9317 (2) - Laserfiche WebLink

Home Browse Search

Collecting water samples at an agricultural drain in the Grand Valley. (Photograph by Norman Spahr U.S. Geological Survey.) , Pesticides were commonly detected in agricultural areas of the VeOL, but concentrations were generally low. Most pesticide detections in surface water were not at concentrations of concern. Freshwater aquatic-life guidelines were exceeded occasionally; how- ever, guidelines have not been established for all compounds. Only 5 of the 90 samples collected in the agricultural areas contained pesticides that exceeded estab- lished guidelines. The pesticides that exceeded guidelines for pro- tection of freshwater aquatic life are azinphos-methyl, I of 24 sam- ples at Reed Wash; carbaryl, 2 syn- optic sites, Indian Wash and Orchard Mesa Drain in the Grand Valley; diazinon, I synoptic site, Indian Wash; diuron, I synoptic site, Indian Wash; and gamma- HCH, 1 synoptic site, the drain at Blossom Road in the Uncompahgre Valley. Pesticide detections and con- centrations showed seasonal pat- terns. Periodic sampling at two agricultural streams showed that the total number of pesticides detected was greatest during April through August (Bauch and Spahr, 2000). Pesticide detections per month for Dry Creek are shown in figure 13. Concentrations of atra- zine were found to be greater from May through August than during other periods of the year (fig. 13). The seasonal pattern of concentra- tions was similar for other pesti- cides and reflects the growing season for the Grand' and Uncompahgre Valleys. Pesticides were detected in 40 of 43 agricultural streams. Thirty-one pesticides (21 herbi- cides and 10 insecticides) were detected at least once during a May 1998 agricultural stream synoptic study in the Grand Valley and Uncompahgre Valley areas. Atra- zine and alachlor were detected in more than 50 percent of the sam- ples. Concentrations of atrazine did not exceed aquatic-life guidelines. Guidelines are not available for alachlor. Some insecticides have per- sisted in streambed sediment and fish tissue, although their use has been banned or restricted. DDT was banned in 1972, but DDT or its 10 CJ) 9 w 0 (3 w 8 i= -' c.. CJ) :;; 7 w <( c.. CJ) u. a: 6 0 w a: c.. w 0 5 aJ w :;; f-- 4 :::> () z w Z f-- 3 w <( 0 is 2 w :;; 1 0 0 DRY CREEK == Median number of pesticide detections ~ Median atrazine concentrati6~ -I 1 Agriculture and the San Juan Mountains near Montrose. (Photograph by Norman Spahr, U.S. Geological Survey.) breakdown products, DDE and DDD, were detected in streambed sediments at five of the six agricul- tural sites sampled and in fish tis- sue at all three of the agricultural sites where fish tissue samples were collected (Stephens and Deacon, 1998). Concentrations of DDT and DDE in streambed sedi- ment at two sites exceeded the Canadian Sediment Quality Guide- lines PEL (Canadian Council of Ministers of the Environment, 1999). The DDD concentration at one site exceeded the PEL. Dield- rin (a restricted use insecticide since 1974) was detected in stream- bed sediments at two of the six sites and in fish tissue at all three sites. Insecticides detected in fish tissue and streambed sediments were generally not detected in 0.12 a: w a: c.. w Z f-- 0.10 0 :::; ~ a: w a: c.. 0.08 f-- CJ) Z :;; w <( () a: z CJ 0.06 0 0 () a: w () z ~ 0.04 N <( ~ a: ui ~ -' 0.02 z c.. <( :;; is <( w CJ) 0 :;; J A S NO J FMAM J WATER YEAR 1997 Figure 13. The number of pesticide detections and median atrazine concentrations in surface water are greater during the spring and summer months. Atrazine is commonly used for season-long weed control in corn. 16 Water Quality in the Upper Colorado River Basin