The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the ...
A Preliminary Assessment
The Turks and Caicos Islands (TCI) are located 575 miles southeast of Miami and 100 miles north of Hispaniola. The capital, Cockburn Town, is on Grand Turk Island. These islands are expected to experience a real estate and tourism boom1 and water will be a limiting factor in this expected development.
There is no firm number that automatically classifies an island as being large, small or very small. From a water resources point of view2 any island larger than 2,000 km2 in area "large." Most "small" islands are less than 200 km2 in area, with those islands of less than 100 km2 falling into the "very small" class. Grand Turk, with an area of about seven square miles, is obviously a very small island.
Because of their size, geology, topography and climatic conditions, small islands have serious problems related to the availability of water.3 On limestone islands such as Grand Turk, which are mostly flat and low lying, there are no possibilities for large scale surface storage. In addition, groundwater may be present in thin lens-shaped bodies. These lenses are found in a very delicate equilibrium with saline seawater and can be destroyed by improper extraction, drought or tidal waves. Not having opportunities for surface storage and very limited groundwater reserves, small islands suffer much more from the effects of drought than larger countries.3
Providing a dependable supply of water on small islands is difficult. On most of these islands, water demand has increased in all economic sectors and competition for the limited water resources has developed between different users. Whereas in large continental land masses a single water resource type may be enough to satisfy most demands, to solve water supply problems in small islands it is necessary to develop several types of water resources.3
Investigating and assessing the water resources potential in small islands, planning a rational exploitation of the resources and designing, constructing and operating water supply systems poses a huge challenge to these islands. While it may require the support of the international community, the development of national human resources and the establishment of cooperation networks among these islands, due to the singularity and the similarity of their water resources problems, is paramount.
Sustainability must be the goal of water management for small islands. Current water policies and management practices are not sustainable from any perspective: social, economic or environmental.4 Fortunately, an international consensus has emerged about the principles for improving water management. These principles have been endorsed at conferences on water and the environment in Dublin and Rio de Janeiro.5,6 Some of these principles are
Two concepts are essential to translate these principles into practice: integrated water resources management and water demand management.
Integrated Water Resources Management is a process that aims to ensure the coordinated development and management of water, land and related resources by maximizing economic and social welfare without compromising the sustainability of vital environmental systems.7
Water Demand Management comprises technical, regulatory and economic measures that affect water use and reduce waste and losses. In general, these options enable rising levels of consumption to be met without major new investment, and they also avoid serious environmental costs.8
In such a very small island as Grand Turk, the application of these concepts is critical due to the complexity of the context in which its water resources are managed.
Water Resources Development and Demand
Due to Grand Turk's topography and geologic conditions, surface water is excluded as a possible source of water. The importation of water is used only as an emergency measure. However, this could be an interesting possibility as shown in the "Medusa Bag" project.
The water distribution system currently is being developed. It consists of storage tanks and related catchment areas, the reverse osmosis plants and a main pipe linking all the tanks. This pipe has been used primarily to supply tanker loading points, stand points for the population and for transferring water from one part of the island to another. The connections to this system are increasing considerably and a new main pipe is being laid to increase the reliability of the system and to supply other areas of the island on a 24 hours basis.
The main sources of water in Grand Turk are rain water; reverse osmosis (RO) desalination plants; non-potable water resources such as saline, brackish and "gray" water; and groundwater.9
Rain water harvesting is an ancient and sometimes forgotten practice.10 In Grand Turk it is still the main water source and is widely developed at the household and community level. It is one of the treasures in water management in Grand Turk that should be carefully preserved.
Almost all buildings and houses have their own rainfall collection systems composed of roof catchments, gutters and cisterns. A roof frequently also covers these cisterns for catchment purposes.
The water supply system also is supplied largely by rainfall. There are 27 tanks with a storage capacity of almost 5 million gallons supplied by three major ground catchments, two minor ones and others fed by roof catchments.9 RO plants also play an important role in the water supply.
Despite the advantages that could be obtained from the use of the airport runway as a ground catchment area for rainfall, it is not used for this purpose as it is in Kwajalein and Majuro, in the Marshall Islands and Antigua in the Caribbean.3 Stanley Associates Engineering Ltd.11 carried out an excellent and detailed study of the possibilities of all airfields in TCI, estimating that in practice, a yield of about 4,050 m3 per year could be obtained from Grand Turk's airfield. However, the quality of the water obtained from this source would be affected by oil spills and other strange matters. This water could be treated or used for non-potable uses. For example, the existing car washing facilities next to the airport could be supplied by this source.
Since there is no guarantee that rain will fall in the volume needed to fill all storage systems (both public and residential) Turks and Caicos Islands Government took steps to meet this shortfall by purchasing a total of three additional RO units.9 These units have a total production output of 180,000 gallons per day and are operational. The operation and maintenance of these plants is a very complex task12 and requires constant qualified surveillance that cannot be guaranteed by the crew that is currently in charge of the system's operation. It is interesting to remark that the total production of these plants divided by an estimated population of 5,000 inhabitants, gives a per-capita of about 136 liters per day. This amount seems adequate for Grand Turk, considering that the rainwater collection systems are the principal source of water supply and that salt water flushing is being encouraged. Therefore, the emphasis in the management of the water supply system should be to increase the reliability and the efficiency of the operation of these plants rather than installing new ones.
These plants are high-energy consumers, which increases the costs of water supplied (e.g., in Grand Turk one kW-hr is sold at 0.27 USD). In Bermuda, an interesting study was carried out13 for using a municipal solid waste incinerator equipped with heat recovery equipment as a thermal power plant fueled by solid waste. The energy produced could be used to supply RO plants. If this plant was feasible it alsowould contribute to minimizing the environmental impact of solid waste disposal. The most important measure is to look for means to reduce the energy consumption of RO plants such as installing energy recovery turbines in those plants that do not have these devices.
Non-Potable Water Resources
Non-potable water resources may be used in different ways to alleviate the stresses placed on potable sources. They can be used for many purposes3 including toilet flushing, fire fighting, recreation (i.e., seawater swimming pools), cooling in industrial plants and air conditioning, electric power generation and freezing (especially in fishing industries). Typically, non-potable water is derived from seawater or brackish groundwater, treated wastewater or "gray" water.
In Grand Turk some zones (Palm Grove, South Base and the Ridge) are partially supplied with salt water for flushing purposes. Since plastic pipes and fittings are used for the system, the risk of corrosion is minimal. The houses and buildings with these pipes have experienced important savings of fresh water. This saving has stimulated the decision to build a parallel distribution system of salt water. Wells are planned to be bored in each community for extracting water and through small-pressurized systems, homes and other buildings could be supplied with salt water for flushing toilets. In some poor communities, this supply is conveniently considered to be provided free of charge to reduce the health risks from currently poorly constructed pit latrines.9 Salt water is found freely throughout the island.
It also is being considered to make mandatory that all households requiring a connection to potable water also have an unmetered salt water connection. A parallel salt water distribution system also would have to be looked at to complement fire-fighting needs.
There already are some houses in which there is a separate system of "gray" water for garden's watering purposes.14 It is a very positive feature, derived from the use of a separate system of salt water for flushing, that automatically creates the conditions for implementing the gray system. However, this gray water must not be used for edibles such as vegetables and the systems should be designed to minimize health risks.15
Groundwater is not used for a fresh water supply in Grand Turk. Previous studies discarded the possibility of this source.11 Some dug wells produce brackish water, used for cattle watering. The quality and potentialities of these sources should be assessed to find a more convenient use for them. Some simple treatments or a controlled mixture with the water of the system could be a way to develop them.
Some investigations16 about fresh water lens behavior have established the factors that influence its formation. These factors include the amount and distribution of rainfall; the amount and type of vegetation; the island size, especially width from ocean to lagoon side; hydrogeological factors (permeability, solution cavities and unconformities); tidal movement; the height of the island above the sea level and the width of the reef.
A very simple method to assess the existence of a fresh water lens was developed by Falkland.16 It establishes a relationship between the island width, annual rainfall and depth from water table to the mid point of the transition zone (denoted as lens depth). This relationship is expressed by the following equation.
Lens depth (m) = Annual rainfall (m) [6.94 log island width (m) - 14.38]
An island with a width of 300 m and an annual rainfall of 1.5 m should yield a depth to the transition zone mid-point of only 4.2 m. In these islands the transition zone is often thick or thicker than the freshwater zone. Therefore, 300 m is considered a good indicator of the minimum width for a fresh water lens to develop.
There are some zones of the island that meet this minimum and the assessment of its potentialities for developing freshwater lens should be explored. The exploitation of these lenses in coastal zones should be ruled by sustainability17, which in this case means basically to avoid seawater intrusion. PÚrez Franco18,19 has developed new and more effective methods for the calculation of the maximum flow to be abstracted from wells and trenches for preventing seawater intrusion, that should be considered in the design of these works.
There also is an interesting possibility that is associated with wastewater reuse. In Bermuda, recharge from unsewered urbanized areas appears to be about twice than the one occurring under naturally vegetated areas.3 In the exploration and assessment of the groundwater potentialities of Grand Turk, this should also be taken into account.
Evaluation of Water Demand
The evaluation of water demand requires a knowledge of current use patterns; the extent to which current demand for water is being met; and projected trends in water demand resulting from population increases, development projects, increases in tourism, etc.3 It often is difficult to determine projected increases with any degree of accuracy.
Because it will not always be possible to supply water to meet all demands, priorities must be set as to which demands are to be satisfied first. On most islands, domestic and municipal water supply represents the greatest water use. In Grand Turk, irrigation for agriculture is almost absent. Consumers of domestic and municipal water supply include domestic dwellings; tourist accommodations; offices; commercial outlets and institutions; and industry. On many small islands water use due to tourism is the largest proportion of the total.
Domestic water use is a function of the population to be served and the per capita use. These numbers can vary greatly from island to island. A detailed analysis of water use made in Maldives for 19763 is shown in Table 1.
One observation from these figures and from other islands is that domestic water increases by about 50 percent for toilet flushing. This has stimulated the use of non-potable water for this purpose.
Despite the systematic record of the consumption of the several users connected to the water supply system in Grand Turk, there are no estimates of the demand. It led Reed20 to make some rough estimates for his studies about the water distribution system of Grand Turk. Studies in this field should be carried out immediately. In the meantime it may be considered that minimum requirements of 50 liters per capita per day for domestic uses are appropriate in most cases.3 An absolute minimum of 20 liters per capita per day could be applied in the case of short-term emergency supplies.
Water quality problems on small islands are extensive and varied. In many cases they are similar to the ones experienced in larger islands and continental land masses but often are more severe, because of the more limited opportunities for water supply development.
While the water quality of many rainwater systems is good, pollution can and does occur. Physical, chemical and biological pollution of rainwater systems are common when inferior materials are being used, or when maintenance of roofs and other catchment surfaces, gutters, pipes and tanks is lacking. Debris washed off roofs into open tanks can cause physical pollution, leading to potential problems with pipework, pumps and filters. Some chemical ion concentrations, particularly those derived from roof and tank materials, can exceed desirable or even acceptable drinking water standards. Rain water tanks or cisterns are particularly susceptible to biological pollution. This is due to the difficulty of keeping roofs and gutters clean and the relative long retention time in such storages.3
Another common problem is the presence of foreign matter such as dead insects, birds or small animals, or litter from nearby trees, that may have entered through defective screens or covers, affecting taste, odor and color. A particular source of taste and odor problems is the casuarina tree, of which the fine needle-like leaves can enter through screens into storage tanks. Roofing should be well built with sufficient slope to prevent forming local depressions that cause the build-up of debris and possible pollution. A first (or foul) flush by-pass system can prevent the major roof debris and contaminant loads from entering storages.
In many cases, steel-based roofs and tanks cause high concentrations of iron and lead ions. Paints with relative high content of lead have been used. If paints are not allowed to dry and harden properly, highly alkaline water can result. That is why it is recommended not to paint roofs and the inside of gutters.
If tanks are not well maintained, they may contain water with higher than acceptable bacteriological counts or mosquito-borne viral diseases.
In ground catchments, care should be taken to prevent pollution from debris and other sources. For this reason, trees always should be removed from the immediate vicinity of the catchment area, and some screening should be considered.
In Grand Turk roofing materials are suited for catchment purposes. Since there is no tradition of first flush by-pass systems, it would be very difficult to implement one now. Many of the roofs of the houses' tanks and catchments are iron-based and sometimes corroded. Therefore, high iron ion concentrations must be expected in the water. One of the first measures taken when funds are available should be to replace the most damaged roofs. Less damaged roofs should be painted with lead-less paint. The inlets should be provided with screens and subjected to a maintenance program including a regular cleaning every 6 to 12 months. An educational campaign on household rainwater collection systems also would help preserve water quality.
An interesting feature of the water distribution system is that high quality water produced by the RO plants is mixed with lower quality rainwater supplied by the system and at its final destination, with the building's rainwater collection system. Therefore, the rainwater must be subjected to treatment.
Considering that the storage capacity seems to be more than enough, some of the tanks could be operated as settling tanks with minor adjustments. Filtration also must be considered9 including the possible restoration of some existing facilities.
Disinfection is necessary to ensure adequate microbiological quality. However, the study of Water Resources and Water Supply for TCI carried out by Stanley Associates Engineering, Ltd.11 stated that the disinfection of public water supplies is not practical mainly due to the mixture of water from different sources and qualities in the consumption points. The solution is to guarantee disinfection at all levels of consumption.
Ultraviolet irradiation and ozonation have been considered attractive in Grand Turk since there are no taste or odor effects. However, both processes have high power requirements. There also is a lack of a residual to provide additional protection and permit monitoring to be performed.
Since in Grand Turk vending kiosks and stand points for cistern trucks coexist with the water distribution system, the residual is necessary to prevent contamination in the pipe network and in the transportation from the kiosks to the household. This residual is produced only by chlorination. This residual also is necessary because this water is mixed with the household storage water. While the population in Grand Turk complains about the taste and smell that chlorination leaves, there seems to be no other choice. Maybe it is possible that a combination of ozone with chlorination would imply a reduction of Chlorine injection and that it would help to obtain non-Chlorine tasting water.21
Drinking water safety is a problem that must be carefully and urgently studied in Grand Turk. Because of the mixing with the household water, quality standards may be difficult to control. Alternatives comparing the current cost of importing bottled water for drinking purposes versus the possibility of a local production associated with the RO plants or stimulating the installation of household purifying systems should be studied. At the household level there are examples of local disinfection, but public health and public works departments should coordinately devise a system to encourage and facilitate local disinfecting of all dwellings' water storage.
A water-quality monitoring network should be implemented in coordination with public health authorities. The necessary laboratory facilities need to be installed as soon as there are funds available.
A study of the morbidity to water borne diseases in Grand Turk should be coordinated. These figures should be constantly correlated with the results of the water quality monitoring of the system and with the progress in the building of the system and in the implementation of water treatment facilities.
In Grand Turk there is no sewerage system. A recent report to the WHO roughly estimated that about 52 percent of the dwellings use septic tanks for wastewater treatment and the rest pit latrines.
For a list of references, go to our article archives at www.waterinfocenter.com.