Herbicide Loading to Shallow Ground Water beneath Nebraskaâ•Žs Management Systems Evaluation Area

Atrazine is the most widely detected pesticide in the nation’s ground water and the USEPA has set 3 g L 1 Better management practices can counter deterioration of ground as the maximum contaminant level (MCL) in drinkwater quality. From 1991 through 1996 the influence of improved irrigation practices on ground water pesticide contamination was assessed ing water. at the Nebraska Management Systems Evaluation Area. Three 13.4-ha Generally, nonpoint-source loading of pesticides in corn (Zea mays L.) fields were studied: a conventional furrow-irrigated shallow ground water beneath agricultural fields is visufield, a surge-irrigated field and a center pivot–irrigated field, and a alized as a complex, nonuniform network of macropores center pivot–irrigated alfalfa (Medicago sativa L.) field. The corn fields conveying contaminants through the vadose zone to the received one identical banded application of Bicep (atrazine [6-chlorowater table where high input concentrations are parN-ethyl-N -(1-methylethyl)-1,3,5-triazine-2,4,-diamine] metolachlor tially masked by vertical and radial dilution. The major[2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) ity of loading has been attributed to the transfer, mixing, acetamide]) annually; the alfalfa field was untreated. Ground water application, and disposal or “routine use” of pesticides samples were collected three times annually from 16 depths of 31 multiin agriculture. Hallberg (1989), however, points out that level samplers. Six years of sample data indicated that a greater than 50% reduction in irrigation water on the corn management fields lowthe contamination may be attributable to a wider specered average atrazine concentrations in the upper 1.5 m of the aquifer trum of management variables that, while standard pracdowngradient of the corn fields from approximately 5.5 to 0.5 g tices for the user, are beyond the scope of officially L 1. Increases in deethylatrazine (DEA; 2-chloro-4-amino-6-isopropylrecognized normal use and more likely to impair ground amino-s-triazine) to atrazine molar ratios indicated that reducing wawater quality. The effects of structures for the retention ter applications enhanced microbial degradation of atrazine in soil and reuse of water especially in furrow-irrigated agriculzones. The occurrence of peak herbicide loading in ground water was ture have been overlooked and could be responsible for unpredictable but usually was associated with heavy precipitation much of the shallow ground water loading of pesticides within days of herbicide application. Focused recharge of storm runoff routinely applied to fields. While ground water assessthat ponded in the surge-irrigated field drainage ditch, in the upgraments have become the basis for USEPA’s regulatory dient road ditch, and at the downgradient end of the conventionally irrigated field was a major mechanism for vertical transport. Sprinkler strategy of developing state Pesticide Management Plans irrigation technology limited areas for focused recharge and promoted for atrazine, cyanazine, simazine, alachlor, and metolasignificantly more soil microbial degradation of atrazine than furrow chlor (Federal Register, 1996), they have done little to exirrigation techniques and, thereby, improved ground water quality. plain the transport mechanisms of pesticides to shallow ground water and the large temporal variability of the concentrations. Long-term field-scale studies addressing T and acetamide herbicides are used heavthe effects of irrigation water management practices and ily in corn and soybean [Glycine max (L.) Merr] proseasonal runoff on water quality beneath row-cropped duction and their mobility and moderate persistence cause fields are best-suited to understanding the complexities them to be the most frequently reported pesticides in of downward movement of pesticides and the temporal ground water in agricultural areas (Roux et al., 1991; Kolvariability of shallow ground water pesticide concenpin et al., 1996; Barbash et al., 1999) including Nebraska trations. In a long-term field study on nonirrigated deep (Spalding et al., 1989). Atrazine and its transformation loess soils in western Iowa, Steinheimer and Scoggin products together with cyanazine [2-[[4-chloro-6-(ethyl(2001) documented the relative rapid transport of atraamino)-1,3,5-triazin-2-yl]amino]-2-methylproprionitrile], zine and metolachlor to ground water. simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], alaFor two decades the presence of pesticide transformachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxytion products was ignored largely because the focus was methyl)acetamide], and metolachlor and their transforparent pesticides with regulated concentrations in drinkmation products are the herbicides most commonly ing water, and because improved analytical schemes for detected in row-cropped regions (Barbash et al., 1999). degradate analysis were still evolving. Especially prevalent in ground water are the two triazine metabolites deethylatrazine and deisopropylatrazine (DIA; 2-chloroR.F. Spalding and D.D. Snow, Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583-0915. D.G. Watts, Dep. of 4-amino-6-ethylamino-s-triazine), which frequently are Biological Systems Engineering, Univ. of Nebraska, Lincoln, NE quantified in the same analytical scheme as atrazine and 68583-0726. D.A. Cassada, Water Sciences Laboratory, Univ. of Nethe acetamide herbicides. braska, Lincoln, NE 68583-0844. M.E. Exner, School of Natural Resource Sciences, Univ. of Nebraska, Lincoln, NE 68583-0759. J.S. Schepers, Dep. of Agronomy, USDA-ARS, Univ. of Nebraska, LinAbbreviations: D2R, molar ratio of deisopropylatrazine to deethylcoln, NE 68583-0934. Received 12 Feb. 2002. *Corresponding author atrazine; DAR, molar ratio of deethylatrazine to atrazine; DEA, de([email protected]). ethylatrazine; DIA, deisopropylatrazine; MSEA, Management Systems Evaluation Area. Published in J. Environ. Qual. 32:84–91 (2003).