Characteristics of groundwater in arid lands

Groundwater in arid lands is considered to be a stable water resource not influenced directly by year-to-year climatic variation. This may be true if it is used to a renewable degree, but may not be true if the abstraction rate exceeds the natural recharge rate. Deep groundwater being used in most arid lands was recharged under climatic and hydrological conditions in the past that were much wetter and very different from those of the present.

The mean residence time of groundwater in the world is about 1,000 years, which is far longer than the residence time of about 10 days for river water or the water vapour in the atmosphere. The mean residence time may also be defined as the ratio of the total water storage to the annual recharge rate for the hydrological system concerned. Longer residence time of groundwater in a groundwater basin implies a larger amount of groundwater storage in the groundwater basin, and a smaller rate of annual recharge to it. If the abstraction rate from a groundwater basin exceeds the annual recharge rate to it, the water-table will decline, indicating a decrease in total storage as experienced in many groundwater basins around the world.

Figure 4 Changes in the Electric Conductivity (E.C.) of Groundwater in Sri Lanka from Puttalam in the Dry Zone to Kandy in the Wet Zone (Source: Song and Kayane 1996)

The groundwater in the Great Artesian Basin in arid central Australia may be the oldest groundwater currently being used in the world. Its age, dated by the radioactive isotope of chlorine-36, is estimated to be older than 1 Ma (Bentley et al. 1986; Torgersen et al. 1991). The main groundwater resource in the Nubian sandstone in the eastern Sahara dated by carbon-14 was about 25 ka, although the groundwater in some oases was found to be of recent age recharged from the Nile Valley (Munnich and Vogel 1962). The above two cases are typical examples of groundwater resources in arid lands. The deep groundwater in arid lands is "fossil" and being "mined" in the same sense as petroleum.

The water quality of old groundwater is another issue to take into account. Generally speaking, the longer the residence time, the higher the concentration of dissolved ions in groundwater. Groundwater tends to evolve chemically toward the composition of sea water during the course of flow. It was found by Chebotarev in the Great Artesian Basin that this evolution is normally accompanied by the following regional changes in dominant anion species (Freeze and Cherry 1979):

The sequence above is termed the Chebotarev sequence. Therefore, the deeper groundwater is older and more saline than shallow groundwater. However, very saline groundwater is sometimes found in shallow aquifers in arid lands. Groundwater we investigated in the dry zone of Sri Lanka with marked dry and wet seasons is one such example. Figure 4 shows changes in electric conductivity (E.C.), which increases with increasing amounts of dissolved ions, in a groundwater profile from Puttalam in the dry zone to Kandy in the wet zone. The shallow groundwater has higher E.C. than the deep groundwater. Predominant ion species in the shallow groundwater in the dry zone of Sri Lanka are Cl^- in anions and Na+ in cations (Song and Kayane 1996).

The dry zone in Sri Lanka is not arid land in the strict climatological sense. Natural vegetation in the dry zone is tropical jungle. The annual rainfall is 9001,500 mm, but the annual evapotranspiration is as high as 1,300 mm, so that the annual run-off is small compared with the annual rainfall. Once the natural forest which had shaded the soil surface had been cleared by humans, the rate of soil evaporation during the dry season became very high. Then the soil water infiltrated during the preceding rainy season is easily evaporated, leaving dissolved salts near the soil surface. When the next rain comes, the infiltrated water dissolves the accumulated salts during the process of percolation to the water-table. These processes have been continuing for a long time, ever since the land was first deforested for agricultural use.

Salinity of shallow groundwater is very high in some localities of arid lands, although its age is young, as in the dry zone of Sri Lanka. The evolution of groundwater there does not start from the initial stage of the Chebotarev sequence, but starts from its last stage: i.e. the groundwater is highly saline there from the beginning. This is an example of groundwater-quality deterioration caused by humans in arid lands. Integrated management of soil and water is essential in arid lands.