Runoff and Leaching

The hydrologic cycle governs the rate of water transfer and movement in wetlands. Water transfer also governs the removal of toxic organics from wetlands. Water enters wetlands primarily from rainfall and surface runoff from the adjacent watersheds or upland areas. Tidal waters are a major water source in coastal wetlands. Generally, over time the quantity of water leaving a wetland is compensated by water entering the system, especially in lakes, streams, and estuaries. 

Toxic organics found in water in wetlands, where the mean residence time (MRT) of water is low, would be rapidly removed or transported to another ecosystem. If the MRT of water in a wetland is high (e.g., backwater swamp), water-soluble toxic organics would remain in the system for extended periods of time. The solubility of toxic organics in water and the MRT of water in wetlands govern the rate of transport and removal from the system. 

The MRT for wetlands can be calculated if it is assumed that the input is equal to the output and if the mass of the water in the wetland and the rates at which water is entering and exiting the wetland are known  

Transport of toxic organics from wetlands to surface water is dependent on MRT and solubility of the organic compound. Solubility of an organic can vary a billion fold. Solubility of xenobiotics in water is influenced by temperature, ionic strength, pH, and presence of other organic chemicals. 

Groundwater movements in wetlands are a function of gravitational forces and porosity of sediment substrate. Groundwater movements are faster in coarse-textured sediment (e.g., sand) compared to heavy clay substrates. 

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Up to 47 mg/kg of CP-transformation products such as o- and />-hydroxylated phenols (Knuutinen et al., 1990), as well as chloroanisoles such as 2,3,4,6-TeCA and PCA (Palm et al., 1991), have often been found in CP-contaminated soils. An additional route for contamination is the CP-treated utility poles and railway ties that contaminate nearby environments via runoff and leaching (Wan, 1992). 

Specific management practices influence triazine runoff and leaching, including fertilizer type, tillage crop residues, and previous crop history, as well as triazine application, formulation, and placement (Baker and Mickelson, 1994). Tillage systems affect various soil properties, such as soil moisture, temperature, pH, organic matter, water flow, and microbial populations, especially at and near the soil surface. These factors can affect transformation, retention, and transport of herbicides in soil. Interactions of and compensations between these processes can influence our prediction of triazine transport in soil. Therefore, triazine movement is usually studied under one management practice at a time. 

Davis-Carter, J.G. and B. Burgoa (1993). Atrazine runoff and leaching losses from soil in tilted beds as influenced by three rates of lagoon effluent. J. Environ. Sci. Health B, 28 1-18. 

Most pesticide movement in water is across the treated surface (runofO or downward from the surface (leaching). Runoff and leaching may occur when ... 

Competitive ion displacement can represent an important means by which arsenic is released to the aqueous phase, where it is subject to transport. Displacement and mobilization of As by phosphates is of particular concern (Manning and Goldberg, 1996 Reynolds et al., 1999 Violante and Pigna, 2002 Dixit and Hering, 2003), and regions where fertilizer or pesticide runoff and leaching... 

The occurrence of heavy rainfall soon after apphcation of biosohds is hkely to increase the potential for losses of organic contaminants by surface runoff and leaching. The complexation of organic chemicals with dissolved organic matter and colloidal material may facilitate their movement through soil profile and surface runoff [82-86]. 

Indirect emissions are caused by atmospheric degradation of precursor compounds. Atmospheric degradation of precursors is likely the major source of pollution in remote areas [30, 31]. Municipal WWTP effluents and infiltration of urban runoff and leaching piping [6, 32] are probably the major source of diffuse pollution to rivers and groundwater aquifers. 

Prevention of runoff and leaching. Appropriate t5rpes and degree of controls to prevent runoff and leaching should be implemented. Natmal barriers or manufactured liners placed between the waste material and the groundwater help control leaching. 

There are numerous sources of PAH in surface water, including municipal and industrial effluent, atmospheric fallout, fly-ash precipitation, road runoff, and leaching from contaminated soils. Overall, forest fires and combustion of coal are the most important sources of PAH in the atmosphere. Although automobiles, particularly those with diesel engines, formerly produced large quantities of PAH, recent environmental restrictions have significantly reduced their emissions. This will, however, probably be... 

Land disposal sites result in soil contamination through leachate migration. The composition of the substances produced depends principally on the type of wastes present and the decomposition in the landfill (aerobic or anaerobic). The adjacent soil can be contaminated by direct horizontal leaching of surface runoff vertical leaching and transfer of gases from decomposition by diffusion and convection. The disposal of... 

The concentration levels observed in sewage sludge show that UV filters originate mainly from private households, but besides this, surface runoff and industrial discharges may be considered as additional sources. This indicates that in addition to sunscreens, cosmetics and other personal products, UV filters from plastics, textiles, and other materials can be released to the environment by either volatilization or leaching. 

The transport processes that may move disulfoton from soil to other media are volatilization, leaching, runoff, and absorption by plants. Volatilization of disulfoton from wet soil may be greater than from relatively dry soil (Gohre and Miller 1986). Like other pesticides, disulfoton in soil partitions between soil-sorbed and soil-water phases (Racke 1992). This latter phase may be responsible for the volatilization of disulfoton from soil however, due to the low Henry s law constant value, the rate of disulfoton volatilization from the soil-water phase to the atmosphere would be low. 

Most agriculturally based pollutants probably return to the environment at points that are difficult to identify and are called nonpoint source emissions. Pesticides leave farms as runoff or leach through the soil. Isolating and treating them is not often possible. While techniques such as ditching and basins are being adopted, the problem of treatment in the dilute state in which pollutants will be isolated is a serious concern. 

Nitrates and phosphates are two important nutrients that have been increasing markedly in natural waters since the mid-1960s. Sources of nitrate contamination include fertilizers, discharge from sewage treatment plants, and leachate from septic systems and manure. Nitrates from fertilizers leach readily from soils, and it has been estimated that up to 40% of applied nitrates enter water sources as runoff and leachate. Fertilizer phosphates, however, tend to be absorbed or bound to soil particles, so that only 20% to 25% of applied nitrates are leached into water. Phosphate detergents are another source of phosphate, one that has received much media attention in recent years. 

Losses of DDT by drift and surface runoff, by leaching, and by removal of crops thus account for only a small percentage of the pesticide applied. The two remaining sources of loss—volatilization and degradation—must account for the rest. The role played by each of these factors probably defies evaluation. 

However, the relative importance of scenarios has been yet been recognized in what may be termed the validation of generic exposure assessments such as examining the leaching potential of a compound in the EU using the FOCUS groundwater scenarios [20] or running US EPA s standard scenarios for runoff and drift [25]. 

The nutrients that the plants take from the soil to grow and produce fruit need to be replaced by replenishing the soil and by the application of fertilizers. The nutrient uptake of an apple orchard in full production is relatively low. If the yield is between 25 and 50 t/ha, the trees take up 20-30 kg N, 5-15 kg P2O5, 50-80 kg K2O, 17-20 kg CaO and 6-8 kg MgO (Greenham, 1980). The nutrients supplied by fertilizers are not fully available for use by the fruit plants, as nutrients may be lost as a result of leaching, runoff and fixation. On the other hand, nutrients are constantly being made available to the plants through mineralization and the action of weather on the soil. 

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