November 2007, Vol. 19, No.11
Freshwater Supplies Threatened in Central Pacific
Population growth may mean that fresh groundwater sources are reaching their limit of sustainable supply in urban South Tarawa in Kiribati, an island nation in the central Pacific. An international team, including members of the Australian National University (Canberra), Ecowise Environmental (Fyshwich, Australian Capital Territory), the government of the Republic of Kiribati, the French Agricultural Centre for International Development (Paris), and the Pacific Islands Applied Geoscience Commission (Suva, Fiji Islands), has been studying the impacts of natural and human-induced changes on groundwater in Kiribati since 1996.
Researchers have found that very limited land areas and extremely permeable coral soils in atolls reduce surface runoff to insignificant amounts and decrease the potential for surface storages of water, reports the Soil Science Society of America (SSSA; Madison, Wis.). This means thin lenses of fresh groundwater floating over seawater are the major source of reliable fresh water for people in many atolls. The team found that both the quantity and salinity of atoll groundwater are extremely vulnerable to frequent El Niño–Southern Oscillation-related droughts. Droughts can last as long as 4 years and occur with a frequency of one significant drought, coupled to La Niña events, every 6 to 7 years. In long droughts, domestic water wells are often too salty to drink, and some communities have to rely on large groundwater lenses or on coconuts, according to an SSSA news release.
Groundwater can become salty due to overpumping or inappropriate methods of pumping, SSSA notes. Long, horizontal infiltration pumping galleries, or “skimming wells,” placed just beneath the groundwater table provide the best method of skimming off lower-salinity groundwater. The study team tested the impact of infiltration galleries on lowering the water table and on salinity. The researchers also used the results to examine how the permeability of the coral sands varied across islands and found that surface contaminants could reach shallow fresh groundwater within an hour of being spilled on the soil surface.
The team proposed several strategies to help increase the resilience of small island communities to water-related climate and human changes. These included providing a sound institutional basis for the management of water and sanitation; improving community participation in water and related land planning and management; increasing the capacity of villagers and local agencies to manage water and sanitation under variable climates; improving knowledge of available water resources and demand for them; improving water conservation and demand management and reducing leakages; increasing the use of rainwater by households and communities; protecting groundwater source areas from contamination; improving sanitation systems to minimize water use and groundwater pollution; and ensuring that water aid programs are long-term partnerships that foster local engagement and ownership of solutions.
The research team’s work is published in a special issue (August 2007) of Vadose Zone Journal, available at vzj.scijournals.org.
Wider Buffers Are Better
Scientists from the U.S. Environmental Protection Agency (EPA) found that establishing riparian buffers is considered a best management practice by state and federal resource agencies for maintaining water quality, and they may be especially critical in controlling amounts of human-produced nitrogen.
EPA scientists, according to a news release from The American Society of Agronomy (ASA; Madison, Wis.), collected data on the buffers, as well as nitrogen concentration in streams and groundwater, to identify trends between nitrogen removal and buffer width, water flow path, and vegetation. They found that wide buffers — more than 50 m wide — removed more nitrogen than buffers that were less than 25 m wide. Buffers of different vegetation types were equally effective, the news release notes, but herbaceous and forest vegetation were more effective when wider. Removal of nitrogen within the water was efficient but not related to buffer width; however, removal on the water surface was related to buffer width.
In addition, researchers found that such factors as soil saturation, groundwater flow paths, and subsurface chemical–organism relations are important for governing nitrogen removal in buffers. Vegetation type also may be an important factor in certain landscapes and hydrologic settings where forested buffers may prevent nitrogen in deep groundwater or contribute more organic carbon in streams, according to ASA. Riparian buffers of herbaceous vegetation or a mix with forest vegetation were found to be effective only when wider.
Because buffers are often degraded or removed due to land-use change (such as agriculture and urbanization), there is need for further research to identify the most effective methods for restoration, according to ASA. This could lead to the enhanced nutrient removal and optimal riparian areas needed for restoration to have the greatest impact with minimum resources spent.
Results from the study are published in the July–August 2007 issue of the Journal of Environmental Quality; see jeq.scijournals.org/cgi/content/abstract/36/4/1172.