No abstract

As nonpoint source pollution of water becomes more evident, more concern is being focused on the effects of agricultural practices on water quality. This study evaluated the effects of conventional tillage (CT), chisel‐plow tillage (CP), and no tillage (NT) on the quality of runoff water from a Maury silt loam soil (fine, mixed, mesic Typic Paleudalf) near Lexington, KY. The mean runoff rate, total runoff volume, mean sediment concentration, and total soil losses were significantly less for NT than for CP and CT. Concentration of NO−3, NH+3, and PO3−4 in the runoff water from NT were greater than from CP or CT. Concentration of atrazine [6‐chloro‐N‐ethyl‐N′‐(I‐methylethyl)‐1,3, 5‐triazine‐2,4‐diamine] in the runoff water tended to be higher from CP than from NT or CT. Total losses of NO−3, NH+4, PO3−4, and atrazine in runoff water were generally in the order CT > CP > NT. The sum of all chemicals lost was less than 3% of the total amount of each applied.

Colloid migration in subsurface environments has attracted special attention lately due to its suspected role in facilitating transport of contaminants to groundwater. This study was conducted to evaluate the stability and potential transport of water‐dispersible colloids (WDC) through intact soil columns, and the properties of colloids and soil columns facilitating or retarding colloid stability and transportability. Water‐dispersible colloids were fractionated from six representative soil samples with diverse mineralogy and physicochemical characteristics. Their stability was evaluated from settling‐rate experiments at different pH levels. The results demonstrated that colloid stability was pH dependent. Colloid transportability was assessed by introducing colloid suspensions at a constant flux into intact soil columns representing upper Bt horizons of a Maury (fine, mixed, mesic Typic Paleudalf) and a Loradale (fine‐silty, mixed, mesic Typic Argiudoll) soil and evaluating characteristics of the suspensions that were eluted. After five pore volumes of leaching, colloid recovery in the eluents ranged from 35 to 90% depending on type of colloid, initial concentration in the influent, and soil column. The mineralogical composition of the colloid, which was correlated with particle size, appeared to have a profound effect on colloid transportability, following the sequence smectitic > mixed > kaolinitic. Total exchangeable bases (TEB) and pH of WDC also significantly influenced colloid transport. Increasing colloid concentration in the influent slightly increased colloid transportability. Soil columns with better macroporosity and less surface charge (Maury) transported more colloids than soil columns with less macroporosity and higher surface charge (Loradale).

This study evaluated the potential of water‐dispersible soil colloids with diverse physicochemical and mineralogical characteristics to adsorb and mediate the transport of atrazine through macropores of intact soil columns. The soil columns were taken from the upper solum of an Alfisol (Maury) and a Mollisol (Loradale) in the Bluegrass region of Kentucky. Six different colloid suspensions mixed with atrazine [2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino)‐s‐triazine] were introduced at a constant flux into the undisturbed soil columns. The eluents were collected and analyzed periodically for colloid and herbicide concentrations. Colloid recovery in the eluents ranged from 45 to 90%. The presence of colloids enhanced the transport of atrazine by 2 to 18%, depending on colloid type and mobility. Colloids with higher pH, total exchangeable bases (TEB), cation‐exchange capacity (CEC), organic carbon (OC), surface area (SA), and electrophoretic mobility (EM), showed greater mobility, greater affinity for atrazine, and greater potential to co‐transport atrazine. In contrast, increased levels of kaolinite, Fe, and Al inhibited atrazine adsorption and transport. Soil columns with higher hydraulic conductivity, well‐developed macropore system, and lower surface reactivity (Maury) also transported more atrazine. In spite of the increased transportability of atrazine by the presence of soil colloids, the colloid‐bound herbicide fraction contributed ≤1% to the observed increase. This suggests that matrix‐surface‐site exclusion and blocking mechanisms induced by the presence of colloids are more important than colloid sorption in promoting atrazine mobility in subsurface soil environments.

This study evaluated the role of water dispersible colloids with diverse physicochemical and mineralogical characteristics in facilitating the transport of metolachlor through macropores of intact soil columns. The soil columns represented upper solum horizons of an Alfisol in the Bluegrass region of Kentucky. Three different colloid suspensions tagged with metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l-methylethyl)acetamide] were introduced at a constant flux into undisturbed soil columns. The eluents were 949 950 SETA AND KARATHANASIS collected and analyzed periodically for colloid and metolachlor concentrations. Colloid recovery in the eluents ranged from 54 to 90 %. The presence of colloids enhanced the transport of metolachlor by 22 to 70 % depending on the colloid type and mobility. Colloids with higher pH, organic carbon, cation exchange capacity (CEC), total exchangeable bases (TEB), surface area (SA), and electrophoretic mobility (EM), showed better mobility, greater affinity for interaction with the herbicide and, thus, greater potential to co-transport metolachlor. In contrast, increased level of kaolinite, Fe, and Al inhibited metolachlor adsorption and transport. In spite of the increased transportability of metolachlor by the presence of soil colloids, the colloid bound herbicide portion accounted for a very small part of the observed increase. This suggests that surface site exclusion mechanisms and preferential sorption induced by the presence of colloids are more important than ion exchange phenomena in promoting herbicide mobility in subsurface environments.