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about 1,200 feet downgradient before being eliminated from the aqueous phase. • Microbial degradation processes, both aerobic and anaerobic, are probably the most significant mechanisms in contaminant attenuation. Sequential degradation of phenolic compounds has been documented, and continuing research is describing anaerobic degradation of the substituted aromatic compounds. Most of the compounds composing the water-soluble fraction of creosote appear to be degradable, through intermediate organic compounds, into methane and carbon dioxide. • Dissolved gases (methane and carbon dioxide) and inorganic constituents (hydrogen sulfide, ammonia, and iron) are indicative of some of the byproducts resulting from breakdown of selected organic contaminants. In contaminated parts of the water-table zone, water is anaerobic within a few feet of the water table. Unusually high values of stable isotope ratios of carbon and sulfur are byproducts of microbial degradation. For instance, the lighter carbon-12 isotope preferentially forms methane (a gas), whereas the heavier carbon-13 enriches the inorganic carbon (aqueous) phase. The “C/*C ratio is greatest beneath the impoundments and decreases with distance away from the source of contamination. The presence of an iron-rich clay in the contaminated aquifer possibly is a result of interactions between the clay and organic contaminants. • Sorption does not appear to be a significant process in most of the aquifer, which is composed of clean quartz sands. Interactions, including sorption, between minerals in the clay lenses and organic contaminants may be quite important in the attenuation of selected compounds. • Organic compounds have been found in sediments beneath Pensacola Bay. Benthic organisms and sediments from near the site have been collected and analyzed for bioaccumulation of organic compounds indicative of creosote contamination. Some evidence of bioaccumulation has been found, although at very low concentrations.
Interrelated physical, chemical, and microbial processes must all be evaluated when attempting to understand and predict
the effects of contamination on the subsurface environment. Because some combination of these processes will occur in all aquifers, there may be significant transfer value from this investigation to other sites of ground-water contamination. Data on wastewater migration from surface impoundments into the subsurface clearly indicate that surficial aquifers are highly susceptible to contamination from a variety of sources. The wide variety of potential contaminants, combined with the locally complex subsurface geohydrologic environment, however, results in uniquely contaminated systems that are difficult to document. After contaminants leak into a surficial aquifer (a relatively rapid process), the generally slow rates of attenuation, chemical reactions, and physical mixing, combined with the large volumes of contaminants in the subsurface, indicate that, after the source of contamination is removed, hundreds of years would pass before the water in the aquifer would be restored, by natural processes, to its precontamination quality. Both the investigation of the extent of organic contaminants in a contaminated ground-water system and the cleanup and restoration of that aquifer are complex and time-consuming tasks.
Fate and Transport of
The presence and effects of hazardous organic compounds in the aquatic environment have been documented nationwide in recent years. These compounds are released to the environment from a variety of sources such as industry, agriculture, urban runoff, oil and gas exploration activities, and atmospheric fallout. The lower Calcasieu River in southwestern Louisiana (fig. 6) is a prime example of an aquatic system that has been affected by hazardous organic compounds.
Figure 6. Study area and waterquality sampling sites of the lower Calcasieu River, Louisiana.
The lower Calcasieu River is a tidally affected stream that lies within Louisiana's Coastal Plain. The upper reach of the Calcasieu River is characterized by hardwood forests, cypress, and related vegetation. Rice and soybeans are the principal crops grown in the upper and middle basin, including the upper part of the study area. Long-term water-quality data (1968 to present) collected at the National Stream Quality Assessment Network (NASQAN) site at Kinder (fig. 6) indicate that the upper reach of the Calcasieu River is a dilute, colored-water stream. Specific conductance ranges from 13 to 187 microsiemens per centimeter (p.S/cm) at 25°C. Dissolved organic carbon (DOC) concentrations are 5 to 6 milligrams per liter (mg/L).
Streamflow at the Kinder site ranges from
plants are located in this 14-mile section of the river. The specific conductance of river water in this section ranges from 200 to 33,000 p.S/cm. The DOC ranges from 3 to 4 mg/L in the middle reach of the river. Petrochemical and agrichemical plants use the lower Calcasieu River in this section for water supply, navigation, and disposal of wastes. Byproducts such as oil and grease, phenols, and metals are discharged into the river by chemical industries in the Lake Charles area. The second section is bordered upstream by the Intracoastal Waterway and downstream by the Gulf of Mexico. Commercial and sport fishing and oil
and gas drilling support facilities are major industries in this reach of the river. Previous studies have attributed the occurrence of hazardous organic compounds in the water, bottom material, and aquatic organisms of the lower Calcasieu River to industrial activity in the basin. However, none of the studies determined the processes that control the fate and transport of these organic compounds in relation to the physical characteristics of this river. The U.S. Geological Survey began a study in 1985 of the lower Calcasieu River, therefore, to determine the processes that control the fate and transport of organic compounds in the industrial
Sediment Transport of Pollutants in the
A multidisciplinary project to investigate the movement, mixing, and storage of sediment-related pollutants in the Mississippi River was begun in 1987. This project is focused on the 1,243 miles of the river between St. Louis, Missouri, and New Orleans, Louisiana, and will require at least 3 years (1988–90) of intensive field sampling. In addition to providing a comprehensive assessment of the water quality of the Mississippi River, the project will improve understanding of the ways in which rivers process sediments and the pollutants associated with sediments. Among the specific research issues to be addressed are the partitioning of pollutants between particulate, adsorbed, and dissolved phases; the mixing of different types of pollutants and mineral suites that are contributed by the major tributaries; and the storage and remobilization of sediments and their associated pollutants at seasonal and longer time scales. Samples will be collected during repeated cruises that will begin at a section of the river above St. Louis and will cover a 1,119-mile reach of the Mississippi and its major tributaries, including the Ohio. Laboratory analyses will be conducted on a wide range of organic and inorganic constituents (natural and pollutants) of the suspended sediment as well as the river water. By repeating measurements and samplings every 4 months or so, data will be obtained on the downriver routing and mixing of water, sediment, and pollutants and on how water-quality conditions of the Mississippi River vary seasonally. The Mississippi River drains about 40 percent of the conterminous United States and discharges an average of 650,000 cubic feet per second (420,000 billion gallons per day) into the Gulf of Mexico. Nearly half of the water in
the system comes from the Ohio River drainage (and half of the Ohio contribution is provided by the combined discharges of the Tennessee, Cumberland, and Wabash Rivers). The Upper Mississippi and Missouri Rivers contribute only 10 to 15 percent each of the total water discharged to the Gulf. The most striking feature of the lowermost part of the Mississippi system is the diversion of part of the discharge out of the mainstem and down the Atchafalaya distributary. The proportion of the discharge that goes down the Atchafalaya is held at 30 percent of the total by the Old River Control Structures, completed by the U.S. Army Corps of Engineers in 1963 and 1987.
The present-day discharge of suspended sediment to the Gulf of Mexico by the Mississippi River (including the Atchafalaya) averages about 210 million tons per year. Although most of the principal tributary sediment loads have been measured to a reasonable degree of accuracy, the estimate of the sediment discharge of the Ohio River represents a major uncertainty. No comprehensive suspendedsediment data have ever been collected from the Ohio mainstem.
The sediment regime in the Mississippi has been altered drastically during the last 200 years, especially during the last 40 years when large dams were built across the major western tributaries. The greatest changes have been the reductions in the sediment loads of the NMissouri, Arkansas, and Red Rivers that followed the Construction of dams and reservoirs. The greatest apparent increase has been in the sediment load of the Ohio; if this increase is real, it may represent the accelerated erosion brought about by deforestation, crop farming, and coal mining in the Ohio basin.
Figure 7. Concentration of bromoform under moderate wind conditions, May 1985, and low wind conditions, August 1985,
in the lower Calcasieu River.
reach and in the transition zones between brackish water and freshwater. Two classes of organic compounds, volatiles and acid-base/neutral extractables, were selected for study on the basis of results of reconnaissance sampling trips at lower Calcasieu River sites completed in 1985 (sites sampled are listed in table 1). The two classes of organic compounds were found to move in distinctly different ways in the aquatic environment and were associated with different media. Volatile organic compounds (VOC's) were found primarily in the river water and were selected for study on the basis of the widespread odor of organic compounds detected in the air during the first reconnaissance sampling trip. Detection of these compounds at low concentrations in water samples collected during this trip led the USGS to investigate the effects of wind speed, sampling techniques, and compound density on the presence and concentration of these compounds in the lower Calcasieu River. The presence and movement of VOC's in the lower Calcasieu River appear to be a function of wind speed and water density. Analysis of river-water samples collected under different wind conditions during four sampling trips indicated that wind speed, and the turbulence it created, was a primary factor in controlling the concentrations of VOC's. For example, water samples were collected during moderate (15.3
Table 1. Location of water-quality sampling sites for the lower Calcasieu River, Louisiana
Sampling site no. Location (See fig. 6) 1 Calcasieu River east of Moss Bluff 2 Bayou Serpent east of Moss Bluff 3 West Fork Calcasieu River westnorthwest of Goosport 4 Calcasieu River at Buoy 130 at Lake Charles 5 Calcasieu River at Buoy 114 at Lake Charles 6 Calcasieu River at Bayou d'Inde 7 Calcasieu River 3.9 miles south of Hollywood 8 Calcasieu River at Burton Landing 9 Calcasieu River at Devil's Elbow 10 Calcasieu Lake northeast of Hackberry 11 Calcasieu River at buoy 47 southwest of Cameron 12 Bayou d'Inde 0.25 mile upstream from an industrial outfall 13 Industrial outfall at Bayou d'Inde 14 Bayou d'Inde 0.25 mile downstream from an industrial outfall 15 Bayou d'Inde 0.50 mile downstream
from an industrial outfall
miles per hour) and low (7.5 miles per hour) wind conditions in May and August 1985 from a reach of the lower Calcasieu River extending from Lake Charles to Burton Landing (fig. 6). Samples were collected throughout the water column under similar temperature and specific-conductance profiles in the water column. Results showed that only four VOC's were detected at two sites (sites 5 and 6) during moderate wind conditions, compared with six compounds detected at four sites (sites 5 through 8) during low wind conditions. Also, VOC's such as bromoform (fig. 7), chloroform, 1,2-dichloroethane, and chlorodibromomethane were found in concentrations as much as 5 times greater during low wind conditions than during moderate wind conditions. Thus, sampling for volatile organics during moderate to strong wind conditions can lead to erroneous conclusions about the presence and potential impact of these compounds in the aquatic environment. Water samples also were collected at different depths in the water column from the same reach to determine the effects of different densities of organic compounds and specific-conductance profiles on the distribution of VOC's in the lower Calcasieu River. This sampling was conducted to ascertain if the density of a VOC determines where in the water column that compound might be concentrated. Position of a compound in the water column is very important in determining the type of sampling gear and methods needed to accurately describe the presence and concentration levels of organic compounds in the river. For example, VOC's such as bromoform, which has a density of 2.9, would be expected to be found in greater concentrations lower in the water column than lighter VOC's such as 1,2-dichloroethane, which has a density of 1.3. If this in fact occurred in the river, then depthintegrated samples collected throughout the water column or from near the surface would indicate an abnormally low concentration of bromoform in the river, while water samples collected at specific depths (point samples) would tell where the bromoform was concentrated in the water column and therefore would have the greatest environmental impact. Accordingly, point samples were collected in May 1986 during low wind conditions (6.9 miles per hour) and April 1987 during moderate wind conditions (17.0 miles per hour). Results from the May 1986 sampling (fig. 8) indicated that the different densities of organic compounds had little effect on the vertical distribution of these volatiles in the water column. Concentrations of VOC's at different depths were similar for the six sampling sites (fig. 8), except for site 4. Bromoform, for example, showed vertical distribution similar to 1,2-dichloroethane and trichloroethylene (density 1.5) and occurred in similar concentrations at all three depths sampled. This observation indicates that, under most conditions, samples collected throughout the water column and from surface water can accurately represent the concentration levels of VOC's in the Calcasieu River. The exception to the above observation occurred at site 4. Vertical differences in concentrations observed at site 4 can be explained by the results of flow, dye, and specific-conductance profile studies. Flow studies indicate that the slow-moving water currents in the river preclude much vertical mixing in the water column owing to the small vertical-velocity gradients. Therefore, during periods of saltwater intrusion, the denser saltwater moves almost exclusively near the bottom. The saltwater migrates some distance up
stream owing to the longitudinal salinity gradient between the relatively freshwater upstream and the denser saltwater in the Gulf of Mexico. Dye studies completed in 1979 and 1987 indicate slow vertical mixing in the water column when differences exist between surface and bottom specificconductance values. These differences in specific-conductance profiles were most pronounced at site 4, where the bottom sample had the highest concentration of VOC's. This high concentration of volatiles occurred in a part of the water column that had a specific-conductance value (12,500 pS/cm) similar to that of downstream sites. In contrast, surface samples that had lower concentrations of VOC's occurred in relatively low specific-conductance water (2,500 p.S/cm) compared with downstream sites (sites 6, 13, and 14). This phenomenon also was observed in samples collected during moderate wind conditions (17.0 miles per hour) in April 1987. These results indicate that (1) longitudinal movement of VOC's occurs along the bottom when salinity gradients exist in the lower Calcasieu River, (2) VOC's detected in the bottom water sample at site 4 originated downstream, and (3) presence of the salinity gradient at site 4 inhibited movement of VOC's from the bottom of the water column to the top of the water column. Data from dye and volatile-organic studies also indicate that salinity gradients inhibit vertical movement of VOC's in the water column. Presence of VOC's at site 4 and their movement upstream with the salt wedge could not have been determined without point samples. Thus, sampling for VOC's in areas where salinity gradients exist, especially freshwater-saltwater, should include Some point samples to determine an accurate picture of the vertical distribution of these compounds in the water column. Acid-base/neutral extractable organic compounds also were studied in the lower Calcasieu River. One group of compounds detected in water, bottom material, and biotic samples was the halogenated organic compounds (HOC's), which include the haloarenes, a class of chlorinated aromatic compounds that are hydrophobic (nearly insoluble in freshwater) and toxic to aquatic organisms. These compounds attach strongly to sedimentary organic material and accumulate in fatty tissues of aquatic Organisms. Haloarenes are even less soluble in saltwater and precipitate out as