|
|
Water systems research group, University
of the witwatersrand
P.O. Box 44961, Linden, 2104, RSA
Tel: +27 11 716 2516 (o/h)
Fax: +27 11 339 1762
EMail: Bruce@Civen.Civil.wits.ac.za
Many regions in Southern Africa have
water shortage problems. Due to the
financial constraints of the region,
adding new water supply structures is virtually impossible. Populations dependent on these water sources
are rapidly expanding, putting greater pressure on an already overstressed
system. Conjunctive use schemes can
increase the safe yield of such water supply systems, supplying additional
water at a lower cost, than if a whole new water supply system was constructed. Provided suitable aquifers are located,
groundwater reserves can be recharged and used to supplement surface water
resources.
A model that simulates a general
conjunctive use scheme, utilising groundwater and artificial recharge is
currently being developed and will apply to any general conjunctive use project
that utilises surface water as the major water source, with groundwater to
supplement it.
Keywords: Conjunctive use, Artificial
recharge, Ground Water, Increased yield, Computer model
Introduction
The average rainfall in Southern African
is about 600mm per year, varying from over 1000mm/year in the east to less than
125mm/year in the west. With an
ever-increasing population, many of the existing water supply systems are
becoming, or have become inadequate, and are failing to meet the demand placed
on them. Although many potential dam
sites have been identified, exorbitant construction costs prevent most of these
projects from getting off the ground.
South Africa's minister of Water Affairs has stated publicly that no new
reservoirs should be constructed and the emphasis placed on more efficient use
of the existing resources. Namibia and
Botswana are in a position where the lack of water within their borders is
restricting their population and economic growth. Botswana is in the process of constructing the "North South
Carrier", a pipeline that transports water from the Letsibogo Dam to Gabarone,
a total length of over 800km. Namibia
is looking at taking water from the Okavango river, causing both political and
environmental problems for the sensitive Okavango Delta area. The Lesotho highlands water project has been
constructed, which delivers water from Lesotho to Gauteng. Additions to this project are envisaged and
a scheme to take water from the Zambezi river is also being looked at. A canal from the Zambezi river is also
planned to augment Bulawayo's dwindling groundwater supply.
Although conjunctive use is not a
solution to all water shortage problems, many can be alleviated or even
eliminated. South Africa, in particular,
is very dependant on surface water, and with many sites available for
groundwater abstraction, conjunctive use can be used in a number of areas to
alleviate water shortage problems.
why use groundwater?
Groundwater has, in
the past been predominantly used for agricultural purposes, but is fast
becoming an important industrial, commercial and residential water source. Most groundwater is relatively pollution
free, sediment free and, if stored in dolomite formations, is harder,
containing more minerals and salts.(Walton, 1970) Many dolomite formations contain massive underground caverns,
that can be used to store large amounts of water, providing a relatively cheap
alternative to surface reservoirs, with no evaporation losses or siltation.
(Paling, 1985)
Good aquifer management is essential if damage to aquifers is to be avoided. Before groundwater can be used, extensive tests should be done to determine aquifer properties such as yield characteristics, hydrogeological and hydraulic properties, the effects of permanent abstraction of water and the safe yield of the well and the aquifer. These together with other parameters give an indication of how effective a well, or well field will be. Over extraction or mining can have serious and sometimes fatal consequences. Groundwater replenishment in the form of artificial recharge can prevent such potential disasters, and both man and the natural community could benefit from these groundwater reservoirs.
Artificial
recharge
Artificial recharge works in the same
way, except in a more controlled environment.
Water used for recharging can come from a number of sources such as
storm runoff, river water, overflow from existing reservoirs, imported water
from distant water sources, or wastewater.
Treated wastewater is a good recharge source due to its constant
guaranteed flow and consistent quality.
Recharging began in Europe and the United States as early as the 1800's
and since then recharge projects have steadily increased throughout the
world. Recharge basins form an integral
part of many Swedish municipal water supply systems (Jansa, 1952) and is a
practice widely used in Germany to meet industrial and municipal water
demands. In the Netherlands, water
supply systems for Amsterdam, Leiden, and The Hague include basins for
recharging surface water into coastal sand dunes. (Biemond, 1957) Today, in California alone, some 276
artificial recharge projects operate in areas
where groundwater has been extensively exploited. (Task Group on
Artificial Groundwater Recharge, 1963)
Many countries such as Australia have strict laws preventing any form of
discharge into rivers, regardless of the quality (Mathew, 1982) and using
treated wastewater to recharge aquifers is an effective means of waste water
disposal.
Artificial recharge using
wastewater
As discussed previously, wastewater can
be used as a recharge source, with the soil acting as a filter, removing
pathogens. Pathogens are disease
causing agents, that have to be removed from a water source before it can be
classified fit for drinking. These
pathogens include; nitrogen and phosphorous in their various forms, B.O.D.,
bacteria and viruses, heavy metals, boron and fluoride. The level of pre treatment that is
necessary, to avoid contamination of the groundwater is determined by the
loading rate, the type of soil that is present at the recharge site and the
infiltration rate. The length of flooding is dependent on how much ammonia will
be allowed to enter the groundwater with the minimum period determined by the
denitrification process. The rate of
denitrification is determined by the temperature and the availability of
organic carbon present in the sewage effluent.(Mathew, 1982) Organic carbon, in the form of primary
effluent or another source, can however be added to improve the denitrification
process shortening the flooding period while treating the same quantity of
effluent. The duration of the
denitrification process is governed by the infiltration rate and the distance
to the water table. Essentially, the
longer the nitrate remains in the region above the water table, the longer the
denitrification process will last, as no further denitrification occurs below
the water table. This is due to the low
pH and availability of organic carbon. (Mathew, 1982)
Water rationing, although often necessary, can be
damaging to a country's economy and in extreme cases, damaging to human
health. It is essential that water
rationing be minimised, and available resources used optimally. In areas where these resources are fully
developed, the conjunctive use of alternate water resources can increase the
firm yield of such systems, reducing the need for costly water rationing.
A common application of conjunctive use
is the use of groundwater to supplement surface water. Surface storage in reservoirs behind dams
supplies most annual water requirements, while groundwater can be retained
primarily for cyclic storage to cover years of subnormal precipitation. Thus groundwater levels would fluctuate, being
lowered during a cycle of dry years and being raised during an ensuing wet
period. During period of above normal
precipitation, surface water is utilised to the maximum extent possible and
also artificially recharging into the ground to augment groundwater storage and
raise groundwater levels. Conversely,
during drought periods limited surface water resources are supplemented by
pumping groundwater, thereby lowering the groundwater levels. The feasibility of a conjunctive-use
approach depends on operating a groundwater basin over a range of water levels;
that is, there must be space to store recharged water, in addition, there must
be water in storage for pumping when needed.
Computer model
Most conjunctive use models are written
for specific applications. The
advantages of this is that many of the unknowns and variables associated with
such projects become known, making modelling easier and more accurate. The user does not have as many decisions to
make, and more comprehensive solutions are calculated by the model. A model that has been designed for general
applications have many more variables and unknowns, and produces a solution
that requires more interaction and involvement by the user, and is usually less
comprehensive. There exists a trade off
between generality and comprehensiveness.
Various computer modelling software has
been written in the past to model various aspects of a conjunctive use
schemes. Such programs include "ROPT",
which was written to produce operating rules for the Min Der reservoir in
Taiwan, and "RAFFLER", which calculates river runoff from rainfall data.
A computer model that simulates all
aspects of a conjunctive use scheme, has been partly written. It has been written in Delphi, a Windows
based, object orientated, Pascal based programming language. All changes and calibrations are done
through the user interface, making it a user friendly, visual package. Provision is made for the program to be used
beyond the borders of South Africa, with a choice of currencies and metric or
imperial units of measurement that will be used throughout the program.
The program is divided into several
modules, each being able to run separately, or together in the optimisation of
the system. The "Raffler" module is
used to calculate river runoff from rainfall, and incorporates "RAFFLER "(D.
Stephenson and W.A.J. Paling, 1992) which has been modified and rewritten from
the original GW Basic code into Delphi.
The required information is put into the program with the rainfall data
read from a user specified file. The
output file is user defined and the runoff data is read to this file for
analysis at a later stage. The user is
able to print the output in an easily legible format. The "Surface Reservoir", uses Gould's Modified Matrix method to
help the user optimise the operating rules and maximise the yield of the
reservoir, with provision being made for artificial recharge from excess
surface water. The "Subsurface
Reservoir" module is dedicated to groundwater and aquifers. Results from two types of pump tests,
time-drawdown and distance-drawdown may be used to determine relevant aquifer
properties. Provision is made for
artificial recharge, with the program able to calculate the cost of the whole
process from pumping and transport costs to treatment costs if necessary. The program, after taking figures such as
pipe sizes, leakage, inflation rates and economic factors into account, uses
built-in cost curves to calculate the cost of transportation, if necessary, of
the recharge water. These costs are
added to various other costs such as collection costs and treatment costs to
get the overall cost of recharge. Once
all of the above information has been processed, along with the well field
arrangement, the maximum groundwater yield is calculated. Both the surface and the groundwater yields,
along with relevant economic considerations are then put into another
optimisation model that determines the optimum operating rules for each water
resource, providing the maximum yield.
A built in optimisation model will be
written, eliminating the need for third party models. This will ensure faster optimisation. All output is in graphical and text based format, making easy,
accurate reading.
Model Flow
The principal aim of the model is to
obtain two curves:
1.
The
Aquifer Volume Vs Groundwater Yield, shown in figure 1
2.
The
Surface Reservoir Vol Vs Surface Water Yield, shown in figure 2
The combination of these two curves
produces a third curve (Figure 3) that when combined with the draft, describes
periods of recharge and supplementation. From these, a fourth curve can be
generated that describes the operating rules of the whole conjunctive use
project. See figure 4. The two recharge operating rules shown
(A.R.1 & 2) represent different quantities of excess water that can be used
for artificial recharge, depending on the prevailing groundwater volume. The curve also describes the quantity of
groundwater supplementation needed (G.W.) to supplement surface water (S.W.).
A simplified flow diagram illustrating
the flow of the model is shown in figure 5.
Conclusion
When a country no longer has the
resources or space to construct further reservoirs, cheaper alternatives of
water storage must be found.
Groundwater reservoirs provide ideal storage facilities with no problems
associated with siltation or evaporation losses. With the ever increasing populations water treatment is becoming
more and more important. This is
particularly so in developing countries where overpopulation is a problem. Recharging
of groundwater reserves using wastewater will alleviate some of the problems
relating to water treatment in developing communities.
The software being developed will be used in any general conjunctive use project that utilises surface water as the major water source, with groundwater to supplement it.
References
1.
Biemond,
C., Dune water flow and replenishment in the catchment area of the Amsterdam
water supply, Jour. Inst. Water Engrs., v. 11, pp 195-213, 1957.
2.
Jansa,
V., Artificial replenishment of underground water, Intl. Water Supply Assoc.,
Second Cong., Paris, 105 pp., 1952.
3.
Mathew,
K, Newman, P.W.G., Ho G.E. "Groundwater recharge with secondary effluent"
Australian Government Publishing Service, 1982.
4.
Paling,
W.A.J. "System analysis of conjunctive use of groundwater, wastewater and
surface water for the Witwatersrand". Water systems research program,
university of the Witwatersrand, 1985.
5.
Task
Group on Artificial Groundwater Recharge, Artificial groundwater recharge,
Jour. Amer. Water Works Assoc., v. 55, pp. 705-709, 1963.
6.
Todd,
D.K., Groundwater Hydrology, John Wiley and Sons, 1980.
7.
Walton,
W.C. "Groundwater resource evaluation".
McGraw-Hill, Inc, 1970.
Fig 1.
Illustration of groundwater yield Vs aquifer volume
Fig 2.
Illustration of surface water yield Vs aquifer volume
Fig 3. Illustration of total
yield Vs total vol. and periods of recharge and supplementation
Fig 4. Illustration of the
model's operating rules
Fig 5 Simplified flowchart of
operations for the proposed model