Surface soil Terms

Total microbial populations are often higher in no-till soils. In a study comparing surface soils from long-term no-till and conventional tillage plots at seven United States locations, counts of aerobic microorganisms, facultative anaerobes, and denitrifiers in no-till soils were 1.14-1.58, 1.57, and 7.31 times higher, respectively, than in the surface of plowed soils (Doran, 1980). 

The first term on the right-hand side of equation (2.14), termed the rate factor, represents the direct contamination of herbage by fallout during the growing season. The second term, lag rate factor, is the contribution from the previous year s fallout. This includes the contribution of uptake from the surface soil and matt and also the effects of carry-over of silage and other feeding stuffs from one year to the next. The third term, soil factor, represents the contribution of root uptake, allowing for radioactive decay and reduced availability as nuclides move down the soil profile and become fixed to clay minerals. 

When rain falls over land some drain off the surface directly into surface water courses in surface runoff. A further part of the incoming rainwater percolates into the soil and passes more slowly into either surface waters or underground reservoirs. Water held in rock below the surface is termed groundwater, and a rock formation that stores and transmits water in useful quantities is termed an aquifer. Water that passes through soil or rock on its way to a river is chemically modified during transit, generally by addition of soluble and colloidal substances washed out of the ground. Some substances are removed from the water for example, river water often contains less lead than rainwater one mechanism of removal is uptake by soil. 

Cooper et al. (1994) have reported re-suspension studies on soils contaminated with plutonium during nuclear weapons tests by use of a mechanical dust-raising apparatus. Airborne dust was analysed in terms of mass and Am activities for particle sizes less than 7 pm. The AMAD was determined as 4.8-6 pm for re-suspended soil. Also, surface soil was characterised in the laboratory by means of sieving and microparticle classification, yielding mass and "Am activity distribution with respect to size. Data indicate the granularity of plutonium contamination at both major and minor trial sites. Depth profile analyses for undisturbed areas demonstrate that most (74% on average) of the americium and plutonium activity is found in the top 10 mm of soil. Plutonium and americium activities were found to be enhanced in the inhalable fraction over their values in the total soil, and the enhancement factors were similar in re-suspended dust and surface soil. Observed enhancement factors ranged from 3.7 to 32.5. 

Weathering is the general term used to describe the ways in which rock is broken down at or near Earth s surface. Soils are the result of weathering and the activities of plants and animals. 

One-Dimensional Simulation of DBCP Movement at Kunia. DBCP distribution three years after the pesticide spill was simulated by the one-dimensional analytical model with exponential decay source term at the surface (1.) predicted concentrations are shown in Figure 2. Measured concentrations from Boreholes 2, 3 and 5 are also shown in Figure 2 for comparison with simulated results. The three measured DBCP concentration profiles are quite variable, both in shape and in magnitude of concentrations. The reason for the variation in measured profiles is not known, but may be due to differences in the amount of DBCP which entered the soil at each location and variation in soil properties between borehole sites. There appears to be a correlation between the sorption values in Table 1 for Boreholes 2 and 3 and the retention of DBCP near the surface at these two locations, ie. high sorption in the surface soil at site 3 resulted in high retention of DBCP, in contrast to site 2. 

Considering surface soil at old contaminated sites, volatilisation and degradation will have already occurred in the field. Volatilisation and degradation are usually not issues for screening tests however, if there is a possibility that these processes confound results, then they should be considered in the test set-up, in particular for long-term tests (duration of several weeks). 

Figure 1. Different patterns of biodegradation resulting from microbial adaptation. (A) Adaptation to p-nitrophenol in Lulu aquifer samples at 529 ng/mL, but not at 14 ng/mL (41). (B) Adaptation to the triazinone-ring of metribuzin, evidenced by an increasing mineralization rate over time in the surface soil, but not in the subsurface soils (351. (C) Adaptation to EPTC after long-term EPTC use (12). (Reproduced with permission from Ref. 12. 15, 43. Copyrights 1988, 1989, 1987 Weed Science Society of America, American Society of Agronomy, and American Society for Microbiology, respectively)...

The reaction is written to illustrate the formation of the ionic species most prevalent at soil pHs. While urea is extremely stable at ambient conditions, soil urease converts urea rapidly often urea fertilizer additions are completely hydrolyzed in 2 to 3 days, a very rapid reaction in soil terms. The rate of hydrolysis depends on the soil temperature, moisture, how dispersed the urea is, and the amount of urease present. Very cold or dry soils hydrolyze urea more slowly than warmer or moist soils. The enzyme activity can be significantly different in different zones of a particular soil. High enzyme activities can develop on a moist soil surface because of algal and other microbial growth on the soil surface. 

The effects of environmental threats posed by DU residues cannot be assessed only on the basis of direct measurements of surface soil, drinking water, and air samples, but long term (100-1,000 years and longer) migration of uranium from the target missing projectiles and ricochets deposited in surface soil should be also taken into account. 

In view of the low mobility of uranium in the soil and the small transfer factors into biological material, long-term effects on the environment should not be expected. Analysis conducted with the use of RESARD computer model simulations led to the conclusion that in the Balkans, on a long-time scale (several hundreds of years) the main risk for the population from deposited DU penetrators can be connected with possible contamination of drinking water in local wells which critically depends on various hydrogeological parameters which need to be evaluated at the site. On the other hand, it seems to be a reasonable assumption that a main exposure pathway for Kuwait s and southern Iraq s inhabitants is the uptake of uranium into the body by inhalation due to resuspension of DU present in surface soil or sand in the specific dry climatic conditions. 

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