JEQ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (5)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Seol, Y.
Right arrow Articles by Lee, L. S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Seol, Y.
Right arrow Articles by Lee, L. S.
Agricola
Right arrow Articles by Seol, Y.
Right arrow Articles by Lee, L. S.
Related Collections
Right arrow Water Use
Right arrow Agricultural Pesticides
Right arrow Animal Waste
Journal of Environmental Quality 30:1644-1652 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Organic Compounds in the Environment

Coupled Effects of Treated Effluent Irrigation and Wetting–Drying Cycles on Transport of Triazines through Unsaturated Soil Columns

Yongkoo Seol and Linda S. Lee*

Department of Agronomy, Purdue Univ., West Lafayette, IN 47907

* Corresponding author (lslee{at}purdue.edu)

Received for publication August 31, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
The physical and chemical parameters controlling the movement of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine; 98.8%) and prometryn [N,N'-bis(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine; 99.5%] were investigated in columns infiltrated with treated effluent under unsaturated transient conditions and subjected to drying events at 22 or 60°C followed by rewetting. Three soils varying in soil pH and texture and three solutions were used. The infiltrating solutions consisted of either a CaCl2 matrix (CC), a swine waste–derived lagoon effluent (SW), or a simulated buffer solution (SB) representative of the element composition and pH of the SW but with no dissolved organic matter. Several parameters were monitored including leachate triazine concentrations, pH, dissolved organic carbon (DOC), inorganic carbon, and flow rates. Compared with CC, application of SW and SB increased column leachate pH, enhanced dissolution of organic carbon and particle dispersion, and decreased average flow rates, which allowed for increased desorption time. The coupled effect of these processes enhanced movement of triazines in some cases, with SW generally having the greatest effect. The individual effect of increased pH was more pronounced for prometryn (pKa = 4.05) versus atrazine (pKa = 1.66), and most dramatic for the soil with the lowest initial pH. High-temperature drying, which simulated intensive evaporation, further enhanced the dissolution of soil organic matter and the reduction in leachate flow rates with SW and SB applications; however, the net effect under the experimental conditions employed varied with soil type. Relative to low-temperature drying, high-temperature drying in the silty clay loam–packed columns reduced pesticide migration.

Abbreviations: CC, solution consisting of a CaCl2 matrix • DOC, dissolved organic carbon • DOM, dissolved organic matter • IC, inorganic carbon • MI, initial pesticide mass applied • ML, pesticide mass measured in leachate fractions • MC, pesticide mass remaining in the soil column • SB, simulated buffer solution • SOM, soil organic matter • SW, swine waste–derived lagoon effluent


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
REUSE of treated effluents for irrigation has given rise to environmental concerns regarding their effect on transport of surface-applied pesticides. Solution characteristics that may affect pesticide transport include pH, electrolyte composition, and the presence of dissolved organic matter (DOM). For weakly basic pesticides, solution pH may affect pesticide speciation, and thus sorption by soil. Solution pH and electrolyte composition may cause soil dispersion, thus affecting pore velocities and residence times. Seol and Lee (2000) demonstrated that effluent-induced changes in soil–solution pH could reduce sorption of weak organic bases such as atrazine and prometryn, thus increasing mobility. In the latter case, the relative potential for enhanced mobility is a function of the pesticide's pKa and the soil and effluent pH and buffer capacity.

The potential for pesticides to complex with DOM may also lead to the enhanced mobility of pesticides under effluent irrigation. In a field study on the transport and fate of atrazine, Graber et al. (1995) demonstrated that atrazine penetrated to a much greater depth in the effluent-irrigated soil profiles relative to the fields irrigated with high-quality water. They attributed facilitated transport to complexation of atrazine with dissolved organic matter (DOM) in the treated effluent. Other studies conducted in saturated batch systems suggested that the association of weakly hydrophobic compounds such as atrazine with effluent or water-borne DOM was not significant for enhancing pollutant mobility (Clay et al., 1988; Ding and Wu, 1993; Seol and Lee, 2000). Likewise, Seol (1998) did not observe DOM-facilitated transport of either atrazine or prometryn in steady-sate saturated column studies where the column influent contained more than 150 mg/L dissolved organic carbon (DOC). However, Williams et al. (2000) observed DOM-facilitated transport of napropamide [(RS)-N,N-diethyl-2-(1-naphthyloxy)propionamide], a pesticide similar in polarity to atrazine and prometryn, during the initial wetting event in laboratory-packed soil columns. Further studies by Nelson et al. (2000) showed that drying events can enhance the potential for DOM to facilitate transport of even moderately polar pesticides.

Numerous parameters such as weather conditions, shading, landscaping, soil properties, and rainfall affect soil temperature, thus natural drying events (Krarti et al., 1995). In arid environments where air temperature varies from 14.4 to 45.1°C, soil surface temperature shows a wide range from 24.5 to 63°C (Gupta et al., 1981; Lodha and Solanki, 1992). The National Climatic Data Center has collected periodically since 1982 soil temperature data at various depths in selected regions. Soil temperature data in Arizona show a temperature range in July of 43 to 52°C at a depth of 5 cm and 38 to 42°C at a depth of 10 cm (National Climatic Data Center, Environmental Research Laboratories, and the National Weather Service, 1995). These observations suggest that soil temperature could be more than 60°C at the soil surface and that soil may repeatedly cycle through extremely hyperthermal conditions throughout the summer season.

Given the intermittent nature of natural precipitation and agricultural irrigation, the application of information on pesticide sorption and transport obtained under saturated equilibrium or steady-state conditions is limited in its application to understanding the potential effects of treated effluent irrigation. Investigations under unsaturated and nonsteady-state conditions that include the coupled effects of drying and wetting events are essential to a better understanding of the potential effect of irrigation with treated effluent relative to clean water. The overall objective of this study was to assess the effect of effluent irrigation on triazine transport in unsaturated soil columns of varied soil types subjected to low- and high-temperature drying events. Parameters monitored included column leachate pH, DOM, dissolved inorganic carbon, pesticide concentrations, and flow rates, as well as column moisture contents.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Pesticides
Two s-triazine herbicides obtained from Chem Service (West Chester, PA) were selected for this study: atrazine (98.8%) and prometryn (99.5%). Selected pesticide properties are given in Table 1.


View this table:
[in this window]
[in a new window]
  		Table 1. Selected physical and chemical properties of pesticides.

 
Solutions
Lagoon effluent was collected from the last lagoon in a three-lagoon system used as a biological treatment process for swine (Sus scrofa) production wastes at the Baker Farm (a Purdue University Animal Science Research Center in West Lafayette, IN) where the lagoon effluent is often applied to nearby croplands. The collected raw effluent was centrifuged at 4000 rpm for 4 h, and filtered with 0.2-µm membrane filters (Supor-200; Gelman, Ann Arbor, MI) to exclude all the suspended materials. The lagoon effluent (SW) was then analyzed for dissolved organic carbon (DOC), inorganic carbon (IC), pH, and element composition (Table 2). Carbon content was analyzed with a Shimadzu (Kyoto, Japan) 5000A total organic carbon analyzer.


View this table:
[in this window]
[in a new window]
  		Table 2. Chemical properties of solutions. CC, 0.005 M CaCl2; SB, simulated electrolyte buffer solution; SW, swine effluent.{dagger}

 
Based on the measured chemical properties of the lagoon effluent, a simulated buffer solution (SB) was prepared that represented the pH and elemental composition of the lagoon effluent but with no DOM. The use of a simulated buffer solution helps to differentiate the role of DOM from other effluent properties that may have an influence. A 0.005 M CaCl2 solution (CC) was used for simulation of high-quality water irrigation.

Soils
Soil samples for this study consisted of one sandy loam, Bloomfield (sandy, mixed, mesic Lamellic Hapludalf), and two silt loams, Toronto (fine-silty, mixed, superactive, mesic Udollic Epiaqualf) and Drummer (fine-silty, mixed, superactive, mesic Typic Endoaquoll). They were collected from Ap horizons, air-dried, and passed through a 2-mm sieve. Soil characteristics are listed in Table 3 for pH, cation exchange capacity, particle size analysis, and organic matter content.


View this table:
[in this window]
[in a new window]
  		Table 3. Physical and chemical properties of selected soils.

 
Column Preparation
Open-top glass columns (3.5 cm i.d. x 15 cm long) were used to conduct leaching studies. The bottom end of the columns tapered into a 0.5-cm-i.d. tube. The end funnel was filled with glass wool, which had been washed with methanol and deionized water to remove any contamination. To support the soil bedding on top of the glass wool, a 1-cm-thick sand layer was placed to prevent soil from passing through the glass wool. The sand had been thoroughly washed with methanol, 0.1 M HCl, and deionized water. Soils were wetted to approximately 75% of water saturation at field capacity with 0.005 M CaCl2 solution before being packed in columns. Wet packing aids in more uniform packing, and minimizes overpacking. Soil was placed in the column in small fractions and gently packed after each addition with a rubber stopper of similar diameter as the column. The top surface of each soil layer was slightly stirred before packing the next fraction to minimize stratification. Each column was packed to a clean soil bed height of approximately 7 cm. For each soil, 12 columns were prepared to accommodate leaching of two pesticides by three different solutions with two drying temperatures. For leaching of prometryn through the Bloomfield soil, six additional columns were prepared for replication with each of the three influent solutions. A 1-cm layer of pesticide-contaminated soil was placed on the top of the clean soil bed. The pesticide-contaminated soil was prepared by mixing a methanol solution containing the selected pesticide with air-dried soil to give soil pesticide concentrations of 6.5 to 8.2 mg/kg and 7.6 to 8.3 mg/kg for atrazine and prometryn, respectively. The concentrations were selected based on recommended application rates of the active ingredient reported by Beste (1983) (atrazine: 7.6–15.2 mg/kg soil; prometryn: 4.5–10.5 mg/kg soil). Pesticide-contaminated soils were air-dried for 2 d at room temperature (22 ± 1°C) to evaporate the methanol prior to packing. Soils were then wetted with a 0.005 M CaCl2 solution similar to the clean soil and placed in the columns on top of the clean bed, followed by another 1-cm layer of sand to facilitate uniform flow. Columns were weighed empty and after each packing step. Columns were packed to ensure that for each designated layer (clean, pesticide-impregnated, and sand) the soil mass and resulting height was the same for that layer across columns packed with the same soil. Ranges in bulk density, porosity, and pore volume resulting from the column packing of each soil type are summarized in Table 4. All columns were covered with aluminum foil to prevent pesticide photolysis during experiments.


View this table:
[in this window]
[in a new window]
  		Table 4. Selected properties of laboratory-packed soil columns.

 
Pesticide Displacement Experiments
Experimental conditions were chosen to simulate the uppermost layer of the soil profile (ca. 10 cm), which is where pesticide is initially applied and soil is most susceptible to evapotranspiration. Soil columns were subjected to wetting and drying cycles. The first wetting event consisted of an application of approximately 0.75 pore volumes of solution, which did not result in any leachate. The pore volume of each column (the volume of space not occupied by soil) was estimated using column bulk density and assuming a particle density of 2.75 g/cm3 for the Bloomfield soil and 2.6 g/cm3 for the other two soils. One set of columns was dried in a oven by slowly increasing temperature to 60 ± 5°C for 12 h followed by a slow cool-down to room temperature while the other set of columns remained at room temperature (22 ± 1°C). High-temperature drying was used to simulate long periods of air-drying, high evapotranspiration, and hot temperatures between wetting events associated with arid regions. All columns were weighed before and after each wetting and drying event to monitor changes in moisture content.

In each of the next three wetting events, approximately 1.5 pore volumes of solution was ponded on the soil. Drainage from the ponding event ceased in less than 1 h for the Bloomfield soil columns, and in about 12 h for the Toronto and Drummer soil columns. For each wetting cycle, three fractions of approximately 20 mL each were collected from each column in preweighed 20-mL glass vials for pesticide, pH, DOC, and IC analyses. Leachate pesticide concentrations from the Bloomfield soil column were analyzed directly. Leachate from the Drummer and Toronto soil columns contained much lower pesticide concentrations, thus required liquid–liquid extraction and concentration steps prior to analysis. Aqueous leachate was extracted 2:1 with hexane. A 3-mL aliquot of the hexane was taken, hexane was evaporated, and the precipitate redissolved with 1 mL methanol. Liquid–liquid extraction efficiencies were greater than 98% as determined by spiking in a known amount of pesticide into an aqueous phase followed by liquid–liquid extraction and mass balance calculations. Pesticide concentrations were measured using a Shimadzu HPLC/UV (Supelco [Bellefonte, PA] ABZ+, 254 nm, acetronitrile and water, 65:35 [v/v], 1.5 mL/min). Fraction collection time and weights were recorded for each column to estimate drainage rates. Wetting–drying cycles were repeated three times for 3 d followed by removal of soil for residual pesticide analysis.

Soil was removed from columns in 1- to 2-cm increments. Each increment was mixed and split for pesticide extraction and moisture content. Soil was extracted with 15 mL of methanol in a 35-mL centrifuge tube with a Teflon-lined cap for 12 h on an end-over-end shaker. Tubes were centrifuged at 2500 rpm for 30 min (1000 x g) and the supernatant was analyzed for pesticide concentration as described earlier, and corrected for extraction efficiency. Methanol extraction efficiencies had been determined in a previous study with the same soils and solutions, but under saturated conditions (Seol, 1998). Soils were equilibrated at 22°C with 14C-labeled and 12C-labeled pesticide in either CC or SW solutions for 48 h followed by three sequential 24-h extractions with either CC or SW. Soils were extracted with methanol at the end of the adsorption–desorption study in the same manner as the column increments, and the methanol-extracted soil was oxidized for residual 14C (Packard [Meridan, CT] Model 307). Methanol extraction efficiencies estimated from the high performace liquid chromatography (HPLC) and UV determinations of atrazine ranged between 70 and 97% for the various pesticide–soil–solution combinations. Mass balance for the 14C-label ranged between 98.3 and 104% for the various pesticide–soil–solution combinations, indicating that no pesticide was mineralized and any pesticide remaining was due to an inefficient extraction by methanol or irreversible binding or transformation of the parent compound. Measured atrazine concentrations in the methanol extracts of the soil column increments were corrected for extraction efficiency using the values estimated from the saturated batch studies. Note that the extraction efficiencies employed do not account for any additional irreversible binding processes that may have occurred in soils subjected to elevated temperatures.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Soil columns packed with Bloomfield, Toronto, or Drummer soil were leached with 0.005 M CaCl2 (CC), a simulated electrolyte buffer solution (SB), or swine-derived lagoon effluent (SW) and subjected to four wetting–drying cycles with drying cycles of either 60 ± 5°C (high temperature) or 22 ± 1°C (low temperature), followed by removal of soil in 1- to 2-cm increments for residual pesticide analysis. Trends in pesticide leaching and/or the post-leaching pesticide distributions within each column were evaluated relative to the initial pesticide mass applied (MI). The sum of the pesticide mass measured in the leachate fractions (ML) and remaining in the soil column (MC) relative to MI is summarized in Table 5 for each pesticide–soil–solution combination. Regardless of influent type or the temperature of the drying cycles, ML was highest from the Bloomfield soil columns for both pesticides. The Bloomfield soil has the lowest soil organic carbon content and cation exchange capacity (CEC), and has a pH much greater than the pKa of either pesticide. At pH >> pKa, organic bases such as atrazine and prometryn exist as neutral species where the primary sorption domain is organic carbon (Talbert and Fletchall, 1965; Weber et al., 1969). At pH <= pKa, a substantial amount of the pesticide can exist as a cation and undergo cation exchange. Prometryn compared with atrazine is more hydrophobic (greater log Kow) and more basic (greater pKa), and thus has a higher affinity to organic matter and a higher chance of existing as a cation and undergoing cation exchange compared with atrazine, respectively (Weber, 1966; Weber et al., 1969). Consistent with these differences in the chemical properties of atrazine and prometryn, lower percentages of prometryn were consistently recovered in column leachate compared with atrazine for similar soil column–influent combinations. The pesticide mass recovered in the column leachate tended to be greater under application of SB or SW solutions for both pesticides relative to what was observed with CC for the soil columns subjected to a low-temperature treatment. Similar trends were not observed from the columns subjected to high temperatures.


View this table:
[in this window]
[in a new window]
  		Table 5. Recovery (%) of pesticides from column leachate and soil extraction relative to mass applied. CC, 0.005 M CaCl2; SB, simulated electrolyte buffer solution; SW, swine effluent.

 
The total amount of pesticide recovered relative to what was applied varied from 26 to 110% (Table 5) and was a function of the influent solution type and drying temperature. Total pesticide recovery was lower in the high-temperature treatment, most likely due to larger volatilization losses. Loss due to mineralization is also plausible; however, in a previous 5-d adsorption–desorption study with the same pesticide–soil–solution combinations under saturated conditions, no mineralization was observed (Seol, 1998). Mass recovery was highest in columns where most of the pesticide mass was recovered in the leachate during the earlier wetting events, thus a smaller amount of pesticide was available for volatilization during subsequent drying events.

Further assessment of the effect of solution types and drying treatments can be gleaned from the trends in the pesticide leaching profiles (ML/MI versus pore volume) and post-leaching column distributions (MC/MI versus soil column depth). Atrazine leaching profiles from all columns subjected to low-temperature drying cycles as well as from the Bloomfield soil columns subjected to high-temperature drying are shown in Fig. 1. Prometryn leaching profiles from the Bloomfield soil columns for both temperature treatments are shown in Fig. 2. Very little to no leaching of prometryn was observed in the Toronto and Drummer soil columns (Table 5). Residual atrazine and prometryn distributions in Drummer and Toronto soil columns for both drying temperatures are shown in Fig. 3 and 4, respectively.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 1. Atrazine displacement with infiltrating solutions consisting of either a CaCl2 matrix (CC), a swine waste–derived lagoon effluent (SW), or a simulated buffer solution (SB) plotted as mass in leachate (ML) relative to initial mass applied (MI) versus pore volume from Bloomfield soil columns subjected to drying events of (A) 22°C and (B) 60°C, and from (C) Toronto soil columns and (D) Drummer soil columns subjected to 22°C drying events.

 


View larger version (26K):
[in this window]
[in a new window]
  		Fig. 2. Prometryn displacement with infiltrating solutions consisting of either a CaCl2 matrix (CC), a swine waste–derived lagoon effluent (SW), or a simulated buffer solution (SB) plotted as mass in leachate (ML) relative to initial mass applied (MI) versus pore volume from Bloomfield soil columns subjected to drying cycles of (A) 22°C and (B) 60°C. The symbols and bars represent the average and standard deviations, respectively, observed between duplicate columns.

 


View larger version (23K):
[in this window]
[in a new window]
  		Fig. 3. Atrazine distribution resulting from displacement with infiltrating solutions consisting of either a CaCl2 matrix (CC), a swine waste–derived lagoon effluent (SW), or a simulated buffer solution (SB) plotted as atrazine mass extracted from soil (MC) relative to initial mass applied (MI) as a function of depth in Toronto soil columns subjected to drying events of (A) 22°C and (B) 60°C; and in Drummer soil columns subjected to drying events of (C) 22°C and (D) 60°C.

 


View larger version (23K):
[in this window]
[in a new window]
  		Fig. 4. Prometryn distribution resulting from displacement with infiltrating solutions consisting of either a CaCl2 matrix (CC), a swine waste–derived lagoon effluent (SW), or a simulated buffer solution (SB) plotted as prometryn mass extracted from soil (MC) relative to initial mass applied (MI) as a function of depth in Toronto soil columns subjected to drying events of (A) 22°C and (B) 60°C; and in Drummer soil columns subjected to drying events of (C) 22°C and (D) 60°C.

 
Atrazine leaching profiles with all solutions for both drying temperatures are similar for the Bloomfield columns (Fig. 1A and 1B). For Toronto and Drummer soil columns subjected to low-temperature drying, atrazine leaching with respect to influent type is as follows: SW > SB > CC (Fig. 1C and 1D). For the same soil types under high-temperature drying, the amount of atrazine measured in each column effluent fraction was less than 3% of the total atrazine mass, with no observable pattern (not shown). For prometryn in the Bloomfield soil columns (Fig. 2), SW and SB resulted in substantially more pesticide leaching relative to CC under both high- and low-temperature drying cycles. Post-leaching pesticide distributions in the soil columns subjected to either low- or high-temperature drying events followed similar trends for both triazines, with SW solutions generally mobilizing more pesticide further down the column than either SB or CC solutions (Fig. 3 and 4). The exception to this trend was with the atrazine–Bloomfield soil columns, where little difference was observed (not shown). Effects of solution type were generally more pronounced for prometryn in all cases.

Pesticide transport may be affected by the pH and electrolyte composition of the influent, and the presence of DOM. Influent-induced changes in soil pH may affect pesticide speciation, and changes in pH and electrolyte composition may cause soil dispersion, thus affecting influent flow and residence times. Increase in soil pH may increase the dissolution of soil organic matter (SOM). Both SOM dissolution and soil dispersion may be further enhanced with wetting and drying events. Application of both SW and SB solutions resulted in substantial changes in leachate pH, DOC, and IC. Trends in these leachate parameters between solution type and drying temperature were similar for all soil–pesticide combinations. Representative leachate profiles for pH, DOC, and IC after each drying event are exemplified in Fig. 5 for the atrazine–Bloomfield soil columns. Discussion of the individual effects of solution effluent and drying events on pH, DOM, and leachate flow characteristics and their subsequent effect on triazine transport follows.



View larger version (58K):
[in this window]
[in a new window]
  		Fig. 5. Leachate pH, dissolved organic carbon (DOC), and inorganic carbon (IC) measured from Bloomfield–atrazine unsaturated columns infiltrated with solutions consisting of either a CaCl2 matrix (CC), a swine waste–derived lagoon effluent (SW), or a simulated buffer solution (SB) and subjected to drying events of 22°C (left) or 60°C (right).

 
Solution pH
Application of SB and SW resulted in increases in soil–solution pH relative to CC as exemplified in Table 6, with pH of 1 g to 1 mL slurries of each soil–solution combination and of the leachate from the last leaching event for the columns subjected to low-temperature (22°C) drying. The SW solution increased solution pH more than SB in all three soils, indicating that SW had a higher buffer capacity than either SB or CC. Changes in leachate pH during each leaching event for Bloomfield columns are shown in Fig. 5 for both temperature treatments. The pH increased almost 1 pH unit in the first 1.5 pore volumes of SB and SW, whereas pH with CC ranged between 6.2 and 6.4 over the course of the entire experiment. Similar trends of increasing pH were observed in Toronto and Drummer columns, although increases were less significant.


View this table:
[in this window]
[in a new window]
  		Table 6. pH of soils slurried with solutions consisting of either a CaCl2 matrix (CC), a swine waste–derived lagoon effluent (SW), or a simulated buffer solution (SB) and pH of leachate from the last leaching event for the columns treated with low-temperature (22°C) drying cycles.

 
For weakly basic pesticides such as triazines, a change in soil–solution pH due to the application of high-pH effluents may result in speciation changes, and thus influence pesticide adsorption. At a low pH, a basic pesticide would be protonated, forming a cation, resulting in cation exchange being a significant adsorption mechanism. As pH increases, resulting in the neutral species of the pesticide becoming dominant, other adsorption mechanisms such as hydrogen bonding, van der Waals forces, and hydrophobic interactions would govern. The magnitudes of sorption by the latter mechanisms are small relative to cation exchange. As a result, as pH increases, adsorption of weakly basic pesticides would decrease. The soils in this study have pH values between 4.2 and 6.1 (Table 3); therefore, prometryn with a pKa = 4.05 and the lower-pH Toronto soil would be the pesticide–soil combination with the highest sensitivity to influent-induced pH changes. The latter is clearly observed when comparing the post-column leaching profiles of prometryn from the Toronto and Drummer soils (Fig. 4), which have similar properties with the exception of pH. Similar trends were observed in saturated batch adsorption–desorption studies (Seol, 1998).

Dissolved Organic Matter
As exemplified in Fig. 5, DOM measured on a carbon basis (DOC) was highest in the first leachate fractions collected after a wetting event of any solution, with the elevation in DOC concentrations being large after a high-temperature drying event. The SW influent contained approximately 150 mg DOC/L, whereas CC and SB influent had only a few mg/L DOC (Table 2). However, infiltration of SB through the columns after a drying event resulted in much higher DOC concentrations than CC infiltration. The SW influent already contained approximately 150 mg DOC/L, but initial wetting with SW after a drying event resulted in DOC concentrations higher than the initial influent DOC concentrations. The elevation in DOC concentrations between influent and initial column leachate was comparable for the SB and SW, demonstrating the role of pH and electrolyte composition in SOM dissolution. Upon further infiltration after a drying event, DOC concentrations in the SW column leachate returned to concentrations that were similar to or less than the initial SW influent concentration. For SB column leachate, DOC concentrations decreased with infiltration, but still remained elevated over initial SB influent concentrations. Both Williams et al. (2000) and Nelson et al. (2000) observed DOM-facilitated transport of napropamide during the initial wetting event in laboratory-packed soil columns. In steady-state saturated column studies where soils were water-saturated prior to addition of the pesticide followed by application of the pesticide dissolved in various influents, effluent DOM-enhanced triazine transport was not observed. These results suggest that during the initial wetting phase, soil organic matter (SOM) containing sorbed pesticide is solubilized followed by rapid mobilization before equilibrium can be attained.

Leachate Flow Rates
The range in flow rates averaged across the six samples taken for each wetting period for a given soil–solution combination (three each from the atrazine- and prometryn-contaminated columns) is summarized in Table 7. The SW and SB solutions were observed to reduce leachate flow rates for all soil types relative to CC, which was used to represent high-quality irrigation water. High pH and high exchangeable monovalent cation concentration relative to multivalence cations can result in the dispersion and transport of soil colloids (SOM and silicate clay) from the soil (McBride, 1994). Both organic matter and inorganic carbon (IC) constituents were observed to be mobilized in leaching events following drying events (Fig. 5), with IC being substantially greater when SW was the infiltrating solution.


View this table:
[in this window]
[in a new window]
  		Table 7. The range in average leachate flow rates (mL/min) for columns dried at 22 and 60°C. CC, 0.005 M CaCl2; SB, simulated electrolyte buffer solution; SW, swine effluent.

 
Although SB was designed to simulate the electrolyte composition of SW, NH+4 was inadvertently not included in the preparation of the SB matrix, which is most likely a major reason why the electrical conductivity (EC) of SW is much higher than that of the SB (Table 2). According to monitoring results of the swine waste–derived lagoon, NH+4 concentrations averaged more than 300 mg/L (Reaves, 1985), which will contribute to the potential for soil dispersion. Dispersed soil colloids may block microscopic pores, resulting in a reduction in flow rate. Retarded flow would extend the residence time of pore water with adsorbed pesticides, allowing more time for desorption, which could result in increased pesticide concentrations in pore water if desorption processes are rate-limited (Gilchrist et al., 1993; Wauchope and Myers, 1985).

Given the inverse correlation between desorption kinetics and sorption equilibrium (Brusseau and Rao, 1989), the greatest effect on pore-water pesticide concentrations would be expected for the system where pesticide sorption was the greatest, and likewise, the least effect noted for the system where sorption was the smallest. Differentiating influent-induced effects on pore-water residence times from pesticide speciation and DOM release is difficult. However, comparison between the atrazine–Bloomfield and atrazine–Drummer systems may provide insight. In these systems, where the initial soil pH is much greater than the pKa of atrazine, speciation effects are expected to be minimal. The soil–water distribution coefficient of atrazine is seven times greater on the Drummer soil relative to the Bloomfield soil. Flow rates were reduced by approximately a factor of two in the Bloomfield columns; however, enhanced atrazine mobility was not observed (Fig. 1), suggesting that near equilibrium was already attained at the higher flow rates. On the other hand, for the atrazine–Drummer system, enhanced transport of atrazine was observed (Fig. 3C and 3D). Also, compared with the Drummer soil columns, the potential for effluent-borne and effluent-induced DOM to facilitate transport was greater in the Bloomfield soil columns, where elevation of DOC concentrations was greater. However, atrazine mobility was enhanced with treated effluent infiltration in the Drummer but not the Bloomfield soil columns, suggesting that effluent-induced decreases in leachate flow rates resulting in increased residence times can contribute to enhanced pesticide transport.

High-Temperature Drying
High-temperature drying at 60°C simulated not only the long periods of air-drying between wetting events but also the hyperthermal climatic conditions common to arid regions. As previously mentioned, leachate pH and DOC concentrations after the high-temperature drying cycles increased more than observed for the columns subjected to the lower-temperature drying cycles. Dissolved organic C was concentrated in the higher-temperature drying cycle due to high water evaporation, which had the greatest effect with DOC-containing SW solutions. Inorganic carbon decreased significantly after each drying cycle due to carbonate precipitation, followed by a rise in concentration with subsequent redissolution. Precipitation was greater for high-temperature drying because of the higher water evaporation. A reduction in flow rate was also more significant in the columns treated with the high-temperature drying cycles, which is hypothesized to result from migrating colloids dispersed from the soil surface and carbonate precipitation. The leachate from the latter was very brown and suspended particles in the leachate were visually observed, which lends support to this hypothesis.

Unlike low-temperature drying, effects of high-temperature drying on the mobility enhancements with SB and SW varied with soil type. In the Bloomfield columns, high-temperature drying enhanced prometryn mobility (Fig. 2). On the contrary, high-temperature drying reduced movement of both pesticides in the Toronto and Drummer soil columns relative to low-temperature drying (Fig. 3 and 4). As drying takes place and capillary forces cause pore water to rise, pesticides in the pore water also move upward. A sandy soil has larger pore sizes, lower capillary rise forces, and greater infiltration rates relative to silty clay loams; therefore, upward movement of water would be less pronounced. Upward movement of pesticides with the pore water also enhances the potential for volatilization losses, especially at higher drying temperatures.


   SUMMARY
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
The physical and chemical parameters controlling the movement of atrazine and prometryn in columns infiltrated with treated effluent under unsaturated transient conditions and subjected to repeated wetting and drying events were investigated. Several parameters were monitored, including the pesticide concentrations, pH, DOM, and IC in the column leachate as well as leachate flow rates. The high pH and buffer capacity of the effluent did induce increases in soil–solution pH, which resulted in speciation changes, and thus influenced pesticide mobility in some pesticide–soil combinations. Prometryn, with a pKa = 4.05, showed highest sensitivity to pH changes, and thus the greatest pH-related effect on enhanced movement in all three soils. The low pKa for atrazine (1.66) resulted in a pH-related effect being feasible for only the lowest-pH soil. Dissolution of soil organic matter was enhanced upon application of SW and SB, with DOC concentrations being highest during the early wetting phase. The enhanced release of SOM did appear to increase the downward movement of sorbed pesticides. Carbonate precipitation during drying events after application of SW and SB solutions and particle dispersion during SW and SB infiltration resulted in a reduction in leachate flow rate, thus increasing desorption time, which contributed to enhanced pesticide movement. For pesticide–soil systems where the pH–pKa combination was sensitive to speciation changes, the reduction in flow rates intensified the potential for enhanced pesticide transport. High-temperature drying resulted in greater reductions in leachate flow rates and greater dissolution of soil organic matter relative to low-temperature drying, with the largest effect observed with SW solutions. However, high-temperature drying often resulted in reduced triazine mobility, which was attributed to greater upward movement of pesticide-containing pore water and subsequent pesticide volatilization. The magnitude of this effect was dependent on soil texture and the affinity of the pesticide for the soil. Given the multiple and coupled processes that may contribute to the effect of effluent irrigation on pesticide mobility, additional studies are needed, preferably at the field scale, to better assess the long-term effect of effluent irrigation on ground water quality.


   ACKNOWLEDGMENTS
 
This research was funded in part by the Purdue University Research Program and USDA-BARD under Contract IS-2384-94.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 




This article has been cited by other articles:


Home page
 J. Environ. Qual.Home page
T. Ilani, E. Schulz, and B. Chefetz
Interactions of Organic Compounds with Wastewater Dissolved Organic Matter: Role of Hydrophobic Fractions
J. Environ. Qual., March 1, 2005; 34(2): 552 - 562.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (5)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Seol, Y.
Right arrow Articles by Lee, L. S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Seol, Y.
Right arrow Articles by Lee, L. S.
Agricola
Right arrow Articles by Seol, Y.
Right arrow Articles by Lee, L. S.
Related Collections
Right arrow Water Use
Right arrow Agricultural Pesticides
Right arrow Animal Waste

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome