Abstract
Lignitic mine soils represent a dual-porosity medium consisting of a technogenic mixture of overburden sediments that include porous fragments embedded within a mostly coarse-textured matrix. Flow and transport process in such soils are not sufficiently understood. The objective of this study was to identify the most appropriate conceptual model for describing small-scale heterogeneity effects on flow based on the physical structure of the system. HYDRUS-1D was used to simulate water flow under field conditions. We compared a dual-porosity (mobile–immobile) model simulation of the field soil water with field monitoring results. The predicted and observed water content were in good agreement. Since the heterogeneity of the lignitic mine soil may lead to preferential flow, Coomassie brilliant blue dyes were applied to the reclaimed surface, revealing preferred flow paths through macro-pores surrounding the numerous, large rock fragments.
Zusammenfassung
Kippenböden in ehemaligen Braunkohletagebauen stellen ein Medium mit doppelter Porosität dar. Sie bestehen aus einer technogenen Mischung von Abraumsedimenten, die poröse Fragmente in einer Matrix meist grober Textur enthält. Fließ- und Transportprozesse in solchen Böden sind bisher nicht ausreichend verstanden. Das Ziel der Studie war die Identifikation des am besten passenden konzeptionellen Modells zur Beschreibung der Effekte kleinskaliger Heterogenität auf Fließprozesse, basierend auf der physikalischen Struktur des Systems. HYDRUS-1D wurde zur Simulation des Wasserflusses unter Feldbedingungen benutzt. Wir verglichen eine Simulation des Bodenwassers mittels eines Doppel-Porositäts-Modells (mobil-immobil) mit Ergebnissen des Monitorings im Feld. Simulierter und beobachteter Wassergehalt stimmten gut überein. Da die Heterogenität der Kippenböden zu bevorzugten Fließbahnen führen kann, wurde Coomassie-Billiant-Blau als Farbtracer auf die rekultivierte Bodenoberfläche appliziert. Es zeigten sich bevorzugte Fließwege durch Makroporen an den Grenzen von Kohle- und Gesteinsfragmenten.
Resumen
Suelos de minas ligníticas representan un medio de porosidad dual consistente de una mezcla tecnogénica de sedimentos que incluye fragmentos porosos embebidos dentro de una matriz mayormente de textura gruesa. Los procesos de flujo y transporte en tales suelos no están suficientemente estudiados. El objetivo de este estudio fue identificar el más apropiado modelo conceptual para describir los efectos de heterogeneidad de pequeña escala sobre el flujo basado en la estructura física del sistema. HYDRUS-1D fue usado para similar el flujo de agua bajo condiciones de campo. Comparamos los resultados obtenidos con un modelo de simulación de porosidad dual (móvil-inmóvil) con los resultados obtenidos en el monitoreo a campo. Los contenidos de agua predichos y observados fueron semejantes. Como la heterogeneidad del suelo de minas ligníticas puede tener un flujo preferencial, colorantes azul brillante Coomassie fueron aplicados a la superficie regenerada, revelando pasos preferenciales de flujo a través de macroporos alrededor de numerosos fragmentos grandes de roca.
摘要
露天煤矿排土场的矿山土是由剥离工艺排弃的煤岩屑和土的混合物沉积形成。它是一个双重(双域)介质的土壤。由于在矿山土粗质结构的基质中含有大量的煤岩屑,使矿山土内孔隙多,易于水的渗透。但矿山土内水及溶质的运移规律尚不清楚。本文研究的目的是根据矿山土的物理结构来找到恰当的物理概念模型刻画描述矿山土内小尺寸的异质性对矿山土中水的流动的影响。论文运用HYDRUS-1D软件中双域模型(动-不动模型)模拟了现场试验地矿山土内的水的流动,并将模拟结果和现场试验结果进行了比较。结果表明,模型的预测值和实际观测值很吻合。由于露天煤矿排土场矿山土的异质性可能会导致优先流的产生,本研究还在排土场复垦地进行了考马斯亮蓝的示踪试验。试验结果表明复垦地的矿山土中由于大量的煤岩屑存在,在其周围的大孔隙中产生了优先流。
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Introduction
In opencast mining, overburden sediments are first removed from the coal seams using, for instance, dragline excavators. This destroys the original sediment layers; the sediments are then transported to a site where they are dumped, thereby creating large initially unsaturated spoil heaps. At our study sites, the resultant mine soil is a mixture of predominately sandy, overburden sediment with lignitic fragments of various geometries and sizes. The fragments are highly porous, so the mine soil represents a dual-porosity sandy soil system.
Despite a number of experimental (Gerke et al. 2001b; Hangen et al. 2004, 2005) and modeling attempts (Buczko and Gerke 2005, 2006; Buczko et al. 2001; Gerke et al. 1998, 2001a), flow and transport in heterogeneous mine soils is not well understood. In contrast to naturally developed soils, a finger-type preferential flow process (Hangen et al. 2004) seems to result from several interacting and temporally varying causes. Among these are the effects of the lignitic fragments, funneling along inclined structures, and finger-type flow regions influenced by spatially and temporally changing water repellence (Gerke 2006a, b; Gerke et al. 2001b).
Classically, water flow through variably-saturated soils is described by Richards’ equation with a uniform flow domain (Simunek and van Genuchten 2008). Recently, efforts have been made at simulating contaminant transport under the influence of preferential flow using dual-porosity models (Gerke and van Genuchten 1993, 1996; Jarvis 2007; Simunek and van Genuchten 2008; Simunek et al. 2001, 2003). These dual-porosity models assume the coexistence of two separate pore domains: fractures (or inter-aggregate pores, cracks, and macro-pores); and matrix pores, with water exchange between the two domains. The two-domain concept, assuming either mobile–mobile or dual-porosity model flow domains, i.e. a mobile–immobile model (MIM), has mostly been used to describe flow and solute transport in aggregated or otherwise structured soils under variably saturated conditions. Our objective in this paper was to use HYDRUS-1D as a tool to understand the effect of preferential flow on water flow in mine soil containing porous, lignitic components. For the analysis of water movement, we compared 1D dual-domain (i.e. MIM) approaches with field monitoring data. We focused on conceptually understanding and describing the flow processes rather than on parameter optimization or stochastic model analysis.
Materials and Methods
Soil Site Description and Experimental Data
The mine soil samples were from the reclaimed Shenshan mine soil heap (39°45′N, 110°11′E; 1,476 m above sea level), which is located approximately 23 km west of the city of Ordos (Inner Mongolia, China). The soil heap was created in 2009 by removing quaternary overburden sediments from nearby opencast lignite mines. The annual mean temperature is 8.7 °C and average annual precipitation rate is 357 mm.A 1 m deep and 2 m long trench was excavated for experiments and soil sampling at a location where quaternary sediments dominate. The soil profile had a clearly separate top soil and subsoil. The sandy tertiary sediments contained differing amounts of lignitic fragments. Large fragments can be identified as dark spots in Fig. 1. Volumetric water content was measured with time domain reflectometry (TDR). Fourteen TDR probes were installed along the trench at four locations spaced 50 cm apart laterally, at seven depths of 5, 15, 25, 40, 50, and 60 cm. Rainfall was measured and recorded continuously near the trench across a catchment area of 200 cm2. The trench was filled after all devices were installed. Field measurements started on 3 Aug 2012. We used 48 days of data.
Flow Modeling
To consider the significant effect of preferential flow on the mine soil with embedded lignitic components, a dual-porosity flow model (based on mass transfer driven by differences in soil water pressure head) was selected from HYDRUS-1D. The dual-porosity formulation for water flow is based on a mixed formulation of the Richards equation to describe water flow in the macropores (mobile water region) and a mass balance equation to describe moisture dynamics in the matrix (immobile water region), as follows (Simunek et al. 2003):
The subscripts m and im refer to the mobile and immobile water regions, respectively; θ = θ m + θ im and is the volumetric moisture content, S im and S m are sink terms (root water uptake) for both regions [T−1], and Γ w is the transfer rate for water exchange between macro-pores and matrix [T−1]. We assumed that root water uptake was preferentially from macro-pores, so that S im = 0.
In the dual porosity flow model based on mass transfer driven by differences in soil water pressure head, the exchange rate of water between the macro-pores and matrix regions, Γ w , was assumed to be proportional to the difference in pressure heads between the two pore regions (Gerke and van Genuchten 1993; Simunek et al. 2003):
Here, ω is a first-order mass transfer coefficient (L−1 T−1). Since pressure heads are now needed for both regions, this approach requires estimating retention curves for both pore regions. That means each region has its own values of: θ r , θ s , α, and n. As a result, soil hydraulic properties are now described by six parameters for macropores (θ r , θ s , α, n, K s , l), four parameters for the matrix (θ r−im , θ s−im , α im , n im ), and a parameter (ω) for mass transfer between the two zones (Simunek et al. 2003).
Soil Hydraulic Properties
Soil hydraulic parameters of both domains are described (van Genuchten 1980) using
where: θ s (−) is saturated and θ r (−) residual water content parameter, α(L −1), n(−), and m(−) are empirical coefficients, Ks is the value of K at water saturation, and S e = (θ − θ r )/(θ s − θ r ) is the reduced water content.
Measured Hydraulic Parameters
Because of the destructive method of soil block excavation, hydraulic parameters could not be determined directly for the analyzed 2D cross-section. Instead, mine soil water retention characteristics were measured using samples from a profile a few meters from the study site. In May 2012, 35 undisturbed soil cores (100 cm3) were sampled from depths of 5, 15, 25, 40, and 50 cm. Water retention characteristics (Fig. 2) were determined using ceramic suction plates (Romano et al. 2002) in the suction range of 0–745 cm at 20 different pressure steps.
Estimation of Hydraulic Parameters
Hydraulic conductivity and the mine soil matrix water retention function were estimated using pedotransfer functions in which the parameters were calibrated for mine soil properties (Buczko et al. 2001). Here, available data from the Shenshan study site were utilized and the effect of lignitic fragments and particles on the hydraulic functions was considered. The pedotransfer function approach for the lignitic mine soil is based on the Arya–Paris-model (Arya and Paris 1981) for the water retention [ψ(θ)] function. For estimating ψ(θ), the Arya and Paris (1981) model was applied in a first step to the mineral fraction, V s (<2 mm) (i.e. the ‘fine’ soil excluding V s (>2 mm) of lignitic fragments and gravel) of the spoil heap. The pore radius, r i [L], corresponding to the particle size class, i, is calculated as:
with R i , the mean radius of particles of particle size class i[L], M i is the mass fraction of particle size class i, ρ s is the density of particles [ML−3], and α AP is a scaling parameter (the subscript AP denotes Arya and Paris). The matrix potential, ψ i [L], is calculated from the pore radii using the capillary rise equation:
Here, σ denotes surface tension of pore water [MT−2], β is the wetting angle of the water menisci on the pore walls, and ρ w the density of water [ML−3]. The volumetric water contents, θ i for the matrix potentials, ψ i , and the drained pore sizes corresponding to the ith particle size class are obtained by summing up the volumes of all pores, V P,i , of the size class, i, as:
In a second step, the water retention curve for the lignitic particle (Fig. 3) was derived from the volumetrically-weighted difference between the mean measured water retention curve in Fig. 2 (i.e. 100 % soil volume comprising both mineral particles and lignitic fragments) and the mean curve of the mineral soil matrix obtained with the Arya–Paris approach. It reflects the distinctly different pore size distribution and the much higher water retention capacity of the lignite fragments as compared to the mineral soil matrix.
In this paper, the saturated hydraulic conductivity, K s was estimated from a pedotransfer functions (PTFs) (Wosten 1997). Wosten (1997) presented a function for determining K s , as follows:
where x for sandy soil is:
and where BD is bulk density in g cm−3, CS is the sum percentage of clay and silt, and O m is the percentage of organic matter.
The HYDRUS-1D model (Simunek et al. 2008) was used to simulate flow at the field site using hydraulic parameters obtained with PTFs. The HYDRUS-1D software (Simunek et al. 2008) was used to run the simulation. This software gives options to run simulation with the van Genuchten Eq. (4). The unsaturated hydraulic conductivity (K) function was calculated using Eq. (5) for the van Genuchten–Mualem model.
A separate simulation run was performed with HYDRUS-1D for each PTF. Water retention was estimated by applying the PTFs to the soil properties for each individual layer at the Shenshan field site (0–10, 10–20, 20–30, 30–50, and 50–70 cm; see Table 1).
Results and Discussion
Observed and simulated soil water contents are shown in Fig. 4. The topsoil (5, 15, and 25 cm) exhibited more weather-related variation in water content than the subsoil or mine soil (40, 50, and 60 cm), and the water content in the subsoil or mine soil (40, 50, and 60 cm) was greater than that of the topsoil. Figure 4 also shows that the mine soil water flow lagged behind initiation of rainfall. Yet in the second event, water flow responded very quickly to rainfall, which indicates that the mechanism of mine water flow must be dominated by rapid preferential flow through macro-pores in the mine soil rather than saturated flow through the mine soil matrix.
Discussion of Flow Paths
Still, it remains unclear how to explain how water in the lignitic fragments can be more mobile than in the matrix domain. The assumption that the lignitic fragments as a whole form a continuous pore domain would be difficult to verify. Nevertheless, the model analyses suggest the existence of non-equilibrium conditions and of a continuous pore system in which rapid water movement may take place. Observations show that the fragments itself can be heterogeneous and may not be homogeneously distributed; other observations indicate that a more continuous pore domain may be formed by lignitic components along surfaces of the fragments and smaller structures formed by lignitic in the mine soil matrix (Fig. 5). The integration of macro-pores in the mine soil horizon provides a mechanism for rapid downward movement of water through the unsaturated mine soil matrix.
In an attempt to verify the above formulated hypotheses on local flow paths in an unsaturated mine, we applied brilliant blue dye tracer in the waste dump of the Hei Dai Gou opencast coal mine, Ordos (Inner Mongolia, China). The Hei Dai Gou opencast coal mine is not at the same location as the Shenshan spoil heap, but they are only about 60 km apart. One liter of water containing Coomassie Brilliant Blue dye was slowly poured directly onto the surface of the mine soil. The dye readily penetrated into cracks surrounding surface fragments and into cracks beneath the fragments to an additional depth of 10 cm. Where cracks were absent, the dyed water penetrated <1 mm into the mine soil. The general vertical depth of penetration was 10–12 cm. In general, dyed water tended to flow along narrow, discrete paths, the length of which depended on the abundance and proximity of coarse fragments.
Infiltration of dyed water into the macro-pores and subsequent excavation revealed preferential flow paths along macro-pores developed around and connecting coarse fragments within the mine soil matrix (see Fig. 5). The integration of surface and subsurface cracks into a mine soil macro-pore network provides an important pathway for water flow to occur.
Most dyes consist of relatively large organic molecules and, as such, interact to some degree with the solid matrix in soils and aquifers. Dyes have the ability to stain the travel paths of water and solutes in soils and are thus useful for visualizing water flow patterns. In general, organic dyes have amphiphilic characteristics, i.e. the molecule has both hydrophobic and hydrophilic properties. In addition, the functional groups of organic dyes can protonate and deprotonate depending on pH, thereby changing the net charge of the molecule. Because of these characteristics, interaction of the dye molecule with soil surfaces is rather complex. Sorption of dyes to solid surfaces involves one or a combination of the following interactions: hydrophobic, van der Waals, ion exchange, covalent bonding, and hydrogen bonding. The Coomassie Brilliant Blue dye is zwitterionic (amine and sulfonic acid groups) and has a solubility of 200 kg m−3. It is nontoxic and, depending on pH, the dye is either neutral or dissociates to a mono- or bivalent anion (Flury and Flühler 1995). The Coomassie Brilliant Blue dye is a charged organic molecule that consists of polar and non-polar portions; sorption studies indicate that hydrophobic interactions are much less important than electrostatic or sorbate-mineral surface interactions (Flury and Flühler 1995; Ketelsen and Meyer-Windel 1999). The non-conservative nature of the Coomassie Brilliant Blue tracer is probably enhanced by relative slow volumetric flow through the highly absorbent materials in the mine soil.
The pH of the solution determines the degree of dissociation of the sulfonic acid groups and directly influences the net charge of the dye molecule. At the low pH of the soil, the molecule is predominantly in its neutral form, and may be more prone to adsorption in soils than anionic species. Qualitatively, the data are consistent with the presumption of increased sorption at low pH, although pH and soil texture can often interfere with sorption (Judit and Markus 2000). For tracing in field soils, a dye should preferably be mobile, distinctly visible, and nontoxic. The two criteria of visibility and mobility are to a certain degree mutually exclusive, because to stain the flow paths of water or solutes, the compound has to be retained. The advantages of Coomassie Brilliant Blue, especially for mine soil use, are its visibility against the color of the soil and its low toxicity. From the point of view of mobility, however, Coomassie Brilliant Blue is not ideal for tracing the travel times of water, but used in combination with conservative tracers, such as Cl− or Br−, Coomassie Brilliant Blue is useful for detecting flow patterns. We believe that, in terms of toxicity, visibility, and mobility, Brilliant Blue may be one of the best compromises available as a dye tracer in vadose zone hydrological studies.
Discussion of Model Concepts
Figure 5 indicates that the network of flow paths within the mine soil follows a small-scale structure, especially connecting the interface regions surrounding lignitic fragments. A network of flow paths may develop where the fragments are close together and where, with the sandy matrix, large pores are bridged by lignitic particles or by slightly more compacted regions formed locally during the sedimentation process. Further assessment of flow in small-scale heterogeneous media was beyond the scope of this study.
Summary and Conclusions
We studied the effects of local heterogeneity (here in the form of embedded porous lignitic fragments) on observed preferential flow. Such lignitic mine soils represent a typical two-scale dual-porosity medium. The results were used to assess the most appropriate conceptual model for describing small-scale heterogeneity effects on flow. One hypothesis is that at the interface between the sandy matrix and the inner parts of lignitic fragments, a more conductive porous network exists. This ‘interface’ flow domain probably consists of the outer, more weathered and cracked regions of fragments and those sandy regions that have a higher lignitic content. The latter could be due to particle segregation and compaction within the spoil piles during sedimentation, which is one major difference between mine soils and naturally developed and aggregated soils.
This flow model fit reasonably well to the monitoring data (Fig. 4). Our attempts to use HYDRUS-1D (MIM) model simulations based on the Richards’ equations were hence warranted.
Development of macro-pores, predominantly in the mine soil horizon, is the fundamental control on increases in infiltration rates following reclamation of surface-mined land. Macro-pores in the mine soil horizon provide a mechanism for rapid downward movement of water through the unsaturated mine soil matrix.
References
Arya LM, Paris JF (1981) A physic empirical model to predict the soil moisture characteristic from particle-size distribution and bulk density data. Soil Sci Soc Am J 45:1023–1030
Buczko U, Gerke HH (2005) Evaluation of the Arya–Paris model for estimating water retention characteristics of lignitic mine soils. Soil Sci 170(7):483–494
Buczko U, Gerke HH (2006) Modeling two-dimensional water flow and bromide transport in a heterogeneous lignitic mine soil. Vadose Zone J 5(1):14–26
Buczko U, Gerke HH, Huttl RF (2001) Spatial distribution of lignite mine spoil properties for simulating 2D variably saturated flow and transport. Ecol Eng 17:103–114
Flury M, Flühler H (1995) Tracer characteristics of Brilliant Blue FCF. Soil Sci Soc Am J 59:22–27
Gerke HH (2006a) Exploring preferential flow in forest-reclaimed lignitic mine soil. Adv Geoecol 38:380–387
Gerke HH (2006b) Preferential flow descriptions for structured soils. J Plant Nutr Soil Sci 169:382–400
Gerke HH, van Genuchten MT (1993) A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media. Water Resour Res 29(2):305–319
Gerke HH, van Genuchten MT (1996) Macroscopic representation of structural geometry for simulating water and solute movement in dual-porosity media. Adv Water Resour 19(6):343–357
Gerke HH, Frind EO, Molson JW (1998) Modeling the effect of heterogeneity on acidification and solute leaching in overburden mine spoils. J Hydrol 209:166–185
Gerke HH, Molson JW, Frind EO (2001a) Modeling the impact of physical and chemical heterogeneity on solute leaching in pyretic overburden mine spoils. Ecol Eng 17(2–3):91–101
Gerke HH, Hangen E, Schaaf W, Huttl RF (2001b) Spatial variability of potential water repellency in a lignitic mine soil afforested with Pinus nigra. Geoderma 102:255–274
Hangen E, Gerke HH, Schaaf W, Huttl RF (2004) Flow path visualization at a lignitic mine soil using iodine-starch staining. Geoderma 120:121–135
Hangen E, Gerke HH, Schaaf W, Huttl RF (2005) Assessment of preferential flow processes in a forest-reclaimed lignitic mine soil by multicell sampling of drainage water and three tracers. J Hydrol 303(1–4):16–37
Jarvis NJ (2007) A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality. Eur J Soil Sci 58(3):523–546
Judit G-H, Markus F (2000) Sorption of Brilliant Blue FCF in soils as affected by pH and ionic strength. Geoderma 97:87–110
Ketelsen H, Meyer-Windel S (1999) Adsorption of Brilliant Blue FCF by soils. Geoderma 90:131–145
Romano N, Hopmans JW, Dane JH (2002) Suction table. In: Dane JH, Topp GC (eds) Methods of soil analysis, part 4, physical methods, 3.3.2.6, Soil Sciences Soc of America Book Series No 5, Soil Science Soc of America, Madison, WI, p 1692
Simunek J, van Genuchten MT (2008) Modeling nonequilibrium flow and transport processes using HYDRUS. Vadose Zone J 7(2):782–797
Simunek J, Wendroth O, Wypler N, van Genuchten MT (2001) Non-equilibrium water flow characterized from an upward infiltration experiment. Eur J Soil Sci 52(1):13–24
Simunek J, Jarvis NJ, van Genuchten MT, Gardenas A (2003) Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone. J Hydrol 272:14–35
Simunek J, M Sejna, and MT van Genuchten (2008) The HYDRUS-1D software package for simulating the one-dimensional movement of water, heat and multiple solutes in variably-saturated media. Version 4.0, Dept of Environmental Sciences, Univ of California Riverside, CA
van Genuchten MT (1980) A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci Soc Am J 44:892–898
Wosten JHM (1997) Pedotransfer functions to evaluate soil quality. In: Gregorich EG, Carter MR (eds) Soil quality for crop production and ecosystem health, developments in soils science, vol 25. Elsesevier, Amesterdam, pp 221–245
Acknowledgments
We thank Ma Jin-line, vice-president of the Shenshan opencast coal mine, Ordos (Inner Mongolia, China), and Wang Ping-ling, vice-president of the Hei Dai Gou opencast coal mine, Ordos (Inner Mongolia, China) for their permission and cooperation in our conducting this work at those mines. We thank the National High Technology Research and Development Program 863 for funding this research.
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Ma, Ca., Cai, Qx., Wang, H. et al. Modeling of Water Flow in Reclaimed Mine Spoil with Embedded Lignitic Fragments Using Hydrus-1D. Mine Water Environ 34, 197–203 (2015). https://doi.org/10.1007/s10230-014-0299-z
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DOI: https://doi.org/10.1007/s10230-014-0299-z