Protection of stored water

Stored water needs protection from evaporation and seepage loss and from contamination.

Losses are discussed in 5.3 above. Contamination occurs mainly from human or other animal contact. If water is to be used for drinking and food preparation no direct or indirect contact with any animals should be allowed. This includes use of buckets to extract water, which have been touched by human hands. Any human or animal contact with the water, or with any surfaces which contact the water, such as buckets or water jars, or lifting ropes or wires, have potential to transmit bacteria to the water which can then cause disease in the water users. Therefore it is much better to use a pump to extract water from the storage.

This applies to surface storages and tanks as well as to wells. Animals and people need to be kept away from surface storages by surrounding them with secure fencing. As outlined above, animal water can be supplied in a drinking trough. Allowing grazing or work animals to drink directly from a surface pond brings contamination and causes the water to be continually turbid.

Birds should also be discouraged from accessing water stored for drinking purposes.

It is impossible to exclude them, unless the surface is small enough to be covered. However

domestic birds and large flocks of wild birds need to be kept away by fencing and frightening.

Similarly stored water needs protection from disease vectors such as mosquitoes, flies and molluscs, which are discussed in section 5.5.5. The simplest insect protection is to keep the stored water enclosed. This is particularly important for water stored near accommodation, such as in rain-water storage tanks. Having adequate water supply close to one’s house is important, but if frequent incidences of malaria are the result it is probably worthwhile locating the storage at some distance.

5.5.3. Sediments

Most human activity leads to increased soil erosion. Most eroded materials eventually find their way to stream channels and into water storage reservoirs and ponds. The best strategy for dealing with sediments is to prevent erosion. Human activity, including farming and herding, involve removal of vegetation and disturbance of the soil surface. Subsequent wind and rainfall set surface soil grains in motion and those moved by water all travel to the natural drainage system, the river. Increase in sediment delivery changes the character of the river. The sediments build up on the river bed, reducing the channel cross-section and changing the hydraulic gradient. Natural energy processes will then work to develop a new state of equilibrium between bed-slope, channel cross section, discharge and sediment movement. The river channel may widen to accommodate the risen bed level, thus introducing more sediments from eroding banks, and/or the gradient may increase, to provide energy to move the increased sediment load. Whatever occurs, the character of the river channel will change, certainly downstream, but possibly upstream as well, as a result of changed gradients. Property of landholders downstream may be greatly affected. Banks may be eroded and the river may even change its course. During floods large amounts of sediment may be deposited on the floodplain, which may be beneficial, but is more likely, at least in the short term, to be unwelcome. This discussion does not apply just to perennial rivers. It applies equally to rivers which flow for only short periods, such as the wadies, and for most of the time are characterised by dry channels.

The more obvious problem produced by sediments is the loss of storage capacity of reservoirs. It is not uncommon for storages to be completely filled with sediment within a few years of construction. Loss of a storage in this manner is a very serious matter. All investment in construction of the storage is lost, but more seriously, one of the few possible locations in a region for siting of a storage is also lost, probably for ever. Where large amounts of water are available it may be possible to manage silt inflows with spillway gates and sediment sluices, but in areas of water scarcity this is not possible.

As discussed in 5.4.2 above the best means of protection of a water resource from sediment problems is protection of the land surface from erosion. This is the only measure that can be taken for reservoirs that have large catchments. For small reservoirs it may be possible to construct a sediment trap upstream of the reservoir as discussed in 5.4.2. The sediment trap will consume some of the available water resource but this may be a small price to pay for increasing the reservoir life from just a few years to decades. After an inflow event it may be possible to save the water held in a sediment trap. Within a few days most suspended sediments will be deposited on the bottom of the trap and the clear surface water can be pumped to the main reservoir. If this is not done a water volume equal to the

capacity of the sediment trap will be lost from each runoff event. The shallow character of the sediment trap means the water held will all be removed by evaporation within a few weeks, before the occurrence of the next runoff event. Since most runoff events are very small, perhaps producing little more than the capacity of the sediment trap, transfer of sediment trap water to the main storage after each runoff event could significantly increase water availability. This practice is not viable in areas where erosion and sediment yields are very high such as the Loess Plateau in Northwest China. When such unfavourable conditions occur, one of the only means of capturing surface water is to construct rock dikes across valleys. These fill with sediment and provide a flat surface for crop growing, particularly trees with deep roots to reach the water stored in the trapped sediments, as was mentioned earlier for the jessours in North Africa, or for what are locally called caags in Somalia and warpings in China.

5.5.4. Water quality

Water quality issues are of utmost importance where the water is for human consumption.

However some aspects of water quality are of importance for almost all water uses. For human consumption bacterial contamination is of great importance whereas chemical quality is of importance for almost all uses. In regions of water scarcity where there is little or no choice of water source it is important to protect the quality of the available water, and in some cases to prevent its contamination.

Most bacterial contamination results from contacts between the water and animals, particularly humans. Human and animal wastes need to be totally separated from the water resource. It is quite common for much of the bacterial contamination of water to occur at or after its collection from its natural source. For example, water collected in jars becomes contaminated by hands which lift the jars or which haul on ropes used to raise the water to the jars. Similarly at the collection point any spilled water which drains back to the water body is almost certain to contain bacterial contaminants. Great care (and education) is needed to prevent these and many other simple causes of drinking water contamination.

In many situations household wastewater is returned to the water course without any treatment. When this wastewater becomes a significant part of the total flow (say more than about 5%) the bacterial quality of the resource quickly declines, to a point where it is of little value to any user and it can become a serious health hazard. It is imperative that human wastes not be allowed to enter streams, but that dry pit toilets be used or that simple water treatment systems such as septic tanks be interposed between households and the stream system. Without universal use of such methods the scarce water resource of a region can be completely destroyed by a few careless or selfish households.

Bacterially contaminated water can be disinfected by introduction of a small amount of bleach or by boiling. However if it is the household’s responsibility to carry out its own disinfection, then health problems are likely to be frequent because disinfection is expensive and needs careful attention to detail, meaning that the protection system is likely to be much less than perfect.

Chemical contamination has many causes. The most common are contact between the water and deposits of chemicals on or within the soil and rocks over and through which the water flows. Various salts are highly soluble in water. Unfortunately, around the world,

there are communities whose water supply is contaminated with naturally occurring toxins such as arsenic, nitrates and fluorides, to name just three. These often occur in water supplies at levels too low to have an objectionable taste, but high enough to cause severe health effects in the long term. In situations where chemical contamination is a possibility the population needs to be encouraged to make alternate arrangements for drinking water, such as collecting roof runoff, perhaps in pots and jars. In the arid zone and regions with long seasonal dry spells quite large storage is required – enough to supply 10 litres/day/household member for the duration of the dry season, i.e. about 2000 litres per person.

Fine sediments, particularly those of colloidal size which produce turbidity, are often a source of contamination. The sediments themselves may be harmless but many forms of contaminants, both bacterial and dissolved salts often adhere to the microscopic particles.

Allowing water to stand for a few days and then removing for consumption only the clear water from the top can avoid many of the dangers. More effective is filtering the water through sand or a membrane. Filtering can be very effective in removing all particulate contamination but it is expensive and needs use of some specialised equipment. However filtering is totally ineffective in removing dissolved contaminants such as various salts (NaCl, CaCO3) which have not adhered to the surfaces of particulates.

Stored water, particularly if located near a residence, needs protection from insects and other vermin. This can be achieved by either totally enclosing the stored water or by installing and carefully maintaining insect screens.