Elsevier

Geoderma

Volume 109, Issues 3–4, October 2002, Pages 245-268
Geoderma

Multiple tracing of fast solute transport in a drained grassland soil

https://doi.org/10.1016/S0016-7061(02)00178-7Get rights and content

Abstract

Fast transport of fertilizers and other agrochemicals into subsurface drainage systems has been recognized as a serious threat to surface waters. We report on a tracer experiment carried out on a 7.3×20 m2 plot on a loamy grassland soil to determine the flow paths to a tile drain at 1 m depth. The experiment consisted of a series of consecutive tracer applications including seven solutes and liquid manure that were applied either on the entire plot or on limited bands. Based on the discharge behavior under natural conditions, we estimated the effective hydraulic conductivity of the subsoil to be in the order of 8–29 cm day−1. Under experimental conditions, the soil transmitted 120 mm day−1 into the subsurface drain and two vertical profiles without producing surface runoff. Only part of the soil water, corresponding to 6–27 mm of the soil depth, contributed to the fast hydrological response. The transport of the tracers was very fast. Within 7–16 h after application of the conservative Br, Cl and HDO and the slightly sorbing substances brilliant blue (BB) and amino-G-acid (AG), these tracers reached relative concentrations in the outflow between 19% and 35% of the input concentrations. From the mass balance for water and solutes, it follows that the tracers were quickly transported over lateral distances of several meters. The manure constituents dissolved reactive P (DRP), NH4+ and Cl, applied as liquid manure on the surface on a 1 m wide band above the tile drain, reached the drain within 5 min after application. After the early peak of DRP and NH4+, their concentration in the drain decreased quickly to background levels, whereas Cl exhibited a second peak. Despite the fast transport and the small soil volume conducting water and solutes, the interaction between irrigation water and soil matrix was intimate enough to retain the two sorbing tracers. From the stained flow paths, the hydrologic behavior of the field under natural conditions and the hydrometric data during the experiment, it follows that the fast lateral tracer transport occurred mainly close to soil surface and not through the subsoil. Only in the immediate vicinity of the tile drain and of two lateral pits at the edge of the experimental plot water was redirected downwards and discharged from the tile drain and the bottom parts of the profiles, respectively. Hence, effluent from tile drains may not be representative for water reaching the subsoil or shallow ground water in undisturbed soils.

Introduction

The flow patterns occurring in natural and managed soils are in many, if not most cases, highly irregular. This irregularity is often called preferential flow, which means that part of the water and solutes move rapidly through a small portion of the soil without much exchange with the surrounding soil matrix over a substantial length of the flow domain (Flühler et al., 1996). The flow velocities are large and even strongly sorbing solutes like pesticides Flury, 1996, Flury et al., 1995, Kladivko et al., 2001, phosphorus Addiscott et al., 2000, Heckrath et al., 1995, Hergert et al., 1981a, Hergert et al., 1981b, Sims et al., 1998, Stamm et al., 1998 or radionuclides (Bundt et al., 2000) may reach groundwater or subsurface drainage systems before being sorbed or biologically degraded.

Due to the large sampling volume, subsurface drainage systems are sometimes taken as ideal experimental systems for transport phenomena in soils Czapar and Kanwar, 1991, Flury, 1996, Gish et al., 2000, Lennartz et al., 1999. Such studies rely on the assumption that sampling large volumes averages out the small-scale spatial heterogeneities. Classical drainage theory assumes a vertical transport through the unsaturated and a predominantly lateral flow through the saturated zone towards the drain. If this concept was true and preferential transport into the drains observed an interconnected system of vertical and lateral preferred flow paths should exist. This has been shown, e.g., for some heavy clay soils Inoue, 1993, Ruland et al., 1991 or for forest soils (Luxmoore et al., 1990).

We have found preferential P transport into subsurface drains in weakly structured loamy soils (Stamm et al., 1998). Based on infiltration experiments, we concluded that vertical worm burrows are the main macropore structures in our study region. In one case, we observed lateral transport of a dye tracer and we could attribute this to a dense network of well-preserved ancient root channels in the saturated zone. Such structures are rather exceptional features. Therefore, it is not evident what the lateral preferred flow paths towards a tile drain are in weakly structured soils. The purpose of this paper is to investigate the fast transport into a subsurface drain in such a soil and to study how the lateral distance to the drains affects the fast solute transport. We carried out a sprinkling experiment on a drained grassland plot. The flow paths were assessed based on the breakthrough behavior of spatially separated tracers, hydrometric data and infiltration patterns in the soil.

Section snippets

Site description

The study site is situated in a subcatchment (“Kleine Aa”) of Lake Sempach in the central Swiss Plateau at an altitude of 585 m a.s.l. (Schweizerische Landestopographie, coordinates 659,100/221,000). The mean air temperature is about 7.5 °C and the mean annual precipitation amounts to 1200 mm. The soil has developed from glacial till (Würm glaciation) and is classified as a loamy, frigid to mesic Oxyaquic Eutrochrept (Soil Survey Staff, 1992), characterized by periodic waterlogging as well as

Spatial and temporal variation of the water table in the field

As expected, the level of the water table was substantially higher between two drains than in the immediate vicinity of the drains (Fig. 4). The drainage system was functional despite its age of about 50 years. During February 1996, snow (62.5 mm) fell on frozen ground and melted at the end of the month. The melting water raised the water table from February 6 to 28 close to the surface followed by a pronounced decrease during the dry period from February 28 to March 11. The dynamics of the

Discussion

In our experiment, solutes were quickly transported vertically as well as laterally over considerable distances of several meters. This fast transport affected not only a small percentage of the solutes but also up to 20–40% of the applied tracer mass. Because the hydrometric data showed that the contributing water volume was small—in the order of few millimeters to centimeters only—one may ask where this transport took place within the soil.

In an earlier experiment at a smaller scale (2 m2),

Conclusions

Often, surface runoff and subsurface flow are treated as separate processes in the sense that water or solutes are thought to be transported to open waters by either one of the two. Considering our findings, this clear distinction gets blurred. Instead of two parallel processes, transport may be a sequence of two where water may (first) move laterally as (near-) surface runoff that is intercepted by preferred flow paths in the vicinity of subsurface drains.

This has also consequences for the

Acknowledgements

We would like to thank the many people who helped in the field or in the laboratory: F. Denoth, H. Feyen, M. Flühler, F. Funk, G. Gal, H. Hoffmann-Riehm, J. Hollinger, A. Keller, B. Kulli, S. Lampert, H. Läser, P. Lehmann, A. Mares, R. Meuli, B. von Steiger, B. Studer and V. Vouets. R. Höfling helped us by analyzing the deuterium samples. P. Lazzarotto provided the precipitation data of the Sempach weather station. We want to thank also the farmer and landowner, Mr. Rindlisbacher, who agreed on

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    Present address: ProCert Safety AG, Thunstr. 17, 3000 Bern 6, Switzerland.

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