The Hydrological Cycle and Water Quality
Dr. Harriet Nash
Honorary Research Fellow
UNESCO Chair on Aflaj Studies – Archaeohydrology
The quantity of water available to sustain terrestrial life is determined by the hydrological cycle, illustrated in Figure 1. In broad terms, the cycle begins with the evaporation of seawater, which leads to precipitation in the form of fog, rain, or snow. Once this precipitation reaches the ground, it infiltrates,or percolates,through the soil to recharge groundwater, providing supply to wells, as well as to ayni and da’udi aflaj. Groundwater then discharges to surface water systems, such as wadis supplying ghayli aflaj, natural springs that feed ayni aflaj, and various wetlands, before ultimately returning to the sea through evapotranspiration or surface flow.
Although rainwater is relatively pure, it is not chemically identical to distilled water. As it falls through the atmosphere, it absorbs carbon dioxide and other substances, making it slightly acidic and giving it a dissolved-solids content of around 7 mg/l, in stark contrast to the approximately 35,000 mg/l found in seawater. Upon reaching the ground, rainwater absorbs additional carbon dioxide from the unsaturated zone, further increasing its acidity. As it infiltrates downward, it dissolves minerals from the soil and rocks it encounters. The eventual quality of the water we use is therefore largely determined by the composition of the geological material it interacts with and the duration of that contact.
The most common dissolved minerals include calcium, sodium, magnesium, potassium, bicarbonate, chloride, and sulphate. Recently recharged shallow groundwater tends to be relatively high in calcium bicarbonate. As water moves further from the recharge area, both in distance and depth, its salinity increases and the dominant composition shifts toward sodium chloride. In areas underlain by limestone, dissolution processes often produce karst features, including caves, such as those found in the northern Oman mountains. These systems discharge through numerous springs, many of which are tapped by ghayli aflaj. In places where groundwater circulates to greater depths along fractures,particularly where the Earth’s crust is warmed by underlying magma,thermal springs may emerge. A well-known example is Ayn Al Kasfah in Rustaq, which feeds Falaj Al Hammam. There, water temperatures reach around 45°C and the spring contains sulphur believed to have healing properties.
Highly soluble evaporite minerals, such as halite and gypsum, can lead to extreme salinity in areas where groundwater is lost primarily through evaporation. Closed basins like Umm Samim, the main discharge zone of the regional Umm Er Radhuma aquifer, can reach groundwater salinities exceeding 100,000 mg/l.
Some naturally occurring minor constituents of groundwater can pose significant health risks. Arsenic and fluoride are notable examples. Widespread arsenic contamination in West Bengal, India, and western Bangladesh came to light in the 1990s. Decades earlier, many shallow tubewells had been installed to provide safe drinking water and reduce waterborne diseases from contaminated surface sources. At the time, the initiative was deemed a success, but arsenic was not anticipated and therefore not tested for. Subsequent analyses revealed that over a quarter of the wells exceeded the national drinking-water standard of 50 μg/l, and nearly half exceeded the WHO limit of 10 μg/l. This has been linked to approximately 9,000 deaths per year and numerous miscarriages. The arsenic originates from river sediments derived from the Himalayas, where it is associated with sulphide minerals such as galena and pyrite in granitic and metamorphic formations, as well as with silicate minerals like muscovite, biotite, and chlorite.
Similarly severe issues arise with fluoride, particularly in the East African Rift Valley, where Ethiopia and Kenya record some of the world’s highest concentrations. The primary sources are volcanic and alkaline rocks and the hydrothermal fluids associated with them. In Kenya’s Rift Valley, fluoride levels in groundwater often far exceed the WHO limit of 1.5 mg/l, reaching as high as 180 mg/l in some shallow aquifers. In parts of the Ethiopian Rift Valley, deep wells and thermal springs may contain more than 28 mg/l. In the Awash Valley, concentrations of around 20 mg/l occur in wells and springs in lava bedrock, with slightly lower levels in sediments near the bedrock. Long-term consumption has resulted not only in dental fluorosis but also in skeletal fluorosis, particularly among sugar plantation workers, leading to painful deformities and an increased risk of fractures.
Groundwater circulation times vary enormously, as shown in Figure 2. In highly karstified aquifers, residence times may be only a few days, and these systems respond to rainfall almost as quickly as surface water. Elsewhere, groundwater may remain underground for months, years, or even millennia. The shorter residence times,ranging from months to years, are typical of shallow aquifers recharged in the Dhofar Mountains and draining toward the Salalah Plain, as well as shallow systems in northern Oman. In contrast, the Umm Er Radhuma aquifer system was largely recharged over 10,000 years ago in the Dhofar mountains. Further west, toward the interior, much of this groundwater is effectively fossil water, no longer actively replenished.
Residence times in the unsaturated zone can also be considerable. In parts of the UK, where clay-rich superficial deposits dominate, water may take decades to reach the underlying aquifer. As a result, improvements at the surface,such as reductions in nitrate contamination from agricultural fertilizers,can take decades to manifest in groundwater quality.
These varied residence times are reflected in groundwater chemistry, which ranges from around 500 mg/l in the mountain regions of northern and southern Oman to several thousand milligrams per litre farther downgradient. In areas lacking groundwater-level data, increasing salinity can often be used as an indicator of the direction of groundwater flow.
