Variations in Pesticide Leaching Related to Land Use, Pesticide Properties, and Unsaturated Zone Thickness

Abstract

Pesticide leaching through variably thick soils beneath agricultural fields in Morgan Creek, Maryland was simulated for water years 1995 to 2004 using LEACHM (Leaching Estimation and Chemistry Model). Fifteen individual models were constructed to simulate five depths and three crop rotations with associated pesticide applications. Unsaturated zone thickness averaged 4.7 m but reached a maximum of 18.7 m. Average annual recharge to ground water decreased from 15.9 to 11.1 cm as the unsaturated zone increased in thickness from 1 to 10 m. These point estimates of recharge are at the lower end of previously published values, which used methods that integrate over larger areas capturing focused recharge in the numerous detention ponds in the watershed. The total amount of applied and leached masses for five parent pesticide compounds and seven metabolites were estimated for the 32-km2 Morgan Creek watershed by associating each hectare to the closest one-dimensional model analog of model depth and crop rotation scenario as determined from land-use surveys. LEACHM parameters were set such that branched, serial, first-order decay of pesticides and metabolites was realistically simulated. Leaching is predicted to be greatest for shallow soils and for persistent compounds with low sorptivity. Based on simulation results, percent parent compounds leached within the watershed can be described by a regression model of the form e(-depth) (a ln t1/2-b ln K OC) where t1/2 is the degradation half-life in aerobic soils, K OC is the organic carbon normalized sorption coefficient, and a and b are fitted coefficients (R2 = 0.86, p value = 7 x 10(-9)).

Pesticide occurrence in groundwater and the physical characteristics in association with these detections in Ireland

“…Binary logistic regression revealed that pesticide occurrence was related to depth to the water table, soil organic matter content, and particle size distribution. Webb et al (2008) used the Leaching Estimation and Chemistry Model (LEACHM) and observed that pesticide leaching was greatest for shallow soils and persistent compounds with low sorptivity. Logistic regression was also used by Malaguerra et al (2012) to reveal that the factors geology, redox conditions, and well depth affected pesticide frequency of detection in drinking wells.…”

The Geochemistry of Pesticides

“…Examples include investigations of the transport and fate of DBCP in groundwater (Burow et al, 1999), as well as atrazine, metolachlor, and glyphosate in the vadose zone (Barbash and Voss, 2007;Bayless, 2001;Bayless et al, 2008;Webb et al, 2008).…”

Simulation of branched serial first‐order decay of atrazine and metabolites in adapted and nonadapted soils

“…The present study also provides a description of the methods used by Webb et al [18], who used the Nebraska model results to predict the leaching of atrazine and its metabolites beneath agricultural fields in Maryland where soils, land use, and agricultural management practices are similar. In that study, the Leaching Estimation and Chemistry Model (LEACHM) [19,20], which was designed to use only linear first-order decay, was modified to simulate BSFOD decay and the leaching of pesticides and metabolites to groundwater over several growing seasons.…”

“…These production fractions were then used as parameters in LEACHM to estimate the flux of atrazine and metabolites beneath cornfields with similar soils in Maryland [18].…”

“…Half-lives used for the first iteration of the optimization exercise were 69 and 48 d for atrazine and DIA, respectively [11], 241 d for DEA (258 d [24,24] adjusted to 258C), and 121 d for HYA [26]; The range for the percentage of atrazine degrading to DIA was restricted to 0.1 to 20%; previous studies [11,24,25,30] indicate DEA production is three to six times that of DIA. A multiplier of 3.91 was selected here to match the ratio used by Webb et al [18]. Therefore, using the previous constraint, the percentage of atrazine degrading to DEA was limited to the range from 0.391 to 78.2%; The percentage of atrazine degrading to HYA was calculated as 100% minus the percentages of atrazine degrading to DEA and DIA and limited to the range, using the previous constraints, from 1.8 to 99.5%.…”