Two commonly used herbicides in corn fields of the Rhode River Watershed were atrazine (2‐chloro‐4‐ethylamino‐6‐iso‐propylamino‐1,3,5‐triazine) and alachlor (2‐chloro‐2′,6′‐diethyl‐N‐methoxymethyl acetanilide). Although alachlor was applied in larger quantities, atrazine was detected more frequently in runoff waters and had greater concentrations than alachlor (0–40 µg/L vs. 0–6 µg/L). Atrazine was more persistent and more mobile in watershed soils. Linear regression analysis of herbicide loading rates and percentage agricultural land‐use did not give a direct relationship. Runoff waters from forest watersheds where herbicides were not directly applied, were contaminated with herbicides. During the 3‐y study period (1976–1978), a maximum of 10 µg/L of atrazine, and up to 0.5 µg/L alachlor were discharged in winter runoff waters from the eight experimental watersheds. In addition to quantity of herbicides directly applied to land surface, residual levels in runoff waters must be influenced by other important factors such as topography, location of croplands in relationship to drainage channel, etc. A major portion of atrazine was found in dissolved aqueous form in runoff‐water samples collected during storm events. Percolation in subsurface flow and dissolution in overland flow were believed to be important transport mechanisms.

Water samples from the Rhode River, an estuary situated on the western shore of the Chesapeake Bay, were analyzed for atrazine residues twice a week for 2 yr. Precipitation samples, which included dryfall, rainfall, and snowfall were collected with wide-mouth stainless steel collection pans situated about 20 m above ground in an open space. A total of 68 precipitation samples was collected from December 1976 to February 1979. Atrazine residues were detectable in estuarine water and in rainwater year-round. Atrazine residues in estuarine water were generally 6 to 190 ng 1-t; atrazine residues in rainwater (bulk precipitation) were 3 to 2190 ng 1 1. Atrazine residues in rainwater samples collected during the winter season (January to April 1977) were unexpectedly high (e.g., 3 to 970 ngl-1). The highest atrazine concentration of 2190 ng 1-1 was detected from a 0.76 cm rainfall event collected on May 19, 1977. Intermittent spraying operations of atrazine within the cornfields were generally done during May of each year. Rain samples collected during May of 1978 also showed higher atrazine residues than the rest of the 1978 growing season, but at levels much less than those detected in 1977 rainwater. Aithough high atrazine concentrations were detected in winter rainfall, these did not result in similarly higher atrazine concentrations in estuarine receiving waters. Our data showed a decline of atrazine concentrations in estuarine water in October and November which continued until a rainfall following Spring herbicide applications. Atrazine is enriched at the microsurface layer of estuarine water, but direct atmospheric input of atrazine did not seem to contribute significantly to the enrichment mechanism. Atrazine is believed to be transported long distances in polluted air masses. The estuarine microsurface layer could be a source of atmospheric atrazine, but the importance of the source is yet to be determined. Atrazine was quantitatively determined by GC using a nitrogen specific electrolytic detector and was confirmed by GC/Mass. Water, Air, and Soil Pollution 15 (1981) 173-184.

Atrazine and alachlor are commonly used in Maryland corn fields (Zea mays L.), primarily as pre‐emergence herbicides. This research was part of a watershed program designed to monitor nonpoint source pollution due to runoff of chemicals from agricultural lands. Atrazne (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) and alachlor (2‐chloro‐2′,6′‐diethyl‐N‐methoxymethyl acetanilide) were sprayed on a corn field at rates of 1.7 kg/ha and 2.3 kg/ha, respectively. Soil cores were taken from sites at four elevations in the field. At the highest elevation, atrazine underwent a decrease in the surface 5‐cm layer, but a fraction of atrazine was leached into the deeper soil profile. At the middle altitude, atrazine concentrations in the 2.5‐ to 30‐cm soil profile changed very little during the growing season. At the lowest elevation, where soil moisture was rather high, atrazine had an even distribution in the soil profile. Thus, the data on field soil indicated both downward and lateral movement of atrazine. Alachlor remained in the top 8 cm of the soil. Low teachability of alachlor in field soil is believed to result from a strong affinity of alachlor molecules for soil organic matter. Runoff losses were detected at a weir situated near the bottom of the drainage channel of the watershed. During the 1976 growing season, 17 g/ha of atrazine and 3.6 g/ha of alachlor were discharged. About 60% of the discharged herbicides were in the dissolved phase. Flow‐weighted mean herbicide concentrations in the runoff water were 16.9 µg/liter (atrazine) and 0.6 µg/liter (alachlor). Atrazine carryover detected in the corn field soils was generally from 0.09 to 0.56 µg/g. Between 5 and 13% of the atrazine applied to the fields was estimated to be carried over to the next growing season.

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