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The three most commonly detected herbicides at the two agricultural monitoring sites in the UCOL (one site each in the Grand and Uncompahgre Valleys) also were among the top 10 herbicides detected in surface-water samples at 62 agricultural sites in 35 nationally distributed NAWQA Study Units. The percentages of samples with detections for atrazine, alachlor, and metolachlor are shown below and are based on more than 1,550 sam- ples for the national sites and 39 samples at the UCOL sites. Atrazine, alachlor, and metolachlor are commonly used for weed control in corn. Alachlor and metolachlor are also used for weed control in dry beans. Other herbi- cides that were frequently detected (present in over one-third of the sam- ples) in the UCOL but not shown in the graph include trifluralin, DCPA, 2,4-0, cyanazine, and deethylatrazine z (a breakdown product of atrazine). 0.045 ;- ffi These herbicides also were commonly 0.040 ~ ~ detected at other agricultural sites 0.035 g: ffi across the Nation. Median concentra- z 0.. 0.030 ~ ~ tions of the commonly detected herbi- 0.025 6 ~ cides in the UCOL were less than or 0.020 ~ 8 similar to the median concentrations ~o ~ for the national sites. Concentrations 0.015 :2 0.010!:li of atrazine and metolachlor were less than the Canadian guidelines for the protection of aquatic life [1.8 llglL for atrazine and 7.8 llglL for metolachlor (Environment Canada, 1999)]. Guide- lines have not been established for alachlor. EXPLANATION o 62 Agriculture sites from NAWQA Study Units o 2 Agriculture sites from the UCOL . Median concentration -+- Median less than 0.002 IlglL 90 I I- ~ C/) UJ -l 0.. C/) 60 :2z ;JjQ lL I- 50 o&:l UJ I- 40 <.9 UJ ~ 0 30 z UJ u a: UJ 0.. - r- . - ~ - - ~ ~ . . - -+- . . 80 70 20 10 o Atrazine HERBICIDE water samples from the same site, probably because these compounds are relatively insoluble in water. Occurrence of organochlorine pes- ticides, even at low concentrations, is becoming increasingly relevant because of recent evidence linking these compounds to endocrine dis- ruption (Goodbred and others, 1997). Nutrient concentrations in streams and rivers reflect point and nonpoint sources. National background concentrations have been determined for some forms of nutrients: total nitrogen in streams (1.0 mglL), nitrate in streams (0.6 mg/L), and total phosphorus in 0.050 0.005 o Alachlor Metolachlor streams (0.1 mglL) (U.S. Geologi- cal Survey, 1999). Relative concen- trations of nutrients in surface water are linked to the amounts and types of substances used and dis- charged in different land-use set- tings. These substances can then reach the stream through point sources (such as wastewater dis- charge) or nonpoint sources (such as precipitation or runoff from agri- cultural areas). Estimated amounts of fertilizer applied during 1997 in Delta, Mesa, and Montrose Coun- ties were about 14,100,000 pounds of nitrogen and 1,800,000 pounds of phosphorus (Jeffrey Stoner, U.S. Geological Survey, written commun., 2000). Fertilizer use in these counties accounted for about 77 percent of the total estimated fertilizer usage for the UCOL. Nutrient concentrations in areas of agricultural land use were generally greater than in areas of other land uses. Median concentrations of ammonia, nitrite plus nitrate, total phosphorus, dis- solved phosphorus, and orthophos- phate were greater at agriculture sites than at the Colorado Plateau reference and mixed land-use sites (fig. 14). The Colorado Plateau reference site has some limited agriculture upstream (livestock! hay meadows), so is not representa- Major Findings 17