EFFICIENT WATER-SUPPLY DEVELOPMENT AND MANAGEMENT FOR SMALL, ARID, OCEANIC ISLANDS BASED ON WATER USE

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EFFICIENT WATER-SUPPLY DEVELOPMENT AND MANAGEMENT
FOR SMALL, ARID, OCEANIC ISLANDS BASED ON WATER
USE

• Espen Ronneberg,
• UNDESA

Allen L. Zack, Hydrologic Consultant for UNDESA
towards the sustainable development of water and
the reduction of waste
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Many SIDS have inadequate freshwater supplies,
yet lack the financial and technical resources to
implement seawater desalination for all of their
population.Non-potable water uses have been
sought for brackish and gray wastewater to
moderate the demand for potable water.The
sustainable development of water and the
reduction of wastewater in SIDS can be improved
by matching appropriate water-quality requisites
to the various water-use sectors in order to
accommodate both potable and non-potable water
supplies.
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There can be no single strategy for appropriate
water-quality partitioning based on use.The
amount of saltwater (chloride concentration) or
other contaminants present in the water supply
would dictate which non-potable use of the water
can be considered.Existing and evolving
technologies can be considered to provide
adequate, affordable, and sustainable water for
all sectors with minimal environmental
disturbance.
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Water-quality requisites for intended use
Drinking water less than 250 mg/L chloride
Industrial uses vary widely
Agriculture can accept higher salinity, but depends on crop tolerance and rainfall
Golf-course irrigation graywater, up to 500 mg/L chloride, treated sewerage effluent
Toilet flushing saltwater, in dual water-distribution lines
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Wastewater discharge to the environment is
reduced by optimizing freshwater production and
matching water quality to other water uses
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Islands of the Caribbean Region
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The availability of freshwater resources depends
upon the geomorphologic history of the islands
and rainfall accumulation
Greatest availability large islands,
sufficiently elevated to have orographic effects
and exhibiting extensive coastal accumulation of
sediment
St. Lucia
Lowest availability small, low-lying islands
having less than 50 centimeters of yearly
rainfall, without coastal embayments
Middle Caicos, TCI
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Fresh groundwater lenses subject to vertical
saltwater intrusion (upconing) during well
pumping.
Small, arid, limestone/coralline platforms or
atolls extending a few meters above sealevel,
exhibiting subterranean drainage
Case study I
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Freshwater lens typical of small, low-lying
carbonate islands showing disruption of the
freshwater/saltwater interface by pumping
wells Minimal, unsustainable freshwater recovery
using trench-and-skimming, radial wells,
gentle/intermittent abstraction
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SCAVENGER-WELL COUPLES

• Only hydraulic formula for stabilizing the
  interface
• Greater quantities of fresh groundwater
  continuously
• Scavenger-well effluent must be discharged to the
  sea or deep wells or used for some non-potable
  water use

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Hydraulic maintenance of the freshwater lens by
operating the scavenger well
Production well withdrawing a mix of freshwater
and saltwater
Scavenger well withdrawing saltwater
Production well withdrawing freshwater
Pre-pumping conditions
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PROVIDENCIALES, TURKS AND CAICOS ISLANDS
The Bight, freshwater lens
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Improvement in freshwater abstraction by pumping
the scavenger well
scavenger well as production well is pumped
scavenger well pumping alone
production well pumping alone
detectability limit (500 mg/L cl)
USEPA limit (250 mg/L cl)
production well as scavenger well is pumped
Time (decimal days from beginning of pumping)
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COZUMEL, QUINTANA ROO MEXICO
Cozumel wellfield
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Simultaneous pumping of production and scavenger
wells for pozo 4, eje 6200
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Majuro Atoll, Republic of the Marshall Islands
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• Efficient utilization of scavenger-well effluent
• flushing toilets
• washing vehicles
• filling swimming pools

• Final wastewater discharge
• the sea
• deep wells, screened far below the
  freshwater/saltwater interface

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Intermittent streamflow recharges the wedge of
fresh groundwater residing in coastal sediment.
During droughts, saltwater migrates inland
horizontally from the sea, displacing abstracted
or naturally discharged freshwater.
Elevated islands of relatively small size, having
high rainfall, intermittent surface drainage and
sedimentary coastal embayments
Case study II
Tortola, BVI
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Fresh groundwater withdrawals from coastal
embayment aquifers can initiate the horizontal
migration of saltwater toward pumping centers.
Retention structures placed at appropriate
downstream locations in ephemeral drainages
enhance groundwater recharge by retaining
rainfall runoff for greater periods of
time.Although retention structures have been
used throughout history to increase groundwater
storage, no engineering studies have been
conducted to relate storage to aquifer
diffusivity, rainfall, basin evapotranspiration,
and surface-water head.
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However, wastewater issues are somewhat more
problematic in islands having retention
structures because there are fewer options for
environmentally compatible discharge.
Wastewater will be of higher quality in the
elevated islands having coastal retention
structures because of its origin it can often be
considered for agricultural or golf-course
irrigation.
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Annual rainfall accumulation for Caribbean
islands with the longest period of record
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Islands without freshwater resources
Desalination

• Flash distillization
• Efficient reverse osmosis
• (Clark pump)

Catchments

• Rooftop
• Roadway
• Runway

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International forum of hydrologic scientists in
cooperation with AOSISDemonstrating,
documenting, and publicizing new
technologiesSmall Island Developing States
Information Network (SIDSNet)
Application of new technologies to improve
freshwater development and reduce wastewater
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Support provided by U.N. Development
Program University of the West Indies Center for
Environment and Development U.N. Department of
Economic and Social Affairs U.S. National
Weather Service, National Oceanic and Atmospheric
Administration Programa de Modernización del
manejo del agua (Mexico), U.N. World
Meteorological Organization