Many different physical, chemical, and biological parameters can be used to measure water quality. The water quality parameters chosen for the 2008 EPI, which are from the Water Quality Index (WATQI), were selected for two reasons. First, they are good indicators of specific issues relevant on a global basis (eutrophication, nutrient pollution, acidification, and salinization). Second, they are the most consistently reported.
The United Nations GEMS/Water Programme maintains the only global database of water quality for inland waters. GEMStat is the online global database of water quality maintained by GEMS/Water that has almost 4 million entries for lakes, reservoirs, rivers, and groundwater systems from more than 3,000 monitoring stations. While the GEMS/Water database is the most comprehensive global database of water quality, there are still gaps in country coverage.
Five water quality parameters were chosen for the 2008 EPI: Dissolved oxygen, pH, Conductivity, Total nitrogen, and Total phosphorus. Dissolved oxygen is the measure of free (i.e., not chemically combined) oxygen dissolved in water. It is essential to the metabolism of all aerobic aquatic organisms and at reduced levels has been shown to cause both lethal and sublethal effects. pH, which is the measure of the acidity or alkalinity of a water body, is an important parameter of water quality in inland waters in that it can affect aquatic organisms both directly through impairing respiration, growth and development of fish, and indirectly, through increasing the bioavailability of certain metals such as aluminum and nickel. Conductivity is a measure of the ability of water to carry an electric current, which is dependent on the presence of ions. Increases in conductivity can lead to changes that reduce biodiversity and alter community composition. (Weber-Scannell and Duffy, 2007). Nitrogen and phosphorus are naturally-occurring elements essential for all living organisms and are often found in growth-limiting concentrations in aquatic environments. Increases in nitrogen and/or phosphorus in natural waters, largely as a result of human activities in the drainage basin (e.g., from agricultural runoff from manure and synthetic fertilizers and from municipal and industrial wastewater discharge), can result in increased biological productivity of a water body.
The Water Quality indicator is a proximity-to-target composite of water quality, adjusted for monitoring stations’ density in each country, with the maximum score of 100. Data were available to compute indicator values for 94 countries. For countries where no values could be computed using available data, a regional imputed value was used. Water Quality was imputed for a total of 138 countries.
Each dot in the histogram is a single country’s proximity-to-target score for this indicator. The tinted box represents the inner quartile range of country values, or points lying between the 25th and 75th percentiles. The small triangle marker at the bottom of the chart indicates the median of country scores.
