutrients, and pesticides. Trace elements and VOC's also may be monitored.
Land-use studies attempt to characterize the quality of shallow ground water associated with a particular land-use setting. Land-use studies emphasize recently recharge ground water so that the influences of land-use practices and environmental settings can be assessed. About 30 wells are randomly selected within a selected land-use setting for an aquifer. Results from land-use studies will be compared with results from study-unit surveys to determine the effect of land use on ground-water quality.
Flow-path studies are intended to help identify and quantify processes controlling shallow ground-water movement and quality. These studies, performed in selected study units, are designed to trace chemical changes that occur in water as it enters the ground-water flow system, travels along a flow path, and eventually discharges into surface waters.