As far as quality is concerned, in Mexico, the quality of water in surface bodies, coastal areas and groundwater can be classified as:

The previous categories are established based on three indicators, which are:

BOD5 and COD are used to determine the amount of organic matter present in bodies of water coming mainly from wastewater discharges, of municipal and non-municipal origin. On the other hand, TSS originate from wastewater and soil erosion. Increased SST levels cause a body of water to lose its ability to support the diversity of aquatic life. These parameters allow us to recognize gradients that range from a relatively natural condition or without influence of human activity, to water that shows signs or important contributions of municipal and non-municipal wastewater discharges, as well as areas with severe deforestation.

These three indicators are currently measured in Mexico in hundreds of surface water sites through the Water Quality Monitoring Network[13]​ of the National Water Commission, an administrative body with authority over the quantity and quality of water and its management in Mexican national territory, designated as such in the LAN.[1]​ Each body of water evaluated by the Monitoring Network[13]​ will have values for each of the three water quality indicators, one for each indicator and, data curious, without implying that it is classified with respect to any specific use, but rather considering a gradient from natural condition to contaminated body of water, classifying surface waters as shown below.

For some time, in Mexico, the Water Quality Index (WQI)[15]​ was used as a parameter to rate the state of water bodies. However, its use became complicated since the determination of 18 parameters was expensive and the results obtained did not allow us to actually distinguish the state of the water bodies. Furthermore, many bodies of water could not be classified as suitable for use, due to the greater importance given to the parameters aimed at the protection of aquatic life; for example, greater weight was given to dissolved oxygen (O). Thus, starting in 2004, water quality is verified through the three indicators already mentioned: BOD5,[10]​ COD[11]​ and TSS.[12]​.

On the other hand, another reference widely used in Mexico to evaluate water quality refers to the Ecological Criteria of Water Quality CE-CCA-001/89.[16] These criteria were initially published in the Ecological Gazette published by the Secretariat of Urban Development and Ecology (SEDUE) and, later, on December 13, 1989 in the Official Gazette of the Federation "Diario Oficial de la Federación (Mexico)") (DOF). These criteria can be consulted by searching by publication date in the DOF after registering for free.

Based on the Ecological Water Quality Criteria CE-CCA-001/89, bodies of water can be classified as suitable, or not, to be used as:.

The publication of the Ecological Water Quality Criteria in Mexico was inspired by the publication of the Quality Criteria for Water (EPA 440/5-86-001, 1986), published in 1986 by the Environmental Protection Agency of the United States of America (USEPA). The latter arose in response to the Clean Water Act of the United States of America, which required the EPA to publish and periodically update water quality criteria in the document recognized as The Gold Book. These criteria should accurately reflect the latest scientific knowledge regarding the type and extent of identifiable health and environmental effects that could be expected from the presence of contaminants in bodies of water, including aquifers. In addition, they had to include the latest knowledge regarding the concentration and dispersion of pollutants, or their derivative products, through biological, physical and chemical processes. Finally, point out the effects of contaminants on biological communities, productivity and stability, including information on the factors that affected the rates of eutrophication and organic and inorganic sedimentation. Although the Water Quality Criteria were not laws, nor did they have regulatory impact, they did present scientific data, but, above all, they offered (and continue to offer) guidance on the environmental effects of contaminants. This information could be useful in formulating standards based on considerations of water quality impacts. It was also established that when new data became available, said criteria would have to be updated to reflect applicable recommendations for the protection of aquatic life and human health; especially at the regional level. A report on the monitoring and evaluation of streams and rivers, the National Water-Quality Assessment, is now published.[17]​.

Apparently, this same spirit was, in some way, installed in the publication of The Ecological Criteria in Mexico. As in the United States, the Ecological Criteria in Mexico did not have (and do not have) a regulatory nature, but are normally applied by government and academic sectors to qualify water resources. An example of this, just to mention one, is evident in the assessment of water quality in coastal ecosystems, where, in the absence of normative reference values, the use of the Ecological Water Quality Criteria CE-CCA-001/89 is used to evaluate Water Quality in the Coastal Systems of Mexico.[18]​ However, some limits must be reviewed since analytical techniques are not sensitive enough to detect them. Especially in the case of compounds that are found in trace or very small concentrations such as hydrocarbons and pesticides.

Currently, in Mexico, following international examples, we seek to have reference sites, defined as those that can serve as comparison due to having natural conditions or very low alteration within a basin. These sites are very useful in stream classification studies to estimate their capacity to dilute and assimilate pollutants, establish maximum discharge limits and indicate quality goals for different periods in the declarations of article 87 of the LAN[1]​ as well as to associate them with the conditions that allow the existence of aquatic communities in ecological flow studies.

Water quality for public supply

Regarding water from public supply sources, water is considered to be of good quality when it is free of substances and microorganisms that are dangerous to consumers and free of substances that transmit unpleasant sensory sensations for human consumption, such as color, odor, flavor or turbidity. Quality standards for this type of use are established on the basis of microbiological, physical and organoleptic characteristics, content of chemical constituents and radioactive characteristics. The World Health Organization (WHO)[19] sets maximum permissible limits for various contaminants for drinking water and primary contact, which are considered the basis for regulations in various countries.

Although each country establishes its regulations, in general microbiological, physicochemical aspects, content of organic contaminants and in some cases, radioactive contaminants are regulated, as can be seen when comparing the regulations in force in countries such as Peru, Colombia, El Salvador and Mexico. In the case of Mexico, the "MODIFICATION to the Official Mexican Standard NOM-127-SSA1-1994,[20]​ Environmental health. Water for human use and consumption. Permissible limits of quality and treatments to which water must be subjected for its purification" is the standard that regulates the bacteriological, physical and organoleptic, chemical and radioactive characteristics of water for human consumption. This standard can be consulted in the DOF, dated November 22, 2000.

When the regulations of different countries are reviewed in detail, the contrast is interesting. For example, regarding the maximum permissible limits (MPL) for organic contaminants, Peru[21]​ establishes two global or general parameters, carbon chloroform extract and extractables in petroleum ether, while Colombia (Colombian Institute of Technical Standards and Certification, 2013) regulates some specific compounds, such as the pesticides parathion, methylparathion, carbamates and toxaphene, in addition to the persistent chlorinated pesticides also included in Mexican regulations. The vanguard is the La República del Salvador brand[22]​, which specifically indicates limits for a total of 32 pesticides such as atrazine, carbofuran and simazine, among others. This may be due to the fact that its regulations are more recent, from 2009. To date, Mexican regulations[23]​ only include seven chlorinated pesticides. However, there is a draft standard entitled "Water for human use and consumption. Maximum permissible limits of water quality, control and surveillance of supply systems", which proposes regulating a significant number of pesticides, thirty to be precise, among which are chlorinated compounds, phosphorus compounds, carbamates, urea derivatives and chlorinated herbicides. However, until now it has not been published, that is, it has not been made official.

Water quality for agricultural irrigation

The quality of water for irrigation is determined mainly by the type and quantity of dissolved salts but also by the tolerance of crops to salts, soil properties, soil and water management conditions, and climatological conditions. Salts dissolved in water are carried to the soil when crops are irrigated and if they are not absorbed by them, they remain in it when the water evaporates. As the salt content of the water increases, various problems may develop, both in the soil and in the crops, and it may become necessary to apply special management practices to maintain acceptable crop yields. Water quality or suitability for agricultural use is judged based on the potential severity of problems that could develop during long-term use, including restriction of crop yield.

The Food and Agriculture Organization of the United Nations, FAO, through its publication Water Quality for Agriculture[24]​ presents, among other topics, a guide that helps determine the degree of water restriction based on the aforementioned factors.

The International Commission on Irrigation and Drainage,[25]​ ICID (for its acronym in English) has also indicated water quality guide values ​​for different crops of regional importance in the world.

On the other hand, in Mexico the reuse of wastewater is allowed for planting, growing and harvesting agricultural products, except legumes and vegetables that are consumed raw. The water quality that the water must meet for this purpose is regulated by the Official Mexican Standard NOM-001-SEMARNAT-1996 Archived on September 25, 2015 at the Wayback Machine.[26]​ which establishes basic physicochemical parameters, such as floating matter and fats and oils among others, in addition to cyanides and some metals, and indicators of contamination by pathogens and parasites.

Water quality for the self-sufficient industry

A self-sufficient industry is one that is supplied directly from the country's rivers, streams, lakes or aquifers. The main business branches that make up this group are the mining, food, sugar, beverages, textile, paper, chemical and steel industries. In Mexico there are no regulations regarding water quality for self-supplied industry; rather, the water quality needs will be governed by the requirements of the process and other uses for which it is intended. For example, in the food industry, water can be used, in addition to being an ingredient, to wash and disinfect fruits and vegetables, in the process of cleaning and sanitizing surfaces, in personal hygiene, in cooling processes and in the production of steam. In this case, then, the water must meet at least all the water quality parameters established by the Modification to NOM-127-SSA1-1994,[20]​ except for the last two uses in which water with a stricter dissolved solids content is required to avoid scale in the cooling and steam generation systems.

Water quality for thermoelectric plants

Water in thermoelectric plants is mainly used in the cooling system for steam condensate. The main component of such systems are cooling towers where hot water exchanges heat with air. The basis of cooling is evaporation, but as the water evaporates, the ions that make it up do not do so and remain in the circulation water, where they concentrate to excessive levels. When the concentration of some species exceeds certain limits, there is a risk that the water will be corrosive or that some species will precipitate and deposit on the heat transfer surfaces, causing the water to acquire fouling characteristics. The chemical species that cause these problems are mainly: calcium carbonates, calcium sulfate, magnesium and calcium silicates and phosphates. The Electric Power Research Institute[27]​ (EPRI) of the United States has established quality parameters for the water used in cooling towers to minimize operational problems and these also serve as guidelines internationally.

Despite the deficiencies that currently exist in Mexico regarding regulations in relation to water, this is extensive, and includes both [28]​ and standards, some of mandatory observance, (NOMs)[29]​ and others of voluntary application Mexican Standards Archived on September 25, 2015 at the Wayback Machine. (NMXs). A summary of these and some others is presented concisely in Chapter 5.5 of Water Statistics in Mexico Archived June 1, 2015 at the Wayback Machine.[30] classified into the following groups:

Water quality and human health

The importance of water quality lies in the fact that water is one of the main means for the transmission of many diseases that affect humans. According to descriptive note N.330 Archived September 25, 2015 at the Wayback Machine.[54]​ published by the WHO in April 2013, diarrheal diseases cause 760,000 deaths in children under five years of age annually. In Mexico, this painful situation is reflected in the statistics published in the Water and Health icon of the SINA,[6]​ in 2014, where around 9,300 infant deaths are recorded in 2010.

For that same year, the United Nations formally recognized the human right to water and sanitation through Resolution 64/292,[55]​ which calls on States and international organizations to provide financial resources, promote training and technology transfer to help countries, particularly those in development, to provide sufficient, healthy, acceptable, accessible and affordable drinking water to all. The document Report on the human right to water in Mexico[56] illustrates the vision that exists about this right in Mexico.

In terms of sanitation, Mexico maintains a significant gap between purification and sanitation, and between the provision of these services in urban and rural areas, treating only percentages less than 60% of municipal wastewater.[6]​ Therefore, it was established as a goal within the Millennium Goals[57]​ to reduce by half, by 2015, the proportion of people without sustainable access to drinking water and basic sanitation services in order to guarantee environmental sustainability.

Therefore, knowing the quality of water is important not only in terms of disease transmission but also for:

As far as quality is concerned, in Mexico, the quality of water in surface bodies, coastal areas and groundwater can be classified as:

The previous categories are established based on three indicators, which are:

BOD5 and COD are used to determine the amount of organic matter present in bodies of water coming mainly from wastewater discharges, of municipal and non-municipal origin. On the other hand, TSS originate from wastewater and soil erosion. Increased SST levels cause a body of water to lose its ability to support the diversity of aquatic life. These parameters allow us to recognize gradients that range from a relatively natural condition or without influence of human activity, to water that shows signs or important contributions of municipal and non-municipal wastewater discharges, as well as areas with severe deforestation.

These three indicators are currently measured in Mexico in hundreds of surface water sites through the Water Quality Monitoring Network[13]​ of the National Water Commission, an administrative body with authority over the quantity and quality of water and its management in Mexican national territory, designated as such in the LAN.[1]​ Each body of water evaluated by the Monitoring Network[13]​ will have values for each of the three water quality indicators, one for each indicator and, data curious, without implying that it is classified with respect to any specific use, but rather considering a gradient from natural condition to contaminated body of water, classifying surface waters as shown below.

For some time, in Mexico, the Water Quality Index (WQI)[15]​ was used as a parameter to rate the state of water bodies. However, its use became complicated since the determination of 18 parameters was expensive and the results obtained did not allow us to actually distinguish the state of the water bodies. Furthermore, many bodies of water could not be classified as suitable for use, due to the greater importance given to the parameters aimed at the protection of aquatic life; for example, greater weight was given to dissolved oxygen (O). Thus, starting in 2004, water quality is verified through the three indicators already mentioned: BOD5,[10]​ COD[11]​ and TSS.[12]​.

On the other hand, another reference widely used in Mexico to evaluate water quality refers to the Ecological Criteria of Water Quality CE-CCA-001/89.[16] These criteria were initially published in the Ecological Gazette published by the Secretariat of Urban Development and Ecology (SEDUE) and, later, on December 13, 1989 in the Official Gazette of the Federation "Diario Oficial de la Federación (Mexico)") (DOF). These criteria can be consulted by searching by publication date in the DOF after registering for free.

Based on the Ecological Water Quality Criteria CE-CCA-001/89, bodies of water can be classified as suitable, or not, to be used as:.

The publication of the Ecological Water Quality Criteria in Mexico was inspired by the publication of the Quality Criteria for Water (EPA 440/5-86-001, 1986), published in 1986 by the Environmental Protection Agency of the United States of America (USEPA). The latter arose in response to the Clean Water Act of the United States of America, which required the EPA to publish and periodically update water quality criteria in the document recognized as The Gold Book. These criteria should accurately reflect the latest scientific knowledge regarding the type and extent of identifiable health and environmental effects that could be expected from the presence of contaminants in bodies of water, including aquifers. In addition, they had to include the latest knowledge regarding the concentration and dispersion of pollutants, or their derivative products, through biological, physical and chemical processes. Finally, point out the effects of contaminants on biological communities, productivity and stability, including information on the factors that affected the rates of eutrophication and organic and inorganic sedimentation. Although the Water Quality Criteria were not laws, nor did they have regulatory impact, they did present scientific data, but, above all, they offered (and continue to offer) guidance on the environmental effects of contaminants. This information could be useful in formulating standards based on considerations of water quality impacts. It was also established that when new data became available, said criteria would have to be updated to reflect applicable recommendations for the protection of aquatic life and human health; especially at the regional level. A report on the monitoring and evaluation of streams and rivers, the National Water-Quality Assessment, is now published.[17]​.

Apparently, this same spirit was, in some way, installed in the publication of The Ecological Criteria in Mexico. As in the United States, the Ecological Criteria in Mexico did not have (and do not have) a regulatory nature, but are normally applied by government and academic sectors to qualify water resources. An example of this, just to mention one, is evident in the assessment of water quality in coastal ecosystems, where, in the absence of normative reference values, the use of the Ecological Water Quality Criteria CE-CCA-001/89 is used to evaluate Water Quality in the Coastal Systems of Mexico.[18]​ However, some limits must be reviewed since analytical techniques are not sensitive enough to detect them. Especially in the case of compounds that are found in trace or very small concentrations such as hydrocarbons and pesticides.

Currently, in Mexico, following international examples, we seek to have reference sites, defined as those that can serve as comparison due to having natural conditions or very low alteration within a basin. These sites are very useful in stream classification studies to estimate their capacity to dilute and assimilate pollutants, establish maximum discharge limits and indicate quality goals for different periods in the declarations of article 87 of the LAN[1]​ as well as to associate them with the conditions that allow the existence of aquatic communities in ecological flow studies.

Water quality for public supply

Regarding water from public supply sources, water is considered to be of good quality when it is free of substances and microorganisms that are dangerous to consumers and free of substances that transmit unpleasant sensory sensations for human consumption, such as color, odor, flavor or turbidity. Quality standards for this type of use are established on the basis of microbiological, physical and organoleptic characteristics, content of chemical constituents and radioactive characteristics. The World Health Organization (WHO)[19] sets maximum permissible limits for various contaminants for drinking water and primary contact, which are considered the basis for regulations in various countries.

Although each country establishes its regulations, in general microbiological, physicochemical aspects, content of organic contaminants and in some cases, radioactive contaminants are regulated, as can be seen when comparing the regulations in force in countries such as Peru, Colombia, El Salvador and Mexico. In the case of Mexico, the "MODIFICATION to the Official Mexican Standard NOM-127-SSA1-1994,[20]​ Environmental health. Water for human use and consumption. Permissible limits of quality and treatments to which water must be subjected for its purification" is the standard that regulates the bacteriological, physical and organoleptic, chemical and radioactive characteristics of water for human consumption. This standard can be consulted in the DOF, dated November 22, 2000.

When the regulations of different countries are reviewed in detail, the contrast is interesting. For example, regarding the maximum permissible limits (MPL) for organic contaminants, Peru[21]​ establishes two global or general parameters, carbon chloroform extract and extractables in petroleum ether, while Colombia (Colombian Institute of Technical Standards and Certification, 2013) regulates some specific compounds, such as the pesticides parathion, methylparathion, carbamates and toxaphene, in addition to the persistent chlorinated pesticides also included in Mexican regulations. The vanguard is the La República del Salvador brand[22]​, which specifically indicates limits for a total of 32 pesticides such as atrazine, carbofuran and simazine, among others. This may be due to the fact that its regulations are more recent, from 2009. To date, Mexican regulations[23]​ only include seven chlorinated pesticides. However, there is a draft standard entitled "Water for human use and consumption. Maximum permissible limits of water quality, control and surveillance of supply systems", which proposes regulating a significant number of pesticides, thirty to be precise, among which are chlorinated compounds, phosphorus compounds, carbamates, urea derivatives and chlorinated herbicides. However, until now it has not been published, that is, it has not been made official.

Water quality for agricultural irrigation

The quality of water for irrigation is determined mainly by the type and quantity of dissolved salts but also by the tolerance of crops to salts, soil properties, soil and water management conditions, and climatological conditions. Salts dissolved in water are carried to the soil when crops are irrigated and if they are not absorbed by them, they remain in it when the water evaporates. As the salt content of the water increases, various problems may develop, both in the soil and in the crops, and it may become necessary to apply special management practices to maintain acceptable crop yields. Water quality or suitability for agricultural use is judged based on the potential severity of problems that could develop during long-term use, including restriction of crop yield.

The Food and Agriculture Organization of the United Nations, FAO, through its publication Water Quality for Agriculture[24]​ presents, among other topics, a guide that helps determine the degree of water restriction based on the aforementioned factors.

The International Commission on Irrigation and Drainage,[25]​ ICID (for its acronym in English) has also indicated water quality guide values ​​for different crops of regional importance in the world.

On the other hand, in Mexico the reuse of wastewater is allowed for planting, growing and harvesting agricultural products, except legumes and vegetables that are consumed raw. The water quality that the water must meet for this purpose is regulated by the Official Mexican Standard NOM-001-SEMARNAT-1996 Archived on September 25, 2015 at the Wayback Machine.[26]​ which establishes basic physicochemical parameters, such as floating matter and fats and oils among others, in addition to cyanides and some metals, and indicators of contamination by pathogens and parasites.

Water quality for the self-sufficient industry

A self-sufficient industry is one that is supplied directly from the country's rivers, streams, lakes or aquifers. The main business branches that make up this group are the mining, food, sugar, beverages, textile, paper, chemical and steel industries. In Mexico there are no regulations regarding water quality for self-supplied industry; rather, the water quality needs will be governed by the requirements of the process and other uses for which it is intended. For example, in the food industry, water can be used, in addition to being an ingredient, to wash and disinfect fruits and vegetables, in the process of cleaning and sanitizing surfaces, in personal hygiene, in cooling processes and in the production of steam. In this case, then, the water must meet at least all the water quality parameters established by the Modification to NOM-127-SSA1-1994,[20]​ except for the last two uses in which water with a stricter dissolved solids content is required to avoid scale in the cooling and steam generation systems.

Water quality for thermoelectric plants

Water in thermoelectric plants is mainly used in the cooling system for steam condensate. The main component of such systems are cooling towers where hot water exchanges heat with air. The basis of cooling is evaporation, but as the water evaporates, the ions that make it up do not do so and remain in the circulation water, where they concentrate to excessive levels. When the concentration of some species exceeds certain limits, there is a risk that the water will be corrosive or that some species will precipitate and deposit on the heat transfer surfaces, causing the water to acquire fouling characteristics. The chemical species that cause these problems are mainly: calcium carbonates, calcium sulfate, magnesium and calcium silicates and phosphates. The Electric Power Research Institute[27]​ (EPRI) of the United States has established quality parameters for the water used in cooling towers to minimize operational problems and these also serve as guidelines internationally.

Despite the deficiencies that currently exist in Mexico regarding regulations in relation to water, this is extensive, and includes both [28]​ and standards, some of mandatory observance, (NOMs)[29]​ and others of voluntary application Mexican Standards Archived on September 25, 2015 at the Wayback Machine. (NMXs). A summary of these and some others is presented concisely in Chapter 5.5 of Water Statistics in Mexico Archived June 1, 2015 at the Wayback Machine.[30] classified into the following groups:

Water quality and human health

The importance of water quality lies in the fact that water is one of the main means for the transmission of many diseases that affect humans. According to descriptive note N.330 Archived September 25, 2015 at the Wayback Machine.[54]​ published by the WHO in April 2013, diarrheal diseases cause 760,000 deaths in children under five years of age annually. In Mexico, this painful situation is reflected in the statistics published in the Water and Health icon of the SINA,[6]​ in 2014, where around 9,300 infant deaths are recorded in 2010.

For that same year, the United Nations formally recognized the human right to water and sanitation through Resolution 64/292,[55]​ which calls on States and international organizations to provide financial resources, promote training and technology transfer to help countries, particularly those in development, to provide sufficient, healthy, acceptable, accessible and affordable drinking water to all. The document Report on the human right to water in Mexico[56] illustrates the vision that exists about this right in Mexico.

In terms of sanitation, Mexico maintains a significant gap between purification and sanitation, and between the provision of these services in urban and rural areas, treating only percentages less than 60% of municipal wastewater.[6]​ Therefore, it was established as a goal within the Millennium Goals[57]​ to reduce by half, by 2015, the proportion of people without sustainable access to drinking water and basic sanitation services in order to guarantee environmental sustainability.

Therefore, knowing the quality of water is important not only in terms of disease transmission but also for: