In areas where surface water is not readily available (located far from areas of need), groundwater is the primary water source. Groundwater aquifers supply an estimated 20% of the global population living in arid and semi-arid regions. Despite their widespread presence, groundwater aquifers in arid areas receive only limited or seasonal recharge, making such aquifers susceptible to rapid depletion. The Northern Sahara Basin Aquifer, for example, was exploited at almost twice its replenishment rate during the 1990s, causing many of its springs to stop flowing (Jackson et al., 2001). The rapid shift of populations to urban areas is causing ever-greater demands on groundwater resources, particularly in the developing world.
Where cities are located above productive aquifers and are far from surface water supplies, groundwater is usually the primary freshwater source. It is primarily exploited through hand-dug wells or drilled boreholes (Foster et al., 1998). Although urban aquifers meet the growing water demands of several major cities today (Merida, Madras, Bangkok, Hat Yai, Santa Cruz, Dakar), major problems are being caused by unregulated groundwater exploitation, and the disposal of solid and liquid wastes above or into these aquifers. A growing number of large urban centre aquifers are facing pollution from organic chemicals, pesticides, nitrates, heavy metals and waterborne pathogens.
The level of water and wastewater service provision can also radically alter aquifer replenishment mechanisms, affecting not only the dynamic equilibrium between increased recharge availability and pumped withdrawals, but also the magnitude of the pollutant load and the rate of aquifer contamination. All these problems occur, to a certain degree, in towns and cities, depending on their type of groundwater supply.
Cities Experiencing Major Groundwater Problems
- Hat Yai, Thailand: The mixing of unpolluted regional groundwater and canal seepage has occurred in this busy border city. It was discovered that the most polluted urban groundwater has high chloride concentrations, indicating that canal water seepage has occurred at groundwater abstraction points and where downward leakage is greatest.
- Merida, Yucatan Peninsula, Mexico: With no main sewerage system, the majority of Merida’s wastewater is disposed of directly to the ground via septic tanks, soak-aways, and cesspits. The fissured nature of the local limestone means that water movement to the water table is frequently rapid, and the vadose zone provides virtually no attenuation capacity because the aperture of the fissures is many times larger than the size of pathogenic micro-organisms. Not surprisingly, the shallow aquifer has been grossly contaminated, with fecal coliforms (FC) typically in the range of 1,000-4,000 per 100 ml. The permitted concentration set by WHO for drinking water is less than 1 per 100 ml.
- Santa Cruz, Bolivia: This low-rise, relatively low-density, but fast-growing city derives its water supply from wellfields within the city limits, which extract from deep semi-confined alluvial aquifers. Although groundwater in the deeper aquifer, below 100 m, is of excellent quality, the uppermost aquifer above 45 m has begun to show substantial deterioration, with elevated nitrate and chloride concentrations under the more densely populated districts. These are caused by effluent disposal to the ground, mainly from on-site sanitation units. This urban recharge is drawn downwards in response to pumping from the deeper semi-confined aquifers. Dissolved oxygen in the urban recharge is low, being consumed as the carbon in the organic load is oxidized to carbon dioxide, which, in turn, reacts with carbonate minerals in the aquifer matrix to produce bicarbonate. The oxidation of the high organic load also mobilizes naturally occurring manganese from the aquifer matrix, and some of the production boreholes in the main wellfield have started to show concentrations above 0.5 mg per litre, leading to taste and laundry problems (Lawrence et al., 1997).