Saturated zone degradation

Laboratory Studies. A variety of laboratory studies have been performed including degradation rate studies with actual samples from the saturated and unsaturated zones, oxidation mechanism studies, saturated zone degradation mechanism studies, potential for sulfoxide or sulfone reduction studies, and distilled water hydrolysis studies. The experimental methodology in these experiments varied according to the study objectives. 

Saturated Zone Degradation. Because of the rapid oxidation process in the root zone, parent aldicarb is rarely detected in the saturated zone. In rare instances where transport from the soil surface is rapid, aldicarb may be present at less than five percent of the total residues found. In the saturated zone, residues are usually a mixture of aldicarb sulfoxide and aldicarb sulfone in an average ratio of 3 2. 

In all of the workshops, but especially in the FAT and Exposure Assessment workshops, the need for better understanding and model representation of soil systems, including both unsaturated and saturated zones, was evident. This included the entire range of processes shown in Table II, i.e., transport, chemical and biological transformations, and intermedia transfers by sorption/desorption and volatilization. In fact, the Exposure Assessment workshop (Level II) listed biological degradation processes as a major research priority for both soil and water systems, since current understanding in both systems must be improved for site-specific assessments. 

Location of the contaminants in the subsurface. Degradable compounds must be dissolved in groundwater or adsorbed onto more permeable matrix materials within the saturated zone. 

Aerobic bacteria complete most of the petroleum bioremediation applications, particularly those above the groundwater table. Aerobes are those bacteria that require an oxygen source as their TEA. Conversely, anaerobic species require the absence of oxygen (anoxic conditions) for their respiration. In situ anaerobic bioremediation is typically only conducted in the saturated zone because of the difficulty in maintaining a strict anaerobic environment. In some instances, facultative anaerobes are utilized because they can alter the respiration to be metabolically active under both anaerobic and aerobic conditions. As such, the type of TEA available will dictate the metabolism and subsequent degradation mode. The most common TEAs used for bioremediation are listed in Table 2. Careful selection of microbe-TEA combinations can enable a specific degradation pathway to facilitate cometabolism and prevent undesired degradation by-products. 

Saturated-zone (groundwater) Movement Upward Lateral fP Downward Transport Degradation (chemical/biological) Soil Retention Transport. 

Research conducted on the movement and degradation of aldicarb residues in the unsaturated and saturated zones has shown that it is a complex process affected by soil and hydrogeological properties, climatic conditions, and agricultural practices. This paper presents the results of unsaturated and saturated zone field studies conducted in 16 states over a period of six years in which approximately 20,000 soil and water samples have been collected. Results from laboratory degradation studies are also included. 

Many factors influence the degradation of aldicarb residues to biologically inactive compounds. Some of the more important include temperature and pH, and the presence of moisture and microbial populations. The following paragraphs discuss these in more detail for degradation occurring in both the unsaturated and saturated zones. 

If aldicarb residues leach into the saturated zone, or groundwater, degradation of the residues continues, mainly by chemical hydrolysis in cold areas and by both chemical hydrolysis and microbial degradation in warm areas. Factors which tend to increase the degradation rate are high temperature and high pH. Microbial populations may provide a significant contribution to... 

TABLE II. Results of Laboratory Studies Measuring Degradation Rates of Aldicarb Residues in Saturated Zone Samples... 

The movement of aldicarb residues is quite complex, depending on a number of interacting factors. In most aldicarb use areas, residues degrade completely before moving through the unsaturated zone and into the saturated zone. In the few areas Where aldicarb residues have entered the saturated zone, residues are usually located in shallow groundwater near treated fields. 

In groundwater a vertical flux in the water unsaturated zone as well as a horizontal flux in the water saturated zone has to be stated and, consequently, an associated transport of dissolved and particle bound contaminants can be observed. Concurrently, corresponding aerobic and anaerobic zones have to be differentiated with respect to the microbial degradation processes. 

The three main compartments in which the fate of herbicides is investigated are the root zone, the unsaturated zone, and the saturated zone. In the first zone, volatilization, biodegradation, and sorption processes take place. In the second and third zones, other degradation and sorption reactions occur, but with lower kinetics and dissociation constants. 

The only study located regarding the degradation of 1,1,2-trichloroethane in soil involved subsurface samples taken from the margin of a floodplain near Lula, Oklahoma (Wilson et al. 1983). These samples were obtained both above the water table of a shallow aquifer and in the unconsolidated material in the saturated zone. A portion of the soil was sterilized and slurries were made and test chemical added. Manipulations made with samples from the saturated zone were carried out under nitrogen. After 16 weeks of incubation, no degradation of 1,1,2-trichloroethan was observed in the samples from above or below the water table. These results are in conflict with other studies (Wilson et al. 1983). It has been suggested that the time frame for the experiment may have been insufficient for resident microorganisms to have become acclimated to the chemical (Newsom 1985). 

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