Soil and Water Quality An Agenda for Agriculture (1993) / Chapter Skim
Currently Skimming:
10 Salts and Trace Elements
10 Salts and Trace Elements
Pages 361-398
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 361... ...
About 5.2 million ha (14 million acres) of this irrigated land is currently affected by salt.
|
From page 362... ...
Adding to the difficulties in managing irrigationinduced water quality problems was the discovery of selenium and other toxic trace elements in subsurface drainage waters in the San loaquin Valley's west side (National Research Council, 1989b)
|
From page 364... ...
Thus, all irrigators must cope with salinity and all irrigated fields need drainage, be it natural or provided by producers. The drainage water tends to be substantially more saline than the original irrigation water.
|
From page 365... ...
The disposal of the drainage water, however, creates a new problem; and the cost of providing an outlet is often excessive. A less costly solution is found by switching to a flexible cropping system in the recharge area (the area of land that is the source of water for the seep)
|
From page 366... ...
This reservoir was in fact a set of shallow ponds used to store and evaporate agricultural drainage water (National Research Council, 1989b)
|
From page 368... ...
Reducing the Impacts of Trace Elements Irrigation practices can be altered in numerous ways to minimize the adverse effects of salinity on agricultural lands, and similarly, management can reduce water quality problems (Suarez and Rhoades, 1977~. The same can be said for trace elements, although the specific management or corrective practices may differ.
|
From page 369... ...
Society must weigh the benefits accrued from irrigation against the disadvantages associated with and the costs of reducing soil salinization and water pollution (van Schilfgaarde, 1990~. SOURCES AND Etif;CTS OF SALINITY Nature of Salinity The salinity in soils and waters is made up of dissolved mineral salts.
|
From page 370... ...
Moreover, soil salinity is a dynamic property since soluble salts are highly mobile in the soil profile. Soil salinity is typically measured in the laboratory by obtaining an extract from a soil sample that is moistened with distilled water to a reference saturated soil water content.
|
From page 371... ...
Water penetration problems can develop when a seal with very low hydraulic conductivity forms at the soil surface. Seals are generally considered to be wet, and they reduce infiltration and increase runoff and erosion.
|
From page 372... ...
The extent of salt accumulation in the lower portion of the crop root zone is dependent on the leaching fraction, which is defined as the ratio of the depth of drainage water past the root zone to the depth of infiltrated water. For soils cropped under shallow water table conditions, there is a tendency for salts to accumulate nearer to the soil surface with a rise in the water table because of the upward evaporative flux rather than downward leaching flux in deeply drained soils (Namken et al., 1969~.
|
From page 373... ...
Salinity increases with depth Salinity increases with depth FIGURE 10-1 Typical salt accumulation patterns in surface soils for various methods of water application. Salinity ranges from low (unshaded)
|
From page 375... ...
Effects of Water Relationships Salt in the root zone decreases the osmotic potential of the soil solution and therefore reduces the availability of water to plants. If the osmotic potential of the soil becomes lower than that of the plant's cell, the latter would suffer osmotic desiccation and loss of turgor pressure (turgor pressure is the pressure within a plant cell)
|
From page 376... ...
Effects of Salinity oil Crop Yields Crop plants respond to salinity in widely ranging manners because of differences in their abilities to adjust osmotically, enabling them to extract water from saline soil solutions (Figure 10-3~. Typical examples of saltsensitive crops are bean, onion, almond, peach, orange, and grapefruit; moderately sensitive crops include corn, alfalfa, clover, cabbage, lettuce, potato, and grape; moderately tolerant crops include safflower, soybean, wheat, barley, tall fescue, squash, and olive; and tolerant crops include cotton, sugar beet, Bennuda grass, asparagus, date palm, and guayule.
|
From page 377... ...
triggered research into the environmental hazards of mercury in aquatic systems (Adriano, 1986~. More recently, the discovery in the early 1980s of selenium poisoning of fish and waterfowl in the Kesterson National Wildlife Refuge has further heightened awareness of the potential hazards of naturally occurring trace elements in agricultural drainage waters (National Research Council, 1989b)
|
From page 378... ...
These soil minerals serve as reservoirs for the trace elements, which are typically released at a slow rate into soil solutions and waters through a number of chemical weathering mechanisms. Anthropogenic sources of trace elements (Adriano, 1986)
|
From page 379... ...
Reactivities and Mobilizes of Trace Elements The presence or accumulation of trace elements in agricultural drainage waters and groundwaters is influenced by a number of factors, including the nature and sources of trace elements, the particular trace element and its reactivity, and mobility and transport processes. The first item was addressed above.
|
From page 380... ...
The solubilities of minerals containing cationic trace elements typically increase as the pH decreases, whereas the mobilities of those containing anionic trace elements typically decrease as the pH decreases. The half-lives of reactions (Langmuir and Mahony, 1985)
|
From page 381... ...
. Drainage water disposed into Kesterson Reservoir had an average selenium concentration of about 300 ,ug/liter (300 ppb)
|
From page 382... ...
from 1984 to 1989 in the San Joaquin Valley. Source: San Joaquin Valley Drainage Program.
|
From page 383... ...
Final Report. Sacramento, Calif.: San Joaquin Valley Drainage Program.
|
From page 384... ...
Zinc (pg/g) 0 50 100 150 0 200 FIGURE 10-6 Heavy metal contents in Greenfield sandy loam treated with composted sludge from 1976 to 1981.
|
From page 385... ...
Table 10-2 presents the recommended maximum concentrations of 15 trace elements in irrigation waters for long-term protection of plants and animals (Pratt and Suarez, 1990~. These concentrations should be considered as guidelines designed to protect the most sensitive crops and animals from receiving toxic amounts of trace elements.
|
From page 386... ...
Copper 0.20 Concentrations of 0.1 to 1.0 mg/liter in nutrient solutions have been found to be toxic to plants, but soil reactions usually precipitate or adsorb copper, so that soluble cop per does not readily accumulate. Fluoride 1.0 This concentration is designed to protect crops grown in acidic soils.
|
From page 387... ...
1990. Irrigation water quality assessments.
|
From page 388... ...
Davis: University of California, Division of Agriculture and Natural Resources, UC Salinity/ Drainage Task Force. cropping systems, irrigation and drainage systems, water rights, institutional infrastructure, and drainage water disposal practices differ.
|
From page 389... ...
American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America. Source Control Measures The principal aim of source control is to use water and land resources efficiently with off-farm and on-farm measures that minimize salinity and trace element problems.
|
From page 390... ...
by computing the soil-water balance. The first method involves measuring the soil water content or matrix potential (a measure of how tightly water molecules are bound to soil particles)
|
From page 391... ...
If, however, the principal source of salts is naturally occurring salts in the soils, soil salinity needs to be monitored and water in excess of crop water needs to be applied periodically to control salinity in the root zone. Because of the nonuniformity of both application of water and infiltration rates, subsurface drainage water and associated pollutants will be produced even if producers implement best-management practices.
|
From page 392... ...
Biological and Physicochemical Processes The San loaquin Valley Drainage Program sponsored research on the removal of selenium and other trace elements from agricultural drainage waters, including biological and physicochemical processes. An anaerobic bacterial process used methanol as a source of carbon for microbes to reduce selenium, microfilters to remove fine suspended solids, and
|
From page 393... ...
The physicochemical treatment processes that have been investigated include chemical reduction and surface adsorption of selenium onto hydroxylated surfaces. Selenium can be reduced and precipitated from drainage waters by using heavy doses of ferrous hydroxide.
|
From page 394... ...
_ _ J ~ _ _ _, ~ _, . Drainage Water Disposal Options for disposing of agricultural drainage waters include (1)
|
From page 395... ...
Because of these new findings on the hazardous nature of evaporation ponds (Skorupa and Ohlendorf, 1991) , this practice of drainage water disposal is expected to be severely curtailed for drainage waters containing potentially toxic amounts of trace elements.
|
From page 396... ...
Thus, pretreatment appears to be a necessary process for deep-well injection of agricultural drainage waters. The estimated cost for this option is $132 to $172/103 m3 ($164 to $213/acre-foot)
|
From page 397... ...
It is not unusual, however, to have an entity that delivers irrigation water and another entity that manages drainage water in a given region but with different boundaries. There is a need for joint planning and management of irrigation and drainage waters.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.