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Toxic substances

Dissolved salts may be present in high enough concentrations to be toxic (eg naturally occurring selenium in the soils of the Central Valley, California and boron in Southern Peru). However, pesticides are a more common source of poisons associated with irrigation schemes. They are poisonous to plants, fish, birds and mammals including humans. Persistent chemicals are a threat to aquatic systems even when not soluble, as many bond chemically to soil particles and may be transported by erosion. Persistent organochlorine insecticides (eg DDT, dieldrin and endosulfan) are particularly hazardous to aquatic systems and become rapidly concentrated in the food chain. Non-specific herbicides can rapidly affect the supply of food. Pesticide risks are likely to increase if a monoculture is practiced, so that weeds and pests are not controlled by rotation, or if the method of agricultural management requires high applications, such as low tillage methods.

TABLE 6 Guidelines for interpretation of water quality for irrigation¹ (Source: Ayers and Westcot, 1976)

¹ Adapted from University of California Committee of Consultants 1974.

² ECw mean electrical conductivity, a measure of the water salinity, reported in deciSiemens per metre at 25°C (dS/m) or in units millimhos per centimetre (mmho/cm). Both are equivalent. TDS means total dissolved solids, reported in milligrams per litre (mg/l).

³ SAR means sodium adsorption ratio. At a given SAR, infiltration rate increases as water salinity increases. Adapted from Rhoades 1977, and Oster and Schroer 1979.

⁴ For surface irrigation, most tree crops and woody plants are sensitive to sodium and chloride. Most annual crops are not sensitive. With overhead sprinkler irrigation and low humidity (<30 percent), sodium and chloride may be absorbed through the leaves of sensitive crops.

⁵ NO₃ - N means nitrate nitrogen reported in terms of elemental nitrogen (NH₄ - N and Organic -N should be included when wastewater is being tested).

Chemicals have become an essential part of agricultural production and the benefits are enormous. However, when misused, the adverse impacts can be extensive.

Contamination of soil by the following metals is of particular concern: aluminium, arsenic, beryllium, chromium, cadmium, mercury, nickel, antimony and tin. Other elements are of ecotoxicological importance but are also plant nutrients, namely: boron, cobalt, copper, iron, manganese, molybdenum and zinc. This is a specialist subject and local knowledge will be important.

The use of water for irrigation containing sewage or industrial wastes should be of particular concern in an EIA and the WHO Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture (1989) will be very helpful.

The industrial processing of crops, or preparation of agricultural inputs, may involve or produce toxic substances, the safe disposal of which should fall within the remit of any EIA. The International Programme on Chemical Safety (IPCS), a joint WHO/ILO/UNEP programme, produces standards and guidelines on safety.

Agrochemical pollution

A high nutrient level is essential for productive agriculture. However, the use of both natural and chemical fertilizers may result in an excess of nutrients which can cause problems in water bodies and to health. Nitrates are highly soluble and therefore may quickly reach water bodies. Phosphates tend to be fixed to soil particles and therefore reach water courses when soil is eroded. Phosphate saturated soils and high phosphate level groundwater are now found in some developed countries.

TABLE 7 Inorganic constituents for drinking water quality
(Source: WHO, 1993)

TABLE 8 Water quality for freshwater fish (temperate zone excluding salmonids)

Notes:

1 The two parameters to which fish are most sensitive are temperature and dissolved oxygen. Oxygen is less soluble in water at higher temperatures. Also more non-ionized ammonia, which is toxic to fish, moves into solution from as the temperature rises as well as with an increase in pH. The higher the ambient temperature, the closer fish are living to their upper tolerance limit and the less able they are to tolerate changes to their environment. Organic pollution will reduce the dissolved oxygen content of the water.

2 A wide range of heavy metals, industrial pollutants and agrochemicals are toxic to fish.

3 More information may be obtained from various FAO Fisheries Technical Papers.

Source: EC Council directive (78/659/EEC) on the quality of fresh waters needing protection or improvements in order to support fish life.

High levels of nitrates in drinking water can cause health problems in small children. However, the transport of pathogens resulting from the use of excrete as a fertilizer or from poor sanitation causes widespread health problems from viruses, bacteria and protozoans capable of causing a range of diseases from minor stomach upsets to cholera and hepatitis.

A high nutrient level is toxic to some aquatic life and encourage rapid rates of algae growth which tends to decrease the oxygen level of the water and thus lead to the suffocation of fish and other aquatic biota. Clear water enhances the effect as it enables increased photosynthesis to take place: reservoirs and slow-moving water are therefore most at risk. Some algae produce toxins, and if deoxygenation is severe, eutrophic conditions occur.

Reservoirs with a high level of organic pollution, including human waste, provide an ideal habitat for the breeding of culicine mosquitos that transmit filariasis.

Anaerobic effects

Most anaerobic conditions in water bodies are the result of an over-supply of nutrients, as discussed above, resulting in eutrophication. In reservoirs, anaerobic conditions may occur in the deeper areas as organic material on the bed decays in an environment with progressively less oxygen. Reservoirs should be cleared of organic matter, prior to impoundment to limit anaerobic decomposition once the dam is filled. Anaerobic conditions also occur when water is so polluted as to kill most aquatic life. Anaerobic decomposition should be avoided as it produces gases such as hydrogen sulphide, methane and ammonia all of which are poisonous and some of which contribute to the greenhouse effect. The production of greenhouse gases may also be produced by irrigated rice fields and this is being investigated by the International Rice Research Institute.

Multi-level outlets may be required for deep reservoirs to ensure that flows are sufficiently oxygenated for downstream aquatic life.

Gas emissions

Irrigated areas can become contaminated by emissions from industry, particularly areas that are close to urban or industrial sites.

Soil properties and safety erects

Soil salinity
Soil properties
Saline groundwater
Saline drainage
Saline intrusion

On-going comprehensive soil studies are essential to the successful management of irrigated areas. A wide range of activities associated with an increased intensity of production can contribute to reduced soil fertility. Soil salinity is probably the most important issue although mono-cropping, without a fallow period, rapidly depletes the soil fertility. A reduction in organic content will contribute to a soil's erodability. The increased use of agro-chemicals, needed to retain productivity under intensification, can introduce toxic elements that occur in fertilizers and pesticides.

Arable land is continuously going out of production at approximately 5 to 7 million hectares per year (approx 0.5%) due to soil degradation (FAO, 1992). On irrigated lands salinization is the major cause of land being lost to production and is one of the most prolific adverse environmental impacts associated with irrigation. Saline conditions severely limit the choice of crop, adversely affect crop germination and yields, and can make soils difficult to work. Careful management can reduce the rate of salinity build up and minimize the effects on crops. Management strategies include: leaching; altering irrigation methods and schedules; installing sub-surface drainage; changing tillage techniques; adjusting crop patterns; and, incorporating soil ameliorates. All such actions, which may be very costly, would require careful study to determine their local suitability. Figure 6 indicates the sensitivity of a range of important crops to soil salinity.

It is important that all evaluation regarding irrigation water quality (see Ayers and Westcot, 1985) is linked to the evaluation of the soils to be irrigated. Low quality irrigation waters might be hazardous on heavy, clayey soils, while the same water could be used satisfactorily on sandy and/or permeable soils.

Soil salinity

There are four main reasons for an increase in soil salinity on an irrigation scheme:

• salts carried in the irrigation water are liable to build up in the soil profile, as water is removed by plants and the atmosphere at a much faster rate than salts. The salt concentration of incoming flows may increase in time with development activities upstream and if rising demand leads to drain water reuse;

• solutes applied to the soil in the form of artificial and natural fertilizers as well as some pesticides will not all be utilized by the crop;

• salts which occur naturally in soil may move into solution or may already be in solution in the form of saline groundwater. This problem is often severe in deserts or arid areas where natural flushing of salts (leaching) does not occur. Where the groundwater level is both high and saline, water will rise by capillary action and then evaporate, leaving salts on the surface and in the upper layers of the soil; and

• the transfer from rainfed to irrigation of a single crop, or the transfer from single to double irrigation may create a "humidity/salinity bridge" in the soil, between a deep saline groundwater and the (so far) salt-free surface layers of the soil. Careful soil monitoring is highly recommended whenever the irrigated regime is intensified, even though the saline layers might be far below the soil surface and the irrigation water applied is of high quality.

FIGURE 6 Yield potential of selected crops as influenced by soil salinity (ECe) (Ayers and Westcot, 1985)

Unless there is some drainage from the scheme, whether natural or artificial, salinity problems will arise with consequent adverse impacts for agriculture.

Soil properties

The accumulation of salts in soils can lead to irreversible damage to soil structure essential for irrigation and crop production. Effects are most extreme in clay soils where the presence of sodium can bring about soil structural collapse. This makes growing conditions very poor, makes soils very difficult to work and prevents reclamation by leaching using standard techniques. Gypsum in the irrigation water or mixed into the soil before irrigation is a practice that is used to reduce the sodium content of sodic soils.

In certain areas, in particular in tropical coastal swamps, acid sulphate soils may be a problem. The danger of potential soil acidification needs to be considered. The transfer from rainfed to irrigated crop production, or intensification of existing irrigated crop production requires a higher level of nutrient availability in the soil profile. If this aspect is not given adequate attention, the irrigation efficiency remains low. High water losses through the profile will result and useful cations may be washed out from the soil-complex. A general lowering of pH may result in a decrease of the plants capability to take up nutrients. The decrease of pH may also result in an increased availability/release of heavy metals in the soil profile. Rectifying soil acidification problems can be very costly. For similar reasons the content of organic material in the soil may decrease. Such decrease leads to a degradation of soil structure and to a general decrease of soil fertility.

Saline groundwater

An increase in the salinity of the groundwater is often associated with waterlogging. An appropriate and well-maintained drainage network will mitigate against such effects. Saline groundwater can be particularly critical in coastal regions.

Saline drainage

Drainage may not be required initially but it should be allowed for if there is insufficient natural drainage. Areas with a flat topography or with water tables that have a low hydraulic gradient are at risk from salinization as are areas with soils of a low permeability which are difficult to leach. Groundwater drains, either pipe (tile) drains or deep ditches, carry out the dual task of controlling the water table and through leaching, counteracting the build up of salts in the soil profile. Normally water is applied in excess of the crop water requirement and soluble salts are carried away in the drainage water although in some areas leaching can be achieved during the rainy season.

An increase in solute concentration from the applied irrigation water to the drain water cannot be prevented. Typically salt concentrations in drainage water are 2 to 10 times higher than in irrigation water, (Hoses and Pearson in Worthington E B (ed), 1977). The quantity of drainage water can be reduced by good irrigation management though this will tend to have the effect of making the quality worse. Reducing salt inputs is one way of improving drain water quality. The safe disposal of salts is of prime importance, either to the sea (using dedicated channels if river quality is threatened) or to designated areas such as evaporation ponds where the negative impacts can be contained. Leaching typically requires an extra 1020% of water.

Saline intrusion

The location of the boundary between fresh and salt water at the coast line is a function of the hydraulic potential of the fresh water. A lowered water table will result in the boundary moving inland as the pressure reduces. Large numbers of people may be affected by a reduction in the quality of their drinking supplies when fresh water is replaced by salty water. Moreover, people may be forced to turn to sources of water whose collection and use have important health risks. The plant life in the area may also change as only salt tolerant species survive. The environmental effects can be irreversible as reversing the movement of a salt water wedge is usually both difficult and very expensive.

Changes to the flow regime may alter the salinity of the estuary. This is likely to have a major impact on the local ecology: a highly productive habitat which is often sensitive to salinity levels.

Erosion and sedimentation

Local erosion
Hinterland effect
River morphology
Channel structures
Sedimentation
Estuary erosion

Upstream erosion may result in the delivery of fertile sediments to delta areas. However, this gain is a measure of the loss of fertility of upstream eroded lands. A major negative impact of erosion and the associated transport of soil particles is the sedimentation of reservoirs and abstraction points downstream, such as irrigation intakes and pumping stations. Desilting intakes and irrigation canals is often the major annual maintenance cost on irrigation schemes. The increased sediment load is likely to change the river morphology which, together with the increased turbidity, will effect the downstream ecology.

FIGURE 7 Factors affecting soil erosion (Petermann, 1993, after Morgan, 1981)

Soil erosion rates are greatest when vegetative cover is reduced and can be 10 to 100 times higher under agriculture compared with other land uses. However, there are a wide range of management and design techniques available to minimize and control erosion. For erosion to take place, soil particles need to be first dislodged and then transported by either wind or water. Both actions can be prevented by erosion control techniques which disperse erosive energy and avoid concentrating it. For example, providing good vegetative cover will disperse the energy of rain drops and contour drainage will slow down surface runoff. See Figure 7 for factors effecting erosion potential.

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