Unsaturated zone depth

Tritium extracted from soil moisture in the unsaturated zone at various depths below the surface has been used to infer the progress of recharge of underlying aquifers. Studies of recharge in arid and semi arid zones where water moves very slowly in a downward direction have been particularly instructive [20, 32-35]. 

Wood, W. 1973. A technique using porous cups for water sampling at any depth in the unsaturated zone. Water Resources Research, 9, 486-488. 

Kerfoot [6] examined the performance of a grab sampling technique for soil-gas measurement analyses, at a site with groundwater known to be contaminated with chloroform. The study assessed the correlation between soil-gas and groundwater analyses with chloroform as a model volatile organic compound. Chloroform concentration in soil gas increased linearly with depth in the unsaturated zone. 

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... 

The maximum depth of liquid hydrocarbon penetration into the unsaturated zone can be estimated by the following equations ... 

DBCP has been found In ground water In Hawaii, California, Arizona, South Carolina, and Maryland (5-7,19,77-79). Typical positives are 0.02-20 ppb. Areas with the highest frequency of positives and the highest well concentrations are the San Joaquin Valley In California and the region southwest of Phoenix, Arizona. The Hawaii contamination has occurred despite several hundred feet of overburden between the basal aquifer and the surface. One set of California soil core results show that ppb amounts of DBCP has leached about 15 m through the unsaturated zone ( ), whereas DBCP was not detected In another set of California soil cores sampled as deep as 10 m and five years after the last application (80). The latter results can possibly be explained by rapid movement of DBCP down the soil profile to depths greater than 10 m. 

The primary application of the BERT technology will likely be to complement active soil vapor extraction efforts by removing residual contaminants after active methods become insufficient. It could also be used on the edge of unsaturated zone contaminant plumes where concentrations of volatile contaminants are low or for enhancement of bioremediation activities. The primary advantages of the technology application are low capital costs and minimal operating costs. The system is well suited for applications in low-risk contaminant settings, where rapid response and remediation are not necessary. Suitable applications include volatile contaminants at relatively shallow depths (less than 20 ft) in the vadose zone, such as ... 

The unsaturated zone, where most of the low-level waste is buried, could probably be used for high-level waste after cooling but it does not lend itself to the regional approach to defining permeability. Additionally, the laboratory methods of evaluating permeability of unsaturated material are extremely difficult to perform consistently. However, an air permeability technique that may prove useful in evaluating the zone between the surface and the water table was developed by Weeks W. The technique uses normal barometric fluctuations measured at different depths in the formation to determine permeability to air which can be used to determine permeability to water. 

According to Box 18.5, Eq. 13, the breakthrough time depends on D,eff, that is, on the mobile fraction of the chemical. Here this fraction is given by fia = 0.05. The layer thickness is equal to the depth of the unsaturated zone, 5 = 4 m. 

Figure 5. Position of subsurface numerical markers during an atmospheric pressure cycle. Simulation is of a 60-m unsaturated zone with air permeability of 10-" m2. Note that soil gas initially deeper than 1,75 m never reaches the surface while soil gas at shallower depths would be flushed from the system. Reprinted from Auer et al. (1996), Copyright 1996, pg. 152, with permission from Elsevier Science.

The initial conditions were typically a mean pressure throughout the depth of the unsaturated zone, but any type of initial pressure profile could be chosen as the initial condition. 

Figure 10. Air pressure at land surface and three depths in the unsaturated zone as a function of time for the August and October 1996 sampling events. Symbols are only used to help distinguish between lines and do not necessarily represent actual data points. Actual data points were collected at 5-minute intervals for all subsurface depths and hourly for surface pressure.

Fig. 42 Vertical cross section of tritium in the unsaturated zone (0-20 m depth) for the time intervals 10, 20, 30, 40 years assuming an impulse-like input during the first ten years...

The concentration of natural, dissolved organic carbon (DOC) content in the soil moisture of shallow soils is often 5 to 10 ppm. This DOC may percolate downward in the unsaturated zone, perhaps in association with colloidal-sized clay particles. Its aerobic bacterial oxidation can produce increasing amounts of CO2 with depth (cf. Wood and Petraitis 1984 Wood 1985). 

Whereas in humid environments the Na is flushed from the soil and unsaturated zones, in more arid climates the concentration of Na in the ground-water is high due to the precipitation of calcite, cation exchange, and the higher ionic mobility of Na with respect to Ca, and Na-montmoriUonite tends to be the stable solid phase in lowland areas and with increasing depth in the groundwater system. 

As the name of the model implies, PRZM models the unsaturated zone, which includes the root zone and a user-specified depth below the root zone within the "vadose" zone. The simulation uses a daily... 

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