Environ. Sci. Technol. 2004, 38, 3272-3278
Herbicide Runoff along Highways. 2. Sorption Control XINJIANG HUANG, THERESA PEDERSEN, MICHAEL FISCHER, RICHARD WHITE, AND THOMAS M. YOUNG* Department of Civil and Environmental Engineering, University of CaliforniasDavis, One Shields Avenue, Davis, California 95616
Controversy remains about the importance of nonlinear sorption isotherms, desorption rate limitations, and aging effects, collectively referred to as nonideal sorption processes, in controlling the fate and transport of organic contaminants. Herbicide runoff from highway soils represents a good test case for assessing the relative importance of nonideal sorption because runoff flow rates are often high, soilwater contact times are short, and significant time is available for contaminant aging after application. This study examines the sorption and desorption of five herbicides with a wide range of properties (isoxaben, oryzalin, diuron, clopyralid, and glyphosate) on soil samples from two roadsides in northern California and uses the results to examine field runoff data from multiple rainy seasons. Nonideal sorption processes do not appear to be significant in determining herbicide runoff at the field sites because (i) sorption isotherms were linear or slightly nonlinear for all compounds but glyphosate, (ii) field runoff concentration ratios between isoxaben and oryzalin were consistent with linear partitioning predictions, (iii) runoff leaving the site appeared to be in equilibrium with local soil concentrations, and (iv) desorption distribution coefficients for aged herbicides on soil samples collected from the field site did not differ substantially from those obtained in shortterm laboratory adsorption experiments. Collectively, these findings indicate that linear equilibrium models are adequate for predicting the concentration of herbicides in runoff in these field settings and that more complicated nonideal models do not need to be invoked. Vegetated slopes effectively reduced the herbicide loads, with average removals of 35-80% occurring as runoff traversed a 3-m segment 1 m from the edge of the spray zone.
Introduction Over the past decade, a significant body of research has evolved that suggests the importance of nonlinear sorption, rate limitations, and aging effects in controlling the fate and transport of organic contaminants in soils and groundwater aquifers (1-5). These phenomena can collectively be referred to as nonideal sorption because they represent deviations from the widely used linear equilibrium model of organic compound sorption. The vast majority of the evidence for the relevance of these phenomena has been derived from careful laboratory sorption studies that reveal (i) significant * Corresponding author phone: (530)754-9399; e-mail tyoung@ ucdavis.edu. 3272
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 12, 2004
sorption-desorption hysteresis, (ii) large decreases in the rate and extent of desorption with increased contact time, and (iii) sorption isotherms that deviate substantially from linearity. However, the generality of these phenomena and their importance as compared to other important fate and transport processes has not been widely studied. In fact, there are very few field-scale experimental investigations of organic contaminant fate and transport that focus on the nonideal sorption behaviors outlined above. In this paper, we examine the contribution of these processes to the runoff of herbicides applied along highways as reported in a companion paper (22). The importance of sorption processes in controlling the fate and transport of hydrophobic pesticides is wellestablished. Sorption affects pesticide transport directly by retarding migration and indirectly by reducing the rates of biotic and abiotic degradation processes because of reduced solution-phase concentrations (6-8). The net result of these effects is that concentrations of pesticides in runoff are often negatively correlated with the soil-water distribution coefficients of the chemicals. For example, Konda and Pa´sztor found that variations in the maximum concentration of atrazine, acetochlor, propisochlor, and chlorpyrifos in runoff could be explained by their respective soil sorption coefficients (9). The larger adsorption capacity of simazine as compared to diuron resulted in lower simazine concentrations and mass loadings in runoff (10). Hansen et al. concluded that alachlor concentrations were lower than cyanazine concentrations in runoff due to a greater degree of sorption and more rapid dissipation (11). Sorption processes play a key role in attenuating off-site herbicide migration, thereby minimizing surface water contamination. Vegetated filter strips adjacent to agricultural fields have been shown to reduce transport of pesticides, sediments, and nutrients in runoff water (12-14). Arora et al. (15) found that the length of these strips has various effects on the removal of herbicides from runoff waters. Briggs et al. (16) compared waterways with grassy or gravel/clay surfaces for herbicide reduction and found that the grassy waterways had higher removal efficiencies. Similar results have been reported for removing a variety of constituents from highway runoff, but these studies have not specifically addressed herbicides. For example, Barrett et al. tested the performance of vegetative controls for treating highway runoff and indicated that strips with side slopes of less than 12% and a length of at least 8 m are effective in reducing stormwater contaminant loads from highways (17). Grassy swales have also been reported to be an effective way to remove metals, total suspended solids, volatile suspended solids (VSS), and chemical oxygen demand (COD) from highway runoff (18). Sorption reactions are typically assumed to attain equilibrium instantaneously during transport unless the flow rate is high (19). Model simulations suggest that the linear equilibrium adsorption assumption usually produces adequate prediction of herbicide runoff concentrations in agricultural applications (20, 21), but few direct tests of this result have been conducted in nonagricultural settings. In a companion paper (22), we demonstrated that the concentration of herbicides in surface water runoff at the edge of a highway right-of-way declined exponentially with time following application and that the amount of a particular herbicide mobilized was strongly related to its solubility with the exception of glyphosate, which is soluble but largely immobile. These findings suggest that sorption plays an important role in mitigating herbicide runoff in these situations but do not clearly distinguish among various 10.1021/es034848+ CCC: $27.50
2004 American Chemical Society Published on Web 05/14/2004
TABLE 1. Selected Soil Properties from Sampling Sitesa soil moisture retention (%)
soil location
pH
CEC (mequiv/100 g)
OC (%)
N (%)
Al (%)
Fe (%)
rock (%)
sand-silt-clay (%)
1/3 atm
1atm
3 atm
5atm
15 atm
Tolay Creek site spray zone middle slope ditch Eel River site
6.5 5.6 5.0 5.7
11.6 28.4 26.6 27.5
2.63 1.61 2.54 2.49
0.17 0.13 0.22 0.20
0.83 0.96 0.93 1.63
2.64 3.12 0.96 2.58
52.1 33.6 13.1 9.3
76-17-7 66-21-13 52-33-15 47-38-15
12.3 19.7 19.2 18.2
10.9 17.6 17.4 16.0
10.5 13.2 13.3 11.4
10.4 12.6 12.2 10.3
10.0 11.7 11.7 9.2
a
All property analyses were based on the soils without rock fraction.
sorption related mechanisms. Runoff of highway-applied herbicides offers an excellent test case for examining the importance of nonideal sorption phenomena at the field scale because of the combination of several factors unique to the highway environment. First, in the Mediterranean climate of northern California there is typically a single application of herbicides (most of which are pre-emergent) just before the onset of winter rains. Each subsequent storm therefore leaches herbicide from successively more “aged” material, allowing the importance of contact time to be assessed in the field. Second, roadsides are engineered to ensure rapid and complete drainage of the road surface and the rightof-way to prevent accumulated water from causing highway accidents. This can create large water velocities passing over the herbicide application zone and may result in short watersoil contact times during runoff. Sorption rate limitations are expected to be especially severe in such situations. Third, the long time between application and the last rainstorms of winter may result in significant changes in the herbicide concentration in soil within the application zone and corresponding changes in the soil-water distribution coefficient for herbicides displaying nonlinear sorption isotherms. Our objective in this research was to test the importance of sorption nonlinearity, rate limitations, and aging effects in a field-scale, real-world system in which these effects would be expected to be significant. To achieve this goal, field data on herbicide runoff in multiple storm seasons at two northern California sites were combined with laboratory sorption and desorption measurements with materials from the monitoring sites. A secondary objective was to examine the role of sorption processes in attenuating herbicide concentrations as runoff water flows across vegetated slopes that are currently being promoted as “best management practices” for treating highway runoff. The study represents one of the first efforts to experimentally measure the impact of nonideal sorption behaviors on the field-scale transfer of organic contaminants to surface runoff.
Experimental Section Field Sampling. Herbicide runoff was monitored from two highway sites for 2 or 3 yr, and the site characteristics, herbicide properties, application methods, and sampling/ analytical procedures are described in a companion paper (22). The soils used for both adsorption and desorption studies were collected from the top 5 cm of soil using a spade, air-dried, passed through a 2-mm sieve, and stored at -20 °C until experiments were performed. Soil properties at the Tolay Creek sampling site were determined at three locations: within the herbicide spray zone, at the middle of the slope, and in the ditch. Soil from the Eel River sampling site was a composite from across the site since no significant differences in properties were observed between subsamples collected from various locations (Table 1). To measure herbicide attenuation as runoff moved down the grassy slope, six runoff collectors were installed at the Tolay Creek sampling site at defined distances from the edge of the pavement along the entire length of the site; three
collectors were 1 m away from the spray zone and another three were 0.5 m from the bottom of the vegetated slope (approximately 4 m from the edge of the pavement). The runoff collectors were metal cans (15 cm i.d. × 20 cm high) built into the ground with a 1-cm opening against the flow direction. The area surrounding the collectors was constructed with concrete to prevent infiltration around the container edges. Aluminum foil loosely covered the metal collector to minimize entry of dust or rainfall. The design ensured that only the surface runoff flowing perpendicular to the highway was collected. The runoff collectors were emptied within 6 h of the conclusion of a storm and cleaned carefully before the next event. Water samples from the collectors were returned to the laboratory and analyzed for herbicides using methods described below. Vegetation samples along the slope were also collected and analyzed for herbicides, but herbicide concentrations were always below the method quantitation limit. Herbicide Adsorption. Adsorption isotherms were determined for the five target herbicides (oryzalin, isoxaben, diuron, glyphosate, and clopyralid) and one degradation product of glyphosate (aminomethylphosphonic acid; AMPA) on surface soils from the field sites. To obtain greater than 50% adsorption and final concentrations above method quantitation limits, soil:water ratios (g/mL) were determined from preliminary experiments (1:1 for clopyralid; 1:10 for oryzalin, isoxaben, and diuron; and 1:80 for glyphosate and AMPA). The time required to attain an apparent equilibrium was also determined in preliminary 3-day rate studies (24 h for oryzalin, isoxaben, and clopyralid; 48 h for diuron, glyphosate, and AMPA). Batch adsorption experiments were conducted by combining herbicide solution in a 0.005 M CaCl2 matrix with soil at the predetermined ratios in 45-mL Teflon centrifuge tubes. Six samples with varying initial herbicide concentrations (0-1000 µg/L) were mixed in triplicate at 23 ( 1 °C on an end-over-end tumbler for the specified contact time and were subsequently centrifuged for 30 min at 3500 rpm. Samples with water and soil only and samples with the initial herbicide solution without soil were included as controls in all cases. A 2-mL aliquot of the supernatant phases was sampled for analysis. Oryzalin, isoxaben, diuron, and clopyralid were determined using a high-performance liquid chromatograph (Hewlett-Packard 1100 series HPLC, Wilmington, DE) with diode array detector. Glyphosate and AMPA were first derivatized and then determined using a Hewlett-Packard 6890 series gas chromatograph coupled to a mass selective 5973 series detector and a 7683 series injector (Hewlett-Packard Co., Wilmington, DE). Analytical method details were described in a companion paper (22). Sorbed-phase concentrations were determined by difference and were corrected for losses in the blank tubes, which averaged less than 1%. Sorptive Reversibility. A combined herbicide adsorption and desorption study was conducted to investigate the reversibility of the sorption process. First, an adsorption experiment was conducted by mixing 5 g of soil with 40 mL of 0.005 M CaCl2 stock solution containing 506.6 µg/L VOL. 38, NO. 12, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3273
FIGURE 1. Sorption isotherms on Eel River site soil for oryzalin (4), isoxaben (O), diuron ([), clopyralid (2), glyphosate (9), and AMPA (0). isoxaben and 421.8 µg/L oryzalin in 45-mL Teflon centrifuge tubes. A total of 21 sample tubes were prepared, tumbled end-over-end for 24 h, and centrifuged for 30 min at 3500 rpm. A 2-mL aliquot of the supernatant was collected to measure contaminant concentrations, and the remaining solution was decanted. After decanting, an average of 2.69 mL of solution remained in each tube. A 10-mL sample of 0.005 M CaCl2 stock solution containing neither herbicide was then immediately added to each tube. Triplicate desorption samples were removed from the tumbler after 10, 30, 60, 180, 360, 1500, and 3000 min. The remaining sampling and analytical procedures were the same as described above. Herbicide Desorption. Desorption of herbicides from spray zone soils collected from the Tolay Creek field site at different times after herbicide application was determined by combining 10 mL of 0.005 M CaCl2 matrix with 5 g of soil in 45-mL Teflon centrifuge tubes. Each treatment was prepared in triplicate and was continuously mixed on an end-over-end tumbler for 30 min at 23 ( 1 °C. Following mixing, samples were centrifuged for 30 min at 3500 rpm. Aqueous samples were collected and analyzed for herbicides as described above. When subsequent desorption steps were performed, additional supernatant was removed until less than 3.0 mL of the initial solution remained. Freshly prepared 10 mL of CaCl2 solution was added, and mixing was resumed. The desorption cycle was repeated four times for each sample.
Results and Discussion Sorption Isotherms. Sorption isotherms for the five herbicides and one degradation product (AMPA) on soil from the Eel River sampling station are shown in Figure 1. As expected from the large range in herbicide aqueous solubilities and octanol-water partition coefficients, significant variation is observed in the extent of sorption at a particular solutionphase concentration, with the highest distribution coefficient observed for glyphosate and the lowest for clopyralid. Each set of isotherm data was fit using both a linear model (Cs ) KDCw) and the Freundlich model (Cs ) KFCnw) where Cs represents the soil concentration (µg/kg), Cw is the aqueous concentration (µg/L), KD is the linear distribution coefficient (L/kg), KF is the Freundlich unit capacity factor (µg/kg)(µg/ L)-n, and n is the Freundlich exponent. The fitted parameters, their standard errors, the number of data points, organic 3274
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 12, 2004
carbon-normalized distribution coefficients (Koc ) 100 × KD/ OC%), and regression R 2 values are summarized in Table 2. Isotherms for oryzalin and isoxaben on all four soil samples are effectively described by a linear sorption model because their Freundlich exponents (n) are not statistically different from unity (p < 0.05). The high sorption capacity of the soils for glyphosate is likely due to the binding of its phosphoric acid moiety to cations on the clay or organic matter (23). Hance indicated that glyphosate sorption is correlated with unoccupied phosphate sorption capacity of a soil and that a Freundlich equation can be used to describe the adsorption of glyphosate to soil (24). Glyphosate isotherms on our soils are nonlinear (p < 0.05) in all cases as might be expected for a compound for which competition for a limited number of cation exchange sites contributes significantly to the sorption process. However, the direction in which the Freundlich n value deviates from unity differs between the samples, with values ranging from 0.74 to 2.06 for different soil samples. AMPA sorption was only measured on the Eel River site sample because this was the only location where it was detected in the field experiments. The isotherm for AMPA was nonlinear as for its parent compound but exhibited smaller extents of sorption over the tested range of aqueous concentrations. Diuron sorption also exhibited nonlinear isotherms on all of the soil samples (n e 0.93), and the deviation from linearity was statistically significant (p < 0.05) for all but the Tolay Creek ditch sample. Nonlinear sorption of phenylurea herbicides, including diuron, on soils has been widely reported (25). Clopyralid isotherms were linear on all the soils, and the linear distribution coefficients were the smallest of the tested herbicides. Little or no adsorption of clopyralid on diverse soils has been reported previously by other researchers (26-28). As is evident from the lower R 2 values, the quality of the clopyralid, glyphosate, and AMPA isotherms is not as high as those for the other compounds. In the case of clopyralid, this is caused by the limited extent of sorption, even in tests with high soil-water ratios. For glyphosate and AMPA, the data scatter results from difficulties in the derivatization process required for their analysis via GC-MS. The similarity of soil properties at the two sampling sites (Table 1) results in similar sorption behaviors. Organic carbon-normalized distribution coefficients (Koc) reported in Table 2 are generally within a factor of 2 of those from the literature of 1100, 570, 400, 0.4-60, and 20-100 L/kg for oryzalin, isoxaben, diuron, clopyralid, and glyphosate, respectively (29). Glyphosate and AMPA exhibited the greatest extents of sorption of the tested herbicides, and this finding is consistent with the failure to observe these compounds at significant concentrations in the stormwater runoff from the sites despite their high aqueous solubilities. AMPA sorption on the Eel River soil was about 40% as large as that of its parent compound, glyphosate, a finding that explains its penetration to soil depths where glyphosate was not detectable (37-50 cm) and greater persistence in runoff. Clopyralid is clearly the compound with the least tendency to sorb to soil of those studied. This finding helps to explain why nearly 40% of the applied clopyralid was transported off site during the first storm of 2001-2002 at the Tolay Creek site as compared to less than 5% of the oryzalin, isoxaben, and diuron during the same event. The event mean concentrations reported in a companion paper (22) were based on dissolved concentrations of the herbicides, but it is worth considering whether significant transport may occur in a suspended solid-bound form. When isotherms are linear and equilibrium is attained rapidly, the fraction of herbicide transported in the dissolved phase is (1 + TSS × KD)-1where TSS is the total suspended solid concentration in the runoff (in kg/L). As noted by Rao and
TABLE 2. Sorption Isotherm Parameters linear model herbicide oryzalin
isoxaben
diuron
clopyralid
glyphosate
AMPA a
Units in L/kg.
b
Freundlich model
soil location
KDa
R2
Koca
log KFb
n
R2
Nc
TC,d spray zone TC, middle slope TC, ditch Eel River site TC, spray zone TC, middle slope TC, ditch Eel River site TC, spray zone TC, middle slope TC, ditch Eel River site TC, spray zone TC, middle slope TC, ditch Eel River site TC, spray zone TC, middle slope TC, ditch Eel River site Eel River site
49.4 (1.07)e 31.2 (0.92) 38.8 (0.97) 43.8 (1.11) 13.4 (0.32) 12.3 (0.36) 16.6 (0.45) 16.2 (0.29) 8.2 (0.10) 10.7 (0.34) 12.9 (0.34) 15.5 (0.29) 0.06 (0.004) 0.04 (0.004) 0.08 (0.01) 0.07 (0.01) 167 (11.1) 486 (31.2) 808 (36.5) 697 (41.3) 268 (18.6)
0.976 0.955 0.969 0.967 0.968 0.962 0.963 0.982 0.993 0.929 0.953 0.982 0.907 0.832 0.861 0.799 0.862 0.822 0.894 0.848 0.775
2680 1940 1530 1760 730 765 653 651 447 666 508 624 3.26 2.48 3.21 2.81 9060 30200 31800 28000 10800
1.62 (0.04)e 1.47 (0.10) 1.47 (0.13) 1.66 (0.03) 1.05 (0.04) 0.98 (0.12) 1.37 (0.14) 1.14 (0.04) 1.09 (0.03) 1.34 (0.12) 1.33 (0.14) 1.64 (0.03) -0.75 (0.22) -1.52 (0.18) -0.53 (0.17) -1.58 (0.47) 2.82 (0.12) 0.76 (0.49) 3.13 (0.07) 2.12 (0.19) 2.89 (0.10)
1.05 (0.03)e 1.01 (0.05) 1.06 (0.07) 1.00 (0.02) 1.05 (0.02) 1.05 (0.06) 0.93 (0.07) 1.05 (0.03) 0.93 (0.02) 0.87 (0.05) 0.90 (0.07) 0.83 (0.01) 0.76 (0.12) 1.07 (0.11) 0.77 (0.08) 1.16 (0.20) 0.74 (0.06) 2.06 (0.29) 0.88 (0.05) 1.42 (0.12) 0.81 (0.05)
0.984 0.980 0.966 0.990 0.986 0.978 0.955 0.987 0.992 0.970 0.957 0.993 0.789 0.939 0.918 0.778 0.951 0.820 0.974 0.931 0.954
24 10 10 24 27 10 10 24 27 10 10 27 12 8 11 12 9 13 12 12 14
Units in (µg/kg)/(µg/L)n. c Number of observations.
Davidson (30), significant transport in a sediment-bound form therefore requires either a large value of KD (e.g., greater than 100 L/kg) or a significant sediment load (>0.001 kg/L) or both. Since all sorption coefficients in our study were below 50 L/kg except for glyphosate and AMPA and average event TSS values were below 100 mg/L for both sites (data not shown), the vast majority of herbicide transport should occur in the dissolved phase. An upper limit on particlebound transport can be obtained by considering the distribution of glyphosate (KD )808 L/kg), in runoff with TSS ) 100 mg/L. In this case the solid-phase accounts for 7.5% of the glyphosate mass load in runoff. Therefore, in all cases studied here more than 90% of the herbicide will be transported in dissolved form. Transport of glyphosate with eroded soil particles may play a significant role in the initiation of its surface runoff since TSS content in the herbicide application zone is likely far higher than in the runoff after passage through vegetation along the slope. Adsorption to suspended solids followed by rapid settling/ interception of suspended solids may help to explain why little glyphosate was detected in the final runoff. Herbicides were detectable in suspended solids collected from runoff but never above our quantitation limits, which confirms that particle-bound transport was not significant in this case. The proportionality between herbicide losses and runoff flow throughout each event documented in a companion paper (22) arises, in part, from the minor role of particle-bound herbicides. Field Observations of Sorption Equilibria. Proving that local sorption equilibrium is (or is not) attained in a dynamic field setting like a roadside during a rainstorm is challenging. However, two lines of evidence suggest that a linear local equilibrium sorption model could adequately describe the results for most of the monitoring events in each year. The first line of evidence involves a comparison between two herbicides applied simultaneously that exhibit different sorption equilibria. Consider the case of isoxaben and oryzalin, which are always applied in combination by Caltrans. If both compounds are present in surface runoff at concentrations in equilibrium with the local soil concentration (19), then the ratio of their aqueous concentrations will
d
TC, Tolay Creek site. e Standard error in parentheses.
be given by:
where KD is the distribution coefficient, Cs is the solute concentration in soil, Cw is the solute concentration in water, and the subscripts o and i represent oryzalin and isoxaben, respectively. For the initial storm event following application Cso/Csi may be approximated as the ratio of the herbicide application rates. For subsequent events this approximation will be reasonable for herbicide pairs such as isoxaben and oryzalin that do not differ too significantly with respect to their transport or degradation characteristics. Table 3 summarizes the results of this analysis for oryzalin and isoxaben where the observed Cwi/Cwo is the ratio of the event mean concentrations (EMC) for each herbicide in each event. This analysis was not conducted for other pairs of herbicides because they were either applied alone (e.g., diuron during 2000-2001 at the Tolay Creek site) or were not detected throughout the entire sampling season (e.g., clopyralid during 2001-2002 at the Tolay Creek site). Although the EMC values for each compound differ significantly between events and monitoring seasons, the variability in the runoff concentration ratio (EMCi/EMCo) is far smaller. The ratio of seasonal average EMC values, which represent flow-weighted averages of the EMCs from each event, match the aqueous concentration ratios predicted from eq 1 quite closely, with the exception of the first season at Tolay Creek (T1). This discrepancy may result from an overestimation of the initial soil concentration ratio (Cso/ Csi). At the Tolay Creek site in 1999-2000, the first sampling event occurred 2 weeks after herbicide application on December 9, and the remaining four sampling events were between January 15 and 23; it was sunny during much of this period. Oryzalin is relatively more susceptible to photodecomposition than isoxaben (29, 31, 32), and the mass ratio of these two herbicides in the spray zone soil may have been VOL. 38, NO. 12, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3275
TABLE 3. Ratios of Oryzalin and Isoxaben Event Mean Concentrations for Two Sites Compared with Concentration Ratios Predicted from Local Linear Equilibrium Theory Tolay Creek site T1 (1999-2000) EMC (µg/L) oryzalin isoxaben event 1 event 2 event 3 event 4 event 5 event 6 event 7 event 8 event 9 event 10 event 11 season averagea (Cwi/Cwo)eq a
Eel River site T3 (2001-2002)
(Cwi/ Cwo)obs
1.13 0.54 1.08 0.45 0.26
0.82 0.90 1.60 0.67 0.42
0.73 1.67 1.48 1.49 1.62
0.87
1.23
1.40 0.71
EMC (µg/L) oryzalin isoxaben 6.03 3.60 3.17 1.44 1.27 2.25 1.51 1.35 1.89 1.22 0.52 2.23
3.15 1.46 1.05 0.56 0.54 0.82 0.68 0.46 0.44 0.39 0.25 0.85
(Cwi/ Cwo)obs 0.52 0.41 0.33 0.39 0.43 0.36 0.45 0.34 0.23 0.32 0.48 0.38 0.39
EMC (µg/L) oryzalin isoxaben
E2 (2000-2001) (Cwi/ Cwo)obs
EMC (µg/L) oryzalin isoxaben
(Cwi/ Cwo)obs
13.11 14.16 10.71 12.74 10.48 14.08
8.55 9.09 5.69 6.86 5.79 8.82
0.65 0.64 0.53 0.54 0.55 0.63
43.13 18.09 11.73 18.68 9.69 11.05 13.18
14.43 6.99 4.15 6.86 2.95 3.46 4.15
0.33 0.39 0.35 0.37 0.30 0.31 0.31
11.76
6.81
0.58 0.68
11.41
3.82
0.33 0.34
Σ(seasonal herbicide load)/Σ(seasonal runoff volume).
TABLE 4. Oryzalin Distribution between Soil and Water in the Ditch at Tolay Creek Site during 2001-2002 days after application
concn (Cw) in runoff (µg/L)
concn (Cs) in soil (µg/kg)
Kapp D (L/kg)
5 9 23 31 61
1.56 0.82 0.98 1.27 0.27
46.7 52.7 61.3 97.0 nda
29.9 64.3 62.6 76.4 nab
a
E1 (1999-2000)
nd, not detectable.
b
na, not applicable.
significantly altered by their differential rates of photodegradation. This may explain, in part, the close agreement of event 1 with the predicted concentration ratio but the poor agreement observed in subsequent events. A closely related view of the data is to compare the ratio of cumulative mass loads for isoxaben and oryzalin to the corresponding ratio of linear distribution coefficients for the two compounds. At the Tolay Creek site the value of the loading ratio was 5.20 in season T1 and 2.77 in season T3 while the average value of KDo/KDi for the site was 2.82. At the Eel River site the loading percentage ratio values were 2.40 (E1) and 2.63 (E2), and KDo/KDi was 2.71. If the same approaches are used to analyze the data collected by Hansen et al. (11) for alachlor and cyanazine runoff, the EMC ratio is nearly equal to the application weighted distribution coefficient ratio (eq 1) while the loading percentage ratio closely matches the inverse ratio of the distribution coefficients. A second line of evidence that supports the existence of linear local equilibrium during the runoff process involves comparing herbicide concentrations measured in the last water sample collected by the automated sampler during each storm event and herbicide concentrations in soil samples collected from the ditch near the sampler immediately following selected events. The concentration of oryzalin in the soil samples (Cs) increased progressively from 46.7 to 97 µg/kg from day 5 to day 31 as the herbicide moved down the slope from the application zone, but by day 61 the concentration was below the detection limit (Table 4). Apparent distribution coefficients increased progressively from 29.9 to 76.3 L/kg as the time after application increased, with an average value of 58.3 L/kg. This value does not differ substantially from the measured distribution coefficient for oryzalin in the ditch soil near the runoff collector at Tolay Creek of 38.8 L/kg (Table 2). A similar result was reported by 3276
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 12, 2004
Hansen et al. (11), who showed that alachlor and cyanazine distribution between sediment and solution as measured in the field demonstrated a linear relationship. Sorption Reversibility. To test whether the distribution coefficient measured in the adsorption experiments also applied to desorption, a process more relevant to the generation of herbicide contaminated runoff, a short-term adsorption-desorption experiment was conducted in the laboratory. After 24 h of contact time, the linear distribution coefficient for isoxaben was 10.4 ( 1.23 L/kg (95% confidence interval) and 36.9 ( 2.30 L/kg for oryzalin. Desorption was initiated by centrifuging the tubes, decanting the solutions, and adding fresh herbicide-free solution. For the seven desorption contact times from 10 min to 50 h there was no statistically significant difference between the triplicate KD values at each time point. Since no trend was detected in the distribution coefficient as a function of time, an average KD value was calculated from the pooled values at all time points with a result of 9.48 ( 1.41 L/kg for isoxaben and 40.6 ( 2.57 L/kg for oryzalin. Thus, no statistically significant difference (p < 0.05) was observed between the distribution coefficients for adsorption and desorption during these short contact time experiments. Although the results do not allow us to conclude that the herbicides in the soil-water system investigated in the laboratory had reached sorptive equilibrium, experiments discussed below with field aged soils produced similar apparent distribution coefficients supporting the notion that approximate equilibrium was established rapidly. Herbicide Desorption from the Spray Zone Soils. Another sorption-related mechanism that might contribute to the declining concentration of herbicides in runoff as the storm season progressed is the “aging effect” in which desorption rates decrease and apparent short-term distribution coefficients increase as the soil-chemical contact time increases. In the field experiments described in a companion paper (22), herbicide was in contact with spray zone soil for more than 2 months prior to the last monitored storm of the season. To examine herbicide release from these aged soils, samples were collected from the spray zone at Tolay Creek at selected time intervals following application. On the basis of the preceding desorption data indicating that desorption equilibrium was achieved within 10 min, a desorption contact time of 30 min was selected. This was viewed as a practical maximum for the amount of time that runoff water would be exposed to a particular portion of the site during a storm event. A total of four sequential desorption cycles was
TABLE 5. Herbicide Attenuation along Roadside Slope at Tolay Creek Sitea 2000-2001
2001-2002
diuron
oryzalin
isoxaben
diuron
clopyralid
event
1m
4m
EMC
1m
4m
EMC
1m
4m
EMC
1m
4m
EMC
1m
4m
EMC
1 2 3 4 5 6 7 8 9 10 11
32.3 11.6 36.1 11.9 12.7 na 6.9 5.8
7.8 4.8 3.7 2.3 1.8 2.2 1.0 1.0
10.8 2.2 2.3 1.0 1.7 0.9 0.3 0.7
60.5 9.5 7.1 4.4 13.1 9.4 41.2 29.6 2.9 5.9 2.1
9.7 4.4 2.4 1.4 3.9 2.1 4.3 6.4 1.6 3.5 0.5
6.0 3.6 3.2 1.4 1.3 2.3 1.5 1.4 1.9 1.2 0.5
8.4 4.7 1.3 1.2 2.6 1.6 1.7 1.7 1.5 1.3 0.3
3.6 2.9 0.9 0.7 1.2 0.8 1.0 1.5 1.2 1.2 0.2
3.2 1.5 1.0 0.6 0.5 0.8 0.7 0.5 0.5 0.4 0.2
4.4 0.7 0.3
1.6 0.5 0.1
2.5 0.4 0.4
10.4
5.3
7.1
a
na, not applicable. Values in the table represent herbicide concentrations in µg/L.
FIGURE 2. Oryzalin desorption from the soils collected at the Tolay Creek site (2001-2002). performed following this procedure, and the apparent distribution coefficient was calculated for each desorption cycle and contact time (Figure 2). The apparent KD values are nearly identical for each of the aging times with the exception of the sample collected on day 31, which may be caused by soil heterogeneity at the sampling site or an unidentified error in sampling or analysis. Small, but generally not statistically significant, increases observed in the Kapp D values for each subsequent desorption cycle suggest the possibility of some desorption rate limitations, but these are small effects. All of the Kapp D values are close to the laboratory-determined distribution coefficients and to the fieldderived Kapp D as discussed above. Taken together, the laboratory and field data suggest that for these herbicides on the roadside soils at the two sites that sorption and desorption are rapid, that aging effects are minimal, and that the system can therefore be described using a linear local equilibrium sorption model. The decline in event mean herbicide concentrations is therefore primarily a result of herbicide dissipation from the near-surface source zone via washoff, infiltration, and degradation rather than arising from nonideal sorption processes. We do not conclude that nonideal sorption processes are absent, but only that they are not the primary determinant of the change in runoff concentration with time. The slower dissipation rate observed for herbicide in the source zone soil as compared to runoff samples in a companion paper (22) supports the notion that changing sorption dynamics may make the herbicides marginally less available with time after application. Although most previous reports of nonideal sorption processes reducing aqueous concentrations below those expected from linear local equilibrium predictions have focused on more hydrophobic contaminants than those studied here, some studies have documented the importance of aging effects for more soluble compounds (33, 34). Scribner et al. (33) showed that pore
water concentration of simazine approximated their equilibrium values only immediately after application, while Steinberg et al. (34) showed that apparent distribution coefficients for desorption of field aged 1,2-dibromoethane were substantially higher than those derived from shortterm adsorption experiments. The difference between our results and those of previous investigators may relate to either the nature of the soil organic matter (roadside vs agricultural) or the different compounds studied, but our work clearly establishes that nonideal sorption processes are not universally significant in controlling field-scale organic contaminant transport. Herbicide Attenuation with Distance. The central role of soil sorption processes, whether linear or nonlinear, equilibrium or rate limited, in controlling runoff concentrations has led to the use of vegetated buffer strips as best management practices for mitigating herbicide impacts on adjacent surface waters. The required length of such strips and their effectiveness for herbicides with a range of physical/ chemical properties has only been the subject of limited investigation (15). Runoff collectors installed at three locations parallel to the road surface at 1 and 4 m from the spray zone at the Tolay Creek site allowed the attenuation of herbicide concentrations in runoff due to transport over vegetated slopes to be assessed. Herbicide concentrations in the runoff collectors, averaged for the three collectors at the same distance from the road surface, during 2001-2002 are summarized in Table 5. Limited runoff collector data are reported for clopyralid and diuron during 2001-2002 because the concentrations of these herbicides dropped below detection limits within the first few storm events. No results are available for glyphosate since it was never detected in runoff collectors. Significant attenuation of all of the herbicides in Table 5 with travel distance is apparent, with average attenuation of isoxaben, oryzalin, and diuron during travel over the last 3 m 100 (1 - C4 m/C1 m) ranging from 35 to 80%. Evidence that samples in the runoff collectors contained representative herbicide concentrations is found in the close similarity between the event mean concentrations collected at the end of the Tolay Creek ditch and those measured in the 4-m runoff collectors about 0.5 m distant from the ditch (Table 5). We would expect the EMC values to be similar to but less than the concentrations in the runoff collectors, as is generally observed. The largest discrepancies between EMC and the 4m runoff collectors occur for events with a significant subsurface contribution to the runoff (i.e., those with runoff coefficients well above 100%) or events immediately afterward that might have residual shallow groundwater flows. Differences in collection channel design between the two sampling sites may also contribute to the lower event mean VOL. 38, NO. 12, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3277
concentrations observed at the Tolay Creek site. At both sites the runoff traveled over/through a vegetated slope, but at the bottom of the Eel River site a concrete channel conveyed the runoff to the automated sampler while at the Tolay Creek site a vegetated ditch carried flow to the automated sampler. Previous research suggests, as expected, that herbicide adsorption by concrete road surfaces is negligible (35) while grassy swales have been proven to be an effective way to remove contaminants (18). Our findings suggest that designing roadsides to encourage shallow subsurface flow of runoff might be of even greater benefit than overland flow over vegetated surfaces in reducing herbicide concentrations in runoff. A variety of processes act in concert to determine the concentration of herbicides in runoff. Infiltration, biotic and abiotic degradation, and previous runoff combine to determine the residual herbicide available for subsequent surface runoff. Within the short time-scale of a precipitation event, desorption of the herbicides from source soils within and beyond the application zone is clearly the factor controlling herbicide mobility and the resulting aqueous concentrations. This research has demonstrated that, although the herbicides display some evidence of nonideal (i.e., nonlinear and incompletely reversible) sorption behaviors in the laboratory, ideal sorption models (i.e., linear isotherms and completely reversible) are likely to be sufficient for describing the sorption component of this process in the field.
Acknowledgments This research has been funded by the National Science Foundation (BES 9733621), the National Institute of Environmental Health Sciences (5 P42 ES04699), and the California Department of Transportation (Caltrans) under Contracts 43A0014 and 43A0073. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS, NIH, NSF, or Caltrans. Kuen Tsay, the Caltrans Project Coordinator, has been supportive and helpful throughout the research project. Staff from Caltrans Districts 1 and 4 provided excellent assistance in arranging herbicide applications and in providing information about typical operating practices. John Allen, Heidi Gehlhaar, and Neil Mock from Humboldt State University assisted with sampling at the Eel River site. Suzan Given, Yun Lu, Kimberly Peterson, Claudia Alvarado, Jonathon Leong, Matt Carlson, and Sam Carlson assisted with laboratory analyses; Dr. Peter G. Green provided critical support with analytical methods development and refinement. The authors appreciate the constructive comments of three anonymous reviewers.
Literature Cited (1) Allen-King, R. M.; Grathwohl, P.; Ball, W. P. Adv. Water Resour. 2002, 25, 985-1016. (2) Pignatello, J. J.; Xing, B. Environ. Sci. Technol. 1996, 30, 1-11. (3) Huang, W.; Young, T. M.; Schlautman, M. A.; Yu, H.; Weber, W. J., Jr. Environ. Sci. Technol. 1997, 31, 1703-1710. (4) Johnson, M. D.; Weber, W. J., Jr. Environ. Sci. Technol. 2001, 35, 427-433. (5) Allen-King, R. M.; Groenevelt, H.; Warren, C. J.; Mackay, D. M. J. Contam. Hydrol. 1996, 22, 203-221. (6) Wauchop, R. D.; Buttler, T. M.; Hornsby, A. G.; Augustijn-Becker, P. W. M.; Burt, J. P. Rev. Environ. Contam. Toxicol. 1992, 123, 1-164.
3278
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 12, 2004
(7) Koskinen, W. C.; Harper, S. S. In Pesticides in the soil environment: Processes, impacts, and modeling; Cheng, H. H., Ed.; SSSA Book Series 2; Soil Science Society of America: Madison, WI, 1990; pp 51-77. (8) Wu, L.; Green, R. L.; Liu, G.; Yates, M. V.; Pacheco, P.; Gan, J.; Yates, S. R. J. Environ. Qual. 2002, 31, 889-895. (9) Konda, L. N.; Pa´sztor, Z. J. Agric. Food Chem. 2001, 49, 38593863. (10) Louchart, X.; Voltz, M.; Andrieux, P.; Moussa, R. J. Environ. Qual. 2001, 30, 982-991. (11) Hansen, N. C.; Moncrief, J. F.; Gupta, S. C.; Capel, P. D.; Olness, A. E. J. Environ. Qual. 2001, 30, 2120-2126. (12) Magette, W. L.; Brinsfield, R. B.; Palmer, R. E.; Wood, J. D. Trans. ASAE 1989, 32, 663-667. (13) Dillaha, T. A.; Sherrard, J. H.; Lee, D. Water Environ. Technol. 1989, 1, 418-421. (14) Asmussen, L. E.; White, A. W.; Hauser, E. W.; Sheridan, J. M. J. Environ. Qual. 1977, 6, 159-162. (15) Arora, K.; Mickelson, S. K.; Baker, J. L.; Tierney, D. P.; Peters, C. J. Trans. ASAE 1996, 39, 2155-2162. (16) Briggs, J. A.; Riley, M. B.; Whitwell, T. J. Environ. Qual. 1998, 27, 814-820. (17) Barrett, M. E.; Irish, L. B., Jr.; Malina J. F., Jr.; Charbeneau, R. J. J. Environ. Eng. 1998, 124, 131-137. (18) Wang, T. S.; Spyridakis, D. E.; Mar, B. W.; Horner, R. R. Transport, Deposition and Control of Heavy Metals in Highway Runoff. FHWA-WA-RD-39.10. Report to Washington State Department of Transportation by Department of Civil Engineering, University of Washington, Seattle, 1980. (19) Davidson, J. M.; Chang, R. K. Soil Sci. Soc. Am. Proc. 1972, 36, 257-261. (20) Ma, Q.; Hook, J. E.; Wauchope, R. D.; Dowler, C. C.; Johnson, A. W.; Davis, J. G.; Gascho, G. J.; Truman, C. C.; Sumner, H. R.; Chandler, L. D. Soil Sci. Soc. Am. J. 2000, 64, 2070-2079. (21) Persicani, D. Ecol. Modell. 1996, 84, 265-280. (22) Huang, X.; Pedersen, T.; Fischer, M.; White, R.; Young, T. M. Environ. Sci. Technol. 2004, 38, 3263-3271. (23) Sprankle, P.; Meggitt, W. F.; Penner, D. Weed Sci. 1975, 23, 229234. (24) Hance, R. J. Pestc. Sci. 1976, 7, 363-366. (25) Spurlock, F. C.; Biggar, J. W. Environ. Sci. Technol. 1994, 28, 996-1002. (26) Shang, C.; Arshad, M. A. Can. J. Soil Sci. 1997, 78, 181-186. (27) Cox, L.; Walker, A.; Hermosı´n, M. C.; Cornejo, J. Weed Res. 1996, 36, 419-429. (28) Cox, L. C.; Hermosı´n, M. C.; Cornejo, J. J. Environ. Qual. 1999, 28, 605-610. (29) Roberts, T., Hutson, D. H., Lee, P. W., Nicholls, P. H., Plimmer, J. R., Eds. Metabolic Pathways of Agrochemicals (Part One): Herbicides and Plant Growth Regulators; The Royal Society of Chemistry Information Services: Cambridge, UK, 1998. (30) Rao, P. S. C.; Davidson, J. M. In Environmental Impact of Nonpoint Source Pollution; Overcash, M. R., Davidson, J. M., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1982; pp 23-68. (31) Skinner, R. F.; Thomas, Q.; Giles, J. W.; Crosby, D. G. J. Chromatogr. Sci. 1980, 18, 108-115. (32) Parochetti, J. V.; Dec, G. W. Weed Sci. 1998, 153-156. (33) Scribner, S. L.; Benzing, T. R.; Sun, S.; Boyd, S. A. J. Environ. Qual. 1992, 21, 115-120. (34) Steinberg, S. M.; Pignatello, J. J.; Sawhney, B. L. Environ. Sci. Technol. 1987, 21, 1201-1208. (35) Ramwell, C. T.; Heather, A. I. J.; Shepherd, A. J. Pestic. Manage. Sci. 2002, 58, 695-701.
Received for review July 30, 2003. Revised manuscript received March 16, 2004. Accepted April 1, 2004. ES034848+
