Vadose zone fields

A Desk Reference Guide (EPA/R-93/003b), Vol. 1 Solids and Ground Water Vol. 2 The Vadose Zone, Field Screening and Analytical Methods... 

R. N. Miller, "A Field-Scale Investigation of Enhanced Petroleum Hydrocarbon Biodegradation ia the Vadose Zone Combining Soil Venting as an Oxygen Source with Moisture and Nutrient Addition," doctoral dissertation submitted to the Civil and Environmental Engineering Department,... 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

The airflow equations presented above are based on the assumption that the soil is a spatially homogeneous porous medium with constant intrinsic permeability. However, in most sites, the vadose zone is heterogeneous. For this reason, design calculations are rarely based on previous hydraulic conductivity measurements. One of the objectives of preliminary field testing is to collect data for the reliable estimation of permeability in the contaminated zone. The field tests include measurements of air flow rates at the extraction well, which are combined with the vacuum monitoring data at several distances to obtain a more accurate estimation of air permeability at the particular site. 

A field study was conducted by Larson et al. [31] to characterise the impact of effluent discharges on a sandy soil about 0.5 m below the surface. A 2.5 m thick unsaturated zone and a 3—4 m thick unconfined sand/gravel aquifer underlaid the tile field. LAS concentrations in the effluent plume decreased over a distance of 10 m from 10 000 to 30 xg L-1. A further object of study was a laundromat pond exposed to LAS-containing sewage for more than 25 years. A clay layer separated the natural pond from the vadose zone made up of porous sand. Measurements of LAS levels as a function of soil depth beneath the pond showed a rapid decrease from about 220 mg kg-1 at 30 cm to... 

Moisture Content/Field Capacity Within the vadose zone, moisture content is important since microbial growth is limited by excessively wet or dry soil. Moisture content, expressed as a percentage of the field (or holding) capacity, indicates the ratio of moisture to air in the soil. The recommended range for optimal growth is between 40 and 70%. 

Table IV. Texture, Alkalinity and Moisture Contents of Soil Sanples (Oct. 27-28, 1981) from the Vadose Zone In Potato Fields and Aldlcarb Residue Concentrations (ppb) In the Soil and Soli Solution <"H20" In Last (blumn).

Soil samples collected from the vadose zone also show a variety of aldicarb residue concentration profiles (Table IV). In view of the low Kd value for aldicarb in sandy soils (22) it can be assumed that any aldicarb residues detected in soil samples are entirely dissolved in the soil moisture. If soil residue values are recalculated on a soil-moisture basis, the resulting peak concentrations are closer to those observed in the groundwater or leachate from the lysimeters. However, the pattern of residue concentrations in the vadose zone is also highly erratic and bears no apparent relation to soil texture, moisture content or alkalinity (Table IV). Equally erratic concentration distributions were observed for samples collected on other dates in the same year. Similarly erratic results were obtained for different soil profiles from Long Island potato fields (22). 

Kung, K.-J. S. Preferential flow in a sandy vadose zone 1. Field observation. Geoderma 46, 51-58. 1990. 

Gee, G. W. Ward, A. L. Vadose Zone Transport Field Study Status Report. U.S. Department of Energy Richland, WA, 2001 PNNL-13679. 

In a typical field system, as depicted in Fig. 14.1, compressed air is delivered to a manifold system, which in turn delivers the air to an array of air injection wells. The wells inject the air into the subsurface below the lowest known point of contamination. Due to buoyancy, the air will begin to rise toward the surface and, through a variety of mechanisms, will either remove or assist in degradation of the contamination. As the contaminant-laden air rises towards the surface, it will eventually reach the vadose zone. At this point, either the contaminated air may be collected with a soil vapor extraction (SVE) system or, if vadose zone... 

Hornsby, A. G., P.S. C. Rao, W., B. Wheeler, P. Hkedi-Kizza, R. L. Jones, Fate of Aldicarb in Florida Citrus Soils 1. Field and Laboratory Studies, presented at Characterization and Monitoring of the Vadose (Unsaturated) Zone, Las Vegas, December 8-10, 1983. 

---