Water Quality Index

What is the Water Quality Index?

The Water Quality Index (WQI) for rivers in Puerto Rico is the result of a research performed by personnel of the PRWRERI. This index intends to classify the water quality in a scale from 0 to 100 according to selected water quality indicators. The WQI is determined based on eleven water quality indicators; fecal coliforms, pH, BOD5, dissolved oxygen, nitrates, total phosphorus, ammonia nitrogen, suspended solids, mercury, lead, and temperature. These water quality parameters are indicative of bacterial contamination, nutrients in water and heavy metals pollution. A group of experts in water quality participated in the selection of the water quality indicators considered in the calculation of WQI. The selection of these indicators followed the Delphi process, in which carefully prepared questionnaires where used for this purpose. The use of this process resulted in eleven water quality indicators selected, each one with its corresponding sub-index graph. The WQI was subjected to a strict evaluation where the goodness of fit of each sub-index was determined, and deficiencies and applicability to the local conditions of the rivers in Puerto Rico were identified. The WQI is representative of water quality conditions for the specific area where water samples were collected. The map below shows the WQI calculated for various USGS stations. Clicking on the dots will show the WQI and associated data.

The following table and color code has been prepared for ease of interpretation of the WQI value. (NOTE: This interpretation is the sole appreciation of the author)

WQI Condition Color Code
90-100 Good GREEN
70-90 Moderate BLUE
30-70 Average YELLOW
15-30 Caution ORANGE
0-15 Poor RED


The Puerto Rico Water Resources and Environmental Research Institute is not responsible for, and expressly disclaims all liability for, damages of any kind arising out of use, reference to, or reliance on any information contained within the site. While the information contained within the site is periodically updated, no guarantee is given that the information provided in this web site is correct, complete, and up-to-date.

Water Quality Indicators*

1. Fecal Coliforms (Col./100mL)

A fecal coliform is a facultative anaerobic, rod-shaped bacterium. Intended to be an indicator of fecal contamination; more specifically of E. coli, which is an indicator microorganism for other pathogens that may be present in feces. Presence of fecal coliforms in water may not be directly harmful, and does not necessarily indicate the presence of feces. In general, increased levels of fecal coliforms provide a warning of possible failure in wastewater treatment plants, a break in the integrity of the wastewater collection system, or possible contamination with pathogens. When levels are high there may be an elevated risk of waterborne gastroenteritis.

2. pH (Std. Units)

Defined as the decimal logarithm of the reciprocal of the hydrogen ion activity in a solution. It ranges from 0 to 14. pH of pure water is about 7 at 25 C; this value varies with temperature. When an acid is dissolved in water, the pH will be less than that of pure water. When a base, or alkali, is dissolved in water, the pH will be greater than that of pure water. When chemicals are mixed with water, the mixture can become either acidic or basic. Vinegar and lemon juice are acidic substances, while laundry detergents and ammonia are basic.

3. Dissolved Oxygen (mg/L)

Amount of oxygen dissolve in the water. Oxygen is measured in its dissolved form as dissolved oxygen (DO). Running water, because of its churning, dissolves oxygen. Respiration by aquatic animals, decomposition, and various chemical reactions consume oxygen. If more oxygen is consumed than is produced, dissolved oxygen levels decline and some sensitive animals may move away, weaken, or die. DO levels fluctuate seasonally and over a 24-hour period. They vary with water temperature and altitude. Cold water holds more oxygen than warm water and water holds less oxygen at higher altitudes.

4. BOD5 (mg/L)

Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter in stream water. BOD also measures the chemical oxidation of inorganic matter. A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time (usually 5 days at 20 C). The rate of oxygen consumption in a stream is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water. BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the more rapidly oxygen is depleted in the stream. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die. Sources of BOD include leaves and woody debris; dead plants and animals; animal manure; wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.

5. Nitrates (mg/L as N)

Nitrates are nitrogen-oxygen chemical units which combine with various organic and inorganic compounds. The greatest use of nitrates is as a fertilizer. Once taken into the body, nitrates are converted to nitrites. Infants below six months who drink water containing nitrate in excess of the maximum contaminant level could become seriously ill and, if untreated, may die. Symptoms include shortness of breath and blue baby syndrome.

6. Total Phosphorus (mg/L as P)

Total Phosphorus is an essential nutrient for plants and animals. It is naturally limited in most fresh water systems because it is not as abundant as carbon and nitrogen; introducing a small amount of additional phosphorus into a waterway can have adverse effects. Sources of phosphorus include soil and rocks, wastewater treatment plants, runoff from fertilized lawns and cropland, runoff from animal manure storage areas, disturbed land areas, drained wetlands, water treatment, decomposition of organic matter, and commercial cleaning preparations.

7. Ammonia Nitrogen (mg/L as N)

Ammonia (NH3) is a common toxicant derived from wastes, fertilizers, and natural processes. Ammonia nitrogen includes both the ionized form (ammonium, NH4 +) and the unionized form (ammonia, NH3). An increase in pH favors formation of the more toxic unionized form (NH3), while a decrease favors the ionized (NH4+) form. Temperature also affects the toxicity of ammonia to aquatic life. Ammonia is a common cause of fish kills, but the most common problems associated with ammonia relate to elevated concentrations affecting fish growth, gill condition, organ weights, and hematocrit. Exposure duration and frequency strongly influence the severity of effects.

8. Suspended Solids (mg/L)

It is sediments primarily composed of fine inorganic particles of clay and silt, but also may include fine sand and particulate organic matter, suspended in the water column. Although sediment is a natural part of aquatic habitats, the quantity and characteristics of sediments can affect the physical, chemical, and biological integrity of aquatic ecosystems.

9. Mercury (ng/L)

Mercury is an element naturally occurring in the earth's crust. Humans cannot create or destroy mercury. Pure mercury is a liquid metal, sometimes referred to as quicksilver that volatizes readily. It has traditionally been used to make products like thermometers, switches, and some light bulbs. Mercury in the air eventually settles into water or onto land where it can be washed into water. Once deposited, certain microorganisms can change it into methylmercury, a highly toxic form that builds up in fish, shellfish and animals that eat fish. Fish and shellfish are the main sources of methylmercury exposure to humans.

10. Lead (g/L)

Lead is a naturally occurring element found in small amounts in the earth's crust. While it has some beneficial uses, it can be toxic to humans and animals causing of health effects. Lead can be found in all parts of our environment - the air, the soil, the water, and even inside our homes. While natural levels of lead in soil range between 50 and 400 parts per million, mining, smelting, and refining activities have resulted in substantial increases in lead levels in the environment, especially near mining and smelting sites.

11. Temperature (°C)

Temperature is the concentration of thermal energy in a substance such as water. The phrase "thermal regime" is used when emphasizing the temporal and spatial distribution of temperature. Temperatures in streams and rivers are influenced by many atmospheric and hydrologic processes that influence the movement of heat. In turn, temperature plays a fundamental role in shaping the structure and function of aquatic systems, and is frequently used as a basis for classifying streams (e.g., cold-water, warm-water).

*Information exerted from

Instructions for use:
  1. Please, read the disclaimer located above the Water Quality Index map to the left.
  2. The map shown indicates the available USGS' water quality stations of different bodies of water in Puerto Rico. These stations where used for the calculation of the WQI. Note that not all stations are active; meaning that the collection of water quality data of some stations has been temporarily or permanently discontinued.
  3. In the Water Quality Index Map Application, blue dots represent active stations and light grey dots stand for inactive stations.
  4. When a dot is clicked, a pop-up window opens. This window has three pages of information. The first page contains information about the USGS stations. The second page shows the WQI calculated with the most recent available data and the third window, similar to the second, shows the WQI calculated using the arithmetic mean of all available data for each water quality indicators.
  5. The pop-up window has arrows at the top right corner which are used to navigate between the different windows of the selected station.
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