Increases in population, water use, and urban and agricultural land development affect the quality of surface- and ground-water resources in parts of the lower Tennessee River Basin. State water-quality agencies have identified 109 stream segments and 3 lakes in the study unit that are water-quality impaired with respect to their designated use (Alabama Department of Environmental Management, 1996; Georgia Department of Natural Resources, 1996; Kentucky Natural Resources and Environmental Protection Cabinet, 1996; Mississippi Department of Environmental Quality, 1996; Tennessee Department of Environment and Conservation, 1996). The primary nonpoint sources affecting surface-water quality cited by these agencies were agricultural and urban runoff. In order of frequency of citation, the predominant causes of impairment resulting from nonpoint sources were siltation, organic enrichment and dissolved-oxygen depletion, nutrient enrichment, and pathogens. The primary point sources affecting surface-water quality cited by these agencies were industrial and municipal discharges. In order of frequency of citation, causes for impairment from point sources were pathogens, organic enrichment and dissolved-oxygen depletion, ammonia, and siltation.
These State agencies also have documented contamination of ground water from both nonpoint and point sources. Ground-water studies conducted in the study unit have documented shallow ground-water contamination by pesticides, nutrients, and bacteria from nonpoint sources (Alabama Department of Environmental Management, 1996; Tennessee Department of Environment and Conservation, 1996). Carbonate rock aquifers are common in the study unit and have a high potential to be affected by nonpoint sources. Point sources affecting ground-water quality included underground storage tanks, landfills, and contamination from industrial sites. Volatile organic compounds were the constituents most often associated with point-source contamination of ground-water aquifers.
The changes in water quality as a result of contamination from these sources can affect both human health and the health of aquatic ecosystems. Excessive nitrate concentrations (greater than 10 mg/L as N) in untreated, domestic drinking water can pose a health risk to infants by causing methemoglobinemia, or blue baby syndrome. Nutrient enrichment in surface-water bodies may produce algal blooms that cause taste and odor problems in drinking water. Excessive growth of algae and aquatic macrophytes also can deplete dissolved oxygen in streams and reservoirs, adversely affecting fish and other aquatic life. Degradation of water quality and destruction of instream and riparian habitat threatens freshwater mussels and fish in streams in the study unit. For example, the Muscle Shoals area, near Florence, Alabama, once had one of the most diverse assemblages of mussels in the world. Many species have become extinct and several of the remaining species are now endangered or threatened. Pesticides and other organics can be toxic to aquatic invertebrates and fish or may accumulate in fish tissue and sediment. Six fish consumption advisories have been issued for streams and reservoirs in the study unit. These advisories include DDT in fish in Indian Creek, polychlorinated biphenols (PCB's) in fish in Woods Reservoir and Nickajack Lake, and chlordane in fish in Nickajack Lake (Alabama Department of Environmental Management, 1996; Tennessee Department of Environment and Conservation, 1996 ).
Data for nutrients and bacteria in surface and ground water collected between 1980 and 1996 and available in digital format were summarized by subunit to characterize the water quality of the subunits for constituents that represent major water-quality issues in the study unit. Nutrients and bacteria were selected because these constituents were cited as affecting both surface- and ground-water quality. The U.S. Environmental Protection Agency's STOrage and RETrieval (STORET) data base and the U.S. Geological Survey water-quality data base WATer STOrage and REtrieval system (WATSTORE) were the primary sources for both surface- and ground-water-quality data. Additional data from local ground-water studies conducted by State agencies were included in the data set. Data retrieved from STORET and WATSTORE were screened to select ambient data. Median values were used in this data summary for sites with multiple samples. Data likely to be affected by point sources were excluded from the data set. Only surface-water-quality sites with drainage areas contained within a single subunit were included in this data summary. Ground-water-quality data were limited to sites that represented the shallow ground-water flow system. In most of the subunits, wells with depths less than 300 ft were selected, but in the Coastal Plain and Transition, wells less than 200 ft deep were selected.
Data from STORET and WATSTORE were combined to improve the spatial distribution of sites in each subunit. Combining the data bases resulted in a total of about 520 surface-water sites with water-quality data for at least one of the nutrients (nitrogen and phosphorus) and about 360 surface-water sites with data for fecal coliform occurrence. The distribution of surface-water sites was similar for STORET and WATSTORE, though the total number of sites was considerably higher in STORET (fig. 23). Most subunits had 40 or more surface-water sites with data for nutrients and 30 or more sites with data for fecal coliform. The Eastern Highland Rim had the most surface-water-quality sites. The Inner Nashville Basin and the Valley and Ridge, the smallest subunits, had the fewest number of sites with data for both nutrients and bacteria.
Ground-water data from the two data bases were unevenly distributed across the study unit (fig. 23). Combining data from STORET and WATSTORE and the local ground-water studies yielded about 1,240 wells and springs with data for at least one nutrient. Much of the available ground-water data for nutrients were collected in the Eastern Highland Rim as part of a study by the Alabama Department of Environmental Management, in which about 400 shallow wells located in agricultural areas in four counties in northern Alabama were sampled twice for nitrate and four pesticides (E. Bittner, Alabama Department of Environmental Management, written commun., 1997). About 490 ground-water sites had data for fecal coliform.
Nutrient enrichment was cited as the cause of impairment for 37 of the 109 impaired stream segments and was identified as an important issue for ground-water quality in the lower Tennessee River Basin (Alabama Department of Environmental Management, 1996; Georgia Department of Natural Resources, 1996; Kentucky Natural Resources and Environmental Protection Cabinet, 1996; Mississippi Department of Environmental Quality, 1996; Tennessee Department of Environment and Conservation, 1996). Nutrients were the most commonly analyzed water-quality constituents in surface and ground water in the lower Tennessee River Basin during 1980-96. Nitrogen can be found in surface and ground water in several oxidation states. In its reduced valence states, nitrogen is found primarily as ammonia, ammonium, and organic compounds; in its oxidized valence state, nitrogen is found primarily as nitrate and nitrite. Phosphorus also can occur in several oxidation states, but in natural waters the fully oxidized species (phosphate) is most common, but phosphorus also can be found in organic compounds. Phosphorus in its oxidized state has a low solubility and is often associated with sediment or other particulate material.
Given the various forms that nitrogen and phosphorus can be found in natural waters and the fact that data were collected for different objectives, available nutrient data were stored under a mix of different parameter codes. To obtain adequate spatial coverage across the study unit, data stored with different nutrient parameter codes were combined. For example, data for nitrate and nitrite were commonly reported as nitrite-plus-nitrate, nitrite, or nitrate in both dissolved (filtered) or total concentrations (unfiltered). Generally, nitrite concentrations are small compared to those of nitrate in oxic waters (including shallow ground water), and the concentrations of nitrite and nitrate associated with sediment and particulate material also are small. Therefore, nitrite-plus-nitrate and nitrate, filtered and unfiltered, were combined as an estimate of nitrate-nitrogen, when data for only one of these constituents were available. Ammonia and organic nitrogen were summed as an alternate to total Kjeldahl nitrogen (total ammonia and organic nitrogen) where total Kjeldahl nitrogen was not reported. If more than one minimum reporting level was present in the data, the highest minimum reporting level was applied to all of the data.
Nitrate-nitrogen was the most commonly reported nutrient species in both surface- and ground-water samples between 1980 and 1996. Median concentrations of nitrate in surface water in each subunit were less than 1 mg/L (fig. 24). Median concentrations (0.6 to 0.8 mg/L) were higher in the Outer and Inner Nashville Basins, Eastern Highland Rim, and Cumberland Plateau, than in the remaining subunits (less than 0.3 mg/L). Median concentrations of nitrate in ground water in most subunits were less than 1 mg/L, but were greater than 1 mg/L in samples collected in the Eastern Highland Rim and the Cumberland Plateau (fig. 24). The ninetieth percentile concentration in the Cumberland Plateau exceeded the recommended drinking water maximum contaminant level of 10 mg/L (U.S. Environmental Protection Agency, 1995). Median nitrate concentrations were higher in ground water than surface water in most subunits (fig. 24). In both the Outer and Inner Nashville Basins, however, median concentrations were higher in surface water. In all subunits, concentrations at the ninetieth percentile were higher in ground water than in surface water.
Median concentrations of total Kjeldahl nitrogen were generally less than 0.5 mg/L in each subunit (fig. 24), except those in the Inner Nashville Basin, Valley and Ridge, and Cumberland Plateau which exceeded 1 mg/L. Ground-water data for total Kjeldahl nitrogen were not available in many of the subunits, and data were insufficient in the remaining subunits to make a meaningful comparison. Concentrations for total Kjeldahl nitrogen in ground water, where available, were less than 1 mg/L.
Nutrient concentrations in surface and ground water were compared to the density of agricultural land use in each subunit. The rank of median concentrations by subunit for nitrate and total Kjeldahl nitrogen (fig. 24) in surface water roughly correlates with the percentage of agricultural land use in each subunit (table 2) and estimates of total nitrogen input from nonpoint sources (fig. 22). Concentrations of nitrogen in surface-water samples are highest in the Outer and Inner Nashville Basins, Eastern Highland Rim, and the Cumberland Plateau. Agricultural land use was also greatest for these subunits. However, the Coastal Plain, which had total nitrogen inputs comparable to the Outer Nashville Basin (fig. 22), has small nitrogen concentrations relative to the other subunits (fig. 24). The lower than expected nitrogen concentrations in samples collected from streams in the Coastal Plain may be a result of the type of nitrogen input and hydrogeology of the subunit. The nitrogen input from livestock waste is much lower in the Coastal Plain than in the other four subunits with comparable total nitrogen input (fig. 22). No relation could be identified between nitrogen concentrations in ground water and land use in a subunit. Agricultural land use was highest in the Inner Nashville Basin, but nitrate concentrations in ground water were lowest in this subunit.
Natural setting is likely as important to concentrations of total phosphorus as the distribution of inputs of phosphorus in the lower Tennessee River Basin. Total-phosphorus data were available for about 350 surface-water sites and fewer than 70 ground-water sites in the nine subunits. Median concentrations of total phosphorus for surface-water sites were highest (about 0.3 mg/L) in the Inner and Outer Nashville Basins; median total phosphorus concentrations were about 0.1 mg/L or less in the remaining subunits (fig. 24). Phosphatic limestones present in the Inner and Outer Nashville Basin likely contribute to the elevated phosphorus concentrations there. This natural source also would account for the relatively high concentrations of phosphorus in streams in these subunits compared to other subunits with similar phosphorus inputs (fig. 22). Total-phosphorus data for ground water were limited, and only two subunits were represented by more than four sites. Total-phosphorus concentrations in ground water generally were less than 0.1 mg/L for most sites. No relation was evident between total-phosphorus concentrations in surface or ground water and land use or nonpoint phosphorus inputs.
Pathogens were cited as the cause of impairment for 29 of the 109 impaired stream segments and also were identified as an issue for ground-water quality in the lower Tennessee River Basin by State water-quality agencies (Alabama Department of Environmental Management, 1996; Georgia Department of Natural Resources, 1996; Kentucky Natural Resources and Environmental Protection Cabinet, 1996; Mississippi Department of Environmental Quality, 1996; Tennessee Department of Environment and Conservation, 1996). Fecal coliform bacteria are used to identify contamination to water bodies by waste from warm-blooded animals. Fecal coliform bacteria generally are not disease-causing, but their detection can indicate the presence of other more dangerous pathogens. Of the fecal-indicator bacteria commonly used in water-quality assessments, data for fecal coliform were the most frequently reported in both STORET and WATSTORE for the period 1980-96 and are summarized in this report.
Median counts of fecal coliform were higher in surface water than in ground water for each subunit (fig. 25). The highest median counts in surface water were about 7,500 colonies per 100 milliliters (col./100 mL) in the Valley and Ridge and about 5,000 col./100 mL in the Outer Nashville Basin; however, data were reported for only three sites in the Valley and Ridge. The typical range in counts of fecal coliform in surface water is less than 1 to 5,000 col./100 mL and between 200 to greater than 2,000,000 col./100 mL in fecal-contaminated surface water (Wilde and others, 1997). The criteria for protection of recreational water bodies in Tennessee is 1,000 col./100 mL in a single sample (U.S. Environmental Protection Agency, 1999). Median reported fecal coliform counts for the Inner and Outer Nashville Basin and Valley and Ridge were greater than 1,000 col./100 mL, suggesting that fecal contamination of surface water in these subunits is somewhat higher than in the remaining subunits. Median concentrations of fecal coliform in ground water were well below 200 col./100 mL in all subunits; however, no data were available for the Valley and Ridge. Median fecal coliform counts were highest for the Outer and Inner Nashville Basins and were lowest (less than 1 col./100 mL) for both the Coastal Plain and Transition; however, the number of sites with data from these subunits was very low.
Natural setting is an important factor affecting fecal-contamination of surface and ground water in the study unit. Fecal coliform counts in both surface and ground water generally were highest in subunits underlain by carbonate rocks, and especially the Ordovician carbonate rocks that have thin to absent regolith overlying bedrock. Sources of fecal contamination cited by State water-quality agencies include sanitary and combined sewer overflows, leaking septic systems, and livestock waste. The correlation of fecal counts with the distribution of these sources is not a straightforward analysis, because data are not available to estimate the distribution and magnitude of sewer overflows and leaking septic systems. Population and livestock densities were used as indicators of potential sources within each subunit and were compared to the rank order of fecal coliform counts (fig. 25). In general, higher median fecal coliform counts were related to a greater population and livestock density (figs. 20 and 25). Exceptions to this relation were the Outer Nashville Basin, which ranked sixth in both population and livestock density but ranked first in median fecal coliform counts; and the Cumberland Plateau ranked relatively low in fecal coliform counts in surface water but was ranked highest in livestock density. Fecal coliform counts in ground water from the Outer Nashville Basin also ranked high compared to the relative density of population and livestock.
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