To determine the effects of application rates, grass cover, and formulation type on herbicide losses in runoff, we applied 4.5 kg/ha cyanazine (2‐[(4‐chloro‐6‐(ethylamino)‐1,3,5‐triazin‐2‐yl)] amino]‐2‐methylpropanenitrile) with 0.4 kg/ha sulfometuron‐methyl (methyl‐2[[[[(4,6‐dimethyl‐2‐pyrimidinyl)amino] carbonyl] amino] sulfonyl] benzoate) to 1.2 by 2.4 m plots, using suspension concentrate (SC) and dispersible granule (DG) formulations of cyanazine, and SC and emulsifiable concentrate (EC) formulations of sulfometuron‐methyl. The plots were established on a Tifton loamy sand soil (fine‐loamy, siliceous, thermic Plinthic Paleudults) and had 3% slope. The plots were bare or covered with a mixed stand of common Bermudagrass [Cynodon dactylon (L.) Pers.] and Bahiagrass (Paspalum notatum Flugge var. suarae Parodi). On the day after the herbicides were applied, we simulated rainfall events of 69 mm/h intensity until 2 mm of runoff occurred. The runoff was analyzed for sediment and herbicides. The bare plots required one‐third less rain to produce the same amount of runoff and yielded twice as much sediment as the grassy plots. However, losses of all formulations were 1 to 2% of the amounts applied regardless of grass cover and even though cyanazine rates were 11 times that of sulfometuronmethyl. Total losses of all formulations were sensitive to the length of time between rainfall initiation and runoff initiation, indicating that leaching made herbicide unavailable for runoff. These results suggest that, for these formulations under conditions of similar runoff volumes, losses of pesticides are a fairly constant fraction of the amounts applied, with or without grass cover. For intense storms where the amount of rainfall is similar, chemical runoff from the grassed plots was predicted by computer simulation to be less than half of that from bare soil.

Excessive amounts of dissolved or suspended solids in surface runoff or base flow may degrade the quality of streams, lakes, or other water bodies. Loads of dissolved and suspended solids in streamflow reflect the quality of water entering via surface runoff or base flow. This study was conducted to determine the concentrations and loads of dissolved and suspended solids in Coastal Plain streamflow, to examine relationships between concentrations, loads, and flow rate; and to determine overall streamflow water quality for these parameters. Dissolved solids and suspended sediment concentrations were determined on weekly or high-flow storm event streamflow samples collected at gaging stations on three subwatersheds (B, 334.3 km 2 ; F, 114.9 km 2 ; and K, 16.7 km 2 ) of the Little River Watersheds located near Tifton, GA. Dissolved solids concentrations ranged from 19 to 159 mg L ', and generally decreased as per unit area instantaneous discharge rate increased. Suspended sediment concentrations ranged from 1 to 137 mg L ', and generally increased as per unit area instantaneous discharge rate increased. Regression analyses showed good relations between log transforms of both dissolved solids load (r 2 = 0.97) and suspended sediment load (r 2 = 0.79), vs. total monthly runoff. Mean suspended sediment concentrations during high-flow events were greater than means from the overall data set, while mean concentrations of dissolved solids from these events were reduced relative to the overall data set. The study showed that dissolved solids are the major component of total solids in Coastal Plain streamflow. The mean dissolved and suspended sediment concentrations during the study were 67, 60, and 51 mg L '; and 14,17, and 14 mg L~' for Watersheds B, F, and K, respectively. Overall, the study showed that, as measured on these watersheds, Coastal Plain streamflow is of good quality in terms of both dissolved and suspended solids. This good quality may reflect land-use practices designed to prevent soil erosion, but primarily reflects the Coastal Plain landform shape, which causes sediments eroded from the uplands to be deposited in the riparian zone before they can enter streamflow.

A multiresidue procedure was developed for analysis of cotton pesticide and harvest-aid chemicals in water using solid-phase extraction and analysis by GC-NPD, GC-MS, and HPLC-DAD. Target compounds included the defoliants tribufos, dimethipin, thidiazuron; the herbicide diuron; and the insecticide methyl parathion. Three solid-phase extraction (SPE) media, octadecylsilyl (ODS), graphitized carbon black (GCB), and a divinylbenzene-N-vinyl pyrollidine copolymer (DVBVP), were evaluated. On GCB and ODS, recoveries varied depending on compound type. Recoveries were quantitative for all compounds on DVBVP, ranging from 87 to 115% in spiked deionized water and surface runoff. The method detection limit was less than 0.1 microg L(-)(1). SPE with DVBVP was applied to post-defoliation samples of surface runoff and tile drainage from a cotton research plot and surface runoff from a commercial field. The research plot was defoliated with a tank mixture of dimethipin and thidiazuron, and the commercial field, with tribufos. Dimethipin was detected (1.9-9.6 microg L(-)(1)) in all research plot samples. In the commercial field samples, tribufos concentration ranged from 0.1 to 135 microg L(-)(1). An exponentially decreasing concentration trend was observed with each successive storm event.

The influence of previous herbicide applications on the degradation rate of butylate [S-ethyl bis (2-methylpropyl)carbamothioate], EPTC (S-ethyl dipropyl carbamothioate), alachlor [2-chloro-N-(2,6-diethyl-phenyl)-N-(methoxymethyl)acetamide], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] was measured. Degradation studies were conducted on soils with zero to eight previous applications of butylate, zero and six consecutive annual applications of alachlor, and zero to seven previous applications of metolachlor. Previous applications of alachlor or of metolachlor did not affect the rate of degradation when the same herbicide was reapplied. Soils with previous butylate-use history had more rapid degradation of butylate or EPTC than soils with no previous butylate applications. Because soil sterilization reduced14CO2evolution to a low level, soil microorganisms could be the primary mechanism of butylate degradation. There was no difference between butylate or EPTC degradation rate in soils treated previously with butylate. Butylate could not be detected 28 days after treatment in soils previously treated with butylate, whereas soils not previously treated with butylate still contained biologically active butylate. Dietholate (O,O-diethylO-phenyl phosphorothioate) added to butylate generally reduced the rate of butylate degradation in soils previously treated with butylate, but the butylate concentration always was lower in soils treated previously with butylate than in soils without previous butylate applications. Weed control in the field showed rapid loss of butylate and EPTC in soils with previous butylate treatment and that dietholate reduced the rate of butylate and EPTC degradation.

No abstract