Mitigation options - Exploiting alternative water resources
NOTE: Article from the Geogenic Contamination Handbook
The provision of drinking water from alternative sources that are not contaminated with arsenic and fluoride has proven to be a popular mitigation option. In Bangladesh, for example, “well switching” is most commonly used for mitigation of arsenic contamination. The underlying reason for this is the difficulty, in terms of acceptance, supply, monitoring, maintenance and overall cost, in establishing technologies to remove contaminants. Therefore, before efforts are made to treat contaminated water, it is worthwhile to determine whether alternative water resources are available.
Resource availability is a question of scale and thus of institutional engagement:
Regional-scale solutions may be sought by government agencies that need to provide water not only for drinking, but also for agriculture and industry. This may include the provision of piped drinking water derived from surface water or groundwater. Many water resource tools of differing degrees of sophistication have been developed to support planning and implementation. One central theme is Integrated Water Resources Management (IWRM), a planning and implementation tool for managing water resources for different uses, including agriculture, industry, personal use, recreation and ecosystem protection. See the website of the Global Water Partnership (GWP and UN Water) for more information and downloadable resources.
Local-scale solutions may include rainwater harvesting, making use of uncontaminated groundwater from different locations in the aquifer by “well switching” or the treatment of local surface-water resources, such as rivers, lakes or ponds. Here the focus is on ensuring that microbial contamination does not replace geogenic contamination as a health problem, since groundwater is often selected as a replacement for microbially contaminated surface waters. Water storage is another important issue. Infrastructure is required to collect, treat and deliver drinking water to consumers. “Household Water Treatment and Safe Storage” is a strategy for making surface-water sources safe in resource-poor settings (see section below). Numerous texts provide guidance on the exploitation of surface water, groundwater and rainwater for drinking; see the References section for a small selection.
Contents
Surface water
Surface water is the water found in rivers and lakes. Surface water is replenished naturally by precipitation and is “lost” naturally through discharge to the seas and oceans, by evapotranspiration, by evaporation and by sub-surface seepage. Although the only natural input to any surface-water system is precipitation within its watershed, the total quantity of water in that system at any given time is also dependent on many other factors. These factors include storage capacity in lakes, wetlands and artificial reservoirs, the permeability of the soil beneath these storage bodies, the runoff characteristics of the land in the watershed, the timing of the precipitation and its interaction with groundwater, and local evaporation rates. All of these factors also affect the proportions of water lost.
Although surface water is seldom contaminated by arsenic and fluoride, it nearly always requires treatment to improve the microbial water quality. Pathogens differ in their susceptibility to various treatments. For example, Cryptosporidium cysts may be retained by filters but are resistant to chlorination; the opposite is true of many viruses. Furthermore, all treatment systems are subject to occasional failures which may not be recognised by the operators. The key to developing a robust and reliable system for providing safe water is to implement multiple barriers for pathogen control. Different pathogens can be removed in different stages, according to their particular weaknesses, resulting in water of progressively higher quality. The multiple-barrier approach protects against the transmission of pathogens in the event that one barrier should fail. A typical multiple-barrier system for treating surface water might include sedimentation, some type of filtration (multi-stage filtration, slow sand filtration or coagulation followed by rapid filtration) and disinfection.
Numerous texts provide guidance on the design of treatment plants that can be used for conventional drinking-water treatment. An excellent starting point, available for free download on the internet, is the "Small community water supplies" (IRC, 2002). The IRC in 2006 also produced a detailed report on multi-stage filtration (IRC, 2006).
Groundwater
Groundwater is water that fills the cracks and spaces between underground rocks and sediments. Underground rocks and sediments that hold substantial amounts of water are called aquifers – these can gain water from, or lose water to, surface water bodies. Sometimes it is useful to make a distinction between shallow aquifers that are closely associated with surface water and deep aquifers that are isolated from the surface, containing what is sometimes called "fossil water".
A critical factor in the use of groundwater is that abstraction rates need to be lower than replenishment rates. In arid climates, replenishment rates may be very low. This results in a lowering of the groundwater table.
Because of natural filtration through sediments, groundwater is typically of a much higher microbial quality than surface water. However, groundwater is not necessarily free from pathogens: especially where aquifers are near the surface and water tables are high, sediments contain little silt, and clay and on-site sanitation is widely practised, groundwater is vulnerable to contamination. While groundwater is often distributed and consumed without treatment, safety disinfection (e.g. chlorination) would be recommended in such settings (ARGOSS, 2001).
Since aquifers by their nature allow long contact periods between pore waters and rocks and sediments, groundwater frequently has higher levels of dissolved minerals than does surface water or rainwater. Under the right geochemical conditions, different elements can reach undesirable levels in groundwater. This manual describes contamination with fluoride and arsenic in detail, but other elements commonly found in groundwater can include sodium and chloride (major components of salinity), calcium and magnesium (which make up hardness), and iron and manganese (metals which can stain materials and give an unpleasant taste to water).
Removal of salinity and hardness is complicated and relatively expensive. However, simple sand filters can be optimised to remove iron and manganese, as described in Hartmann (2001).
Even though groundwater extracted from one aquifer may be contaminated with arsenic or fluoride, other aquifers (deeper or shallower) in the same area may provide completely uncontaminated water. This could be due to differences in the mineralogy of the aquifer material or changes in dissolved oxygen concentrations, which can influence the mobility of redox-sensitive contaminants such as arsenic. A classic example of this is the widespread geogenic arsenic contamination in deltaic areas of Bangladesh. Here, shallow wells in young sediments under reducing conditions yield very high arsenic concentrations, whereas deep tube wells usually provide water with a completely different chemistry, with little arsenic (Hug et al., 2011).
A vast number of technologies exist for the abstraction of groundwater. These are described in a range of resources and manuals. A good overview of water-lifting devices is given in WHO/IRC (2003) and Baumann (2000). In addition, UNESCO has produced several documents describing groundwater resources. Particularly useful are “Groundwater resources of the world and their use” (UNESCO/IHP, 2004) and “Non-renewable groundwater resources: A guidebook on socially-sustainable management for water policy makers” (UNESCO, 2006).
Rainwater
Rainwater is the ultimate source of all drinking water in the long term, since it replenishes both surface water and groundwater. Rainwater can also be captured directly and used as drinking water. However, rainwater is highly variable in its spatial and temporal distribution, so the use of rainwater for drinking often requires significant storage or distribution capacity. Whether rainwater harvesting is viable in a certain region depends very much on the yearly amount and distribution of rainfall. Rainwater is a main drinking-water source for relatively few people, but in some settings on ocean shores or islands, it can be the only source of drinking water.
Rainwater is free from pathogens, at least until it reaches the ground, and except in some urban areas, is of excellent chemical quality. When properly collected and stored, rainwater can provide a safe and acceptable source of drinking water for at least part of the year. Rooftop water harvesting has been extensively researched by the Development Technology Unit of the University of Warwick, which has produced an excellent handbook on the topic of “Roofwater harvesting: A handbook for practitioners” (Thomas et al., 2007). A wealth of additional information on rainwater harvesting can be found at the SSWM portal: Rainwater Harvesting (Rural).
Household water treatment and safe storage
Regardless of its source, drinking water can easily become contaminated with pathogens through unhygienic distribution, collection, handling and storage (Wright, et al., 2004). One approach to minimising the adverse health impacts of such contamination is to promote microbial treatment at the household level, or Household Water Treatment, combined with safe storage (HWTS). A growing body of evidence demonstrates that the use of HWTS methods improves the microbial quality of household water and reduces the burden of diarrhoeal disease in users (Fewtrell et al., 2005; Clasen et al., 2007; Waddington and Snilstveit 2009). Several HWTS methods have been proven to improve drinking-water quality significantly, both in the laboratory and in field trials in developing countries (Clasen et al., 2007; WHO, 2011). These HWTS methods include filtration, chemical disinfection, disinfection with heat (boiling, pasteurisation) and the use of flocculants and/or disinfectants. The role of the International Network on Household Water Treatment and Safe Storage (the “Network”) is in part to coordinate the effective implementation of such options. The Network, established in 2003 by WHO, and as of 2011 co-hosted by WHO and UNICEF, includes over 100 international, governmental and non-governmental organisations, private sector entities and university research departments that are actively involved in household water treatment and safe storage policy, research, implementation, monitoring and evaluation. Additional resources can be found in the WHO/UNICEF toolkit (WHO/UNICEF 2012) and at the SSWM portal (Sustainable Sanitation and Water Management Toolbox).
References
For references, please visit the page References - Geogenic Contamination Handbook.
