Water Sources

By Omar Laris

Demand for clean water is exceeding the growth of the world’s population. According to a 2015 report by the UN Department of Economic and Social Affairs, the world population will reach 9.7 billion people by 2050;¹ meanwhile, the UN Environment Programme projects that by 2050 the number of people experiencing water stress and scarcity will reach over 4 billion people across 54 countries.² The consequences of a widespread lack of clean water are severe not only for individuals but also for society as a whole. In fact, the UN Environmental Programme states in a 2010 report that “Over half of the world’s hospital beds are occupied with people suffering from illnesses linked with contaminated water.”³ Clean water is essential for public health and its increasing scarcity is leading to water-related health issues such as dehydration, cholera, and diarrhea. At a broader level, water scarcity affects economic growth because urban growth involves water-intensive sectors such as industry and energy, both of which consume substantial amounts of water (See Figure 1) and are limited by the amount of water available (See Figure 2). However, there are many solutions which can address the problems of water shortages through water augmentation. The main techniques for water augmentation include artificially recharging groundwater, building desalination plants and collecting rainwater.

Figure 1. Global Water Withdrawal by Sector (Food and Agriculture Organization of the United Nations, 2009).

Figure 2. Global Freshwater Availability (Food and Agriculture Organization of the United Nations, 2008).

Artificial Recharge

Artificial recharge refers to the planned, human activity of increasing the volume of surface water which ends up in groundwater.⁴ Artificial recharge is intended to mitigate issues associated with groundwater depletion, or when groundwater extraction rates exceed replenishment.⁵ Groundwater constitutes nearly half of the supply for domestic water usage, 40% of water used by self-supplied industry, and 20% of water used for irrigation.⁵ However, unchecked abstraction has more than doubled the rate of groundwater depletion from 1960 to 2000. Over-abstraction contributes to groundwater depletion by increasing the rate at which water exits aquifers. As aquifers empty, the land above them can sink and permanently reduce the aquifers’ capacity to store water in a process called land subsidence. Additionally, over-abstraction in coastal areas can draw ocean water into groundwater reserves, contaminating them in a process called saltwater infiltration.⁶

Humans are also reducing the amount of water which returns to the ground, referred to as natural recharge.⁷ Urban development in the form of paved roads and parking lots reduce the permeable land area through which water can pass into the ground.⁸ Man-made channels, such as drainage systems meant to alleviate flooding, also prevent water from infiltrating the ground and often dump water laden with urban pollutants into surface waterways.⁸ Artificial recharge, as a broad category of solutions, addresses both problems by moving water into aquifers, either through soil percolation, well injection, or hydrological gradient flow.

Direct Surface Recharge

One method of artificial recharge is direct surface recharge. Direct surface recharge involves moving water to areas where it can directly enter aquifers through the soil.⁴ The water can be sourced from storm runoff or other forms of wastewater such as treated sewage or greywater. Because groundwater is an important source of water, replenishing these reserves is an important way to make sure abstraction rates are sustainable. This method also takes advantage of water filtration by the soil, and the water used should be pretreated if it contains high levels of pollutants in order to avoid groundwater contamination.⁹ Relatively few technical challenges exist for routing water, and this method can be cost-effective and easy to implement. Stanford’s Water in the West project estimates that direct surface recharge costs between $90 and $1,100 per acre-foot of water as opposed to different augmentation techniques such as desalination, which can run from $1,900 to over $3,000 per acre-foot of water.⁷ However, there are also drawbacks to direct surface recharge, namely that land area is not always available for use, biofilms which reduce infiltration rates can build up at the soil surface, and this method only works when aquifers have direct access to percolating waters; they cannot be blocked from the surface by impermeable deposits of sediment.⁹

Direct Subsurface Recharge

An alternative solution for areas in which direct surface recharge is infeasible due to either inaccessibility of the aquifer or limited land area is direct subsurface recharge. Direct subsurface recharge is the practice of moving water directly into aquifers through injection wells (See Figure 3).⁴ This approach allows water to reach aquifers at virtually any depth, requires a small watershed, and immediately contributes to the volume of water contained in the aquifer.⁷ However, there is a high risk of contaminating aquifers associated with this technique if the water has not been pre-treated. Because water pretreatment requires infrastructure and infrastructure incurs both building and maintenance costs, direct subsurface recharge can be more costly with contaminated sources of water. Another drawback of this method is that injection systems require high upfront construction costs, continued maintenance, and are vulnerable to clogging.⁹ Therefore, it stands that this method should only be applied in areas where building well-developed infrastructure is possible.

Figure 3. Diagram of Injection Wells (Direct Subsurface Recharge) and Spreading Basins (Direct Surface Recharge).

Indirect Recharge

The last method of artificial recharge is indirect recharge. Indirect recharge entails a variety of techniques which manipulate the flow of groundwater in a watershed (See Figure 4).⁴ For example, pumping groundwater at a specific location may induce water flow into an aquifer from a previously untapped surface location due to hydrological pressure throughout the water table. Other indirect methods include building slightly raised earthen barriers in streams to slow the flow of water and increase the time water spends infiltrating.⁴ While indirect recharge often provides low-cost and low-maintenance solutions, it is unreliable in terms of the quantity of water recharges as well as its cleanliness because there is no direct control over the water which is infiltrating or the rates at which it infiltrates.⁴

Figure 4. Induced Infiltration from Surface Stream.

Artificial recharge is already widely used in water-stressed areas as a means of managing water supplies, but as groundwater abstraction continues to outpace recharge, it has become even more important to preserve groundwater supply. Consider for example an artificial recharge project in the park of Lodhi Garden, New Delhi. Rains flood the 36-hectare park with 25,000 cubic meters of water each year. 3 recharge pits and 3 injection wells have been built to recharge this water into the underlying aquifer. Water levels in the aquifer are expected to rise 35 cm over a 40 hectare area each year due to this inflow, counteracting abstraction.¹⁰ The net cost of recharging and abstracting this water per cubic meter is 1.34 rupees, about $0.20. The low cost and implementation example of this solution serve as a proof that artificial recharge can be a cost-effective solution.

The costs associated with artificial recharge vary widely depending on the technique used, and feasibility depends both on cost and available land resources. For urban areas with limited surrounding land area, implementing well injection is often the best solution.⁹ Although water must be collected, transported, treated, and then injected, municipal governments are better positioned to fulfill these obligations than rural areas which may lack the resources necessary to implement this type of recharge. However, if a city is near available land area or is unable to afford the costs associated with injection wells, direct surface recharge is the best option. Again, water must be collected and transported, but pre-treatment is often unnecessary unless the source of the water is very polluted. In any case, the treatment required is less than that of direct injection because surface infiltration naturally filters water as it descends into the aquifer. Indirect methods of artificial recharge are less well-defined solutions, and their practicality should be judged on a case-by-case basis.

Desalination

Desalination refers to the practice of producing freshwater from seawater or brackish groundwater so that it may be utilized for human consumption, irrigation and industry.⁴ Salt water contains dissolved salts and other solids which render it unfit for human consumption or industrial use. To make it a viable water source, it must first undergo purification through distillation or reverse osmosis. Distillation involves boiling water and capturing condensation, which leaves behind the impurities.⁴ Membrane filtration passes the water through membranes which capture the impurities and permit only clean water to pass.⁴ Desalination has emerged as an important technology for many areas under extreme water stress such as the Middle East, where there are limited freshwater resources.⁴ Purifying ocean water is the least expensive and most practical way to access water in contrast to the alternative solution of importing water which incurs high transportation costs. According to a 2011 report by the Water Resources Department of Abu Dhabi, long distance water transfer can incur costs up to $2.35 per cubic meter as opposed to only $0.55 for reverse osmosis desalination, a price which is becoming even more competitive as technologies mature.¹¹

The first type of desalination is distillation. Distillation involves boiling, condensing, and collecting water, a reliable but energy intensive process, and energy costs end up factored into water costs. The second type, membrane filtration desalination, is similarly energy intensive and labor intensive due to the maintenance required for the highly specialized membranes and pumping equipment.¹² When comparing the energy usage of both methods, two main factors come into play: whether seawater or brackish groundwater is used, and the technique used for desalination.¹² Seawater is more energy intensive and therefore more costly to desalinate compared to brackish groundwater, and membrane filtration is more efficient than distillation in both cases.

Desalination is only economically feasible if performed in large quantities, limiting the locations of desalination plants to large municipalities with adequate energy production. While piping water out to adjoining areas is steadily becoming more feasible, desalination is primarily a centralized, expensive approach to water reserve augmentation. Therefore, desalination should be considered primarily by dry, coastal cities with low energy costs. A model area would be the Middle East, where desalination has already been widely implemented (Figure 5).⁴

Figure 5. Global Desalination Capacity.

Rainwater Harvesting

Rainwater harvesting refers to the practice of collecting and storing precipitation for human use.⁴ Rainwater harvesting may be the only fresh water supply in areas without centralized supply systems or surface reservoirs and aquifers.¹³ Rainwater collection systems typically involve routing rainwater from catchment areas such as rooftops or land surfaces to a storage area such as barrels, cisterns, ponds and reservoirs.¹³ Rainwater collection can yield large amounts of water even over small areas, making it a dependable source in wet areas and reducing a dependency on centralized water distribution systems when implemented on an individual level.⁴ Consider for example the average 3-bedroom home in an equatorial region such as Brazil. Assuming a conservative estimate of roof area at 100 square meters (1,046 sq ft) and an average annual rainfall of 1700 mm (67 inches), with 30% of gross rainfall volume lost to evaporation, a rooftop catchment system can provide 120 cubic meters of water per year, equal to nearly 31,700 gallons.⁴ Scaling up rainwater collection systems to the size of municipalities can thus be a tremendous resource for centralized water distribution systems which are undergoing water stress.⁴

Rainwater collection is an important technique because it can be implemented when there are no other options available which can supply sufficient water of adequate quality. However, it is not a perfect solution. In urban areas, rainwater can be polluted with airborne contaminants such as soot, nitrogen oxides, sulfur oxides and air-borne microorganisms, rendering the water unfit for drinking.¹⁴ Additionally, if the water collection infrastructure is inadequate, water may be contaminated by leaching of toxic materials such as lead or other heavy metals in roofs or storage tanks.¹⁴ Therefore, it is crucial that water filtration measures are implemented before using collected rainwater. The low costs and accessibility of rainwater collection make it a good option for areas where access to clean water is unreliable, but precipitation is not.¹⁴

Rainwater collection on an individual level is fairly straightforward but scaling up rainwater collection to the size of municipalities transforms rainwater collection into the challenge of handling stormwater and runoff. Rainwater collects large quantities of pollutants as it runs off of urban areas which produce high volumes of water tainted with urban contaminants.¹⁵ Often, it is not feasible to build infrastructure which can directly collect such large volumes, and it is instead routed to natural waterways with minimal treatment. By handling stormwater in this manner, waterways become polluted and water quality degrades.¹⁵ A potential solution is implementing artificial recharge using stormwater. Rather than routing untreated water into the environment, if stormwater were to be minimally treated and subsequently routed to areas where it could infiltrate into the ground naturally, it would be stored underground. Thus, rather than building costly infrastructure to capture rainwater, it suffices to use the pre-existing natural infrastructure of aquifers.¹⁵ Other solutions may involve reducing the impervious land area in cities with more green spaces and permeable pavements.¹⁵ Because many of the solutions for managing stormwater require engineered infrastructure, it is critical that future urban development takes into account the widespread effects of stormwater and its potential for freshwater augmentation.

Conclusion

Addressing the problem of water stress and scarcity through water augmentation will require intimate knowledge of the affected cities. No template for water management can fully accommodate the wide variety of geographic situations around the world. It is thus crucial that relevant factors such as cost, population, precipitation, local hydrogeology, and energy availability be taken into account when engineering solutions. Artificial recharge, desalination, and rainwater harvesting can always be implemented in an intelligent, practical water management system. Artificial recharge proves its value when considering regions with natural groundwater infrastructure and recharge techniques can be implemented in cost-effective manners whether in rural or urban areas. Desalination is particularly well suited when large reserves of saline water are available as well as cheap energy. Finally, rainwater harvesting can serve areas with reliable precipitation but no other reliable sources of water. Through well-tailored implementations of these methods, the world edges closer to adequate water supplies for all.