Flow and transport processes in a macroporous subsurface-drained glacial till soil I: Field investigations
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ELSEVIER Journal of Hydrology 207 (1998) 98- 120
Flow and transport processes in a macroporous subsurface-drained glacial till soil
I: Field investigations
K.G. Villholth,*, K.H. Jensena, J. Fredericiab
aDepartment of Hydrodynamics and Water Resources, Technical University of Denmark, 2800 Lyngby, Denmark bGeological Survev of Denmark, Thoravej 8. 2400 Copenhagen NV, Denmark
Received 5 March 1997; accepted 18 February 1998
The qualitative and quantitative effects of macropore flow and transport in an agricultural subsurface-drained glacial till soil in eastern Denmark have been investigated. Three controlled tracer experiments on individual field plots (each approximately 1000 m*) were carried out by surface application of the conservative chloride ion under different application conditions. The subsequent continuous long-term monitoring of the rate and chloride concentration of the drainage discharge represented an integrated and large-scale approach to the problem. In addition, point-scale determination of macropore structure and hydraulic efficiency, using image analysis and tension infiltration, and of soil water content, level of groundwater table, and chloride content of soil water within the soil profile yielded insights into small-scale processes and their associated variability. Macropore flow was evidenced directly by the rapid (within 10 mm of water input) and abrupt chloride break- through in the drainage water at 1.2 m depth in two of the tracer experiments. In the third experiment, the effect of macropore transport was obvious from the rapid and relatively deep penetration of the tracer into the soil profile. Dye infiltration experiments in the field as well as in the laboratory supported the recognition of the dominant contribution of macropores to the infiltration and transport process. The soil matrix significantly influenced the tracer distribution by acting as a source or sink for continuous solute exchange with the macropores. An average field-determined active macroporosity constituted 0.2% of the total porosity, or approximately 10% of the total macroporosity. 0 1998 Elsevier Science B.V. All rights reserved.
Keywords: Structural soils; Macropores; Subsurface-drainage; Field-scale experiments; Conservative tracer; Flow and transport processes
The predominance and effect of macropore flow processes in soil and groundwater systems have been studied and documented in numerous reports during the past couple of decades (for a review, see Thomas and Phillips, 1979; Beven and Germann,
* Corresponding author.
1982; White, 1985; Villholth, 1994). Macropore flow, or alternatively bypass flow or fracture flow, is attributed to structural pore systems that provide path- ways for relatively rapid transport of water and dissolved or suspended constituents through the porous medium. These express routes (e.g. worm and root channels, and planes and openings created by fracturing and dissolution) are numerous and ubiquitous in most natural soils and many aquifer
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K.G. Villholth et al./Joournal of Hydrology 207 (I 998) 98-120 99
materials. Thus, a better understanding of these preferential flow and transport processes is needed.
In shallow unsaturated soils, which is the main focus of this study, the effect of macropore flow can be manifested through very short arrival times of surface-applied or derived substances to the ground- water. The retarding or buffering capacity of the unsaturated zone is further diminished for many degradable and sorbing chemicals because the reten- tion time and contact possibilities with the porous medium and the resident water are small. Drainage studies in which the leaching of surface-applied chemicals is traced in subsurface drains, have pre- viously been used and recommended as a tool for analyzing transport processes in the upper predomi- nantly unsaturated soil (Bottcher et al., 1981; Everts et al., 1989; van Ommen et al., 1989; Kladivko et al., 1991; Vinten et al., 1991; Jayachandran et al., 1994; Southwick et al., 199.5), including macropore flow phenomena (Richard and Steenhuis, 1988; Utermann et al., 1990). The advantages of the drainage study approach are as follows.
The measured drainage response integrates the effects of spatial variability with respect to flow and transport characteristics within a large soil volume. A measure is obtained of the flux concentration (Parker and van Genuchten, 1984; Utermann et al., 1990) which is of concern when evaluating the contamination load to the shallow ground- water. Existing subsurface drains provide easy access for sampling and monitoring equipment, installed without disturbing the soil volume of interest.
Finally, the agricultural practice of subsurface drainage which is widespread on heavy soils charac- terized by macroporosity provides on its own merits a justification for research into the hydrological and water quality impacts of cultivation and soil management.
In previous reported drainage studies the documen- tation of the effect of macropore flow has primarily been based on observations of rapid breakthrough of surface applied chemicals in the drainage effluent and, only to a smaller extent, has this presumption been substantiated with further experimental documenta- tion. This study aims at extending the conventional
drainage tracing approach in order to improve the interpretation of the flow and transport processes responsible for the movement of water and a con- servative tracer (chloride, Cl-) from the soil surface to the drain and shallow groundwater in an agri- cultural glacial till soil with visible macroporosity. As opposed to previous drainage tracer studies this study also includes:
hydraulic and geometric characterization of the macropores; point-scale observation of important variables, such as soil water content, soil water tension, piezometric head, and concentration of tracer in soil water: continuous monitoring on short as well as long time scales of the drainage response; varying conditions for the soil surface application of the tracer; a qualitative analysis of the main recipient of the tracer (groundwater or surface water): a model analysis of the field tracer experiments with special emphasis on the effect of macropore flow and transport.
The experimental procedure was implemented in order to specifically observe and evaluate the macro- pore flow and transport processes on a small and large scale, spatially as well as temporarily. A comprehen- sive monitoring scheme was designed in order to provide a consistent and relevant data set for the model analysis. The aim of the model application was to assess more quantitatively the significant flow and transport components in the investigated soil on the basis of a numerical, dynamic double- porosity flow and transport modeling concept. The results of this study are reported in an accompanying paper (Villholth and Jensen, 1998). In the present paper, the results of the field investigations are reported.
2. Materials and methods
2.1. Field site
The experimental site is located in an agricultural area in the Syv Creek Catchment (1170 ha) in the eastern part of Denmark, 40 km south-west of
100 K.G. Villholth et al.Noumal of Hydrology 207 (1998) 98-120
Copenhagen (Latitude: W, 40). The Quaternary- deposited till sequence consists of interbedded layers of moraine clay and alluvial sand and gravel resting upon a primary Paleocene limestone aquifer at lo- 15 m depth. The soil is classified as an Inceptisol (Soil Taxonomy, Soil Survey Staff, 1975). The soil texture ranges from silty to sandy loam (FAO, 1977) with a maximum clay content of approx. 30% by weight. The macroscopic soil structure was observed and evidenced from vertical as well as horizontal cross- sections exposed in three large pit excavations. The visible macropores are dominated by predominantly vertical earthworm channels (0.5- 10 mm in diameter, approximately 15 per square decimeter), but also root patterns and a ped and block structure in the till give rise to macropores in the upper approximately 1 m of the soil. Below approximately 0.8 m depth a pseudo gley zoning (mottled grey/brown coloring of the soil) indicates temporary saturation and that the structural pores have generated heterogeneous saturation and oxidation conditions in the soil. Further insight into the macropore structure was gained from dye infiltra- tion studies and image analysis of horizontal cross- sections of excavated soil columns.
The drained field site is relatively flat (
K.G. Villholth et al./Joumai of Hydrology 207
- Drain line 0 Pwometer pi LOT 1
Plot boundary x Soil sampling spot
- Cl application stnp ~3 Soil sampling spot @@ XsfS X MB 0
s23 Access manhole + piezometer X xx xx xx
Cl2 C11 Cl0 4R,,
X x X X
El28 X 825 & O 827
Fig. 2. Schematic plane view of PLOT 1 and PLOT 2.
PLOT 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~. rJ c
i @,g@ r;, % 0 00 s s CXI
0 Soil sampling spot
Fig. 3. Schematic plane view of PLOT 3.
P / I 33
102 K.G. Villholth et al./Journal of Hydrology 207 (1998) 9% 120
laboratory (Standard Methods for the Examination of Water and Wastewater. 198.5).
Besides the drainage monitoring. the held site was instrumented to allow monitoring of local conditions within the soil profile. The monitoring program included: (1) piezometric head at 1.0-2.6 m depth measured manually, or automatically with pressure transducers, in 16 mm diameter piezometers: (2) soil water tension at 0.3 and 0.6 m depth measured with tensiometers; (3) chloride concentration determined by argentometric titration in soil water collected in either porous suction cups (0.3 and 0.6 m depth). in piezometers. or from disturbed soil samples (50 g) collected at O-3 m depth prior to and after the tracer applications (2-12 and 4-8 replications each time prior to and after. respectively (Fig. 4)); and (4) soil water content (down to 2-3 m depth) measured with ,eutron logging equipment and from the collected soil
samples. Figs. 2 and 3 show the layout of the plots including the measurement locations. On-site measurement of rainfall was provided by a recording tipping bucket rain gauge (resolution, 0.1 mm) and an integrating rain gauge at ground level used to correct for aerodynamic effects.
2.2. Field tracer experiments
The experimental work commenced in the fall of 1989 and lasted through the spring of 1992. Drain flow was a prerequisite for the drainage experiments, which dictated that the tracer experiments were initiated in the fall-to-spring period. The tracer experi- ments on PLOT 1 and PLOT 2 were initiated late in the drainage season of 1989-1990 (SEASON 1 ), and on PLOT 3 the tracer application was performed in the 199 1 - 1992 drainage season (SEASON 3). During the intervening drainage season (1990- 199 1, SEASON 2) no tracer applications were performed due to unfavorable weather conditions. The hydraulic response of PLOT 3 was, however, monitored during SEASON 2 (Fig. 4).
The tracer experiments on PLOT 1 and PLOT 2 were considered preliminary tests of the immediate effect of macropore flow on discharge and concentra- tion in the drain and hence the monitoring focused on the drainage outflow in SEASON 1. However, grab samples were taken occasionally during the following two drainage seasons from DRAIN 1. which combines
the response of the tracer applications on PLOT 1 and PLOT 2. In addition, analysis of soil samples collected in the plots between the seasons gave information on the long-term response. The tracer experiment on PLOT 3 was conducted to evaluate. with better temporal and spatial resolution, the short- and intermediate-term response of the drainage effluent as well as the water and solute transport inside the test area. Hence the installation and monitoring program on PLOT 3 during SEASON 3 was quite intensive. A reliable and consistent comparison of tracer movement in different drain plots hinges on an assumption of similar hydraulic characteristics. A linear correlation between the instantaneous drainage flow rate in DRAIN 1 (Qdm,) and the discharge rate in DRAIN 2 (Qdll12) during SEASON 2 and SEASON 3 was deducible (Qdml = 2.49Qd,,: + 0..13 1 min-: R = 0.89: II = 32). The linearity suggests that the hydraulic responses of the adjacent drain plots are similar. In addition, the slope of the line corresponds to the ratio of the drained areas.
The tracer was applied manually on the soil surface in 2 m wide rectangular strips along the drains. The area of the tracer application strips were 236. 142 and 56 m for PLOT 1, PLOT 2 and PLOT 3, respectively. The strips were displaced 1 .O- 1.5 m from the location of the drain line (Figs. 2 and 3). The strategy was to avoid tracer infiltration into the soil volume directly above the drain that could have been disturbed during the past drain installation (Hergert et al.. 1981: Richard and Steenhuis. 1988; Kladivko et al.. 1991: Chow et al., 1993). A fast breakthrough in the drainage water upon tracer application would then be indicative of macropore flow effects in a repre- sentative part of the held soil. At the same time, the close proximity of the tracer application area to the drain was chosen to allow investigation of effects dominated by flow in the unsaturated zone. High tracer input doses were used due to relatively high initial background chloride concentrations in the soil. ranging from 24 to 292 mg ll. Amounts ot 620-790 g Cl (as CaClZ,2H20) per square meter were applied. When mixed with 3 mm of water. as in the experiment on PLOT 3, this corresponded to input concentrations of about 180 g Cl 1-l.
The circumstances of tracer application varied among the three experiments (Table 1). In PLOT 1 and PLOT 2 the calcium chloride was applied in solid
K.G. Villholth et al./Journal of Hydrology 207 (1998) 98-120
PROFILE 1 b PROFILE 2a
gg PLOT 1
m PLOT 2
PROFILE lc PROFILE 3a AUG.1Z
PROFILE ld PROFILE 3b APR.24
qcl SEASON 3
Fig. 4. Experimental sequence for the tracer experiments. Length of bars represents the duration of continuous monitoring in the drain. 4 and date indicate time of tracer application. PROFILE no. and date indicate plot and time of soil core sampling.
Table 1 Tracer application conditions
K.G. Villholth et al./Journal of Hydrology 207 (199X) 9% 120
PLOT I PLOT 2 PLOT 3
Tracer form solid Soil water content field capacity Water input w. tracer none
solid > field capacity
dissolved > field capacity
form as flakes, directly on the ground. In PLOT 2 this application method was immediately succeeded by irrigation of 11 mm...