Water quality describes the chemical, physical, and biological characteristics of water, reflecting its suitability for sustaining aquatic life or for human consumption. Monitoring this quality is a continuous process because water bodies are dynamic systems constantly influenced by natural processes and human activities. Maintaining environmental standards is essential for the health of aquatic ecosystems and the safety of public water supplies. Water quality is assessed using four primary categories of indicators: Dissolved Oxygen, pH Levels, Turbidity and Total Suspended Solids, and Biological Indicators.
Dissolved Oxygen
Dissolved Oxygen (DO) is the amount of free oxygen gas (O2) physically dissolved in the water. This free oxygen is necessary for the respiration of nearly all aquatic life, including fish, invertebrates, and aerobic microorganisms. Aquatic animals absorb this oxygen through their gills or body surfaces to support their metabolic functions.
Oxygen enters the water primarily through diffusion from the atmosphere and aeration caused by rapid water movement. Photosynthesis by aquatic plants and algae is also a significant source, releasing O2 as a byproduct during daylight hours. Warmer water naturally holds less dissolved oxygen than colder water, meaning increasing temperatures reduce the available oxygen supply.
Decomposition of organic matter, such as sewage waste, by bacteria consumes large amounts of oxygen in a process known as biological oxygen demand (BOD). When oxygen consumption exceeds replacement, DO levels drop, causing physiological stress to aquatic organisms. If DO concentrations fall too low, the water body enters a state of hypoxia. Conditions with virtually no DO are termed anoxic and are unsustainable for fish and many macroinvertebrate populations.
pH Levels
The pH level measures how acidic or basic the water is, quantified by the concentration of hydrogen ions (H+). The scale ranges from 0 to 14, where 7 is neutral, lower values are acidic, and higher values are alkaline. Because the scale is logarithmic, a change of one unit represents a tenfold change in acidity. The optimal range for most freshwater aquatic life falls between 6.5 and 8.5.
While extreme pH values can directly stress or kill organisms, its major role as an indicator relates to how it affects the solubility of other substances. Changes in pH alter the chemical state of pollutants, significantly changing their solubility and bioavailability to aquatic organisms. A lower, more acidic pH increases the solubility and mobility of heavy metals, such as copper, cadmium, and aluminum. This increased solubility makes the metals more bioavailable, thereby increasing their toxicity to fish and invertebrates.
Conversely, an increase in pH often causes heavy metals to precipitate out of the water column, reducing immediate toxicity but potentially leading to contaminated sediments. Fluctuations outside the acceptable range cause physiological stress in aquatic species, resulting in decreased growth, reproductive issues, or death.
Turbidity and Total Suspended Solids
Turbidity measures the cloudiness of water, caused by suspended particles that scatter light. Total Suspended Solids (TSS) refers to the physical particles suspended in the water column that do not dissolve, such as silt, clay, and organic matter. Turbidity and TSS are closely related, as high concentrations of TSS directly increase the measured turbidity.
Sources of TSS include soil erosion from agriculture or construction, stormwater runoff, and industrial discharges. These suspended particles significantly reduce sunlight penetration into the water column. Reduced light inhibits photosynthesis by submerged aquatic plants and algae, which reduces the amount of oxygen they produce.
High TSS levels affect aquatic life directly by clogging fish gills, impairing respiratory function and leading to stress or disease. Settled solids can smother benthic habitats, disrupting ecosystems that support bottom-dwelling organisms and fish eggs. Suspended particles can also absorb and transport harmful pollutants like pesticides and heavy metals throughout the aquatic environment.
Biological Indicators
Biological indicators focus on organisms present in the water, emphasizing microorganisms that signal contamination posing a direct human health risk. Fecal contamination from sources like wastewater treatment plants or agricultural runoff introduces pathogens, including bacteria, viruses, and parasites. Because testing for every potential pathogen is time-consuming, agencies test for Fecal Indicator Bacteria (FIB), such as Escherichia coli (E. coli) and Enterococci.
FIB are typically harmless bacteria found in the intestinal tracts of warm-blooded animals and are present in feces. Their presence in water indicates that fecal matter has entered the system, signaling the potential for harmful disease-causing microorganisms. Detecting these indicator organisms provides a simple and fast method for diagnosing sewage pollution and assessing the risk of waterborne illnesses.
Exposure to water contaminated with fecal matter is a serious public health concern, often associated with illnesses such as gastroenteritis. While FIB are useful for identifying fecal contributions, they do not always perfectly correlate with the presence of pathogens due to differing survival rates. However, the use of indicator bacteria remains a valuable tool for monitoring recreational waters and drinking water supplies to safeguard human health.