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

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.

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.

Water-quality requisites for intended use Drinking waterless than 250 mg/L chloride Industrial usesvary widely Agriculturecan accept higher salinity, but depends on crop tolerance and rainfall Golf-course irrigationgraywater, up to 500 mg/L chloride, treated sewerage effluent Toilet flushingsaltwater, in dual water-distribution lines

Wastewater discharge to the environment is reduced by optimizing freshwater production and matching water quality to other water uses

Islands of the Caribbean Region

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 Lowest availability: small, low- lying islands having less than 50 centimeters of yearly rainfall, without coastal embayments Middle Caicos, TCI St. Lucia

Fresh groundwater lenses subject to vertical saltwater intrusion (upconing) during well pumping. Case study I Small, arid, limestone/coralline platforms or atolls extending a few meters above sealevel, exhibiting subterranean drainage

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 Minimal, unsustainable freshwater recovery using trench-and-skimming, radial wells, gentle/intermittent abstraction

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 SCAVENGER-WELL COUPLES

Hydraulic maintenance of the freshwater lens by operating the scavenger well Pre-pumping conditions Production well withdrawing a mix of freshwater and saltwater Scavenger well withdrawing saltwater Production well withdrawing freshwater

PROVIDENCIALES, TURKS AND CAICOS ISLANDS The Bight, freshwater lens

Improvement in freshwater abstraction by pumping the scavenger well scavenger well as production well is pumped production well as scavenger well is pumped scavenger well pumping alone production well pumping alone detectability limit (500 mg/L cl) USEPA limit (250 mg/L cl) Time (decimal days from beginning of pumping)

COZUMEL, QUINTANA ROO MEXICO Cozumel wellfield

Simultaneous pumping of production and scavenger wells for pozo 4, eje 6+200

Majuro Atoll, Republic of the Marshall Islands

Efficient utilization of scavenger-well effluent: –flushing toilets –washing vehicles –filling swimming pools 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 Final wastewater discharge: –the sea –deep wells, screened far below the freshwater/saltwater interface

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. Case study II Elevated islands of relatively small size, having high rainfall, intermittent surface drainage and sedimentary coastal embayments Tortola, BVI

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. 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.

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.

Annual rainfall accumulation for Caribbean islands with the longest period of record

Desalination: Catchments: Rooftop Roadway Runway Islands without freshwater resources Flash distillization Efficient reverse osmosis (Clark pump)

International forum of hydrologic scientists in cooperation with AOSIS Demonstrating, documenting, and publicizing new technologies Small Island Developing States Information Network (SIDSNet) Application of new technologies to improve freshwater development and reduce wastewater

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