Simulation of picloram, atrazine, and simazine leaching through two New Zealand soils and into groundwater using HYDRUS-2D

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Abstract

Two 15 m×15 m field plots, a Te Awa silt loam and a Twyford fine sandy loam, located in Hawkes Bay, New Zealand, were applied with bromide, picloram, atrazine, and simazine. The Te Awa subsoil was a heterogeneous coarse sand and sandy gravel, and the Twyford subsoil was a more homogenous fine sandy loam. The underlying aquifers were composed of alluvial gravels at both sites with the water tables generally between 4–5 m below ground level. The sites were monitored for 2.2–3.5 years at approximately monthly intervals using suction cups in the unsaturated zone and monitoring wells in groundwater. HYDRUS-2D was used to simulate water movement and solute transport in soil and groundwater in a domain with a depth of 10 m and length of 68 m, including a 4.5-m unsaturated zone. The model simulated well the general trend of field observations for soil water content (θ) and potential (ψs), and the values matched better for the soils with less heterogeneity. For the soils with significant surface cracks, the simulated θ values were overestimated. On the other hand, for the soil layer perching on top of a less permeable layer, the simulated θ values were underestimated.

Simulated pesticide concentrations using the “best available literature values” (BALVs) of organic carbon distribution coefficient (Koc) and half-life (T1/2) were generally lower than those observed. At early times in the trails, most simulations using BALVs were still within the same order of magnitude as observed concentrations for the shallow depths. However, at greater depths and later times, there were major differences between observed and simulated concentrations. The model was then calibrated for Koc and T1/2 values using observed data with an aid of the PEST optimisation package. Despite higher organic contents found in the topsoil, optimised Koc values for pesticides were consistently lower for the topsoil than for the subsoil, and were also lower than the BALVs except for picloram, possibly as a result of preferential flow in the topsoil. Compared to the BALVs, picloram was much more persistent but slightly more sorptive in both soil types, and atrazine and simazine were more persistent in the Te Awa soils but less persistent in the Twyford soils. The optimised Koc values for all three pesticides were generally greater, or close to, the BALVs in the subsoil of both sites.

HYDRUS-2D provided a reasonably good link for pesticide transport in both the unsaturated zone and groundwater. Simulated pesticide concentrations in groundwater using optimised values were generally similar to the observed values. Both observed and simulated bromide and pesticide concentrations indicated that solutes leached more quickly through the soils that were coarser and more heterogeneous, but were more diluted in the groundwater system that was more heterogeneous, conductive, and dispersive. Significant levels of picloram were found in groundwater 22 and 53 m down-gradient of the Twyford and Te Awa plot, respectively. Both simulated and observed atrazine and simazine concentrations in groundwater were less than detection limits.

Introduction

Soil and groundwater contamination by pesticides from agricultural activities is a worldwide environmental problem. Monitoring pesticide concentrations in soil and groundwater is generally very expensive, which has led researchers to explore alternative methods of prediction. The use of simulation models is a cost and time effective approach for a preliminary assessment of groundwater vulnerability to contamination, and assists in land-use planning, resource management, and the design of monitoring programs.

Various leaching models have been developed, for example, MOUSE (Steenhuis et al., 1984), GLEAMS (Davis et al., 1990), HYDRUS (Kool and van Genuchten, 1991), and LEACHM (Huston and Wagenet, 1992). However, most of these models are one-dimensional and are limited to the unsaturated zone. Large numbers of field experiments have been undertaken to investigate pesticide leaching through soils, but relatively fewer experiments have involved the groundwater. These have been reviewed by Flury (1996). Most studies reported to date for modelling pesticide transport are limited to the unsaturated zone, generally the root zone, and there has been a lack of linkage for solute transport between unsaturated zone and groundwater from a modelling aspect.

HYDRUS-2D has been recently developed (Šimùnek et al., 1996). It simulates two-dimensional water flow and solute transport in variably saturated porous media, thus providing a linkage between unsaturated and saturated zones. Although there have so far been few published applications of HYDRUS-2D, several publications are available regarding the use of its predecessor, SWMS-2D. Among these published studies, the SWMS-2D code was mostly used for simulating water movement in unsaturated zones Wu et al., 1995, Stolte et al., 1996, Ritsema et al., 1996, Romano et al., 1999, with a few studies of solute transport (Dou et al., 1999) and in the linkage between unsaturated zones and groundwater (Gribb and Sewell, 1998). While some applications involved solute transport, the code sometimes was only used for simulation of water flow and its output was used as the input for a transport model Yang et al., 1996, Gerke et al., 1998. Unlike most other studies, Braganm et al. (1997) used the code for simulating flow and bromide transport only in the saturated zone. To our knowledge, a study using HYDRUS-2D for simulating pesticide transport either in unsaturated or saturated zones has not been reported in the literature.

Numerous studies have been carried out in determining transport parameters of pesticides, particularly organic carbon distribution coefficient (Koc) and half-life (T1/2 or degradation rate λ), which describe mobility and persistence of pesticides, respectively. These are two fundamental parameter inputs for adequate simulation of pesticide leaching (Persicani, 1996). A comprehensive literature review has been undertaken by Wauchope et al. (1992), who selected “best available literature values” (BALVs) of Koc and T1/2 and detailed the range of these parameters in the literature. These BALVs have been widely used in many studies, especially for preliminary estimations, and are referred to throughout this paper. However, these values are mostly derived from laboratory experiments using homogenous soils at room temperature, and their applicability to field conditions needs to be evaluated.

Pesticide leaching models need to be evaluated with actual field observations since one of the major concerns is their capability in simulating complex field conditions. In this study, HYDRUS-2D is evaluated for simulation of pesticide leaching through soil and into groundwater using field data obtained from two experimental sites. These field data were previously simulated using GLEAMS (Close et al., 1998) and LEACHM (Close et al., 1999). The following are substantial differences between this paper and those by Close et al., 1998, Close et al., 1999.

⋅ This study addressed the linkage between unsaturated zone and groundwater, while the earlier studies deal with unsaturated zone only.

⋅ The model used in this paper allowed two-dimensional movement of flow and solutes, while one-dimensional models were used in the earlier studies.

⋅ The transport scale was much larger in this study (vertically 4.5 m for the unsaturated zone and 5.5 m for the saturated zone, and horizontally 68 m for the groundwater) in order to simulate pesticide movement in down-gradient groundwater. In the earlier studies, the greatest depth was only 2.3 m.

⋅ This study estimated Koc values for both top- and sub-soils (possible in HYDRUS-2D), while the models used in the earlier studies permit only a single Koc value for all soil layers.

Taking into account the above differences, the objectives of this paper are (1) to evaluate model applicability for simulating pesticide transport in both the unsaturated zone and groundwater under field conditions, (2) to examine the usefulness of the BALVs for Koc and T1/2, and (3) to determine optimal Koc and T1/2 values of top- and sub-soils, respectively, for the study sites investigated.

Section snippets

Experimental sites

The two experimental sites are located 11 km apart in Hawkes Bay, North Island, New Zealand. The excessively drained Te Awa soil consists of 0.3 m silt loam topsoil over mainly coarse sand and sandy gravels, overlying alluvial greywacke gravels at 1 m depth. The well drained Twyford soil consists of a fine sandy loam with bands of fine sands and silt loam, overlying alluvial greywacke gravels at about 3 m depth. There were some visible cracks in the surface layer at the Te Awa site. These two

Water movement

Simulated and observed soil θ and ψs values for both sites are shown in Fig. 1, Fig. 2, respectively. Solid and dashed lines represent the observed and simulated data, respectively, which also apply to the other figures. The slightly different patterns of θ and ψs distributions between the two sites are thought to be mainly due to different lithology and rainfall/irrigation amounts. There was significantly less rainfall at the Twyford site than at the Te Awa site (Fig. 3), although the two

Conclusions

HYDRUS-2D was capable of simulating the general trend of field soil water contents and potentials in this study. The predicted values of water content were relatively better for the soils with less heterogeneity, providing the soil hydraulic properties were appropriately defined. The discrepancy between simulated and observed water contents reflected non-ideal hydraulic processes. For soil layers with significant surface cracks, the simulated θ values were overestimated. On the other hand, for

Acknowledgements

The authors thank staff of the Hawkes Bay Regional Council, particularly Dr D. Dravid, for assistance with the study and for funding and installation and monitoring wells. This study was funded by contracts CO3410 (ESR) and CO9802 (Landcare) from the Foundation for Science, Research and Technology, New Zealand.

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