PURCHASED BY USDA FOR OFFICIAL USE Reprinted from the Jaumalaf Environmental Quality Volume 26, no. 6, Nov.-Dec. 1997, Copyright 01997, ASA, CSSA, SSSA 677 South Segoe Rd., Madison, WI 53711 USA SpatialandTemporalVariabilityofHerbicidesina Claypan Soil Watershed F. Ghidey,* E. E. Alberts, and R. N. Lerch ABSTRACT or leaching. It encourages the use of site-specific crop The spatial and temporal variability of herbicides during the grow- management, which improves production efficiency by ing season were studied on a 35-ha watershed located in the cIaypan adjusting crop inputs to the local soil conditions within soil region of north-central Missouri. Soil samples were collected from a field (Birrell et aI., 1993). A number of studies have the 0- to 5-em soil depth in 1993, and 0- to 5-, 5- to 10-, and 10- been conducted to evaluate the spatial variability of soil totS-em soil depths in 1995 and analyzed for atrazine (2-chloro-4- properties (Cambardella et aI., 1994; Sudduth et aI., ethylamino-6-isopropylamino-s-triazine) and alachlor (2-chloro-N-(2, 1994; Lascano and Hatfield, 1992; Gajem et aI., 1981) 6-diethl-N-(methoxymethyl) acetanilide) concentrations. The effects and pesticide distribution (Rao and Wagenet, 1985; of rainfall, topography, soil pH, cation exchange capacity (CEC), and Wood et aI., 1987). Several researchers have found that organic matter (OM) content on the spatial distribution of herbicides the behavior of herbicides was related to soil properties were evaluated. There was no spatial dependency between samples taken immediately after herbicide application; however, spatial depen- such as OM content and soil pH (Wood et aI., 1987; dency was observed in the following sampling periods. During the Kells et aI., 1980; Best at aI., 1975; Best and Weber, year, <2.5% of atrazine and 2.0% of alachlor applied to the soil was 1974; Corbin et aI., 1971; Sheets, 1970; Weber, 1970). lost in runoff, and the movement of herbicides below the layer of -Sudduth et aI. (1994) reported that there was significant application was also very low throughout the sampling period. Atra- variability, in water holding capacity, soil nutrients, soil zine and alachlor concentrations in the soil decreased rapidly during pH, top soil depth, crop growth, and yield within fields the growing season. Concentrations measured 4 and 8 wk after applica- and plots located in claypan soil area. However, the tion were <20 and 5% for atrazine, and <10 and 3% for alachlor, spatial variability of herbicides and the effect of soil respectively, of those measured immediately after application. The properties such as OM content, CEC, and soil pH on study showed that the spatial and temporal variability of herbicide concentration was a function of the interaction between soil pH and the variability of the herbicides within these fields and the sorptive capacity of the soil (CEC and OM content). These findings plots have not been studied. are useful for site-specific crop management to increase the efficiency Previous research and modeling efforts indicated that of herbicide application and also to reduce the loss of excess herbicides extraction and transport of chemicals to surface runoff to surface runoff. during a rainfall event occur from the top «2.0 em) soil (Donigan et aI., 1977; Frere et aI., 1980; Ahuja and Lehman, 1983). Early models such as ARM (agricultural HEtransport of nutrients, pesticides, and other agri- runoff management) and CREAMS (chemicals, runoff, Tchemicals in runoff affects the quality of surface and erosion from agricultural management systems) as- water. Claypan soils occupy about 4 million ha in Mis- sumed the existence of a thin layer «1.0 em) where souri and Illinois and are primarily found within major rainwater mixes completely and uniformly with the soil land resource area (MLRA) 113. Claypan soils have a solution. Later, Ahuja and Lehman (1983) found that unique hydrology characterized by the impedance of rainfall extracts chemicals from the soil surface by rain- vertical soil matrix water flow by a restrictive clay layer drop impact after the soil surface is saturated. They (Jamison et aI., 1968). Runoff from claypan soil regions indicated that extraction occurs from depths as great as is relatively high during the seedbed preparation period 2.0 em, but the contribution decreases exponentially when agrichemicals are applied (Ghidey and Alberts, with depth. Because the main concern on claypan soils 1996). As a result, regions with claypan soils have been is surface water quality and since the transfer of chemi- identified as potentially vulnerable areas for pesticide cals to runoff is from the top layer, our study primarily and nutrient contamination of surface water. Surface focused on measuring the chemical concentration within and groundwater samples taken from sites across the the top 5 em of the soil profile. Data collected from this Corn Belt by the U.S. Geological Survey (USGS) indi- study can be used in the future to evaluate water quality cated that surface water is more strongly impacted by models that predict chemical transfer to runoff. herbicides than groundwater (Thurman et aI., 1992; The objectives of this study were to: (i) study the Burkhart and Koplin, 1993). Blanchard et ai. (1995) spatial and temporal variations in atrazine and alachlor reported similar results from a study on claypan re- concentrations in a claypan soil; (ii) evaluate the effects gion soils. of rainfall and topography on the spatial variability of Determining the variability of agrichemicals along herbicides; (iii) evaluate the correlation between soil with other soil properties is important to efficiently man- pH, CEC, or organic matter content, and herbicide con- age crop inputs (particularly fertilizers and herbicides), centration in the soil; and (iv) determine the loss of which helps reduce agrichemical losses to runoff and/ herbicides due to runoff during the growing season. F. Ghidey. Dep. of Biological and Agric. Engineering, Univ. of Mis- souri. Columbia, MO 65211: and E.E. Alberts and R.N. Lerch, USDA- Abbreviations: ARM, Agricultural Runoff Management; CEC, cation ARS. Cropping Systems and Water Quality Research Unit, Columbia, exchange capacity; OM, organic matter; MLRA. major land resource MO 65211. Received 13 Nov. 1996. *Corresponding author area; USGS, U.S. Geological Survey; CREAMS, chemicals, runoff. ([email protected] ). and erosion from agricultural management systems; N-P. nitrogen- phosphorus; LOI, loss of weight on ignition; Gc, gas chromatography; Published in J. Environ. Qual. 26:1555-1563 (1997). UAN. urea ammonium nitrate. 1555 1556 J. ENVIRON. QUAL. VOL. 26, NOVEMBER-DECEMBER 1997 MATERIALS AND METHODS Merr.] rotation agricultural farming system with the goals of maximizing grain yield and profit. The area was cropped to The study was conducted on a 35-ha area (Fig. 1) located soybean in 1992, com in 1993, and soybean in 1994. In 1995, in the Goodwater Creek watershed, a 7250-ha agricultural area in northeast Missouri. Predominant soils are Vdollic Och-. due to high rainfall during the planting period, it was not possible to plant com. Instead, the study area was cropped to raqualfs, Albaquic Hapludalfs, and Vertic Ochraqualfs of the grain sorghum [Sorghum hicolor (L.) Moench]. In 1993, 190 Mexico, Adco, and Leonard series, respectively. Claypan soils are considered poorly drained, partially because of an argillic kg ha-I of N (as VAN), 2.24 kg ha-I of atrazine, and 2.8 kg ha-I of alachlor were applied to the soil. In 1994, because the claypan horizon located 15 to 30 em below the surface. The clay content of the argillic horizon is generally greater than study area was cropped to soybean, only alachlor was applied 50% and is primarily montmorillonite. The outlet of the water- at the rate of 2.8 kg ha-I. In 1995,123 kg ha-I ofN (as VAN), shed was instrumented with a concrete v-notch weir, water 1.9 kg ha -I of atrazine, and 2.8 kg ha -I of alachlor were applied stage recorder, and a refrigerated pumping sampler to measure to the soil. The chemicals were broadcast and incorporated runoff and chemical concentrations in runoff. The sampler with field cultivator before planting. automatically starts collecting when flow over the weir is signif- Soil samples were collected using a 2.5-cm diameter probe. icant and continues to sample until the end of the runoff Eight subsamples were taken from the ridges and furrows of each cell and were mixed and wrapped in aluminum foil. '. period. The samples were stored in a refrigerated cabinet until transported back to the laboratory for chemical and Samples were frozen before being sent to the USDA-ARS sediment analysis. National Soil Tilth Laboratory at Ames, lA, for analysis. Soil The area was divided into 36 equally spaced cells (80 m by samples were extracted with 80% methanol-water for atrazine 120 m), and soil samples were collected from each cell within and alachlor using a robotic system. Atrazine and alachlor the depths of 0 to 5 em in 1993 and 0- to 5-, 5- to 10-, and 10- were isolated and concentrated from the extracts by solid to 15-em depths in 1995. Initial samples were collected 1 wk phase extraction (C18cartridge), eluted with ethyl acetate, and before field operation or chemical application to monitor any analyzed with a HP 5890 capillary gas chromatograph using a chemical residue remaining from previous years' application. nitrogen-phosphorous (N-P) detector. Extraction efficiencies Additional samples were collected 3 d, and 1, 2, 4, 8, and 27 for atrazine and alachlor were approximately 90 and 80%, wk after application in 1993, and 1, 4, 8, and 27 wk in 1995. respectively. Atrazine and aIachlor concentrations were cor- The study area was farmed using a high chemical input .rected for the recovery efficiencies.