In addition to the 47 agricultural pesticides applied to crops during the period 1992-95 and discussed in the previous section, there are many other organic compounds that can pose potential health and environmental hazards. These organic compounds (chemical compounds containing the element carbon) include organochlorine compounds (compounds with both carbon and chlorine); semivolatile organic compounds (SVOCs), which usually are the result of combustion (polyaromatic hydrocarbons [PAHs]), either natural or anthropogenic, or the product of industrial manufacturing (phenols, phthalate esters, amines, and a variety of other compounds); and volatile organic compounds (VOCs) associated with refined gasoline (benzene, toluene, xylene, methyl tert-butyl ether [MTBE]) or solvents, such as trichloroethene and carbon tetrachloride). Many of the organochlorine compounds were used extensively as pesticides (for example, DDT) during the 1950's, 60's and 70's, but were largely banned during the late 1970's and early 1980's because of their carcinogenicity, toxicity, and ability to persist in the environment for many years. They are generally not very water soluble. SVOCs also do not easily evaporate into the atmosphere and are not very water soluble. VOCs, on the other hand, evaporate easily and are very water soluble. With the exception of VOCs, only results of bed-sediment sampling are discussed. Because VOCs are very soluble, they are discussed as a contaminant in ground water.
Trace elements are another class of contaminants in water and sediment that can pose environmental hazards. Trace elements, as discussed in this report, include metals and nonmetals; some elements, such as aluminum and iron, actually are very common in the environment. However, most of the elements analyzed as part of the NAWQA study typically occur at low concentrations in the environment and, thus, are termed "trace" elements. Trace elements can occur locally at elevated concentrations as a result of enrichment due to geologic processes, such as evaporite deposits, or to anthropogenic processes, such as industrial, mining, or agricultural activities. Only the occurrence of trace elements in bed sediment of streams will be discussed in this report.
Many pesticides have very low solubility in water and tend to adsorb (attach) to streambed sediment. Although these compounds, mostly organochlorine pesticides, are usually not detected in the water, they persist in bed sediment and provide a source for bioaccumulation by aquatic organisms. In addition, trace elements accumulate in bed sediments and occur near urban areas as a result of runoff or waste discharges from industrial, municipal, and residential activities. Bioaccumulation of metals, such as mercury, is a concern because trace levels in bed sediment may be concentrated to harmful levels in organisms at the top of the food chain.
Streambed sediments were collected in 1992 at 22 stations and analyzed for 35 organochlorine pesticides, 63 semivolatile compounds, and 44 major, minor, and trace elements (Woodside and Simerl, 1996). DDT was detected in 27 percent, DDD in 40 percent, and DDE in 63 percent of the samples. DDD and DDE are degradation products of DDT. DDT was banned from use in the United States in 1972 yet persists in the environment. In addition, dieldrin was detected in 18 percent and chlordane was detected in 9 percent of the samples. DDT and its degradation products were detected in stations downstream from both urban and rural areas; chlordane was detected at two urban sites.
During a review of historical pesticide and trace-element data for streambed sediment in the Albemarle-Pamlico Drainage Basin for 1969-90 (Skrobialowski, 1996), results were examined from 1,049 samples collected at 301 sites. Chlordane concentrations greater than the effects range established by the National Oceanic and Atmospheric Administration (Long and Morgan, 1991) were detected in the upper Neuse River Basin, and elevated concentrations of DDT, DDD, and DDE were detected at Tar River sites and several sites in the Roanoke River Basin. DDT was detected in 18 percent, DDD in 25 percent, and DDE in 28 percent of the approximately 160 samples analyzed.
Lead concentrations during the 1992 NAWQA sampling were highest in streambed sediments from Ellerbe Creek and Crabtree Creek, both of which are urban basins in the Neuse River Basin. Mercury and cadmium concentrations in streambed sediment were highest in the Trent River--a Coastal Plain stream in the Neuse River Basin. Zinc concentrations were highest in Ellerbe Creek and the Trent River. Concentrations of chromium, lead, nickel, and zinc were higher in streams draining urban areas and near wastewater-treatment plants than in other settings.
SVOCs were most commonly detected in bed sediment of streams draining urban basins. Of 22 streams sampled throughout the Albemarle-Pamlico Basin (Woodside and Simerl, 1996), the Roanoke River downstream from Roanoke, Va., had the greatest number of SVOCs detected (36 of 63 compounds), most of which were PAHs. Of all basic sites, Devil's Cradle Creek (site 6, pages 26 ) had three occurrences of phthalate esters, with one (bis 2-ethylhexyl) phthalate being the highest concentration (870 micrograms per kilogram [µg/kg]) of any SVOC detected. However, phthalate esters are pervasive in the environment because the compounds are used in the manufacturing of common plastics. In general, SVOC concentrations from all other stream sites in the Albemarle-Pamlico Drainage Basin were relatively low and were few in number.
Of the 28 organochlorine compounds analyzed, seven were detected in Asiatic clam soft tissues and whole redbreast sunfish--DDD, DDE, DDT, dieldrin, trans-nonalachlor, PCBs, and toxaphene. Of these, DDE was the most common and widespread, being present at 11 of the 19 sites sampled. DDE is a chemically stable and persistent metabolic breakdown product of DDT and is among the most commonly detected organochlorine compounds nationwide (U.S. Environmental Protection Agency, 1992). More than one organochlorine compound was detected at 6 of the 19 sites. The only detection of toxaphene exceeded the National Academy of Sciences and National Academy of Engineering (NAS/NAE) guidelines for the protection of fish-eating wildlife (Nowell and Resek, 1994). The toxaphene exceedance occurred at Pete Mitchell Swamp--a small, intensively farmed basin in the Coastal Plain. All of the other detected compounds were present in concentrations well below the NAS/NAE guidelines (Smith and Ruhl, 1996).
Compared to the Asiatic clam, redbreast sunfish appear to be better bioindicators of organochlorine compound contamination in aquatic systems. Of the seven compounds detected, all but PCBs were detected in whole redbreast sunfish samples; only three compounds--DDE, trans-nonalachlor, and PCBs--were detected in Asiatic clam samples. Pesticides were detected at all eight sites sampled for redbreast sunfish, whereas only 3 of the 15 sites sampled for Asiatic clams had detections above the minimum reporting level. On the other hand, PCBs were only detected in Asiatic clams, lending support to other studies which have shown that Asiatic clams are good bioindicators of PCB contamination (Peterson and others, 1994).
Collecting benthic invertebrates in the Neuse River Basin, North Carolina.
Although all 10 of the USEPA trace-element priority pollutants were detected in Asiatic clam soft tissues or redbreast sunfish livers, they were present in relatively low concentrations. The sampling methods used do not permit a direct assessment of risks to human health; however, it is unlikely that widespread trace-element contamination poses a significant risk in the streams and rivers of the Albemarle-Pamlico Drainage Basin. Mercury concentrations were highest in blackwater streams and should continue to be monitored on a regular basis (Ruhl and Smith, 1996).
Trace-element concentrations detected in Asiatic clams in the study area were similar to those measured in other NAWQA Study Units in the southeastern United States (Ruhl and Smith, 1996). Mercury was widely detected, being present in 29 of 30 tissue samples, but concentrations did not exceed the FDA action limit of 1 microgram per gram (g/g) wet weight or a risk-based screening value for the general public (3.2 µg/g wet weight) (Cunningham and others, 1992). Mercury concentrations in the study area ranged from 0.02 to 0.33 µg/g wet weight.
The highest concentrations of mercury were detected in redbreast sunfish from the Blackwater River (0.19 to 0.33 g/g wet weight), and Contentnea Creek (0.18 µg/g wet weight). Both are Coastal Plain streams where high concentrations of dissolved organic carbon also were measured. Higher mercury concentrations in fish are often associated with high concentrations of dissolved organic carbon. Studies have indicated that waters with low pH and high dissolved organic carbon concentrations, such as the Blackwater River, promote mercury uptake by aquatic organisms (Gilmore, 1995). Many Coastal Plain streams and lakes in North Carolina and Virginia have naturally low pH and high dissolved organic carbon concentrations and, thus, may be particularly susceptible to mercury bioaccumulation.
Volatile organic compounds (VOCs) were not commonly detected in shallow ground water. VOCs can occur in large concentrations in ground water, usually in urban environments. The most common sources of VOCs are vehicle fuel and industrial discharges of solvents. Most VOCs are known to cause cancer or are toxic; therefore, drinking-water standards have been established for these compounds.
Only three VOCs (chlorobenzene, trichlorofluoromethane, and MTBE) were detected in ground water in four separate wells in the Coastal Plain. All of the detected concentrations were below drinking-water standards. In general, VOCs do not appear to occur widely throughout the Coastal Plain and probably do not pose a concern except in local urban areas. The two highest concentrations detected occurred in industrial/commercial land-use areas. A comparison of concentrations from wells in the Albemarle-Pamlico Drainage Basin with nationwide NAWQA results indicates that VOC detections in ground water were among the lowest in the Nation.