Relation of Landscape Position and Irrigation to Concentrations of

Chapter 15

Downloaded by UNIV OF GUELPH LIBRARY on October 9, 2012 | http://pubs.acs.org Publication Date: June 21, 1996 | doi: 10.1021/bk-1996-0630.ch015

Relation of Landscape Position and Irrigation to Concentrations of Alachlor, Atrazine, and Selected Degradates in Regolith in Northeastern Nebraska 1

2

2

Ingrid M. Verstraeten , D. T. Lewis , Dennis L. McCallister , Anne Parkhurst , and Ε. M. Thurman 3

1,4

1

Water Resources Division, U.S. Geological Survey, 406 Federal Building, 100 Centennial Mall North, Lincoln, NE 68508 Department of Agronomy, University of Nebraska-Lincoln, Lincoln, NE 68588 Department of Biometrics, University of Nebraska-Lincoln, Lincoln, NE 68588 2

3

Concentrations of alachlor, its ethanesulfonic acid degradate, atrazine and its degradates, deethylatrazine and deisopropylatrazine, in the upper regolith and associated shallow aquifers were determined in relation to landscape position (floodplains, terraces, and uplands) and irrigation (nonirrigated and irrigated corn cropland) in 1992. Irrigated and nonirrigated sites were located on each landscape position. Samples were collected from three depths. Canonical discriminant and multivariate analyses were used to interpret data. Herbicides and their degradation products tended to be present in soils with high percent organic matter, low pH, and low sand content. Atrazine was present more frequently on the floodplain at all depths than the other compounds. Atrazine (maximum 17.5 μg/kg) and ethanesulfonic acid (maximum 10 μg/kg) were associated with landscape position, but not with irrigation. Alachlor (maximum 24 μg/kg), deethylatrazine (maximum 1.5 μg/kg), and deisopropylatrazine (maximum 3.5 μg/kg) were not significantly associated with either landscape position or irrigation. Ground-water analytical results suggested that concentrations of these herbicides and degradates in ground water did not differ among landscape position or between irrigated and nonirrigated corn cropland. 4

Current address: U.S. Geological Survey, 4821 Quail Crest Place, Lawrence, KS 66049 0097-6156/96/0630-0178$15.00/0 © 1996 American Chemical Society In Herbicide Metabolites in Surface Water and Groundwater; Meyer, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


15.

VERSTRAETEN ET AL.

Relation of Landscape

& Irrigation to Herbicides

179

Alachlor [2-chloro-2,6-diethyl-A^-(methoxymethyl)acetanilide] and atrazine [6-chloroA^-ethyl-A^Hl-niethylethyO-l^^-triazine^^-diamine] are the herbicides most often present in ground and surface water in the midwestern United States (1-8). They have been detected at concentrations generally less than the U.S. Environmental Protection Agency Maximum Contaminant Levels for drinking water (2.0 μg/L for alachlor and 3.0 μg/L for atrazine) (9). These herbicides are mainly pre-emergent herbicides commonly applied to corn (Zea mays L.) and sorghum (Sorghum bicolor L.) to control annual broadleaf weeds and grasses, respectively. Atrazine in ground water has been associated with irrigated corn cropland in Nebraska (8). Alachlor is a nonionic herbicide with moderate vapor pressure and moderate solubility (10). Atrazine, a moderately persistent pesticide, is a weak base with low vapor pressure and moderate solubility (77, 12). The degradation pathways of alachlor and atrazine include chemical, microbial, and photolytic degradation processes, which depend upon the chemical and physical conditions of the media and microbial populations (13-15). Numerous studies on the degradation pathways of alachlor and atrazine have been conducted (13, 14, 16-20). Baker and others (21) reported the presence of alachlor ethanesulfonic acid [2-[2,6diethylphenyl(methoxymethyl)amino]-2-oxoethanesulfonic acid], a major alachlor degradate, in ground water. Other authors have identified this degradate in ground water by high performance liquid chromatography (HPLC) (22,23) and by an enzymelinked immunosorbent assay for alachlor (21, 22, 24). The half-life of atrazine is dependent upon water holding capacity, acidity, organic matter, the presence of microbial populations, and soil redox conditions (25, 26). Deethylatrazine and deisopropylatrazine mainly are formed through microbial degradation, a Af-dealkylation process principally attributed to fungi (27). Microbial degradation of atrazine occurs frequently in the first meter of the soil profile and to a lesser extent at greater depth (28). Deethylation, rather than deisopropylation, is a more rapid degradation pathway (14, 29). Agricultural chemicals are distributed to surface and ground water by transport as runoff and by leaching through the unsaturated zone. Agricultural chemicals also may volatilize to the atmosphere, degrade, and be adsorbed to and desorbed from mineral surfaces and organic matter. Water and solute transport are associated with the landscape position and intimately related to soil properties and water-balance relationships (30). Transport is controlled by soil biological, chemical, and physical properties, including antecedent soil-water content, the rate and amount of infiltrating water, and rate and amount of water applied during irrigation (30, 31). Hydraulic properties and ability of soil to retain organic chemicals vary across the landscape due to temporal and spatial changes in pedon characteristics, such as organic matter content and clay content, resulting in diverse patterns of water and solute distribution (30). Therefore, concentrations of herbicides and their dégradâtes in the pedon and in ground water may be associated with landscape position and irrigation. The objective of this research was to determine whether: (1) concentrations of alachlor, atrazine, and selected dégradâtes in and beneath soil on uplands, terraces, and floodplains differ across landscape positions, and (2) concentrations of alachlor, atrazine, and selected dégradâtes in and beneath soils differ depending upon irrigation use.

In Herbicide Metabolites in Surface Water and Groundwater; Meyer, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


HERBICIDE METABOLITES IN SURFACE WATER AND GROUNDWATER

;ure 1. Study area and generalized locations of sample sites.


15. VERSTRAETEN ET AL. Relation of Landscape & Irrigation to Herbicides

181


Materials and Methods Site Selection. Sites were located along three transects that crossed the floodplain, terrace, and upland landforms located within a loess-capped glaciated region of eastern Nebraska (Fig. 1). Irrigated and nonirrigated sample sites were located along these transects on each landscape position. Three sampling subsites within each major site were identified. Upper regolith samples (162 samples) were collected using a Giddings probe or spade in May 1992 from three depths: 15 to 31 cm (depth 1), 82 to 115 cm (depth 2), and 137 to 305 cm (depth 3). Soils at the sampling sites were within the fine-silty particle-size class. Samples were not collected from depths less than 15 cm to avoid the presence of herbicides from spring application. Land use was limited to cultivated land, with sprinkler irrigated and nonirrigated land in close proximity, and where alachlor and atrazine were applied within 5 years prior to sampling. In addition, 8 representative ground-water samples were collected from 7 irrigation wells and 1 domestic well completed at depths generally less than 200 feet in unconfined aquifers at or near the 18 upper regolith sample sites in accordance with U.S. Geological Survey ground-water sampling protocols (7). Chemical and Physical Analytical Methods. The upper regolith samples collected for analyses of chemical and physical properties were air-dried and subsequently sieved to less than 2 mm. Particle size (Soil Survey Interim Report, No. 43, 1994), soil pH and excess lime (52), organic carbon (33), exchangeable cations and cation exchange capacity (34), and surface area (35) were determined for each sample. Atrazine, deethylatrazine, and deisopropylatrazine in upper regolith samples were determined using an automated solid-phase extraction, followed by automated GC/MS (gas chromatography/mass spectrometry) analysis of the eluates on a Hewlett-Packard Model 5890 gas chromatograph and a 5970A mass-selective detector (36). The atrazine extraction method was modified for extraction of alachlor and its ethanesulfonic acid through deletion of the anion-exchange procedure during the extraction process. The extract was blown to dryness using a Turbovap and subsequently dissolved in 5.0 mL organic-free water for analysis by an enzyme-linked immunosorbent assay (24). To test this new extraction procedure for alachlor and its ethanesulfonic acid degradate, batch equilibrium experiments were conducted using three samples collected at three depths. Five grams of air dry sample were weighed into a teflon-lined screwtop test tube. Six mL of organic-free water with 0.005 M CaS0 .2H 0 were added to the samples. Zero ng, 10 ng, 100 ng, 500 ng, and 1,000 ng alachlor or its ethanesulfonic acid were added to the samples in separate tubes. The solution was equilibrated for 24 hours utilizing a rotary mixer at 30 rpm. The slurry was centrifuged until the supernatant was clear (generally in excess of 15 min). The clear supernatant was removed with a pipette into a glass vial for evaporation and solid-phase extraction. Samples were analyzed with automated GC/MS for alachlor (37). Samples were analyzed with an enzyme-linked immunosorbent assay (ELISA) for ethanesulfonic acid (24). Percent recovery was 85-125% and reproducibility was about 15% (Diana Aga, U.S. Geological Survey, personal communication, 1994). 4

2


182

HERBICIDE METABOLITES IN SURFACE WATER AND GROUNDWATER


Water samples initially were tested qualitatively using enzyme-linked immunosorbent assay kits for triazines and alachlor (37). If atrazine or alachlor were detected with this technique, the samples were analyzed for alachlor, atrazine, deethylatrazine, and deisopropylatrazine using extraction procedures and gaschromatographic separation as described by Thurman and others (37) and Meyer and others (38). Concentrations of ethanesulfonic acid were determined by automated solid-phase extraction and enzyme-linked immunosorbent assay by the U.S. Geological Survey Laboratory in Lawrence, Kansas (24). Statistical Analytical Methods. Statistical analyses were performed with SAS Version 6.08 and 6.09, SAS Institute, Inc., Cary, North Carolina, 1990. Correlations among upper regolith properties and extracted concentrations of herbicides and selected dégradâtes were calculated to suggest relationships. The data were determined not to be normally distributed. Several transformation were done to normalize the data. Neither log reciprocal, or quadratic transformations significantly improved the distribution of the data, with the exception of a log transformation of the sand variable. Subsequent parametric analyses therefore were done on untransformed data. Consequently the validity of many of the interpretations in the dicussion of results may depend upon the robustness of the statistical procedures. Multivariate analyses of variance, ΜΑΝΟ V A , were used to compare several treatment means (39) with class variables land use, landscape position, and transect. M A N O V A included interpretations of interactions and addition of contrasts. Depth was added to these M A N O V A analyses as a repeated measure. Univariate analyses were conducted to test whether differences in mean concentrations existed for each depth. Herbicides and degradation products were detected in few samples. Therefore, canonical discriminant analyses were conducted to identify relationships between herbicide occurrence and chemical and physical properties (39), and to identify upper regolith factors that can discriminate among the landscape positions by depth. The assumption of equal covariance between discriminant groups was tested and failed. Subsequently, analyses for quadratic discriminant functions were done. As the error for quadratic functions was equal or greater than those for linear functions, linear functions were used as a tool to help understand the results.

Results and Discussion Chemical and Physical Properties of Upper Regolith. In general, chemical and physical properties of upper regolith in floodplains differed from those properties found on terraces and uplands. Upper regolith properties had differences in chemical and physical properties at all depths and landscape position, e.g. sand, silt, clay, pH, exchangeable calcium, surface area, cation exchange capacity, and organic matter (40) (Table I). Differences in upper regolith properties by landscape position and depth were partially confirmed through statistical analyses. Physical and chemical characteristics of the upper regolith implied that (1) organic matter, fine silt, and clay probably contribute most of the herbicide retention sites, especially in the A and Β horizons rich in organic matter or clay, (2) basic organic molecules tend to be adsorbed


15. VERSTRAETEN ET AL. Relation of Landscape & Irrigation to Herbicides

183

in A and B horizons, rich in organic matter and clay with relatively low pH, and (3) soil with deep sandy layers and relatively higher pH (especially common in the floodplain near the rivers) does not adsorb organic molecules as frequently (40).


Multivariate and univariate analyses. Multivariate and univariate analyses can suggest interactions and significant differences in chemical and physical properties (Table Π). Interactions were absent among landscape positions, transects, and depths for all measured chemical and physical properties. A relationship existed between landscape position and the amounts of sand, pH, organic matter, and cation exchange capacity (Table II). Table I.—Summary statistics of alachlor, atrazine, and selected metabolites in upper regolith sampled (N=54) Variable

Units

Depth

Mean

Standard Deviation

Minimum

Maxii

Alachlor

ug/Kg

1

1.3

3.1

Relation of Landscape Position and Irrigation to Concentrations of

Recommend Documents