Pressure indicators

Municipal wastewater discharge

In urban areas, sewage networks are designed to collect wastewater from households, offices and businesses which thereby dispose of organic matter such as human wastes and food scraps, and well as all sorts of chemicals from cleaning and maintenance products including detergents, paper, paints, oils and solvents. Sewage systems also collect rainwater that runs off the pavement, gardens, parks and vacant lots, carrying along organic and inorganic solids, oils, greases, fuels, heavy metals and other substances that leak from cars and storage tanks, dissolved and suspended solids resulting from construction works (e. g. soil, cement, sand, gypsum), plant leaves, fertilizers and pesticides, as well as wastes from dogs, rats and other urban wildlife.

Therefore, municipal wastewater may contain a large amount and variety of agents that are harmful to human health, including toxic chemicals, decaying organic matter, hormones, viruses, bacteria, sediments, synthetic chemicals and pharmaceuticals (Silk and Ciruna, 2004; UNEP, 2007). Municipal wastewater discharge is indicative of the pressure on the quality of water bodies, especially suface water, since wastewaters are discharged directly into them, mostly without any previous treatement (CNA, 2001; Carabias and Landa, 2005). The OECD includes wastewater discharges as part of its Core Set of environmental indicators (OECD, 2008).

Non-municipal wastewater discharge

Surface waters are at high risk of contamination, mainly in developing countries. Pollution sources can be of two types: point and diffuse. The former are related to industrial and municipal waste discharges through pipes. Pollution from diffuse sources is caused by pollutants from various sources that are carried by runoff (UN-WWAP, 2006). Industries are considered important point sources of pollution of surface waters for several reasons (Culp et al., 2003; EEA, 2003; Silk and Ciruna, 2004): high toxicity of some chemicals discharged, such as heavy metals, halogenated organic compounds and phenolics, hydrocarbons and radioactive isotopes (called micropollutants as they are harmful even in small quantities). Many industries also produce organic waste in their processes, which add to the Biochemical Oxygen Demand (BOD) resulting from households, agriculture and livestock-raising activities. Furthermore, some industrial processes generate thermal water pollution, which reduces dissolved oxygen while accelerating oxygen-demanding biochemical processes. The indicator Non-municipal wastewater discharge is, therefore, another important pressure on water quality.

Another form of pressure imposed by industrial activities on water quality is the emission of gaseous pollutants which sometimes even react in the atmosphere to become compounds with a higher polluting potential. Once in the air, these compounds precipitate and are transported by runoff to water bodies, as in the case of sulfur dioxide and nitrogen oxides, which acidify rain, or mercury oxide, which in aquatic systems becomes transformed into monomethylmercury, a highly toxic chemical (Baselice et al., 2002; Swackhamer, 2004; PNUMA, 2009). Pressure Indicators included in the Atmosphere chapter (Final consumption of oil derivatives and Pollutant emissions in urban and industrial areas) also show the impact of industrial and some urban activities, such as transportation, on water quality.

Fertilizer consumption

Some essential nutrients limit the development of life as a result of their scarcity, both in terrestrial and aquatic environments. The most important ones are phosphorus and nitrogen. In natural ecosystems, these elements are replenished through a continuous cycle during growth, death and decay of living organisms. However, in agricultural land this cycle is interrupted by the removal of plants, so that these nutrients have to be replaced by artificial fertilizers as they become depleted, to maintain soil productivity (Leinweber et al., 2002).

Nevertheless, more nutrients are added to agricultural soil that those removed during harvest. The excessive use of fertilizers has led to the contamination of surface and ground waters, as surpluses are transported to them by runoff and infiltration, or by atmospheric precipitation after fertilizers are volatilized or carried by wind (Shortle et al., 2001; Oren et al., 2004; Swackhamer, 2004; EPA, 2006).

Water pollution from excess nutrients leads to an ecological imbalance consisting in the proliferation of populations of algae and aquatic plants known as eutrophication. Excessive amounts of plants and algae increase water turbidity, preventing light penetration for others organisms and, when they die, deplete dissolved oxygen during decomposition. Some algae that are favored by these conditions even contain toxic substances which are harmful for humans, livestock and wildlife. This condition affects water quality and restrains the use of this resource for fishing, recreation, industry and human consumption, among others (Carpenter et al., 1998; Oren et al., 2004; Forján et al., 2008).

The indicator Apparent fertilizer consumption indirectly shows one of the pressures imposed by agricultural activities on water sources, through the potential addition of pollutants to water bodies (Daily et al., 1997; Dudley and Stolton, 2003; Clarke and King, 2004; PNUMA, 2009). This indicator is included in the OECD Core Set of environmental indicators (OECD, 2008), the UN sustainable development indicators (UN, 2007) and the US-EPA's Environmental Indicators Initiative (EPA, 2008).

Livestock population

The increase of livestock-raising activities has led to the production of large volumes of manure. Although nutrients contained in manure can be recycled through its application to agricultural land, the amount generated, particularly in intensive production facilities, often exceeds the use and retention capacity of nearby crops. In fact, these facilities represent point sources of pollution, more similar to those of industrial activities than those of traditional farming (Lanvon, 1994; Chadwick and Chen, 2002; Johns et al., 2007).

Pollution of surface and ground waters by this type of sources may occur as a result of direct discharges to water bodies, leakage and surface runoff, leaching and infiltration from wastewater treatment lagoons where these wastes are concentrated, or by excessive application of manure to crops (Carpenter et al., 1998; Chadwick and Chen, 2002; GWP-CA, 2006).

In addition to its high nutrients concentration, which cause eutrophication of surface waters, manure may contain other polluting agents, including pathogenic bacteria, protozoa and virus such as Salmonella, Giardia and rotavirus, organic matter requiring biochemical oxygen demand, drug residues and even heavy metals such as copper and zinc, which sometimes are used to supplement livestock diet (Chadwick and Chen, 2002). The country’s Livestock population is thus an indirect but meaningful indicator of the pressure that livestock and poultry impose on the quality of water sources.

Extensive cattle-raising makes the soil more vulnerable to degradation processes because of the loss of vegetation cover and compaction derived from overgrazing (see the chapter on Soils). In turn, the different forms of soil degradation affect water quality, because the input of sediments, nutrients and other pollutants is favored.

Municipal solid waste disposal

Municipal solid waste produces leachates, which are fluids that may contain high concentrations of toxic organic or inorganic chemicals, whose nature depends on the type and quantity of the wastes, the local climate and hydrogeological conditions, and the age of the landfill or final disposal site. Rainwater, surface runoff and ground water (when the depth of wastes buried reaches the water table) wash out the compunds derived from waste breakdown and carry them down to water bodies (Fatta et al., 2000; Fetter, 2001; UN-WWAP, 2006). The Final disposal of municipal solid waste in uncontrolled sites is a major source of pollution of ground and surface waters, as those sites may lack the proper hydrogeological conditions and techniques to minimize the impact of leachates. In fact, groundwater pollution is one of the main impacts of solid waste (Ibe and Njoku, 1999). To note, solid waste disposal in humid climates produces large volumes of leachates compared to arid zones, where the shortage of rainfall, runoff and infiltration tends to prevent the pollution of water sources (Fetter, 2001). Eventually, waste disposal causes soil pollution and this, in turn, leads to water pollution. This indicator is developed in the chapter on Municipal Solid Waste.

Apparent pesticide consumption

Pesticides is a generic term that refers to a broad group of chemicals including herbicides, insecticides, fungicides, molluscicides and rodenticides. These are used in various sectors: in healthcare, for controlling disease-bearing fauna; in domestic, urban, commercial and industrial environments, for controlling rodents and other undesirable animals and plants in gardens, parks, roads and facilities; in forestry and agriculture, for controlling pests and plant diseases. Agriculture is the main market for these substances, since they have been very successful in increasing production yields, quality and variety (Gevao and Jones, 2002; Clarke and King, 2004).

Pesticides contain a highly diverse range of active substances, and the harmful effects they cause on human health are equally diverse. These include acute and permanent CNS and lung damage, injury to reproductive organs, dysfunction of immune and endocrine systems, birth defects and cancer (Mansour, 2004; Lo et al., 2007). Water pollution by pesticides, even at low concentrations, besides having a direct toxic effect on human health and ecosystems, in some cases also affects the water organoleptic properties, making it unacceptable for human consumption (OMS, 1970).

Many of these substances are difficult to degrade to harmless moieties, since persistence is one of the characteristics sought in the design of these products to achieve long-lasting effects on pests. The latter, along with other characteristics such as volatility, solubility and mode of delivery, facilitates their dispersion and buildup in the natural environment, particularly in soil, aquatic environments and living organisms (Gevao and Jones, 2002).

The use of agricultural pesticides is also discussed in the chapter on Soils. Like fertilizers, pesticides lead to the chemical degradation of soil, affecting fertility and impairing the quality of water bodies when these substances are carried by surface runoff and infiltrate into subsoil.

The indicator Apparent pesticide consumption quantifies part of the pressure imposed by agricultural activities on the quality of water sources. The UN includes this indicator in its Sustainable Development Indicators (ONU, 2004), whereas the US-EPA uses instead the concentration of pesticides in rivers and aquifers in agricultural land in its Environmental Indicators Initiative (EPA, 2004).

Aquaculture production

Aquaculture is one of the world’s fastest growing productive activities. In the 90's, it more than doubled in terms of production volume and value (Naylor et al., 2001). While this is an important activity for food provisioning, Aquaculture production may have direct and indirect negative impacts on water quality. Untreated wastewater from aquaculture directly contributes to the pollution of the receiving water bodies due to the load of organic matter and nutrients in the unused food and faeces produced by cultured organisms (Naylor et al., 2001; EPA South Australia, 2007; EEA, 2009c). This causes eutrophication, acidification, turbidity and depletioin of dissolved oxygen in water bodies, among other impacts. Aquaculture wastewater may also contain chemical additive residues, pathogens and antibiotics, and may change the water temperature (EEA, 2003; EPA South Australia, 2003; MPCA, 2004). Indirect negative impacts occur when setting-up the facilities involves the removal of riparian vegetation or mangroves. This vegetation provides ecological services relevant for water quality, such as the sequestration and retention of sediments, degradation of pollutants and recycling of nutrients from surrounding land (Daily et al., 1994; Naylor et al., 2001).

Soil erosion

The increase of activities such as agriculture, livestock-raising, urban development and road construction has led to severe losses in vegetation cover in the country (Semarnat, 2009). One consequence of the clearance of vegetation cover is the increase in soil erosion, a process caused by transportation of sediments into surface water bodies by runoff and wind. To note, other natural conditions in addition to the vegetation, such as climate, soil and rock type, topography and tectonic activity, influence the watershed’s vulnerability to soil erosion (Dingman, 2002).

The excessive input of sediments impaires water quality as they increase turbidity and frequently carry other adsorbed pollutants such as natural nutrients and fertilizers, pesticides, salts, pathogens and heavy metals present in soil (Bianchi and Harter, 2002; EPA, 2004). Increased sedimentation also causes silting of dams, lakes and rivers, thereby decreasing their water-storage capacity and increasing the risk of flooding of adjacent land (Daily et al., 1994; Revenga, 2000). In this sense, Soil erosion is an indicator of the pressure imposed on the quality of surface water sources. The US-EPA, in its Environmental Indicators Initiative (EPA, 2003), uses instead a sedimentation index. Soil erosion is also addressed in the chapter on Soils, along with other forms of soil degradation, under the indicator Area affected by soil degradation.