State indicators

Biochemical oxygen demand in surface waters

Dissolved oxygen is one of the factors that determine the quality of water bodies. The presence of this gas in sufficient amounts is essential for aquatic life and the aesthetic quality of rivers, lakes and lagoons. The absence of oxygen in water creates septic conditions and confers bad taste and foul odor, which prevent virtually any use of this resource. Organic matter discharges into water bodies affect the concentration of dissolved oxygen, as microorganisms that breakdown these wastes consume the oxygen available (Dingman, 2002; Delzer and McKenzie, 2003; UN-WWAP, 2006). Water pollution by organic matter is assessed through a laboratory test that estimates the biochemical oxygen demand, that is, the amount of oxygen required for breaking down waste (Dingman, 2002; Delzer and McKenzie, 2003; UNEP, 2007). The indicator Biochemical oxygen demand (BOD₅) in surface waters denotes the quality of surface waters with regard to this parameter. This indicator is also used by the European Environment Agency (EEA, 2005), the OECD in its Core Set of environmental indicators (OECD, 2008) and the UN in its sustainable development indicators (UN, 2007).

Total phosphate in surface water

Water pollution by phosphorus derives mostly from the use of agricultural fertilizers, but is also caused by soil erosion and decaying organic matter discharged by industries, urban centers and farms. Phosphorus has the ability to bind strongly to soil particles which, paradoxically, promotes its excessive application in crops, as it entails little losses from the agricultural standpoint. Nevertheless, the small amounts of this nutrient that are transported by water and wind erosion of soil can have severe impacts on the quality of surface water sources (Leinweber et al., 2002; Swackhamer, 2004; UN-WWAP, 2006).

Phosphorus in water is not deemed toxic for humans and animals; however, it may have indirect effects by causing eutrophication of surface water bodies, which involves algal blooms and the ensuing depletion of oxygen due to the decomposition of dead algae (Carpenter et al., 1998; UN-WWAP, 2006). Some algae are potentially harmful when present in water supplies, due to the presence of endotoxins that cause severe damage to humans and other organisms, ranging from gastrointestinal disorders, neuromuscular damage, and even death. The risk severity will depend on their abundance in the aquatic system. Algal blooms also hamper fishing activities and tourism, clog irrigation canals and hydroelectric turbines, accelerate water loss in dams and reservoirs through evapotranspiration, and contribute to dam silting.

Although most phosphorus is lost from soil as undissolved particles, it eventually converts to phosphate, which is readily available for aquatic organisms (Carpenter et al., 1998). A number of studies have shown that phosphorus often plays a bigger role than nitrogen in the eutrophication of freshwater ecosystems, while the latter is more critical in coastal ecosystems (Howarth et al., 2000; Howarth et al., 2003). The indicator Total phosphate in surface waters describes the state of surface water bodies in Mexico with regard to their phosphorus content. Other ways of measuring phosphorus in surface waters are used by organizations like the OECD in its Core Set of environmental indicators (OECD, 2008), the EEA in its state indicators (EEA, 2009a) and the US-EPA in its  Environmental Indicators Initiative (EPA, 2009).

Nitrate in surface waters

Nitrate is an important component of fertilizers and can also result from the oxidation of ammonium (NH^₄+) and other sources in the organic wastes (Stournaras, 1998; OMS, 2004). Its presence in surface water is associated with both diffuse (e. g. agricultural fields) and point sources of pollution (e. g. direct discharge of municipal wastewater, industrial effluents or open dumps). Nitrate precursors can enter water bodies via runoff or are also deposited from the atmosphere in significant amounts (Swackhamer, 2004; UN-WWAP, 2006).

There is solid evidence of the adverse effect on human health when water with high nitrate concentrations is consumed, particularly in babies under 3 months. In the gut, nitrates are reduced to nitrites (NO^₂-), which oxidize hemoglobin to methaemoglobin, a form that is unable to carry oxygen to tissues, leading to cyanosis and even asphyxiation. Why infants are more susceptible to this condition remains largely unknown (Socolow, 1999; OMS, 2007). Furthermore, consumption of nitrate-polluted water by livestock affects their growth and may cause abortions, as well as a form of anemia similar to methaemoglobinanemia in human babies (Vitousek, 1994; Carpenter et al., 1998; Cabrera and Blarasin, 1999; EPA, 2004; UN-WWAP, 2006). The indicator Nitrate in surface waters describes the pollution status of surface waters as regards nitrate. The US-EPA (EPA, 2008) and the EEA (EEA, 2009a) quantify nitrogen in surface waters for their environmental indicators.

Nitrate in groundwater

Aquifers are an important source of water supply for direct human consumption, agriculture and industry, particularly in arid areas of Mexico where surface water bodies are either absent or scarse. Altogether, these account for about one third of the country’s water supply for different uses (Conagua, 2008a); hence, paying attention to aquifer quality is essential. Another compelling reason for monitoring it is the difficulty to remediate polluted aquifers. In addition to its inaccessibility, groundwater flow tends to be extremely slow, so that pollutants remain in aquifers for a long time (Revenga et al., 2000; Danielopol et al., 2003).

Intensive farming practices, which involve an excessive use of fertilizers and pesticides, and the accumulation of animal, municipal and industrial organic wastes in soil, are potential sources of aquifer pollution. The presence of certain substances in groundwater is associated with the infiltration of leachates from these sources (Stournaras, 1998; OMS, 2008; UN-WWAP, 2006). Nitrate concentration in groundwater is a good indicator of the presence of other pollutants, given the high mobility and stability of these ions in aquifers (Cabrera and Blarasin, 1999). Additionally, at high concentrations, nitrate affects the health of both infants under three months and livestock (Vitousek, 1997; Carpenter et al., 1998; Cabrera and Blarasin, 1999). The US-EPA included this indiactor in its Environmental Indicators Initiative (EPA, 2008). However, since no data are available for this indicator, it has not been included in this publication.

OVEREXPLOITED AQUIFERS, AQUIFERS WITH SEAWATER INTRUSION AND/OR SOIL SALINIZATION OR BRACKISH GROUNDWATER PROBLEMS

A high concentration of dissolved salts is another condition that affects groundwater quality and restrains its use for several human activities. Water salinity can be assessed in therms of the total amount of dissolved solids or of specific ions, such as chloride. WHO (OMS, 2007) has not set guidelines for these indicators, since there is no sufficient information on their health effects; however, this organization points out that amounts in excess of one thousand milligrams per liter of total dissolved solids and 250 mg per liter of chloride lead to poor water taste. Furthermore, WHO reported that high ion concentrations may lead to higher corrosion rates of distribution systems, thereby causing higher metal concentrations. Water with excess salt levels is unsuitable for agricultural irrigation, as it impairs plant growth (Ben-Hur et al., 2001; Tanwar, 2003; Assouline and Ben-Hur, 2003; Ashraf et al., 2008; Torres and Acevedo, 2008).

Groundwater may naturally contain high concentration of salts for various reasons (Fetter, 2001): one is the high content of soluble minerals in the aquifer’s geological matrix, especially if water has remained under such conditions for prolonged periods of time, as in the case of fossil brackish water. Another condition occurs when the aquifer underlies a closed basin or is located in an arid area, where poor drainage and high evaporation cause the accumulation of salts in soil and their migration to shallow aquifers. Also, when the water table is high and reaches plant roots, promotes a high evapotranspiration which also increases the salt concentration in shallow groundwater. Last, in aquifers located in coastal plains, the negative hydraulic gradient favours the inflow of seawater into the continental subsoil, as is the case in tidal areas.

However, mismanagement of aquifers by humans can also affect groundwater quality. In land subjected to excessive irrigation, particularly when poor-quality water rich in dissolved minerals is used, soil salinization can occur along with a rise of the water table (Ben-Hur et al., 2001), with the consequences explained above. Moreover, overexploitation of coastal aquifers reverses the normal hydraulic gradient, causing saltwater intrusion, i.e. inflow of sea water into continental subsoil, where it mixes with the aquifers’ fresh water (Fetter, 2001; Gordon et al., 2002). The mixture of freshwater with a mere 3-4% of seawater hampers many of its uses and, when it reaches 6%, aquifers can only be used for cooling and the disposal of household waste. Salinized aquifers can recover only very slowly (Morris et al., 2003).

When aquifer levels drop due to overexploitation, water has to be extracted from deeper levels. However, the mineral content of deeper water is higher, which limits its potential uses.

The indicator that addresses the issue of overexploitation of aquifers and other forms of mismanagement of groundwater, such as excessive irrigation, is developed in detail in the Availability section of this chapter. However, as these issues are closely related to groundwater quality, it is used as a joint indicator.