No abstract

Accurate input data for leaching models are expensive and difficult to obtain which may lead to the use of "general" non-site-specific input data. This study investigated the effect of using different quality data on model outputs. Three models of varying complexity, GLEAMS, LEACHM, and HYDRUS-2D, were used to simulate pesticide leaching at a field trial near Hamilton, New Zealand, on an allophanic silt loam using input data of varying quality. Each model was run for four different pesticides (hexazinone, procymidone, picloram and triclopyr); three different sets of pesticide sorption and degradation parameters (i.e., site optimized, laboratory derived, and sourced from the USDA Pesticide Properties Database); and three different sets of soil physical data of varying quality (i.e., site specific, regional database, and particle size distribution data). We found that the selection of site-optimized pesticide sorption (Koc) and degradation parameters (half-life), compared to the use of more general database derived values, had significantly more impact than the quality of the soil input data used, but interestingly also more impact than the choice of the models. Models run with pesticide sorption and degradation parameters derived from observed solute concentrations data provided simulation outputs with goodness-of-fit values closest to optimum, followed by laboratory-derived parameters, with the USDA parameters providing the least accurate simulations. In general, when using pesticide sorption and degradation parameters optimized from site solute concentrations, the more complex models (LEACHM and HYDRUS-2D) were more accurate. However, when using USDA database derived parameters, all models performed about equally.

Preferential flow of bromide tracer through two New Zealand soils, Horotiu (Typic Hapludand) and Te Kowhai (Typic Ochraqualf) soils, was examined. These two soils differed in physical, chemical, and drainage characteristics. These soils had been irrigated with meat‐processing wastewater for 38 mo that ceased 3 mo before this study. Impact of heavy rainfall on residual water quality from such wastewater applied soils was examined by measuring the concentrations of anions (nitrate and phosphate), and cations (Na, K, Ca, Mg, and ammonium) in leachates collected. Heavy rainfall did not appear to have any impact on leachate quality, as indicated by low nitrate‐N and phosphate concentrations compared with those measured during wastewater irrigation. While no preferential flow occurred in the Horotiu soil when it was wetted to field capacity, preferential flow occurred to depth in the Te Kowhal soil under similar conditions. Other water quality problems (e.g., pathogens), which were not investigated in this study may arise in Te Kowhai soils under heavy rainfall.

Allophanic soils are widespread around the world, but little research has been done on their transport properties. This study reveals the effect of two soil water potential heads and two water-flow regimes of continuous and intermittent flow on solute transport through undisturbed soil columns of Horotiu silt loam (Typic Hapludand), an allophanic soil. Two different methods--breakthrough curves (BTCs) and time domain reflectometry (TDR)--were employed to determine the extent of preferential solute transport in the topsoil. The TDR data were also used to look at the depth dependence of the transport properties. The convection-dispersion equation (CDE) with the appropriate boundary conditions adequately described the movement of both Br and Cl under the various flow conditions. Although no preferential flow was found under the imposed unsaturated flow conditions, the flow of water and transport of solute became more uniform with depth. The results show that both Br and Cl are retarded in this allophanic soil. Retardation values range from 1.5 to 1.9, and, as the TDR data showed, increase from the depth of 5.0 to 10.0 cm. Intermittent leaching results showed that there was no effect on solute concentrations in the leachate following no-flow periods. This suggests that water and solute transport in this soil were either relatively uniform or that transverse mixing during flow was already fast enough to eliminate concentration gradients between regions of different "mobility."

Quantification of soil processes, such as adsorption, is needed for predicting the fate of agricultural chemicals in soils. Adsorption is affected by soil properties, which vary with depth. We conducted a laboratory study to determine the influence of soil depth on the adsorption of an organic solute (atrazine) in both allophanic and non-allophanic soils, and an inorganic solute (phosphate) in two non-allophanic soils. Adsorption isotherms of atrazine and phosphate were determined using 14 C-labelled atrazine and KH 2 PO 4 , respectively, in batch equilibrium. The adsorption isotherms for atrazine were almost linear, and adequately described by the Freundlich equation. For phosphate the adsorption isotherms exhibited substantially more curvature and were best described by the Langmuir equation for one of the soils. Generally, adsorption of both solutes decreased with depth. While atrazine adsorption appeared related to such factors as percentage clay content, total C, cation exchange capacity (CEC), and surface area, no similar relationships appeared apparent for phosphate adsorption.