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CHAPTER 1

The water quality challenge

Key messages

Water quality continues to deteriorate despite improvements in the control of industrial point source pollution and wastewater treatment. Ongoing water quality problems in OECD countries are characterised by a number of pollutants, none more so than nutrient pollution, primarily from agricultural sources, which leads to eutrophication and harmful algal blooms. As a consequence, the relative importance of diffuse pollution loads is increasing in OECD countries, and increasing treatment and regulation of point source pollution is no longer necessarily the most cost-effective approach to improving water quality. However, maintaining these processes to manage point source pollution is essential and must not be abandoned.

OECD cities face distinct challenges, given that the negative impacts of poor water quality largely fall on cities (e.g. increased water treatment costs, health service costs), as does the value of assets at risk (e.g. corrosion and premature ageing of infrastructure and reduced property values from contaminated water), and the costs of treating pollution (e.g. wastewater and stormwater) before discharging to the environment. Diffuse pollution from stormwater runoff and combined sewer overflows is an ongoing challenge for cities. Climate change will exacerbate existing water quality challenges, due to altered precipitation, flow and thermal regimes, and sea level rise, which will mean water authorities and water and sanitation utilities will be confronted by further economic and operational challenges.

Freshwater of a high quality is also valued for environmental uses, such as the provision of fish habitat and ecosystem health. However, freshwater ecosystems are under immense pressure as a result of a legacy of industrial pollution and alteration of the natural morphology of water bodies, continuing pollution from diffuse sources (agricultural and urban), and an ever-evolving number of emerging pollutants in wastewater. This pollution, coupled with the effects of hypoxia, algal blooms, the introduction of invasive alien species and climate change, are having a devastating impact on freshwater biodiversity. Policy responses to these complex water quality challenges are required to protect freshwater ecosystems and the services they provide.

An introduction to water quality and its impact on the environment and society

Good water quality is essential for human well-being, for use in agriculture, aquaculture, and industry, and to support freshwater ecosystems and the services they provide. What qualifies as "good" water quality depends on the purpose of use and the value society holds for water quality.

Water pollution is defined as anthropogenic contamination of water bodies (e.g. rivers, lakes, groundwater, estuaries and oceans) from the discharge, directly or indirectly, of a substance that changes the functioning of the system (Hanley et al., 2013). Pollution alters the composition and characteristics of a water body and its level of water quality. For example, the discharge of organic waste from sewers to rivers accelerates biological processes, and in the process uses up oxygen which can cause loss of aquatic life. Nutrients from fertilisers and livestock from the agriculture sector can lead to eutrophication of rivers and lakes, and can result in toxic algal blooms and changes in freshwater fauna and flora communities. Furthermore, poor water quality reduces the quantity of useable water and therefore exacerbates the problem of water scarcity.

Figure 1.1 illustrates the global distribution of pollution, which includes the effects of nutrient and pesticide loading, mercury deposition, salinisation, acidification, and sediment and organic loading (Sadoff et al., 2015; Vörösmarty et al., 2010). Pollution "hotspots" are identified in most regions of the world, including OECD countries.

Population growth, coupled with climate change3, are thought to have the greatest effect on water quality (Allan, et al., 2013), placing increasing pressure on the ability of finite water bodies to process wastewater, nutrients and contaminants before they lose their life-supporting function. So much so, that at least half the world's population suffers from polluted water (Jones, 2009). And the situation is set to worsen. Under even the most optimistic economic growth and climate change scenarios, a global and rapid increase in nitrogen (35-46%), phosphorus (15-24%) and biochemical oxygen demand (BOD4) (9-11%) is projected to 2050 (IFPRI and Veolia, 2015). Increases are projected in all regions of the world, but will be felt the greatest in upper-middle and lower-middle income countries, particularly Asia. This will, in turn, increase risks to human health, economic development and ecosystems.

Pollution, over-exploitation and alteration of water bodies as a result of human activities have led to the extinction, or risk thereof, of 10 000 to 20 000 freshwater species (Strayer and Dudgeon, 2010; Vörösmarty et al., 2010) - an 81% reduction in freshwater biodiversity between 1970 and 2012 (WWF, 2016). Of further concern, wetlands, which are biodiversity hotspots that deliver a wide range of ecosystem services including water purification, have declined by 64% globally since 1900 (Ramsar, 2015). Polluted freshwater also has an impact on coastal and ocean waters, for example the formation of eutrophic and hypoxic zones (also known as "dead zones") in the oceans.

Improving water quality is consistently ranked as a top environmental concern in public opinion surveys across most OECD countries (OECD, 2012a). For example, in the United States, an annual national public opinion survey from 1989 to 2014 consistently ranked water pollution as one of the top environmental concerns from a list including climate change, loss of rain forest, extinction of plant and animal species, and air pollution (Gallup Poll, 2014). A similar survey in the European Union in 2012 showed comparable results to those of the United States, with 84% of respondents listing chemical pollution as the greatest threat to a country's water environment, ahead of climate change, changes to the water ecosystem, floods, water scarcity, and other water-related threats (European Commission, 2012a). Challenges to water quality are the primary environmental concern for New Zealanders, ahead of air quality, terrestrial biodiversity, coastal waters and soils, with public attention increasingly focusing on the impact of agricultural runoff (Hughey et al., 2013).

Over recent decades, policy actions and major investment in OECD countries have helped to reduce point source pollution from urban centres, industry and wastewater treatment plants, with substantial gains for the economy, human health, environment and social values linked to water (OECD, 2012b). However, despite these improvements, diffuse pollution loads from agricultural and urban sources, combined sewer-overflows, and emerging contaminants in human and animal wastewater are continuing challenges in OECD countries (OECD, 2012b).

A typology for water pollution: sources, types and pathways

Characteristics and determinants of water quality

The quality of a water body is the function of its physical, biological and chemical characteristics. Physical characteristics relate to temperature, colour, taste, odour, turbidity and salinity, among others. Biological characteristics relate to living organisms, such as bacteria, zooplankton, algae, fungi, invertebrates, worms, aquatic plants and fish, among others. Chemical characteristics relate to pH, biological oxygen demand, and substances that dissolve in water, such as total dissolved solids, dissolved oxygen, nitrates, phosphates and other minerals.

Contaminants from naturally occurring events and human activities alter these characteristics, with a corresponding change in the composition of the waterbody and its level of water quality (Joyce and Convery, 2009). The nature of these alterations is not always linear, and can depend on a combination of variables related to the characteristics, volume and concentration of the pollutants (individually and in combination), the characteristics of the receiving water body, distance to the polluting source, the stochastic environmental conditions and timing (as outlined in Figure 1.2). Pressures from a range of policies and developments can affect water quality, such as water allocation, flood management, urban development, alterations to the natural morphology of water bodies, land and soil management practices, and climate change.

Water pollutants are commonly characterised as point or diffuse, according to their source and pathway to the receiving environment:

• Point sources of pollution are directly discharged to receiving water bodies at a discrete location, such as pipes and ditches from sewage treatment plants, industrial sites and confined intensive livestock operations. The most severe water quality impacts from point source pollution typically occur during summer or dry periods, when river flows are low and the capacity for dilution is reduced, and during storm periods when combined sewer overflows operate more frequently. The "first flush" of a combined sewer system after a dry spell is particularly detrimental to surface water quality. Groundwater quality can also be affected where it interacts with polluted surface water.

• Diffuse sources of pollution (also referred to as non-point) are indirectly discharged to receiving water bodies, via overland flow (runoff) and subsurface flow (including pipeflow) to surface waters, and leaching through the soil structure to groundwater. Examples of diffuse pollution sources include nutrient runoff and leaching from the use of fertilisers in agriculture, atmospheric deposition of nitrogen oxides from energy and transport emissions, and runoff of petroleum hydrocarbons and heavy metals from urban surfaces not serviced by stormwater collection and treatment. The most severe water quality impacts from diffuse sources of pollution occur during storm periods (particularly after a dry spell) when rainfall induces hillslope hydrological processes and runoff of pollutants from the land surface.

The distinction between point and diffuse sources of pollution is also a function of policy and regulation. Point sources of pollution are largely under control in OECD countries because they are easier to identify and more cost-effective to quantify, manage and regulate. In comparison, diffuse sources are challenging to monitor and regulate due to: i) their high variability, spatially and temporally, making attribution of sources of pollution complex; ii) the high transaction costs associated with dealing with large numbers of heterogeneous polluters (e.g. farmers, homeowners); and iii) because pollution control may require co-operation and agreement within catchments, and across sub-national jurisdictions and countries (OECD, 2012a). For these reasons, diffuse sources of pollution and their impacts on human and ecosystem health largely remain under-reported and under-regulated.

The damage caused by pollution disposal depends crucially upon the ecosystem's ability to absorb and dilute pollutants, which depends upon the ecosystem condition. If emissions exceed the assimilative capacity (absorptive or dilution capacity) of the system, they will accumulate and cause damage to the ecosystem. The deterioration of water quality has subsequent knock-on impacts on the functioning of in-stream invertebrates, fish, and aquatic plant communities (Doledec et al., 2006; Ling, 2010). This causes negative feedbacks, particularly the ability of ecosystems to process contaminants, thereby causing pollutants to accumulate in the environment and cause further damage. Conversely, activities that enhance ecosystems can increase their ability to process pollutants. Therefore, in addition to pollutants being classified as point source or diffuse source, pollutants can also be classified by the ability of the ecosystem to adsorb them (Lieb, 2004). The distinction below is relevant from a policy perspective:

• A stock pollutant is a pollutant with a long lifetime and for which the ecosystem has little or no absorptive capacity. Stock pollutants therefore accumulate in the environment. Examples include heavy metals, toxic contaminants, such as dioxins and polychlorinated biphenyls (PCB's), and non-biodegradable plastics. Groundwater aquifers, lakes, reservoirs and estuaries, particularly those with low recharge rates and high residence times, are examples of water bodies where their ability to absorb pollutants is limited. By their very nature, stock pollutants create interdependencies between decisions made today and the welfare of future generations, and the costs of treatment and damages typically rise over time (although advances in technology can reduce costs).

• Conversely, a flow pollutant has a short lifetime for which the ecosystem has some absorptive capacity. For example, suspended sediments washed out by rainfall into rivers only have a short lifetime. Organic pollution can be transformed into lessharmful inorganic matter by bacteria in water bodies, although this process uses up available oxygen and can cause loss of aquatic life. Nutrients (nitrates and phosphates) are required for aquatic plant growth, but in excess can proliferate aquatic weeds and turn waterways eutrophic. Since rivers are flowing, the concentrations of river pollutants decline more quickly than aquifers and lakes once pollution emissions have ceased. For this reason, river pollutants are generally short-lived and are often considered as flow pollutants. It is important to also note that a flow pollutant in one place, such as a river, can result in a stock pollutant elsewhere, such as an estuary, and as such, the source and dispersal of pollutants needs to be looked at systemically.

An overview of the main pollutants

The quality of water resources are affected by a number of pollutants. Table 1.1 summarises the most common pollutants and their sources, and they are individually described in more detail in Annex 1.A1. It is important to note that many of these pollutants may occur in parallel, and may be derived from a number of different sources and actors.

Negative feedbacks on water quality

Other factors that contribute to degradation of freshwater ecosystems, and thus their ability to process contaminants, include the introduction of invasive alien species and anthropogenic geomorphological modifications to river systems. According to the IUCN, invasive alien species constitute the second most severe threat to freshwater fish species (Darwall et al., 2009), and the spread of invasive alien species is projected to increase due to a combination of increasing trade and climate change (Death et al., 2015; Rabitsch et al., 2016; Walther et al., 2009).

Changes in the natural geomorphology and flow of water bodies (e.g. channelised rivers, dams, canals, flood defences) can also have some effects on water quality and the ability of ecosystems to process and retain pollutants (Nilsson and Malm Renöfält, 2008; Wagenschein and Rode, 2008). For example, a study on the Weisse Elster River, Germany, revealed that the nitrogen retention rate is almost 2.4 times higher in a natural section of the river compared with a heavily modified and channelised section (Wagenschein and Rode, 2008).

Links between water quality and water quantity

Water quality and quantity are inextricably linked. Water pollution reduces the quantity of useable water and therefore exacerbates the problem of water scarcity. Water scarcity and droughts reduces the capacity for dilution of point source discharges to surface waters, and additional treatment of wastewater may be required to compensate for the lower dilution capacity of water bodies. Water scarcity also increases water temperatures which can affect freshwater ecosystems and nuisance algal growth. Conversely, high rainfall events and flooding induce diffuse pollution from land runoff (agricultural and urban) and trigger combined sewer overflows into rivers.

There can be competing demands for quality and quantity, driven by the requirements of the users. Different users require different volumes of water at different times and places, and different users are more or less sensitive to water quality. They also require varying levels of certainty regarding the availability and quality of water, and citizens have increasing expectations as regards the quality of water. There may be trade-offs and cobenefits between water quantity and quality management, and other important sectoral policies, such as land, energy, biodiversity, urban planning, health care, waste, construction, transport, and climate change (discussed in Chapter 4).

Ongoing challenges of diffuse pollution sources and eutrophication in OECD countries

Eutrophication and harmful algal blooms in freshwater systems are quickly becoming a global epidemic. For instance, there have been reports of algal blooms in Lake Nieuwe Meer in The Netherlands (e.g. Johnk et al., 2008), Lake Erie in North America (e.g. Michalak et al., 2013), Lake Taihu in China (e.g. Qin et al., 2010), and Lake Victoria in Africa (e.g. Sitoki et al., 2012). Furthermore, the effects of climate change are expected to exacerbate existing eutrophication and algal bloom problems (Bates et al., 2008).

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Excerpted from "Diffuse Pollution, Degraded Waters"
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