Maximization of benefits from Africa’s ground water and aquifers
Introduction
Groundwater and aquifers are among the most under utilized natural resources in Africa. The situation was probably the same some decades ago in most other parts of the world. Groundwater and aquifer represented sources of water for drinking, domestic uses, irrigation and for industrial plants and machinery.
Groundwater/aquifers are versatile resources and cannot be useful only in providing portable water.
There are other uses for groundwater and aquifers which are not related to water supply.
Edworthy and Puri (1986) observed that there are four distinct uses which have been termed to as exotic’ uses.
These are:
i) Fluid storage and waste disposal, including deep well injection
ii) Energy conservation including storage of fossil fuel generated energy (as heat or pressure)
iii) Extraction of solar energy from groundwater
iv) Subsurface in situ leaching of economic minerals
v) Groundwater quality management
There are other possible uses. Groundwater and aquifers are multipurpose resources. Many countries in the world are already deriving benefits from this concept of the resources.
USA appears to be the leader in harnessing these results. There are numerous injection wells in use in that country.
The records indicate that most of African countries are yet to appreciate the concept of exotic’ uses of groundwater and aquifers as well as the other useful applications.
I have vainly urged hydrogeologists to re-channel their expertise and imputes in this direction. The patios of hydrogeologists is bewildering and the professional reflexes are extremely sharp this is especially true of Africa’s hydrogeologists.
This paper attempts to draw the attention of the hydrogeologists and other relevant professionals to the vast benefits of this approach.
Exotic uses for groundwater and aquifers
i) Fluid storage
This involves re-injection of fluids e.g. brine. Air and other gases, surfactants and solvents have also been used for re-injection.
ii) Waste disposal
The manufacture of chemicals as well as processing of metals and radioactive materials produce wastes which are either untreatable or very difficult to treat for conventional surface disposal. It appears that deep well injection is the only economically acceptable disposal option.
iii) Fluid storage for subsequent recovery
The storage of a range of gaseous materials for eventual recovery is a common practice in the USA and Europe.
iv) Energy extraction and conservation
Energy may be extracted from groundwater. This energy resource is unique when compared to fossil fuels as it is naturally renewable. Thermal energy can also be stored in aquifers.
v) Mining of soluble minerals present in porous host rock.
This involves in-situ leaching of metals from the host rocks. The feasibility of mineral leaching is determined by a number of factors, one being the geological complexity of the area. It is important that all the mineral body is accessible to leaching solvent.
vi) Groundwater quality management
A number of process may be contemplated. For example, polluted river water may be treated prior to recharge. Subsequent abstraction has several beneficial effects on the river water.
a) Bacteria and viruses are removed.
b) Soluble organic components are degraded.
c) Peak concentration of dissolved salts are reduced.
Other features
A) Aquifer Petrophysics and Hydrocarbon Evaluation
Worthington (1981) discussed the relationship of aquifer petrophysics to hydrocarbon evaluation. Well log evaluation for fluid hydrocarbons has always drawn upon petropyhsical relationships for the aquifer zone, where the water saturation unity and the resultingly less complex petrophysical regime can be used for control.
The dependence on aquifer petrophysics has traditionally centred on Archies equation, which draws on electrical resistivity concepts and thus valid for only clean sands. Perhaps research could describe more generally acceptable equations.
Hydrogeologists may profitably join geophysicist in the research recommended.
B) Economic design of wells
Stoner et al 1986 made useful suggestions on strategies for reducing costs of water delivered from wells. The procedure involves paying adequate attention to the design of wells and making use of the unique hydraulic properties of the aquifer well configuration. The basic principles involved is that capital investment in a well can affect the hydraulic performance in that capital saved is reflected in higher pumping costs. A good example is that a short screen section in the aquifer will produce a larger draw down than a long screen for a given discharge. Differentiation of the value expression with respect to a design parameter leads to the determination of the optimum value for that parameter.
Action by Africa’s hydrogeologists
The foregoing indicates the need for modifying the objectives in exploiting the groundwater/aquifer resources. The special techniques acquired by hydrogeologists should not be deployed entirely on issues of potable water. some hydrogeologists play’ extensively with terms and cherish creating models or stating and re-stating equations and laws.
A few examples will suffice;
Darcy’s Law
A derived formula for the flow of fluids (not only water) on the assumption that flow is laminar and that inertia can be neglected. [Also Reynold’s] number is relevant.
Darcy Weibsbach Formula
A formula for determining the loss of lead in flowing water. Loss of lead is taken as being equal to a coefficient that varies with the surface roughness of the conduit divide by its diameter, times velocity lead of flowing liquid.
Groundwater equation
The equation attempts to balance the groundwater budget:
R = E + S I
Where
R = rainfall
E = evaporation and transpiration loss
S = water discharge from the area as stream flow
I = recharge
Further currents:
There are other equations and laws as well as an arsenal’ of definitions and techniques. Regrettably, the current approach to groundwater resources involves averaging’ of properties of groundwater/aquifers to suit models or to agree with laws and balance equations. Some of the parameters given are costly to obtain or verify.
Hydrogeologists should provide detailed information on a site specific basis which must embrace geostatistical techniques and various studies of the aquifer such as the sedimentology. Computer simulation which also be useful.
Maximization of benefits
The exotic’ uses of groundwater/aquifers will provide immense benefits to countries in Africa or elsewhere who have not yet put the same in their programme.
A multi-disciplinary approach is essential in maximizing benefits from the same. Hydrogeologists, water engineers, sedimentologists, geophysicists and other professionals need to work together on various relevant parameters.
Hydrogeologists are obvious leaders of the working parties. However, they must deploy additional techniques to the traditional methods of sampling in order to obtain detailed relevant data on a site-specific basis.
Regarding the mining of soluble minerals, there may be potential hydrodynamic and geochemical complexity in the operation in addition to uncertainties of the lithology and structure of the subsurface. In spite of these, more than 80% recovery is possible.
Hydrogeologists and water engineers should employ economic principles to minimize the costs of water delivered from wells. The value of aquifer petrophysics in hydrocarbon evaluation is noted.
Conclusions
This paper may be useful in countries that have not yet developed programmes related to the exotic’ uses of aquifers. The benefits from groundwater and aquifers in such countries will be maximized by programmes that extract water for domestic and industrial uses (potable water supply) as well as exotic’ uses of aquifers. Aquifer petrophysics is of value in evaluating hydrocarbons.
References:
Edworthy K. J and Puri S 1986 Groundwater and aquifers; an overview of exotic’ uses: Quart. Journ. of Eng. Geol. Vol. 19 pp 87-95.
Stoner R. F., Milne, D. M and Lund P. J. 1979- Economic design of wells: Quart. Journ. of Eng. Geol. Vol. 12 pp 61-78.
Worthington P. F. 1986 The relationship of aquifer petrophysics to hydrocarbon evaluation: Quart. Journ. of Eng. Geol. Vol. 19 pp 97-107.
