Rivers and Streams
In many areas rivers are often the most convenient source of water. However, the flow of the river is dependent on rainfall in the catchment area. Consequently, during dry seasons, when most water is required, the flow will be lowest. If too many users take water from the river, the flow may be reduced to such an extent that it silts up. The quality of river water is often poor since domestic and industrial sewage is frequently discharged directly into the water course. In addition, the clearing of vegetation in catchments has resulted in widespread soil loss and increased the sediment load and turbidity in river systems.
Lakes, Dams and Reservoirs
Natural lakes accumulate water when it rains and so provide a source which can be used at other times of the year. Similarly, dams or reservoirs store water from the wet season for use in the dry season. This overcomes the problem of naturally fluctuating flow. Because a lake or dam is a closed system, nutrients and pollutants can accumulate and sometimes reach toxic levels. If there is a build-up of nitrates and phosphates then eutrophication may occur which will further lower the quality of the water.
Piped Supply
Piped water is often treated to a high standard to make it suitable for human consumption. The capacity of many piped water supplies is often insufficient for irrigation purposes, although it may be adequate for general farm or plantation use. Due to this storage tanks are often used. These are filled up overnight so that pumped supplies can be delivered during the day.
Groundwater
Groundwater is obtained from boreholes sunk into water bearing rocks or aquifers. This water is often of great age and so, providing it is not saline, it is often of very good quality. Aquifers normally recharge naturally through time by the slow infiltration of water through the soil and rock from the surface. However, if usage rates are high then eventually the water table may fall and the aquifer dry up.
Water Quality
No natural water is actually pure since it contains a variety of materials, either dissolved or in suspension, as well as micro-organisms.
However, it is normally difficult to determine visually whether a water sample is safe to drink or suitable for irrigation. During drought conditions it becomes acceptable to use water of lower quality, particularly for non-essential uses. Under these circumstances it is even more important to regularly check the water quality since contamination frequently increases during long dry periods.
Water contaminants can be divided into the following categories:
1. Suspended Particles eg sand, clay, humus and iron.
Suspended solids may clog the filters and nozzles of irrigation equipment. Sand particles are abrasive and can cause excessive wear in pumps. When dry these particles may cause an unattractive deposit on crops and flowers so lowering their commercial value. Humus and iron may colour the water.
2. Chemical Salts eg sulphates, carbonates, calcium, manganese, iron
These are present in most water supplies but are not normally found in concentrations high enough to affect crop growth. However, if used for irrigation, water containing these salts can increase the alkalinity of the soil., Excessive hardness of water can lead to pipe and sprinkler blockages. Iron salts can also cause blockages, as well as leading to pipe corrosion through the growth of iron bacteria.
3. Nitrates
Nitrogen is required for both plant and animal growth. It forms 78% of the gas in the atmosphere and becomes incorporated into the soil by biological fixation.
Many crop plants respond to the nitrogen concentrated in the soil and produce higher yields. Due to this both natural and artificial nitrogen fertilisers are used by farmers to increase their yields. However, any excess nitrogen in the soil may be leached from the soil to contaminate surface and ground waters. High levels of nitrates in water are considered to be harmful and the World Health Organisation has an acceptable level of 50 milligrams per litre.
4. Salinity
Few crops and even fewer animals are able to tolerate saline water. Many commercial crops are extremely sensitive to saline water and will either be damaged or produce reduced yields. Irrigation with saline water is also likely to damage the soil.
5. Turbidity, colour, odour and taste
Turbidity arises from suspended or undissolved particles in the water such as clay, silt and organic matter.
Colour may be due to dissolved iron, manganese or complex organic compounds.
Odours arise from microbiological and algae activity, decomposition of biota or formed as a result of chlorination of organic material.
Some substances may be detected by taste principally the inorganic salts of calcium, copper, iron, magnesium, manganese, sodium, zinc and some organic compounds.
6. Metals eg zinc, copper, chromium, borates, lead
Natural water sometimes contains raised levels of metals but they are more commonly associated with sewage sludge or the pollution of other water sources with industrial effluent. These metals sometimes cause direct damage to crop plants but are also often accumulated in leaves. In this way humans or animals may be subjected to a higher than normal intake of these when the crops are eaten.
7. Micro-organisms
Plant, animal and human diseases can be spread by water contaminated by bacteria, viruses cysts and other micro-organisms. Contamination of water supplies with animal and domestic sewage is one of the main problems. Water which is used to irrigate crops which are likely to be eaten raw should be checked for biological contamination before it is used.
8. Algae & Plankton
In themselves, algae are relatively harmless but they may render a dam unsightly, may cause colours, promote bacterial growth and assist in the formation of chloramines. Algae is introduced into a pool by airborne spores, and may be attached to dust particles or contained within raindrops. Blue green algae is a microscopic aquatic plant "Cyanophyta". They are considered to be among the simplest forms of plants.
Plankton is comprised of all the microscopic organisms which are suspended in water and includes small plants (phytoplankton) including algae small animals (zooplankton), and bacteria. When there is enough plankton in the water to discolor it and make it appear turbid, the water is said to contain a plankton "bloom". The phytoplankton uses inorganic salts, carbon dioxide, water, and sunlight to produce its own food. The zooplankton feeds on living or dead plank--ton and other tiny particles of organic matter in the water. Bacteria utilize any type of dead organic matter in the water for food. In addition to encouraging fish growth, plank.-ton prevents the growth of undesirable aquatic weeds through shading.
Heavy plankton blooms usually contain large numbers of blue-green algae which can form scums at the surface. These scums absorb heat during the day and cause shallow thermal stratification. During the night, heavy plankton blooms consume large amounts of dissolved oxygen and may cause oxygen depletion before the nixt morning. Scums of plankton may suddenly die, decompose, cause oxygen depletion and sometimes produce toxins.
9. Other pollutants
• Pesticides sprayed onto fields may drift over considerable areas and reach dams or lakes. Therefore, dams in agricultural areas are often contaminated to some degree with pesticides. Cotton and other non-food crops are often treated with especially toxic and persistent pesticides. In some regions, pesticides which contain heavy metals, such as arsenic and lead, are still used.
Sampling should be arranged to take account of variation in water quality which may occur from time to time; for instance many streams exhibit marked changes in quality throughout the year which may be due to fluctuations in flow, to run of from different parts of the catchment or to changes in the catchment itself.
Selection of sampling points should be done with considerable care.
A record should be made of every sample collected, and every bottle should be identified by a suitably marked tag or label. The record should always include sufficient information for positive identification of the sample at a later date, as well as the name of the sample collector, the date, hour, and exact location, the water temperature, and other data which may be needed in the future, such as weather conditions, water level, and stream flow. Sampling points should be fixed by detailed description, survey, or maps, or with the aid of stakes, buoys, or landmarks so that they can be identified by other persons without reliance upon memory or personal guidance.
Before samples are collected from pipeline, the lines should be flushed sufficiently to ensure that the sample is representative of the supply. In the case of wells or bores, samples from bailing are considered to be of limited value and pumping should continue for some hours before sampling directly from the discharge. If practicable, further samples should be taken at regular intervals to establish whether any change in quality occurs with continued pumping.
When samples are collected from a stream, the quality may vary with depth, discharge, distance from bank, proximity to springs or tributaries, and other factors. In general, a sample taken from the main stream flow is sufficiently representative. Special care must be exercised in tidal reaches or during periods of fluctuating stream flow.
Water quality in lakes and reservoirs may be subject to considerable variation due to such things as seasonal stratification, rainfall, runof and wind. The choice of location, depth and frequency of sampling will depend on local conditions and the purpose of the investigation. Where several samples at different depths are to be taken at the one location, the upper samples should be taken first to avoid disturbance of any layering which may exist.
Hardness
Water hardness is caused by the presence of dissolved calcium and magnesium salts. It comes from rocks such as common limestone (calcium carbonate and magnesium carbonate), gypsum (calcium sulphate), epsom salts (rhagnesium sulphate), calcium chloride, and magnesium chloride.
Hardness is a term originally referring to the soap-consuming power of water. As such it is sometimes also taken to include iron and manganese. High hardness can cause boiler or, pipe scale and failure of reverse osmosis membranes.
The most common water problem in the average home is hardness. Very hard water is recognised by the following symptoms:
1. Reduced water flow and eventual plugging;
2. Premature water heater element burn out;
3. Poor laundry results and economy;
4. Scale ring at water level in toilets and tanks;
5. Permanent water spots and deposits;
6. Poor cooking results;
7. Reduced life of laundered goods;
8. Reduced lathering with soaps and domestic detergents.
High levels of calcium in irrigation waters may cause blockage of micro filtration outlets and spotting on fruit. Deposition will depend upon the pH, bicarbonates and concentrations of calcium, magnesium and sodium. The USDA classication of hardness (ppm as CaCO3) are as follows:
(0-60 Soft) (61 - 120 Moderately Hard) (121 - 180 Hard) More than 180 I Very Hard
.As the water hardness increases so do other dissolved minerals. Drinking water standards normally require a guideline of less than 80ppm.
Calcium hardness is a measure of all the different dissolved calcium compounds found e.g., calcium carbonate, calcium hydroxide, calcium bicarbonate etc. Each of these compounds have different solubilities in water, and the calcium compounds which precipitate first, normally calcium sulphates, cause scaling.
Nitrates
The nitrate ion has the formula NO3 and substances containing these are called nitrates. Similarly the nitrite ion has the formula NO2 and these substances are called nitrites. Ammonia wkich is a nitrogen fertilizer NH3 can be converted by bacteria to nitrate.
The presence of nitrate in a water analysis is a danger signal. Nitrate is an indication that the water supply may be contaminated with bacteria. Even small amounts of nitrate make the water supply suspect.
High nitrate levels may occur in shallow bores close to intensively farmed crops or pastures. This nitrate may arise from excessive inputs of nitrogen fertiliser or from nitrogen fixing leguminous plants.
Nitrates may arise from bacterial decomposition of organic matter or may be present naturally, but analysis for nitrate is usually unnecessary since significant amounts are rarely found. However, certain underground waters are an exception.
Nitrates are dangerous to infants under one year of age, and possibly older children, when excessive ingestion may give rise to methaemoglobinaemia.
Babies drinking high nitrate water develop a blue colour of the skin because the blood has a decreased ability to carry oxygen. Continued drinking of the water will cause the baby to become ill, or may even result in death. Adults are less• susceptible due to their acidic stomachs.
Ruminant farm animals are also susceptible to nitrate with similar problems as human infants.
The quantity of nitrate is reported as nitrate-nitrogen (NO3 N). The maximum allowable limit of nitrate given by the World Health Organisation is 45 mg/I (10 mg/I as N). The acceptable limit for nitrate is 10 ppm for humans and 100 ppm for livestock when reported as nitrate nitrogen.
Since ammonia may be derived from either harmless sources or from pollution, it•alone is not adequate for judging the character of a water. The World Health Organisation places the minimum limits indicative of pollution at '1 mg/1 for total nitrogen (exclusive of nitrate nitrogen) and 0.5 mg/l for ammonia (NH3).
Iron
Four types of iron exist:
1. Soluble -
Soluble iron is easily recognised because the water is clear when first drawn, but soon becomes filled with red rust on standing. It is commonly found in well and bore water supplies.
The main objection to iron -water is that it causes reddish brown stains on plumbing fixtures, porcelain, cooking utensils, and laundry. Iron causes a disagreeable metallic taste and can have a -sewer type odour. Iron causes coffee, tea, liquor, and other beverages to turn inky black. As little as 0.3 ppm is enough iron to cause staining and 0.1 ppm enough to promote iron bacterial growth.
Iron can stain spray-irrigated vegetables and fruit. High levels on leaves cause shading preventing normal functioning while deposits on pasture make it unpalatable to stock and, if eaten, will cause dairy cattle to scour and drop their milk production.
High iron concentrations in stock water may induce copper deficiency in animals. The problem is most prevalent with dairy cattle in areas where copper status is naturally low.
2. Oxidised
Oxidised iron is insoluble in non-acid water. When water containing soluble iron is exposed to air, the soluble iron reacts with the air to form oxidised iron which precipitates if there is sufficient alkalinity present. Usually about 100 ppm of excess alkalinity is necessary for complete precipitation. If surface waters contain iron, the iron is usually oxidised. Well and bore water supplies frequently contain a mixture of oxidised iron and soluble iron because some of the soluble iron becomes oxidised in the pressure tank.
Water containing oxidised iron is filled with red rust when first drawn from the tap. Iron oxide deposits can plug up plumbing and equipment.
The iron sediment formed plus iron bacteria growing in the water can also block pipes and micro-outlets and damage irrigation equipment and small water fittings. Guideline values for drinking water is less than 0.1 ppm-while 5 ppm is considered excessive for irrigation.
Oxidised iron causes less of a metallic taste than soluble iron, but the taste is still objectionable.
3. Colloidal
Colloidal iron is a special case of extremely small particles (less than 0.1 micron) of oxidised iron in water. It is misnamed because the particles are usually combinations of iron and other substances. Like colloids they do not settle out and can't be removed by ordinary filtration. Frequently, colloidal iron is mistaken in appearance for colour rather than turbidity.
When water comes in contact with iron-bearing rock in the presence of decaying vegetation, `conditions for forming this kind of iron exist. Colloidal iron may be present in shallow wells or surface water supplies. It is seldom found in deep bore supplies.
Colloidal iron causes the same problems as other forms of iron.
4. Organic Bound
Iron will frequently combine with tannins and other organics to form complexes which cannot be removed by ion exchange or oxidising filters. Iron which has been sequestered by phosphates will act in a similar manner explaining why phosphate treatments should be placed after a softener if the water is to be softened.
