Soil moisture content

Volatilization. The susceptibility of a herbicide to loss through volatilization has received much attention, due in part to the realization that herbicides in the vapor phase may be transported large distances from the point of application. Volatilization losses can be as high as 80—90% of the total applied herbicide within several days of application. The processes that control the amount of herbicide volatilized are the evaporation of the herbicide from the solution or soHd phase into the air, and dispersal and dilution of the resulting vapor into the atmosphere (250). These processes are influenced by many factors including herbicide application rate, wind velocity, temperature, soil moisture content, and the compound s sorption to soil organic and mineral surfaces. Properties of the herbicide that influence volatility include vapor pressure, water solubility, and chemical stmcture (251). 

Alkali metal dechlorination processes are water sensitive. Soil moisture content can adversely affect the reaction in in-situ soil applications. The application of alkali metal dechlorination is generally expensive. 

The persistence of the N-nitrosamine that may be formed in soil will depend on a host of conditions, such as soil type, organic matter content, clay content, pH, the microflora present in the soil, moisture content and temperature, etc. Superimposed on all these factors will be the chemical nature of the pesticide. The N-nitrosoatrazine ( ) formed in soil from the herbicide atrazine ( ) was shown to be rapidly disappeared (1). Thus, in soil W-nitrosoatrazine was observed after one week, but was absent 4 and 10 weeks later (Table IV). In contrast, N-nitroso-butralin (11 ) persisted much longer than N-nitrosoatrazine (9) under the same conditions (Table V) and was still detectable after 6 months (3). Our studies demonstrated that N-nitrosoglyphosate is persistent in the soil. Fox soil treated with 20 ppm of nitrite nitrogen and 740 ppm glyphosate contained about 7 ppm of N-nitrosoglyphosate even after 140 days (6). 

The effects of physical factors such as temperature, soil moisture content, pH and oxygen availability on microbial survival and activity in soil are well documented (for a review, see Ref. 90). It is also widely acknowledged that these same factors may also influence plant growth and can therefore be presumed to influence both root exudation and rhizosphere microbial populations. 

Rhizodeposition is measured as 0 recovered from soil adhering to the roots or soil not adhering to the roots. Soil moisture content was constant in all treatments. 

Eor certain types of study, e.g., soil dissipation, additional factors may need to be recorded, e.g., soil biomass, specific soil moisture content, soil conductivity, and pH. 

Much work has been carried out recently on the velocity of detoxication of 2,4-dichlorophenoxyacetic acid under a variety of soil conditions—e.g., high soil moisture content, high temperatures, high pH, and high organic matter content (10,12,15,21, 26). 

Tensiometer Measures the matric potential of a given soil, which is converted to soil moisture content Commonly consists of a porous ceramic cup connected to a pressure-measuring device through a rigid plastic tube... 

Modeling The numerical model UNSAT-H was used to predict the annual and cumulative percolation through the cover. The model was calibrated with 12 months of soil moisture content and weather data. Following calibration, UNSAT-H predicted a cumulative percolation of 50 cm for the ET cover and 95 cm for a conventional cover over a 10-year period. The model predicted an annual percolation of approximately 0 cm for both covers during the first year. During years 3-10 of the simulation, the model predicted less annual percolation for the ET cover than for the conventional cover. 

At this point it is important to note that the flow model (a hydrologic cycle model) can be absent from the overall model. In this case the user has to input to the solute module [i.e., equation (1)] the temporal (t) and spatial (x,y,z) resolution of both the flow (i.e., soil moisture) velocity (v) and the soil moisture content (0) of the soil matrix. This approach is employed by Enfield et al. (12) and other researchers. If the flow (moisture) module is not absent from the model formulation (e.g., 14). then the users are concerned with input parameters, that may be frequently difficult to obtain. The approach to be undertaken depends on site specificity and available monitoring data. 

Lead may be taken up in edible plants from the soil via the root system, by direct foliar uptake and translocation within the plant, and by surface deposition of particulate matter. The amount of lead in soil that is bioavailable to a vegetable plant depends on factors such as cation exchange capacity, pH, amount of organic matter present, soil moisture content, and the type of amendments added to the soil. Back-... 

This formulation is based on several assumptions, including one of small soil moisture content (meaning that negligible radon is dissolved in liquid in the pore space). Other simplifying... 

There are a number of other types of measurement made in soil that involve electrodes that are not directly in contact with the soil. An example is the thermocouple psychrometer, which involves a Thomson thermocouple in a ceramic cell buried in soil. The thermocouple cools when a current is passed through it, causing water to condense on the thermocouple. When the electricity is turned off, the condensate evaporates at a rate inversely proportional to the relative humidity in the soil. A voltage generated by the cooling junction is measured and related to the soil moisture content. This moisture content is related to both the matrix and osmotic potentials of the soil being investigated. 

Moisture levels are also expected to influence the degradation process of the silicone surfactants. The degradation of surfactants on soils and peat is known to increase with soil moisture content [61], and this will also be expected for the primary degradation (hydrolysis) of silicone surfactants. Further studies of these compounds under different... 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

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%. 

Groundwater depth Soil moisture content Soil pH... 

Zimdahl and Clark (1982) reported half-lives of 15-38 and 33-100 d for the herbicide in clay loam soil, and sandy loam soil, respectively. They also reported that soil moisture increased the dissipation rate. At 20 °C, the dissipation rates of metolachlor in the clay loam and sandy loam soils at 20, 50, and 80% soil moisture contents were 0.028, 0.053, 0.062, and 0.016, 0.028, and 0.037/day, respectively. The half-lives of metolachlor in soil maintained at temperatures of 30 and 40 °C were approximately 3.85 and 2.75 wk, respectively (Bravermann et al., 1986). The reported half-lives of metolachlor in soil is approximately 6 d (Worthing and Hance, 1991) and 3-4 wk (Bowman, 1988). 

Ismail and Lee (1995) studied the persistence of metsulfuron-methyl in a sandy loam (pH 5.1) and clay soil (3.1) under laboratory conditions. Degradation was more rapid in nonsterilized than in sterilized soil. In nonsterilized soil, the rate of degradation increased with increasing soil moisture content. When the moisture level in the sandy loam and clay soil was increased from 20 to 80% of field capacity at 35 °C, the half-lives were reduced from 9.0 to 5.7 and 11.2 to 4.6 d, respectively. The investigators concluded that the disappearance of metsulfuron-methyl in soil resulted from microbial degradation and chemical hydrolysis. 

The adsorption of contaminants on geosorbents also is affected by climatic conditions reflected in the subsurface temperature and moisture status. Calvet (1984) showed how the soil moisture content may affect adsorption of contaminants originating from agricultural practices. The moisture content determines the accessibility of the adsorption sites, and water affects the surface properties of the adsorbent. The competition for adsorption sites between water and, say, insecticides may explain this behavior. Preferential adsorption of the more polar water molecules by soil hinders... 

Temperature and moisture content are other important factors that control volatilization of organic contaminants in the subsurface. Spencer and Cliath (1969, 1973) showed that a temperature increase from 20°C to 40°C led to an increase in dieldrin vapor density from 45 ng/L to 700ng/L (Fig. 8.7a). It also may be observed that a reduction in the soil moisture content caused a large reduction in the dieldrin vapor densities, even when the pesticide concentration in the moist soil was high enough to yield vapor densities approaching those of the pure compound. These results explain why reduction in pesticide volatilization in dry soils was observed over many years. 

Wolters et al. (2003) observed that volatilization kinetics of the fungicide fenpropimorph express a clear correlation between volatilization rates and soil moisture content. Volatilization rates reached a maximum 24 hr after application... 

Fig. 8.7 Effect of (a) temperature, and (b) soil moisture content on dieldrin vapor density (Taylor and Spencer 1990)...

The residual content of immiscible liquids can be defined by the amount of NAPL remaining in the subsurface when pore geometry permits NAPL flow greater than the retention capacity. In an outdoor pilot experiment. Fine and Yaron (1993) studied the effect of soil constituents and soil moisture contents on the retention of kerosene in the subsurface. This retention is termed the kerosene residual content (KRC). Ten soils were studied, with a broad spectrum of clay and organic matter contents, together with four soil moisture contents corresponding to oven-dried, air-dried. 

Fig. 12.8 Distributions of parathion in (A) biologically-inert (sterile) Gilat soil (20% moisture content, 1.4 g cm bulk density) after 3.03 days, and (B) biologically-active Gilat soil (34% soil moisture content, 1.4 g cm bulk density) after 2, 4 and 7 days. The solid curves were calculated using D= 1.67x 10 cm s the points represent experimental measurements (Gerstl et al. 1979a)...

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