1991-2000 Study Findings

The Chowan, Roanoke, Tar, and Neuse River basins in the Albemarle-Pamlico drainage area contain elevated concentrations of nitrogen and phosphorus. Of the large river basins, total nitrogen concentrations were highest (1-3 mg/L) in the Neuse and Tar basins and lowest (0.2-0.8 mg/L) in the Roanoke and Chowan basins.

The Neuse River and Contentnea Creek, which have the highest yields of nutrients, contribute the largest percentage of nitrogen and phosphorus to the sounds. The Neuse basin alone accounts for almost half of the phosphorus entering the sounds, although it represents only about 20 percent of the total contributing drainage area. Although the Roanoke River contributes 34 percent of the nitrogen, it has the largest drainage area and the lowest yields of both nitrogen and phosphorus.

Agricultural fertilizer and livestock waste are major sources of nitrogen and phosphorus, supplying 50 percent of the nitrogen and 75 percent of the phosphorus originating in the basins. Atmospheric inputs are a source of 27 percent of the nitrogen load and 21 percent of the phosphorus load. Point sources of nitrogen and phosphorus account for less than 5 percent of the nutrient-source inputs. However, these sources go directly into streams and may constitute up to 20 percent of the instream nutrient load.

Phosphorus concentrations generally have declined since 1980. This decrease may be a result of a detergent phosphate ban in North Carolina and Virginia since 1988.

Total nitrogen concentrations generally have declined since 1980, probably as a result of improved wastewater treatment.

Concentrations of suspended sediment, suspended solids, and total solids have decreased throughout the Albemarle-Pamlico drainage system. Decreasing concentrations of suspended solids in the sounds and estuaries may result in clearer water and deeper light penetration in water, which in turn improve conditions for algal blooms in areas with nutrient enrichment. Decreases in sediment and solids concentrations are probably a result of (1) construction of new lakes and ponds in the basin, (2) improved agricultural soil management, and (3) improved wastewater management.

Based on random well samples, nitrate-nitrogen concentrations in the inner Coastal Plain are higher (median = 0.4 mg/L) than concentrations in the outer Coastal Plain (median = 0.05 mg/L). Ground water from shallow wells (less than 100 feet deep) in the Coastal Plain of the Albemarle-Pamlico Study Unit had higher nitrate concentrations and greater variability than wells deeper than 100 feet, indicating the presence of sources of nitrate.

About 10 percent of 20 shallow observation wells in agricultural areas in the inner Coastal Plain had nitrate concentrations that exceeded the U.S. Environmental Protection Agency maximum contaminant level of 10 mg/L, whereas none of the sampled wells in the outer Coastal Plain had nitrate concentrations that exceeded this standard.

Ground water beneath sandy, well-drained soils in the Albemarle-Pamlico drainage basin, and particularly near the Fall Line where nitrogen sources at the land surface are present, generally had higher nitrate-nitrogen concentrations than did ground water beneath poorly drained soils.

Pesticides and pesticide metabolites can be detected in most surface-water samples and in many ground-water samples and aquatic organisms tested. With few exceptions, these detections did not exceed drinking-water or aquatic-health standards.

Metolachlor was detected in 90 percent, atrazine in 80 percent, prometon in 70 percent, and alachlor in 60 percent of 233 stream samples from 65 sites in the Albemarle-Pamlico drainage basin. Only alachlor occurred at levels above the drinking-water standard of 2 µg/L. Standards for drinking water or human health exist for only 50 percent of the individual compounds detected, and no standards exist to evaluate pesticide combinations that typically are detected in the streams.

Herbicides and insecticides generally were not found in ground water, although several breakdown products from these pesticides were detected. No drinking-water standards exist for the breakdown products detected. Of 47 pesticides analyzed, 11 were detected in ground water. Concentrations in ground water were generally less than 0.1 µg/L and never exceeded established maximum contaminant levels for drinking water. Diazinon was the most frequently detected pesticide in ground water, although concentrations were just above the detection limit.

Despite the ban on DDT, the pesticide and its metabolites DDD and DDE persist in the environment. In an examination of existing data from 1969 to 1990, all bed-sediment sampling sites in the Tar River basin had concentrations of DDD, DDE, and DDT greater than the effects range established by the National Oceanic and Atmospheric Administration and associated with adverse biological effects on some benthic invertebrates. Elevated concentrations of DDD, DDE, and DDT also were detected at several sites in the Roanoke River basin. Of 22 bed-sediment samples collected in the Albemarle-Pamlico basin during 1992-93, 63 percent had detectable levels of DDT. No human health standards have been established for DDT in bed sediment.

DDD, DDE, DDT, dieldrin, trans-nonalachlor, PCB, and toxaphene were detected in Asiatic clams or redbreast sunfish at 11 of 19 sites sampled during 1992-93. DDE was the most common and widespread organochlorine compound detected. All but toxaphene were present in concentrations well below the National Academy of Sciences and National Academy of Engineering guidelines for the protection of fish-eating wildlife.

Intensive sampling for organonitrogen compounds in Chicod Creek during 1992-94 (in the Tar River basin) indicated seasonal variations in pesticide concentrations. No relation between streamflow and pesticide concentrations was noted. Concentrations of atrazine increased in late May and early June and decreased gradually until reapplication the following spring.