Modeling field-scale vertical movement of zinc and copper in a pig slurry-amended soil in Brazil
Highlights
► We performed a model validation of Zn and Cu settings in a pig-slurry amended soil. ► A two-site model with reduced kinetic constants present the best fit with field data. ► Future 50-years scenarios related to the pig slurry applications were simulated. ► Downward Zn and Cu solute fluxes will not present risk to groundwater pollution. ► Continuous pig slurry amendments will lead to a high Cu accumulation on soil surface.
Introduction
Soil amendment with organic waste is a common practice in agriculture worldwide. In different regions of Brazil, large-scale animal production generates large amounts of manure. These regions are often characterized by the predominance of family farming, in which each farmer has only a small land area to grow crops. The fertilizing value of organic waste, such as animal manure, is generally recognized, but its quality may vary widely. For example, pig slurry (PS) often has a relatively low nutrient content, making it uneconomical to transport over long distances. Hence, PS disposal criteria are primarily guided by a limited space, increasing risks of soil pollution by nitrate, phosphorus, and trace metals (TMs), notably zinc (Zn) and copper (Cu).
Regulatory agencies in many countries have established environmental guidelines for soil organic waste amendments [1], [2], [3], [4]. Basically, such guidelines restrict, or ban, applications of organic waste to soils where the total TM content exceeds the maximal allowed values. Brazilian laws currently allow for the maximum TM values, which are generally higher than those adopted in the European Union. The Companhia de Tecnologia de Saneamento Ambiental (CETESB) [3], and specifically the state of Sao Paulo, authorizes 2800 and 1500 kg ha−1 of Zn and Cu, respectively, as maximum loads that can be applied in a particular area. Moreover, the maximum soil TM content, when further applications of amendments to agricultural soils are prohibited, are 450 and 200 mg kg−1 for Zn and Cu, respectively [5]. For comparison, the European Union [1] allows the addition of 30 kg Zn ha−1 and 12 kg Cu ha−1 based on a 10-years average. The maximum allowed soil concentrations when further amendments are restricted are 150–300 mg kg−1 for Zn and 50–140 mg kg−1 for Cu, depending on soil pH. Brazil's federal legislation was expanded in the 1990s, based on the experiences of other countries. It was recognized that the total TM levels are not good environmental indicators for making decisions, and that the type of soil management needs to be taken into consideration. For example, the maximum TM levels established by the Brazilian legislation refer to any layer of the soil profile. In southern Brazil, about 50% of agricultural operations (almost 8.5 million hectares) are managed under no-tillage practices. Consequently, the organic waste applied on the soil surface is not incorporated deeper into the soil profile, thus enhancing a rapid increase of TM concentrations in the surface layer.
Soil water movement varies in time and space and determines to a large extent the spatial distribution of TMs in soils. Mathematical modeling of the fate of metals in soils in terms of their retention and transport is helpful to assess their environmental impacts [6]. Mathematical models, simulating water flow and transport and reactions of TMs in soils, provide additional information on risks of the transfer of TMs to the groundwater. Such transfer mainly depends on the amount of water moving through a particular type of soil and on the mobility of TMs in the soil solution. On the one hand, theoretical mathematical models require many parameters to describe complex processes of TMs reactivity with functional groups of the solid phase. On the other hand, empirical models often insufficiently consider physical and chemical soil characteristics or biological activity since such models merely rely on adsorption/desorption data. However, such adsorption/desorption processes are often influenced by various soil factors, which are difficult to measure or individually control.
The Hydrus software package [7] has been widely used to simulate the fate and transport of TMs in soils [8], [9], [10], [11], [12]. Hydrus includes mathematical modules that simulate variably-saturated water flow and solute transport in porous media. Recently, Mallmann et al. [13] demonstrated the effectiveness of the Hydrus two-site sorption sub-model [14] by simulating changes in Zn and Pb concentration profiles in metal contaminated soils over one century. Similar modeling by Rheinheimer et al. [15] predicted that Zn and Pb concentrations in the soil solution will not exceed the reference water quality values of the Council of the European Communities (CEC) [1] during the next 50 years. Additionally, Seuntjens [16] used Hydrus to simulate two remediation scenarios for a soil contaminated with cadmium (Cd). He concluded that Cd concentrations in the soil solution were reduced after adding an organic material and/or an acidity corrector to the soil. He also predicted a decreasing Cd loss from the soil. Moradi et al. [17] simulated Cd movement in a soil amended with a sewage sludge under an arid climate. While they noted that there is little risk of groundwater contamination by leached Cd, they observed an accumulation of TM in the soil surface layer, and an increased risk for TM absorption by plant roots and/or removal by surface runoff.
Here, we use Hydrus-2D to model the past and future movements of Zn and Cu in a Brazilian Alfisol, contaminated by repetitive applications of PS, and compare the obtained results with a non-amended soil. First, we simulate changes in Zn and Cu concentration profiles in the soil using known metal inputs via PS amendments during 8 years. We consider two hypotheses for interactions between the soil solid phase and the soil solution, i.e., either a complete instantaneous equilibrium or a two-site sorption model with both equilibrium and kinetic sorption. The model's validity is assessed by comparing simulated and measured field TM distributions in 2008. Second, we predict the movement of Zn and Cu, using the adopted hypothesis, for the next 50 years under two scenarios of discontinued and continued PS applications.
Section snippets
Experimental site, soil, and other characteristics
The study was conducted on experimental fields of the campus of the Federal University of Santa Maria in the state of Rio Grande do Sul in Brazil (29°42′52″S and 53°42′10″W, 90 m altitude) and involved repetitive applications of PS. Soils, classified as a Typic Hapludalf [18], are well drained, and show a strong vertical clay gradient in the profile. The parent material is upper Triassic sandstone, locally referred to as the Santa Maria Formation [19]. The soil mineralogical composition includes
Simulation of Zn and Cu transport in the soil between 2000 and 2008
Simulations with hypothesis A (100% instantaneous equilibrium) produced a strong Zn and Cu retention in the soil surface layer (0–5 cm), although increasing TM contents were observed in the field down to a depth of about 25 cm (Table 3). In particular, about 97% of added Cu remained in the top 0–5 cm soil layer. Large differences between simulated and measured TM concentrations clearly indicate that the model based on hypothesis A cannot describe observed field metal concentration profiles.
Conclusion
We used Hydrus-2D with a two-site sorption model with a reduced kinetic rate constant and with measured soil hydraulic characteristics to reproduce vertical distributions of Zn and Cu measured in an experimental plot soil profile after 8 years of repeated pig slurry applications. We considered the satisfactory correspondence between simulated and field-observed TM distributions to represent a direct model validation. We then used the validated model to predict Zn and Cu movements during the
Acknowledgements
F.J.K. Mallmann acknowledges CAPES for postgraduate scholarship. The authors thank Reimar Carlesso and Mirta Petry (Sistema Irriga®) for providing daily evapotranspiration and local atmospheric data. They express their special gratitude to Maria Alice Santanna for her relevant contributions to the discussion.
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