Thick Vadose Zone Research Articles

Analytical Modeling of Contaminant Transport through Vadose and Saturated Soil Zones

Screening level analyses for risk-based corrective actions and preliminary remediation investigations and feasibility studies for contaminated sites require simple and conservative models for the transport of contaminants to potential receptor locations. A quasi-analytical method is presented to analyze the transport of contaminants originating at a source located near the ground surface separated from the water table by a relatively thick vadose zone. The method includes one segment to compute source decay rate, a second to simulate vertical transport through the vadose zone, and a third to simulate saturated zone transport to the receptor. Practical application of the method is demonstrated by an illustrative example. The method may be useful and convenient for situations where the objectives of the analysis do not warrant the use of sophisticated numerical models.

Field survey and analysis of hillslopes on tower karst in Guilin, southern China

Limestone dissolution in tropical and subtropical humid southern China created residual hills with steep slopes, a landform that is referred to as tower karst. Two types of tower karst landform feature, fenglin or peak forest (isolated towers) and fengcong or peak cluster (linked-base towers), were identified in Guilin. Previous studies proposed two hypotheses regarding their origin and evolution. One is the sequential evolution model from peak cluster to peak forest. The other is a parallel development model, which postulates that both peak cluster and peak forest have developed simultaneously. Through detailed field survey and analysis of slope forms on tower karst in Guilin, it was found that the mean slope angle of the towers is very high (62·4°) and ranges from 60° to 75°. There is no significant difference in mean slope angle and slope angle distribution between towers in the peak cluster basin and peak forest floodplain areas. Mean slope angle increases with intensified fluvial dissection. Three levels of caves in the towers of the peak forest in Guilin were identified in previous research. The isolated towers of the peak forest as well as scattered residuals of peak cluster are generally distributed in the centre of the Guilin syncline. Favourable circumstances of allogenic water concentration indicate that development of the peak forest resulted from the combined effects of subcutaneous and subterranean dissolution as well as subsequent collapse and recession by fluvial erosion after uplifting. By contrast, peak clusters generally occur on the limbs of the syncline or at the periphery of the Guilin basin with relatively higher elevations. The thick vadose zone and predominantly vertical flow suggests that peak clusters are mainly formed by the combination of intensive uplifting and the enhancement of original dolines. The evidence of slope survey and slope analysis suggests that both isolated towers and linked-base towers developed simultaneously but by different mechanisms of formation and different combinations of development processes. Copyright © 2000 John Wiley & Sons, Ltd.

Spatial Variation in Nitrogen Isotope Values Beneath Nitrate Contamination Sources

AbstractNitrogen isotope ratios of soil water and ground water nitrate have been used to identify or implicate sources; however, lack of data on δ15N in thick (>12 m) vadose zones beneath the source and potential effects of denitrification on δ15N signatures have raised questions about the meaning of δ15N values measured in ground water. In this study, nitrogen isotope ratios (δ15N) were measured on nitrate extracted from 218 core samples removed from the surface to the water table below natural (soil organic matter) fertilizer, onsite sewage disposal systems (septic tank effluent), and animal sources located in Salinas and Sacramento Valleys, California. Additionally, spatial variability of δ15N in the horizontal plane was measured beneath an agricultural field, and δ15N values of native geologic materials (organic‐rich shales) were determined. In general, δ15N values throughout the vadose zone were consistent with the literature and remained fairly constant with depth, indicating little denitrification during transport. The δ15N values from soil organic matter sources varied from about 0 to +4‰, and the mean was not significantly different from that of fertilizer sources. The δ15N values of animal sources varied from about +8 to +20‰ and were dependent on site and animal source. The δ15N of onsite sewage disposal sources varied from about +2 to +12‰, and the mean was significantly different from that of animal sources at a 90% confidence level. δ15N of organic‐rich shales of the Panoche and Moreno Formations (Cretaceous; San Joaquin Valley) and soils derived from these rocks were generally between +4 and +8‰. Thus, it may be difficult to distinguish a geologic‐N source from septic tank or commercial fertilizer sources using δ15N. Little spatial variation was found in δ15N beneath the agricultural field in the horizontal plane.

Linear and Monte Carlo uncertainty analysis for subsurface contaminant transport simulation

Transport simulation can be used to predict subsurface contaminant distributions and the effectiveness of site remediation technologies. Because of the uncertainty inherent in subsurface transport prediction, an integral part of predictive modeling is uncertainty analysis. We have investigated the uncertainty of simulated trichloroethylene (TCE) concentrations due to parameter uncertainty and variation in conceptual model. The transport simulation is performed with T2VOC, a three‐dimensional integral finite difference code for three‐phase (gas, aqueous, non‐aqueous phase liquid), three‐component (air, water, volatile organic compound), nonisothermal subsurface flow. Our modeling is based on an actual site and considers a three‐dimensional system with a thick vadose zone (25 m) into which 35,000 kg of TCE was disposed of in surface trenches over 10 years. Subsurface transport involves TCE moving in the vadose and saturated zones from the source trench toward a nearby residence and includes the processes of advection, diffusion, and adsorption over extended distances and timescales. Uncertainties in the calculated concentrations due to variance in the major transport parameters are quantified using the inverse modeling code ITOUGH2. ITOUGH2 calculates uncertainty in the T2VOC simulation by both first‐order, second‐moment (FOSM) and Monte Carlo analyses. Significant uncertainty in simulated TCE concentrations at a site of potential human exposure is observed owing to uncertainty in permeability, porosity, diffusivity, chemical solubility, and adsorption within a single conceptual model. For the case study considered the linear FOSM analysis generally captures the uncertainty range calculated by the more accurate Monte Carlo method. Calculations also show that significant output uncertainty is introduced by conceptual model variation. Because human exposure and health risk depend strongly on contaminant concentrations, risk assessment and remediation selection based on transport simulation is meaningful only if the analysis includes quantitative estimates of transport simulation uncertainty.

Hydrologic issues in arid, unsaturated systems and implications for contaminant transport

Analysis of unsaturated flow and transport in arid regions is important, not only in water resource evaluation but in contaminant transport as well, particularly in siting waste disposal facilities and in remediating contaminated sites. The water fluxes under consideration have a magnitude close to the errors inherent in measuring or in calculating these water fluxes, which makes it difficult to resolve basic issues such as direction and rate of water movement and controls on unsaturated flow. The purpose of this paper is to review these issues on the basis of unsaturated zone studies in arid settings. Because individual techniques for estimating water fluxes in the unsaturated zone have limitations, a variety of physical measurements and environmental tracers should be used to provide multiple, independent lines of evidence to quantify flow and transport in arid regions. The direction and rate of water flow are affected not only by hydraulic head gradients but also by temperature and air pressure gradients. The similarity of water fluxes in a variety of settings in the southwestern United States indicates that vegetative cover may be one of the primary controls on the magnitude of water flow in the unsaturated zone; however, our understanding of the role of plants is limited. Most unsaturated flow in arid systems is focused beneath topographic depressions, and diffuse flow is limited. Thick unsaturated sections and low water fluxes typical of many arid regions result in preservation of paleoclimatic variations in water flux and suggest that deep vadose zones may be out of equilibrium with current climate. Whereas water movement along preferred pathways is common in humid sites, field studies that demonstrate preferential flow are restricted mostly to fractured rocks and root zones in arid regions. Results of field studies of preferential flow in humid sites, generally restricted to the upper 1–2 m because of shallow water tables, cannot be applied readily to thick vadose zones in arid regions.

Natural Gradient, Surface-Applied Tracer Test at the Ohio Management Systems Evaluation Area

A tracer test was conducted from spring 1991 to winter 1995 to calculate travel times of agrichemicals surface applied in 1991 through a 1.0- to 5.4-m thick vadose zone to an unconfined aquifer. Bromide was applied on a 2.6 hectare agricultural plot coincident with the application of nitrogen, atrazine, and alachlor in the corn phase of a corn-soybean rotation in May 1991. Soil-water and ground-water samples were collected from lysimeters, soil-core extractions, and wells. Transport velocities in the soil based on the conservative behavior of bromide as observed in breakthrough curves from lysimeters were between 60 and 80 cm/year. Because the test occurred under natural recharge conditions, transport velocities varied over the five-year period depending on natural variations in precipitation and the associated transient infiltration rates. The center of mass of the bromide pulse intercepted the water table four years after application. Bromide remained in soil above the water table six years after application. There were no detections of alachlor, atrazine, or the two degradation products of atrazine, desethylatrazine and deisopropylatrazine at the water table during the six-year period of monitoring. This suggests that despite the ability of conservative species to travel to the water table at this site, the pesticides were attenuated in the vadose zone.

Soil‐water flux in the Southern Great Basin, United States: Temporal and spatial variations over the last 120,000 years

The disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil‐water flux and recharge. Soil‐water chemistry and isotopic data are presented from three deep vadose zone boreholes (>230 m) at the Nevada Test Site, located in the Great Basin geographic province of the southwestern United States, to quantify soil‐water flux and its relation to climate. The low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates. Tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated. Stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20,000 to 120,000 years. Secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima, when the climate of the area was considerably wetter and cooler. The stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation, suggesting that recharge under the current climate is not occurring at this arid site. Past and present recharge appears to have been strongly controlled by surface topography, with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated. The data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and, in particular, show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial maxima.

Mesocosms for Subsurface Research

Abstract This article provides an overview of mesocosm research applied to investigations of subsurface processes. The nature of mesocosms and the mesocosm concept are defined and discussed as are the advantages and disadvantages of these systems. In general, a mesocosm meets the requirements of being of sufficient size to contain all the components of interest while permitting “natural” behaviour. It is implicit in the use of mesocosms that they act as a surrogate for the real ecosystem. However, the precise size of a mesocosm is difficult to define without a specific context. Details are provided on the construction, filling and instrumentation of subsurface mesocosms. This article also provides recent examples of applications of mesocosms to studies of biogeochemistry, waste disposal and fate and transport of a herbicide. In addition, a comparison of physical, chemical and microbial data from a mesocosm (diameter 2.44 m × 4.6 m) with a 3-m thick vadose zone and water table and its associated field site is made. Mesocosms provide the advantages of controlled environment, near steady-state conditions, and environmentally acceptable testing of the fate and effects of toxics. However, considerable research is required to optimize their size, instrumentation and expand their application to in situ conditions as well as the laboratory.

Water quality changes during soil aquifer treatment of tertiary effluent

This paper summarizes the results of field studies in Tucson, Arizona, to estimate the soil aquifer treatment (SAT) effectiveness of a 5.7‐ha (14‐acre) water spreading facility, the Sweetwater Underground Storage and Recovery Facility. Groundwater samples collected from the facility during 1989 to 1990 were analyzed for pathogens. A specific basin was selected during the 1990 to 1991, 1991 to 1992, and 1992 to 1993 recharge seasons for sampling source water (tertiary effluent), pore‐liquid samples from the vadose zone, and groundwater during recharge. These samples were analyzed for the nitrogen species, dissolved organic carbon (DOC), and total organic halide (TOX). The results showed that the site provides effective SAT. Enteroviruses were completely removed during travel in the 37‐m (120‐ft) thick vadose zone. No Giardia were detected in any of the groundwater samples. The DOC and TOX were reduced by 92% and 85%, respectively. After leaching of indigenous nitrogen from the vadose zone, total nitrogen was reduced by approximately 47% during recharge. Near‐surface anaerobic conditions promote denitrification.

Analysis of groundwater flow and travel times for a landfill site in an arid region with a thick vadose zone

AbstractGroundwater flows and travel times were analyzed under a landfill site characterized by a thick unsaturated zone, in an arid environment, using two‐dimensional models. Transport in the unsaturated zone is important in arid environments with thick vadose zones and usually determines the travel time of groundwater from the landfill site to the regulatory compliance surface. The combined use of a parallel flow (Dupuit‐Forchheimer) areal model and vertical cross‐sectional model is explored at a site typical in the utility industry. Flow paths and travel times to the compliance zone are obtained for use in landfill site design or environmental impact assessments. Sensitivity analysis shows that boundary conditions are important factors influencing the entire analysis and thus need to be confirmed with field measurements.

Application of VLEACH to Vadose Zone Transport of VOCs at an Arizona Superfund Site

AbstractCleanup standards for volatile organic compounds in thick vadose zones can be based on indirect risk (transport to ground water) when contamination is below depths of significant direct risk. At one Arizona Superfund site, a one‐dimensional vadose zone transport model (VLE‐ACH) was used to estimate the continued transport of VOCs from the vadose zone to ground water. VLEACH is a relatively simple and readily available model that proved useful for estimating indirect risk from VOCs in the vadose zone at this site. The estimates of total soil concentrations used as initial conditions for VLF.ACH incorporated a variety of data from the site. Soil gas concentrations were found to be more useful than soil matrix data for estimating total soil concentrations at this arid‐zone site. A simple mixing cell model was used with the VLEACH‐derived mass loading estimates from the vadose zone over time to estimate the resulting changes in ground water concentrations. For this site, the results of the linked VLEACH/mixing cell simulations indicate it is likely that the federal MCI. for TCE will be exceeded in underlying ground water if remedial action on I he vadose zone is not pursued.