Tile drainage systems are sometimes not sufficient to provide favorable unsaturated conditions in the rootzone. These drainage systems then need to be supplemented with an additional high conductivity material in the trenches above the tiles or by implementing mole drainage. The HYDRUS (2D/3D) model was used to evaluate the impact of such additional measures for heavy clay soil. Three types of drainage systems were simulated: (i) tile drains, (ii) tile drains with gravel trenches, and (iii) tile drains with gravel trenches and mole drains, using either two-dimensional (the former two systems) or three-dimensional (the latter one) transport domains. Three scenarios were considered to test the efficiency of each system: (i) time to drain an initially saturated system, (ii) high intensity rainfall, and (iii) a real case scenario. Different horizontal spacings between tile drains with or without gravel trenches were also compared with the system which included mole drainage. The results showed that the drainage system that included mole drains and gravel trenches was the most efficient. This system provided the largest drainage rate, was the first to reach steady-state in the time to drain scenario, and also efficiently reduced surface runoff. Adding mole drains to a system with tile drains and gravel trenches resulted in a large reduction of surface runoff (75%). Simulations showed that the spacing of tile drains with or without gravel trenches would have to be 40% or 55% smaller, respectively, in order to reproduce the same water table levels as those observed for the drainage system with mole drains. Therefore, introducing mole drains in drainage systems is an efficient practice for reducing waterlogging and runoff. © 2014 Elsevier B.V.

Increases in Zn and Cu concentrations in soils amended with pig slurry (PS) can be described using numerical models. Our main objective was to validate that the HYDRUS-1D model is able to numerically describe profile concentrations and long-term vertical transport of Zn and Cu in a clay soil (Oxisol) cultivated under annual cropping in a no-till system and contaminated by successive doses of PS amendments. We first used a modeling approach that had previously been validated for an Alfisol. Then, we additionally also evaluated the effects of root growth and root water uptake on the transport of trace metals (TMs). Finally, we carried out 50-yr-long prospective simulations for different doses of PS amendments. Consideration of root growth and root water uptake processes in HYDRUS-1D simulations improved the description of measured field Zn concentrations. Although the correspondence between simulated and measured Cu concentrations was not as good as for Zn, we performed prospective simulations for both elements. Future scenarios that considered large PS doses showed large increases in concentrations of both TMs in the soil surface layer. The feasibility of using PS amendments on agricultural Oxisols will be limited by Cu because the soil Cu threshold concentration is exceeded in approximately 29 yr. Moreover, the total loads of both TMs allowed on agricultural soils are reached very fast when large rates are used, especially for Cu (19 yr), indicating that the long-term disposal of PS on agricultural soils should be done at low doses. These conclusions are probably conservative because our model did not consider potential leaching of TMs from the surface soil into deeper soil layers by dissolved organic C facilitated transport.

Long-term applications of organic amendments, such as pig slurry (PS), may represent environmental risk of soil and water pollution by trace metals (TM). Our objective was to examine different soil and manure management scenarios that enhance the long-term agricultural use of soils under repetitive PS applications while avoiding environmental risk. Firstly, we developed a new module for simulating the impacts of soil tillage frequencies in Hydrus-1D. Secondly, we used a previously validated modeling approach to predict the surface accumulation and movement of the TM during the next 100-year in the soil under different PS doses (80 and 40m3ha−1cultivation−1) and tillage frequencies (no-tillage and 20, 10, and 5-year tillage). No-tillage simulations revealed consistent TM surface accumulations, reaching the soil threshold value for Cu in the 0–20cm layer after 86 years of PS amendments at high doses, but in layers 0–5, 0–10, and 5–10cm, this concentration was already reached after 17, 38, and 75 years, respectively. While soil tillage reduced TM concentrations over the top 20cm of the soil profile, it increased their transfer to deeper layers. Periodical soil tillage each 5, 10, and 20 years was found to allow PS applications without reaching the Cu threshold value in soil during 100 years. However, soil solution concentrations of Zn reached the threshold values for groundwater. Therefore, the best manure management practice for the long-term PS disposal with respect to Zn and Cu concentrations in soil is the application of moderate PS rates.