6. Groundwater use and recharge
6.3. Minor aquifers of local importance
6.3.1. Particular aspects of minor aquifers
Items listed above for consideration for major aquifers apply equally well for minor aquifers, but the importance of the problems may be ordered differently. By definition, minor aquifers are of limited extent and have much smaller reserves. Thus, the impacts on a minor aquifer of a prolonged drought, or of an increase of pumping when water demand rises, would be greater in absolute terms than those on a large aquifer, because minor aquifers are more vulnerable to increases in demand.
A minor aquifer would need far more careful management based on studies of the
aquifer and data on water level, estimates of recharge and yield potential, and aquifer performance under pumping. However the chances are that a minor aquifer would not have received such detailed attention, so information on its performance would be limited. Thus, under conditions of drought, if a minor aquifer is over-exploited, the repercussions could be quite severe. The community depending on it could early on experience scarcity of water with the shallow wells becoming dry and the flow of springs reducing. Exploitation and management issues for minor aquifers are listed in Box 6.8.
BOX 6.8. Exploitation and management issues for minor aquifers Exploitation issues:
• Exploitation and management problems for minor aquifers could be more critical than for large aquifers. Therefore, the corresponding operation rules must be well known and accepted by the communities using those aquifers.
• Minor aquifers are more vulnerable to increased demand; similarly, they are more vulnerable to pollution and intrusion of water of inferior quality and susceptible to the side effects of over-exploitation. Rules for protection of the aquifers should be developed and well known by the users.
• Minor aquifers are more sensitive to climatic conditions. Contingency planning to cope with droughts is required.
• More careful management and control are required for minor aquifers since impacts of over-exploitation could be severe. Upper limits for rates of water removal need to be enforced as part of the operation rules.
Management issues:
• Often, the local minor aquifer is the sole reason for existence of certain communities and their continued prosperity. The involvement of the users in management is therefore essential.
• The sufficiency of minor aquifers in years of drought is of the utmost importance especially in isolated and remote areas such as small islands. The repercussions of water scarcity are more pronounced and abrupt, especially on the occurrence of drought.
Contingency planning has to be implemented through appropriate participation of users.
• Good understanding of the geometry and lithological variations as well as of the aquifer characteristics is essential for development of meaningful management practices.
• Monitoring of ground water levels, water quality trends and estimates of recharge, as well as the control and monitoring of pumping, are highly important.
• A favorable water-balance needs to be maintained particularly during drought. At these times it is important to estimate, ahead of time, the quantities that could be pumped.
Monitoring needs to be more carefully and frequently practiced under those conditions.
• Water balance evaluations help the development of water allocation and water conservation measures to be implemented in periods of water crisis. However, it is important to recruit the agreement and commitment of users when water conservation and saving measures and practices need to be implemented.
• Prudent management of the development and use of local minor aquifers is paramount for to the well being of the communities that depend on them.
Minor aquifers are often associated with small isolated developments of dunes and dune complexes commonly located in coastal areas, with alluvial plains and alluvial
deposits along streambeds, with buried river channel deposits, and mostly in arid zones, with piedmont or foot-hill formations such as alluvial fans. Minor aquifers often exist in islands, mainly those of volcanic origin. Small developments of any other geologic formation could similarly be termed minor aquifers.
A minor aquifer is normally an aquifer of small surface area and thus of small recharge capacity. The qualifying term “minor” does not only refer to the size of the formation, but also to the low water transmitting capacity or to the limited water reserves.
The latter could be due to a combination of lithologic characteristics, recharge opportunity, geometry and morphology of the aquifer.
If the aquifer has small reserves, then the impact of increased demand would be immediate and equilibrium conditions would be disturbed with any marginal increase of extraction. On the other hand, if the aquifer has a low transmitting capacity and thus low yield potential, it is very likely that it would be a system of low permeability and small specific yield coefficient. This will result in great water level fluctuations and, in case of drought, the drop of water level will be quite large. This will have a direct impact on the yield of the wells and the flow of springs, and quite often it would quickly result in shallow wells becoming dry or in reduction of yields from boreholes. This would necessitate deepening of the wells or their relocation to a thicker part of the aquifer.
Minor aquifers discharging to the sea or to other neighboring systems will lose most of their water reserves if low recharge conditions prevail over an extended period. On the other hand minor aquifers in contact with water bodies of inferior quality stand to be intruded to a serious degree if increased demand is placed upon them and the hydraulic equilibrium is disturbed. In effect, minor aquifers are very sensitive to climatic conditions and to any extra burden of water demand that might be placed upon them. Similarly, such small systems are more vulnerable to pollution and intrusion of water of inferior quality.
Localized aquifers with low storage coefficient and small sustainable yield are more susceptible to the side effects of over-exploitation, particularly when drought occurs.
6.3.2. Local importance of minor aquifers and management issues
Many communities and settlements have been established, grew and thrived on the basis of their proximity to an available supply of water. The importance of a local aquifer to some communities is that it is the sole reason for existence of that community and its continued prosperity. This is the case for many oases in desert areas.
Many settlements were established on the basis of an existing local minor aquifer.
The growth of the settlement on many occasions has been controlled by the potential of such an aquifer. When demand for water increases beyond its capacity, water becomes particularly scarce. Augmentation of the water supply from alternate, perhaps distant sources is then a matter of costs and technical feasibility.
In isolated and remote areas such as small islands, the dependence of a community on a local minor aquifer for its domestic supply and irrigation requirements is total. The sufficiency of the aquifer in years of drought is of the utmost importance, which creates great concern to the authorities and the community members (Margeta, 1987).
The vulnerability of minor aquifers to deleterious effects from increased extraction or
from lower recharge during droughts is high, and communities that rely on them stand to feel the repercussions much more than others that depend on larger systems. In the latter case, the effect of an extended dry spell may not be noticed until much later or may not be felt at all if the dry period is followed by a wet season. Minor aquifers are also more vulnerable when technological developments, such as electrical pumps are introduced to replace the traditional extraction by hand or with help of animals. Then much more water can be easily pumped and it can be extracted from much larger depths. Therefore, wells that have been exploited for centuries may become dry or nearly dry in a short period causing enormous problems to populations that have traditionally been relying on them. This may also occur in shallow wells in large aquifers, but generally the consequences may not become apparent so quickly and may be remedied when the management of the aquifer is improved. For minor aquifers, over-exploitation due to use of more powerful pumping facilities may create a non-reversible situation.
Usually there is a contradiction between the importance of a local minor aquifer and the control and monitoring of its performance for allowing the necessary management actions to be taken. For communities depending on minor aquifers, the control of the use of the aquifer usually develops naturally and any new or excessive use of water is usually a matter of conflict for discussion among the members of the community. For larger communities, local or regional government usually places high interest on the management of local aquifers, reflecting their importance for the well being of the community.
Good understanding of the geometry and lithologic variations as well as of the aquifer characteristics is essential for meaningful management practice. Monitoring of ground water levels, water quality trends and control of pumping, together with estimation of recharge are quite important. A favorable water balance needs to be maintained, together with evaluations of the quantities that could be pumped in periods of drought. These evaluations will help the development of water allocation and water conservation measures to be implemented during periods of water crisis. Prudent management of local minor aquifers for their protection and use is paramount for to the well being of the communities that depend on them.