South Atlantic Water Science Center - Georgia
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The Apalachicola-Chattahoochee-Flint (ACF) River National Water Quality Assessment (NAWQA) Program
ACF Study Design: Goals of the study design
Much of the historical data and current monitoring programs focus on point-source inputs and their effect on mainstem rivers. Since the effects of nonpoint-source inputs from various land uses is poorly understood, the NAWQA Program has been designed to fill that information gap. Because the ACF River basin is too large to allow for the detailed study of each stream and aquifer, several small watersheds and aquifer systems were selected to represent a predominant land use and/or physiographic area. The term predominant land use is used to describe areas of mixed land uses that are dominated by one land-use type (e.g. 60 percent row-crop agriculture and a mix of other land uses), but are not homogeneous (e.g. 100 percent row-crop agriculture). The ideal study design for comparison of land-use effects on water quality is a system of paired watersheds having different, homogeneous land uses. The goal of NAWQA is to describe water quality in large areas of the country, however, large areas of homogeneous land use generally do not exist in the ACF River basin. Therefore, small watersheds with no point-source inputs and having mixed land use, predominated by the target land use and typical of other small watersheds in the area, were selected for study. These small watersheds, ranging from about 18-105 sq mi in area, represent a medium-sized scale of study that provides the link between small-scale studies (ie. farm-field level studies) and large-scale studies (ie. large tributaries and mainstem rivers draining mixed land uses and physiography, and containing point-source inputs). Because the goal of the study is to document water quality and describe the effects of land uses on water quality in the ACF River basin, it is necessary to study effects of land use at a medium scale, address some specific questions at a smaller scale, but ultimately be able to transfer what was learned at those scales to larger areas of the basin. It is this nested study design that will be described in greater detail in the following sections of this report.
During the development of the study design, the goal was to integrate surface-water, ground-water, and biological components where possible so as to be able to document the current water quality of the study area, to begin to describe the effects of predominant land uses on that water quality, and to lay the foundation for future evaluation of the surface- and ground-water resources as an integrated system. The design primarily focuses on nonpoint-source inputs of nutrients, sediment, and pesticides from agricultural, urban, and forested land uses. The primary agricultural land uses of interest are poultry production in the headwaters of the ACF River basin (Piedmont physiographic province) and production of row crops in the southern half of the basin (Coastal Plain physiographic province). The urban land uses of interest are intensive commercial areas, such as downtown Atlanta, and suburban residential areas, such as those surrounding Atlanta. In some parts of the country forested lands represent large undisturbed areas and are suitable for collection of background information. Forested lands in the ACF River basin generally are being managed silviculturally. But even though the forested lands have been, or are being disturbed, they are the best representation of background water-quality conditions, and their effect on water quality is of value for comparison to other land uses.
Surface-water Study Design
The surface-water system of the ACF River basin was stratified based on physiography and major land uses. Water-quality monitoring locations were then chosen to represent predominant land uses at various scales. The monitoring network reflects the nested study design described earlier, starting with a few fixed monitoring sites (integrator sites and indicator sites, a subset of which were intensive monitoring sites), adding a group of comparison sites, and finally a group of sampling sites on large tributaries and main-stem rivers. Water samples were collected at frequencies varying from hourly to annually, depending on the intended purpose.
Because there already was an extensive monitoring network in the basin, supported by State and Federal agencies, the NAWQA study design focused on three integrator sites located on mainstem rivers; two on the Chattahoochee River upstream and downstream of Metropolitan Atlanta, and one on the Apalachicola River near the mouth. The first two sites were selected to provide an estimate of total load of selected constituents in the Chattahoochee River upstream and downstream of a major source of point and nonpoint inputs to the river. Extensive monitoring of many point-source discharges to the river and tributaries provide data to estimate point-source loads of selected constituents and, by difference, estimate nonpoint-source loads from tributaries draining Metropolitan Atlanta. The site near the mouth of the Apalachicola River was selected to provide an estimate of total load of selected constituents entering Apalachicola Bay. These data are valuable for comparing current and historical water-quality conditions, for documenting current water-quality conditions, and for analysis of long-term trends in the future.
Six indicator sites located on small streams having drainage areas ranging from about 18-105 sq mi, represent target land uses and physiography. West Fork Little River, the most upstream basin, represents water quality in an area of intensive poultry production. The primary issue is nutrient input from poultry litter that is spread on pastures surrounding the production areas. Sope Creek and Peachtree Creek are located within Metropolitan Atlanta and represent water quality in intensive urban and suburban watersheds, respectively. Sope Creek receives runoff from residential areas, and from suburban commercial areas and transportation networks that are less dense than areas within the City of Atlanta. Peachtree Creek receives runoff from dense commercial areas and transportation networks associated with the City of Atlanta, and inputs from combined sewer overflows (CSOs). Snake Creek receives runoff from an area that is predominantly forested. Tracts of land within the basin have been harvested for pulp and lumber and, therefore, the basin does not represent an unimpacted control watershed. However, since about sixty percent of the ACF River basin is forested, and under some type of silvicultural management, Snake Creek is typical and representative of forested basins within the study unit. Lime Creek and Aycocks Creek represent water quality in areas of intensive row-crop agriculture. Unlike parts of the United States where large, continuous tracts of land often are farmed to the stream bank, farming in the Coastal Plain of the southeastern United States is generally limited to well-drained uplands. This results in smaller and more discontinuous farm fields that do not extend to the river bank. Instead, streams generally are protected from overland runoff by natural buffers consisting of forested wetlands and floodplains. However, the potential remains high for the movement of farm chemicals to streams because the climate and availability of ground water for irrigation are favorable for multi-cropping practices that can result in the application of nutrients and pesticides to fields throughout much of the year.
Sampling sites on Sope, Lime, and Aycocks Creeks were selected as intensive sites and were sampled frequently during a 1-year period to provide temporal data that defines the seasonal distribution of nutrients, sediment, and pesticides. Samples collected nearly once each week and several times during storm hydrographs, provided valuable information on the occurrence, magnitude, and distribution of constituents in the stream system. This information not only helps to assess the water quality of those representative basins, but also can be used to adjust future monitoring programs by targeting specific constituents and key times for conducting intensive sampling.
For each of the six indicator sites 5-6 comparison sites were chosen. Three sites were chosen for between-basin comparison and 2-3 for within-basin comparison. Those sites chosen for between-basin comparison met the same criteria as sites in the indicator basin. In theory, if data collected at all targeted land-use sites during basin-wide synoptic surveys indicated that all basins had very similar water-quality characteristics, then information gained through intensive monitoring of one could be transferred to the others. Within-basin comparison sites were selected to represent inputs from tributaries or sub-basins upstream of the indicator site to help define within-basin variability.
Monitoring sites were identified near the mouths of major tributaries, upstream and downstream of major reservoirs, and at additional locations on the main-stem rivers. These sites completed the surface-water monitoring network and assured a more complete spatial coverage than the sampling sites described above could provide alone. Synoptic surveys of the entire monitoring network were conducted three times during the period of study,June 1993, March and May 1994. Surveys were conducted during spring and early summer to coincide with periods of nutrient and pesticide applications to urban and agricultural lands.
Bed Sediment and Tissue Surveys
Two bed sediment and tissue surveys were conducted early in the project to provide information useful to the overall study design. An initial survey of 31 sites consisting of integrator, indicator, selected main-stem river, and reservoir sites was conducted in 1992 to determine occurrence of organic compounds and trace metals in bed sediments and the asiatic clam Corbicula fluminea. A second survey that included resampling many of the same sites and adding about 15 additional sites, primarily in urban and suburban watersheds, was conducted in 1993 to better define the distribution of organic compounds and trace metals throughout the ACF River basin. Additional bed sediment samples were collected at 19 sites in September 1994, following the record flooding caused by Tropical Storm Alberto. Because of the basin-wide distribution of Corbicula fluminea, it was exclusively analyzed to assess the bioaccumulation of organic compounds and trace metals in tissue, except at three locations in the Apalachicola River floodplain where Gambusia affinis holbrooki (mosquitofish) was used for tissue analysis.
Biological samples were collected at least once a year at each of the indicator sites and at their respective comparison sites, and a measure of terrestrial and in-stream habitat made once during the period of study. The most intensive sampling effort was conducted at the six indicator sites. A stream reach 6-10 times as long as the average width that contained replicate examples of the various habitats (i.e. pools, riffles, overhangs, submerged logs) was defined. As cross-sectional areas for sampling were identified, care was taken to minimize disturbance of the cross section. Individual quantitative samples of macroinvertebrates and benthic algae were collected from potentially rich habitats such as cobble and gravel substrates in riffles, and depositional areas such as sand and mud in pools. Qualitative samples were collected from these same habitats and, additionally, from other habitats such as the surfaces of living or dead vegetation, root and leaf mats, and overhanging banks in an attempt to provide data on relative abundance, and to better define a complete species list. Fish were collected using techniques such as electroshocking, seines, dip nets, or combinations of these techniques that provided the most representative sample of the fish community. Measurements of in-stream habitat included stream width, depth, and velocity; size and distribution of substrates; amount and type of submerged and emergent vegetation; and estimates of the percent of pools and riffles. Measurements of terrestrial habitat included bank slope and stability; vegetation type, size, and density; and percent of cover overhanging the stream. A less intensive sampling effort was performed at the comparison sites to conserve funds and human resources. Priority was placed on the collection of a representative sample of the fish community; single, qualitative/semiquantitative samples of macroinvertebrates and benthic algae, and measurements of in-stream habitat.
Because few ground-water data existed within the ACF River basin prior to this study, the ground-water monitoring network was designed primarily to provide information on the occurrence and distribution of a large suite of compounds that can be used to better identify problem areas, and define related questions and issues. Data collected during this first cycle of intensive data collection also will provide a valuable reference for comparison with the data collected during subsequent cycles of intensive data collection.
The study-unit survey was designed to characterize the quality of shallow ground water within the Upper Floridan Aquifer System and to determine the effects of land use. An area of about 3,900sq mi in the southern part of the study unit, known locally as the Dougherty Plain and Marianna Lowlands underlain by the Floridan aquifer system, was selected for the ground-water study-unit survey. The predominant land use in the Dougherty Plain and Marianna Lowlands is row-crop agriculture and orchards. The source of some public and most domestic water supply is the Floridan aquifer system, a highly productive fractured limestone aquifer having karst features. To establish sampling sites within the Dougherty Plain and Marianna Lowlands, the area was subdivided into 30 polygons of similar size and existing wells or, where present, one or two high-flow springs were chosen for sampling from each polygon. Forty two sites were selected for sampling. Depth to water in the monitoring wells ranged from 10-59 ft below land surface (one well was a flowing well with approximately 3 ft of head). Each site was sampled once in August or September 1995. Samples were analyzed for nutrients, pesticides, volatile organic compounds, trace metals, major ions, organic carbon, stable isotopes, and selected radionuclides. On-site measurements of ground-water levels, flow from springs, and field parameters were made at each site.
The agricultural land-use study was designed to determine the chemical quality of shallow ground water that underlies agricultural areas in a 6,700 sq mi area of the southern part of the ACF River basin. Sites for monitoring the surficial aquifer were located randomly using the computer program developed for NAWQA (Scott, 1990), and wells were installed according to NAWQA guidelines (Lapham and others, 1995) adjacent to and downgradient of 37 farm fields. Four reference wells were installed in forested areas to represent background water-quality conditions. The depth to the water table in the surficial aquifer monitoring wells ranged from about 3-67 ft below land surface. Surficial aquifers were selected for sampling rather than deeper aquifer systems because surficial aquifers are the first water-bearing zones to receive recharge from infiltration, and presumably are more susceptible to contamination. Therefore, water-quality conditions in surficial aquifers may serve as an early warning of potential contamination of deeper aquifer systems that are used for drinking-water supply and irrigation. Water samples were collected from all wells during summer 1993 and from most wells during spring 1994. The sample times represented low and high water-table conditions, respectively. The samples were analyzed for nutrients, pesticides, volatile organic compounds, major ions, organic carbon, and selected radionuclides. On-site measurements of water levels and field parameters also were made at each site.
The urban land-use study was designed to determine the chemical quality of shallow ground water that underlies Metropolitan Atlanta within a 350 sq mi area of the surficial drainage to the Chattahoochee River. Sampling sites were established by subdividing the study area into 30 polygons of equal area using the computer program developed for NAWQA (Scott, 1990), and then selecting an existing domestic or observation well, and where present, a spring, from each polygon. Forty locations were selected as sampling sites. Depth to water in the monitoring wells ranged from 2-29 ft below land surface. Each site was sampled once during the period from summer 1994 through spring 1995. Samples were analyzed for nutrients, pesticides, volatile organic compounds, major ions, organic carbon, trace metals, and selected radionuclides. On-site measurements of water levels, flows from springs, and field parameters also were made at each site.
The ground-water flow-system study was designed to track the transport of nutrients and pesticides from a field where they were applied, through the shallow flow system that underlies the field and the adjacent forested floodplain/wetland, to areas of discharge to the surface-water system. During previous studies of a 1,000 acre field, nutrients and pesticides were measured in two shallow ground-water monitoring wells adjacent to and down-gradient from the field. Three generalized flow paths were identified within the study area: 1) shallow ground water collected by a network of tile drains within the field that discharged to ditches; 2) shallow ground water flowing from the farmed upland area and discharging along the toe slope at the edge of the forested flood plain; and 3) shallow ground water flowing from the farmed upland area, through the alluvial sediments within the flood plain, and discharging directly to the stream. Sampling points were located along two transects from edge of field to stream. Shallow monitoring wells were installed at each sample point, including points adjacent to the stream and within the stream bed; lithologic information was recorded; and water samples were collected and analyzed from each well and from the stream. Based on an evaluation of those data, additional sample locations were selected to provide better characterization of the system. These included additional wells installed along the transects between existing locations, nests of wells installed at varying depths at existing locations, springs located along the base of the toe slope that separated the forested upland from the forested flood plain, and three pipes connected to a network of tile drains throughout the field that discharged into two drainage ditches that flowed through the flood plain. The complete network of sample locations included 34 wells, 3 springs, 3 drains, and 2 surface-water sites. The frequency of sample collection and the list of constituents analyzed in water samples varied, but a core of sites were sampled 3 times a year to represent different seasons and flow conditions. Most samples were analyzed for nutrients, pesticides, major ions, and organic carbon. On-site measurements of water levels, flows, and field parameters also were made during each visit.
The study design for the ACF River basin study unit was modified to include four special studies: 1) addition of an intensive network of synoptic sites within the poultry, urban and suburban basins; 2) analysis of sediment cores collected from six of the reservoirs within the study unit; 3) intensive sampling of the Flint River during record flooding; and 4) seasonal sampling of fish to determine community recovery following the record flooding. Each of these activities was pertinent to the assessment of water-quality conditions within the ACF River basin, and also provided information of value to both regional and national synthesis efforts.
A network of surface-water monitoring sites, which included the poultry indicator site, three-between basin comparison sites, and seven additional sites, were sampled during a two-day synoptic survey that represented baseflow conditions in the upper Piedmont region of the ACF River basin. Three surveys were conducted within Sope Creek which included the indicator and within basin comparison sites and an additional twelve sites located within the Sope Creek basin. Two surveys were conducted within the metropolitian Atlanta area. Eleven sites were sampled in May 1995. The second survey, conducted during July 1995, was expanded to include 39 sites within metro Atlanta and Columbus,including integrator, indicator, comparison, large tributaries, two additional mainstem sites, and an additional 19 sites located within the Peachtree Creek basin were sampled during the second synoptic.The purpose for these surveys was to locate areas of ground-water discharge that would provide the basis for a flow-system study, and to better define water quality in small basins affected by urban and suburban land uses. On-site measurements of flow and field parameters were made, and samples were collected and analyzed for nutrients, pesticides, major ions, and organic carbon.
The system of reservoirs within the study unit provided an opportunity to evaluate land-use changes and chemical inputs within the basin, as reflected by changes in the chemical composition of sediments deposited within the reservoirs. Sediment cores were collected from six of the major reservoirs for the purpose of defining changes within each reservoir and differences between reservoirs. Each core was divided into discrete subsamples that were age dated and analyzed for a suite of organic compounds, trace metals, major ions, nitrogen, phosphorus, and carbon.
During July 1994, Tropical Storm Alberto caused record flooding in southwestern and central Georgia, southeastern Alabama, and northwestern Florida. Parts of Georgia received as much as 28 inches of rainfall during the storm. The record flooding provided a unique opportunity to measure concentrations and loads of nutrients, suspended sediments, and pesticides during extreme hydrologic conditions. Water samples were collected from several locations affected by the flood, but the most frequent data collection within the ACF River basin occurred at the Flint River at Newton, Georgia, the most downstream site that was accessible throughout the flood. Nineteen samples were collected during the period July 5-26, 1994.
The record flooding also provided an opportunity to document the recovery of fish communities following the catastrophic event. The pre-flood fish community in Lime Creek, one of six indicator sites, had been documented as a part of the original study design based on samples collected in June 1993 and May 1994. To determine post-flood community structure three samples were collected during the period August 1994 through August 1995.