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