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
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)
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
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 (µg/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.,
*Information exerted from www.epa.gov
Please, read the disclaimer located above the Water Quality Index map to the left.
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.
In the Water Quality Index Map Application, blue dots represent active stations and light grey dots stand for
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.
The pop-up window has arrows at the top right corner which are used to navigate between the different windows
of the selected station.