Tammo S. Steenhuis

MOBILITY AND SOLUBILITY OF TOXIC METALS AND NUTRIENTS IN SOIL FIFTEEN YEARS AFTER SLUDGE APPLICATION

The increased use of sewage sludge as a fertilizer, combined with reports that large fractions of sludge-borne heavy metals cannot be accounted for several years after land application, indicates that more detailed study of potential mobility of these metals in soils is needed. A field site that

Mechanism for finger persistence in homogeneous, unsaturated porous media: Theory and verification

Wetting-front instability, or gravity-driven fingering, can occur during vertical infiltration. Previous studies found that fingers persist over long periods of constant infiltration and in subsequent infiltration cycles. We present a physically based theory to explain finger persistence. The theory is tested using a new technique in which moisture contents are visualized by the transmission of light through unsaturated sand, recorded with a video camera, and then analyzed by video digitizing computer hardware. Theory and experiments show that hysteresis in the moisture characteristic relationship explains the persistence of fingers in time.

A GIS-based variable source area hydrology model

Effective control of nonpoint source pollution from contaminants transported by runoff requires information about the source areas of surface runoff. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runoff in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation-excess overland flow (i.e., surface runoff), and interflow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed stream flow hydrographs and soil moisture measurements were compared to model predictions. Stream flow prediction during non-winter periods generally agreed with measured flow resulting in an average r2 of 0·73, a standard error of 0·01 m3/s, and an average Nash-Sutcliffe efficiency R2 of 0·62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. The model results were most accurate for non-winter conditions. The model is currently used for making management decisions for reducing non-point source pollution from manure spread fields in the Catskill watersheds which supply New York Citys drinking water. Copyright

Modeling and field evidence of finger formation and finger recurrence in a water repellent sandy soil

With prolonged rainfall, infiltrating wetting fronts in water repellent soils may become unstable, leading to the formation of high-velocity flow paths, the so-called fingers. Finger formation is generally regarded as a potential cause for the rapid transport of water and contaminants through the unsaturated zone of soils. For the first time, field evidence of the process of finger formation and finger recurrence is given for a water repellent sandy soil. Theoretical analysis and model simulations indicate that finger formation results from hysteresis in the water retention function, and the character of the formation depends on the shape of the main wetting and main drainage branches of that function. Once fingers are established, hysteresis causes fingers to recur along the same pathways during following rain events. Leaching of hydrophobic substances from these fingered pathways makes the soil within the pathways more wettable than the surrounding soil. Thus, in the long-term, instability-driven fingers might become heterogeneity-driven fingers.

Preferential Paths of Flow under Conventional and Conservation Tillage

Andreini, M.S. and Steenhuis, T.S., 1990. Preferential paths of flow under conventional and conservation tillage. Geoderma, 46: 85-102. Preferential solute movement through conservation and conventional tillage soil profiles was investigated by applying bromide and dye tracers to undisturbed soil columns at an application rate which did not create ponding or runoff. In both tilled and untilled soils preferential flows were established with much of the soil well below saturation. Spatial variations in the solute flows were observed by monitoring the column effluent flowing from a grid lysimeter. Dye in effluent flowing at lower water velocity was more retarded than dye traveling in higher velocity effluent.

Physics of water repellent soils

Although it is generally well known that water repellent soils have distinct preferential flow patterns, the physics of this phenomenon is not well understood. In this paper, we show that water repellency affects the soil water contact angle and this, in turn, has a distinct effect on the constitutive relationships during imbibing. Using these constitutive relationships, unstable flow theory developed for coarse grained soils can be used to predict the shape and water content distribution for water repellent soils. A practical result of this paper is that with a basic experimental setup, we can characterize the imbibing front behavior by measuring the water entry pressure and the imbibing soil characteristic curve from the same heat treated soil. q 2000 Elsevier Science B.V. All rights reserved.

Wetting front instability as a rapid and far-reaching hydrologic process in the vadose zone

Abstract Wetting front instability in the vadose zone causes the formation of fingers which can rapidly transport both water and solute to the phreatic surface. The development of the unstable flow field in laboratory experiments is described for an initially dry, two-layer sand system, in which the top layer has a finer texture than the bottom layer. The effect of repeated infiltration cycles and of initial moisture content at field capacity are presented. Fingers once formed in the dry porous media are found to not change location even after several infiltration events. Only saturation and subsequent drainage alters the finger structure within the chamber. In Eastern Long Island, New York, USA, field infiltration experiments using the combination of two dyes showed that water moved through finger-like structures.

Effect of sludge-processing mode, soil texture and soil pH on metal mobility in undisturbed soil columns under accelerated loading.

The effect of sludge processing (digested dewatered, pelletized, alkaline-stabilized, composted, and incinerated), soil type and initial soil pH on trace metal mobility was examined using undisturbed soil columns. Soils tested were Hudson silt loam (Glossaquic Hapludalf) and Arkport fine sandy loam (Lamellic Hapludalf), at initial pH levels of 5 and 7. Sludges were applied during four accelerated cropping cycles (215 tons/ha cumulative application for dewatered sludge; equivalent rates for other sludges), followed by four post-application cycles. Also examined (with no sludge applications) were Hudson soil columns from a field site that received a heavy loading of sludge in 1978. Romaine (Lactuca sativa) and oats (Avena sativa) were planted in alternate cycles, with oats later replaced by red clover (Trifolium pratense). Soil columns were watered with synthetic acid rainwater, and percolates were analyzed for trace metals (ICP spectroscopy), electrical conductivity and pH. Percolate metal concentrations varied with sludge and soil treatments. Composted sludge and ash had the lowest overall metal mobilities. Dewatered and pelletized sludge had notable leaching of Ni, Cd and Zn in Arkport soils, especially at low pH. Alkaline-stabilized sludge had the widest range of percolate metals (relatively insensitive to soils) including Cu, Ni, B and Mo. Old site column percolate concentrations showed good agreement with previous field data. Little leaching of P was observed in all cases. Cumulative percolate metal losses for all treatments were low relative to total applied metals. Leachate and soil pH were substantially depressed in dewatered and pelletized sludge soil columns and increased for alkaline-stabilized and ash treatments.

Tile drain sampling of preferential flow on a field scale

Abstract Preferential flow phenomena generally exhibit a high degree of spatial variability. In an alternative to geostatistl approaches, which aggregate many small-scale sample results, tile drains were explored as a means to measure solute movement on a scale which integrates spatial variability within the sampling volume. To evaluate the effectiveness of this method, a mass balance accounting of a conservative tracer, chloride, was attempted over a seven-month period. Analysis of the drain effluent indicated rapid breakthrough of surface-applied chloride, with concentrations rising within four hours of rainfall initiation. This traveltime was over two orders of magnitude more rapid than that predicted by homogenous flow models, indicating the importance of preferential flow in the transport of solutes through this field. Close agreement between the amount of chloride initially applied and the cumulative chloride recovered by the end of the experiment indicates the ability of tile drains to intercept solute movement.

Metal mobility at an old, heavily loaded sludge application site.

This study was undertaken to determine the present distribution and mobility of sludge-applied metals at an old land application site. Trace metals concentrations were determined for soils (using 4 M HNO3 extracts), soil leachates (collected with passive wick lysimeters over a 2.5-year period), and plant tissue from a field site which received a heavy loading of wastewater sludge in 1978 and an adjacent control plot. Blue dye was used to indicate preferential percolate flowpaths in the sludge plot soil for sampling and comparison with bulk soil metals concentrations. After nearly 20 years, metals in the sludge plot leachate were found at significantly greater concentrations than in the control plot, exceeding drinking water standards for Cd, Ni, Zn, and B. Annual metals fluxes were only a fraction of the current soil metal contents, and do not account for the apparent substantial past metals losses determined in a related study. Elevated Cd, Cu, and Ni levels were found in grass growing on the sludge plot. Despite heavy loadings, fine soil texture (silty clay loam) and evidence of past and ongoing metals leaching, examination of the bulk subsoil indicated no statistically significant increases in metals concentrations (even in a calcareous subsoil horizon with elevated pH) when comparing pooled sludge plot soil profiles with controls. Sampling of dyed preferential flow paths in the sludge plot detected only slight increases in several metals. Preferential flow and metal complexation with soluble organics apparently allow leaching without easily detectable readsorption in the subsoil. The lack of significant metal deposition in subsoil may not be reliable evidence for immobility of sludge-applied metals.