G. V. Wilson

Physical and chemical controls of preferred path flow through a forested hillslope

Abstract Investigations of the physical and chemical characteristics of macropores and mesopores at two forested sites established for subsurface transport research are summarized. The hydrologically active macroporosity (pores larger than 1 mm diameter) is a very small fraction of the total soil porosity but is sufficient to conduct a large proportion of ponded infiltration. Mesopores (pores less than 1 mm diameter that are generally drained at field capacity) are sufficient to conduct total infiltration during the majority of rain events and have a much higher surface area than macropores. The cation adsorption coefficient of macropore walls was similar to bulk soil for mineral coatings but was higher for organic linings derived from roots. The subsurface outflow from a 0.46 ha subwatershed generated by two precipitation events displayed two contrasting patterns of chemical concentration. At the time of peak discharge, the concentration of Na, K, Mg, Ca and S was close to maximum (high supply); and for Al, Fe and Mn, chemical concentration was highest prior to peak discharge (low supply). A pathlength-supply hypothesis is proposed, based on (1) changing proportions of mesopore flow path lengths with stage of the subsurface hydrograph and (2) the ability of mesopore surfaces and adjacent micropores to continue supplying chemicals to percolating soil water. Macropores are viewed as being important physical conduits in convergent flow zones of watersheds but as having little influence on water quality.

A multiple-pore-region concept to modeling mass transfer in subsurface media

Abstract Recent studies in soil science literature have strongly indicated the need to incorporate pore structures in near-surface mass transport modeling. There is increasing evidence suggesting that pore structures, such as fractures and macropores, facilitate the transport of water and solutes along a preferential flow path while water and solutes are moved into micropores and rock matrices concurrently. This study presents a conceptual model, a multiple-pore-region (or multi-region) concept, to account for pore structures as well as the resultant widely distributed pore water velocities in macroporous media. Pore regions can either be physically identified as discrete features, such as fractures and rock matrices, or be experimentally determined by separation of water retention curves according to pore classification schemes. A multi-region mechanism is proposed to account for the effect of local-scale and field-scale heterogeneities on mass transport under variably saturated conditions. Two numerical codes for subsurface fluid flow and solute transport have been developed with the multi-region concept, in which a firstorder mass exchange model is adopted to simulate the redistribution of pressure heads and solute concentrations among pore regions. The computer codes are used to demonstrate the applicability of the concept to fractured porous media, and to test a three-pore-region hypothesis using laboratory soil column tracer injection data. Based upon the parameters obtained from fitting multi-region and mobile-immobile models to these data, we successfully demonstrated that the former model has the advantage of maintaining consistent conceptual models over the latter under variably saturated conditions.

Hydrology of a forested hillslope during storm events

Abstract Subsurface flow from a 0.47 ha hillslope subcatchment at the upper most reach of the West Fork (38 ha) of Walker Branch Watershed was monitored throughout 1988. Stormflow in the first-order stream draining the West Fork responded simultaneously with the initiation of subsurface flow through 0 to 2.5 m depth from the hillslope. Subsurface flow from the upper hillslope was only 1.3% of total precipitation for 1988, however, individual storms yielded as much as 12.5% of precipitation as subsurface flow. The West Fork had 10.4% of precipitation observed as storm flow with an additional 29% as baseflow. Individual storms exhibited up to 71% of precipitation as storm flow in the West Fork, illustrating the significant increase in percent rain converted to subsurface flow in lower landscape positions as compared to upper hillslopes. Peak subsurface flow preceded peak streamflow and occurred primarily through the 1.0–2.5 m depth (Bt2 horizons) due to perched water tables. Preferential flow from hillslopes through macro- and mesopores was concluded to be the predominant stormflow mechanism. The contributions of the various size pores was reasoned to be dependent upon the position within the soil profile and the stage of the hydrograph. Preferential flow occurred at extreme soil water conditions with the response due to perched water tables under wet conditions and believed to be due to hydrophobic conditions under extremely dry conditions. Intermediate soil water conditions resulted in infiltration into soil matrix pores precluding subsurface stormflow.

Quantifying relative contributions from sediment sources in Conservation Effects Assessment Project watersheds

A technique using the relationship between the naturally occurring radionuclide tracers, 7Be and 210Pbxs, was used to differentiate eroded surface soils and channel-derived sediments in the fine suspended sediment loads of runoff events in five Conservation Effects Assessment Project watersheds. A simple two end-member mixing model was used to determine the relative contribution from each source. Results suggest that eroded surface soils were more prevalent in the suspended load early in a runoff event, but channel contributions dominated the suspended load at later stages. The method proved useful for multiple sites due to a constant proportion of the atmospheric deliveries of the two radionuclides globally. Use of only two radionuclide tracers simplifies the differentiation of sediment sources within a watershed but limits precision.

Unsaturated solute transport through a forest soil during rain storm events.

The unsaturated transport of Br− through two distinct pore classes of an undisturbed pedon was investigated as a function of time and profile depth for a series of intermittent rain events. Water movement was monitored with tensiometers and a neutron probe and solute was collected in coarse and fine fritted glass solution samplers maintained at 2 and 10 kPa suction, respectively. The coarse samplers collected soil solution held at tensions of 0 to 2 kPa and are believed to have monitored predominately macropore and mesopore channels (large pores). The fine samplers are thought to have collected soil solution from predominately matrix mesopores (small pores) which are assumed to be approximately the lower pore size limit for gravitationally mobile water in this soil. The rate of tracer movement through large and small pores was dependent on the intensity and duration of a particular rain event as well as the antecedent soil water content. Small pores retained water and solutes longer and had a greater operational storage for ions than large pores. The data suggested that during the vertical flux of water through the soil profile, solutes were transported by convection and diffusion from small-pore regions to large-pore regions via hydraulic and concentration gradients, respectively, with small pores being a major source for solute transport in large pores. Because of physical and chemical nonequilibrium during rain events, solutes diffused from micropores (believed to have matric potentials > 10 kPa) into large and small pores. Solutes which were rapidly transported to greater depths through large pores increased the solute reserves in adjacent small pores at the same depths through a reversal of the mechanism described above. Solute reserves in small pores were also fed by significant vertical transport processes through this pore class.

Hydrogeochemical processes controlling the transport of dissolved organic carbon through a forested hillslope

Abstract The subsurface transport of dissolved organic carbon (DOC) through a proposed waste burial site during rain events was investigated in order to assess the role of colloid-mediated contaminant mobility. A sub-watershed (0.45 ha) located on a forested hillslope in eastern Tennessee, U.S.A., was instrumented with an isolated soil pedon for one-dimensional transport studies, and a subsurface weir monitoring system for three-dimensional transport studies. The source of DOC in the soils resulted from dissolution of organic matter in the surface horizon during, and between, rain events, as well as from a highly reactive B horizon which stored significant quantities of DOC in small pores. During large storms, the concentration of DOC was similar on ascending and descending limbs of the subface hydrograph, with a maximum concentration occuring at peak flow. During small storms however, chemical interactions with soil solution SO 4 2- caused DOC concentrations to be greater on the ascending limb of the hydrograph, with maximum DOC concenrations occuring before peak flow. Because subsurface lateral flow through preferential paths predominated in the Bt2 and the Bt3 horizons of the soil during storm events, the total cumullative flux of DOC downslope was generally much greater through the lower soil horizons. A significant component of mobile DOC consisted of hydrophobic organic solutes, even though this material was selectively adsorbed with soil depth relative to hydrophilic organic solutes. The implications of these findings on subsurface contaminant transport are discussed.

Using a Multiregion Model to Study the Effects of Advective and Diffusive Mass Transfer on Local Physical Nonequilibrium and Solute Mobility in a Structured Soil

Waste management problems for shallow land burial facilities in the humid eastern United States are usually complicated by slow but continuous movement of wastes through the soil matrix and discrete but rapid pulses of wastes through macropores and fractures. Multiple-pore-region models employed to describe flow and solute transport in the soils usually consist of multiple mass transfer coefficients that cannot be measured experimentally, and their effects on subsurface mass transport are poorly understood. The objective of this research was to study the individual and concurrent effects of interaggregate advection and diffusion on mass transport in a structured soil. The interactions of these two mass transfer processes and local solute concentration equilibrium are examined for a heterogeneous soil. Pore region water retention, hydraulic conductivity, and dispersivities, obtained from independent measurements and published calibration results, were used to test a novel three-pore-region, one-dimensional numerical model. Advective and diffusive mass transfer coefficients were estimated using mass transfer equations and fracture spacings published in the literature. The mass transfer coefficients were then varied systematically, and the sensitivity of local fluid pressure and solute concentration nonequilibrium to interregion mass transfer were analyzed. Our results indicated that time-dependent interaggregate advection and diffusion were important processes controlling solute mobility in heterogeneous media. Under transient flow conditions, interaggregate advection may reduce the significance of interaggregate diffusion that otherwise dominates interaggregate mass transfer under steady state conditions. Nonetheless, the equilibrium of local solute concentrations was 20 times more sensitive to diffusive mass transfer than to advective mass transfer, which suggests that site characterization efforts should be directed more toward the former process. Unfortunately, characterization efforts of this type are not commonplace and if available are frequently ignored because they add a difficult reality to complex waste management problems. Since advective and diffusive mass transfer may be important processes limiting the efficiency of cleanup activities such as pump and treat, it is perhaps time to include the characterization of these processes and quantification of the timescale of physical nonequilibrium in site remediation efforts.

Field-scale transport from a buried line source in variably saturated soil

Lateral subsurface flow through the upper soil layers (stormflow zone) during storm events has been shown to be a dominant mechanism of contaminant transport in forested watersheds. Data bases for multi-region flow and transport modeling for hydrogeologic conditions where stormflow predominates are lacking. Direct measurement of the tracer flux under field-scale conditions are non-existent. The objective of this paper was to evaluate the significance of three hydrologic pore regions to stormflow. Two tracer releases were made from a buried line source during storm events and the spatial and temporal variability in solute concentration and the tracer fluxes were measured. During one of the injections, macropore flow was extremely rapid with solute transport to a downslope trench 65 m from the line source taking just 3.2h. Mesopore flow appeared to be significant for short distances in that tracer movement to solution samplers just 3 m downslope of the line source occurred within 3 h of the release. Soil sampling 6 months after the second release revealed that the tracer plume was refracted in the direction of the fractured bedding plane, and therefore did not coincide with the array of samplers for distances greater than 13 m downslope of the source. Soil sampling data suggested that micropores served as a sink/source for Br− with 47% of the non-reactive tracer remaining immobilized by micropores at the termination of the study. Interaction between the upper 2 m of the stormflow zone and the groundwater system was believed minimal; however, lateral flow below 2 m was concluded to be significant.

TILLAGE AND RESIDUE EFFECTS ON RUNOFF AND EROSION DYNAMICS

The carry-over effects from one year to the next of surface residue and tillage management decisions on runoff and erosion are not clear. The dynamics of runoff and erosion processes during rainfall events are likely dependent on the tillage and residue management system. The objective of this study was to elucidate the effects of tillage practices and residue management, by removal of residue cover, on the properties that describe the dynamics of the runoff and erosion processes. Six-row, 12.2 m long . 5.5 m wide, plots under conventional tillage (CT) or no tillage (NT) corn (Zea mays L.) for nine years were used in this study. Plots had an average slope of 5.7% on a Grenada silt loam (Glossic Fragiudalf) soil. Rainfall simulations were conducted on a 10.7 m . 3.7 m area within each plot at a rate of 65 mm h -1 for 1 h under natural antecedent soil-water conditions (dry run), followed by a 0.5 h simulation 4 h later (wet run), and another 0.5 h application 30 min later (very wet run). The ten treatments consisted of an incomplete 2 . 2 . 3 factorial arrangement of two tillage histories (CTh and NTh), two tillage levels (tilled and not tilled), and three residue management levels (residue left, residue removed just prior to simulated rainfall, and residue removed one year prior to simulated rainfall). The missing treatments were the NTh-tilled and CTh-not tilled with residue left. The time of runoff initiation, maximum runoff rate, flow velocity, and maximum sediment concentration were used to describe differences in runoff and erosion dynamics. Residue removal resulted in significantly sooner runoff with the NT system. There was a significant carry-over effect of residue removal with runoff initiated 35% sooner the subsequent year of removal and sediment concentrations increasing by >100%. Maximum sediment concentrations were lower for the CTh land that was not tilled than for the tilled despite the untilled land experiencing sooner runoff and higher runoff rates. Tilling NTh land resulted in significantly lower sediment concentrations than tilling CTh land, suggesting that the soil quality of NT was not immediately lost when tilled, but these beneficial properties were fully lost within one year of residue removal.

History, residue, and tillage effects on erosion of loessial soil

Studies have shown that no-till (NT) management reduces soil erosion relative to chisel/disk-tillage (CT) and that this benefit may increase over time. There are fewer data, however, to separate the erosion-reduction contributions of surface residue mulch from those of improved soil properties under NT. The objective of this study was to quantify these separate contributions for a silt loam soil (Glossic Fragiudalf) used for corn (Zea mays L.) production in northern Mississippi for five to ten years with either CT or NT. The experiment had ten treatments. Two were normal CT and NT managements in which a crop was planted but had not emerged prior to simulated rainfall. The other eight treatments had surface crop residues removed and comprised a 2 × 2 × 2 factorial arrangement of two tillage histories (CTh or NTh), two levels of tillage immediately prior to rainfall simulation (disturbed or not disturbed), and two levels of residue removal (residue removed just prior to simulated rainfall or residue removed one year prior to simulated rainfall). Simulated rainfall was applied at a rate of 65 mm h-1 in a three-storm sequence on 10.7 × 3.7 m areas. NT exhibited greater runoff but much lower sediment losses than CT. Residue removal doubled erosion for both tillage histories. Surface disturbance decreased runoff from the first storm following tillage but increased total soil loss 26% to 47%. With residues removed, long-term NTh resulted in one-third the soil loss of CTh, and similar benefits were observed with or without surface disturbance. This residual benefit of NTh was lost within one year of fallow after residue removal. These results demonstrate that the erosion resistance of NT areas is due to both residue cover and improved soil quality factors. Although the erosion-resisting soil quality factors developed over several years of NT management may be lost within a single year of fallow management, these factors may protect the soil from excessive erosion if NT fields that must occasionally be tilled are quickly returned to NT management.