Measuring Water Pollution: Methods And Parameters

Water quality is a critical factor in maintaining a healthy ecosystem. It is often measured to determine if the water is safe for drinking, swimming, or other specific purposes. Water quality is assessed through various methods, including direct sampling, advanced instruments, and environmental observations. Scientists measure multiple properties, such as temperature, pH, dissolved solids, dissolved oxygen, and turbidity, to determine water quality and pollution levels. Routine measurements at scheduled intervals are essential for monitoring overall changes in water quality and taking appropriate actions to ensure the health of aquatic ecosystems.

Dissolved oxygen levels

Water quality is a critical factor in maintaining a healthy ecosystem. Measuring water quality provides evidence of ecosystem health and indicates if the water is usable, swimmable, or drinkable. One of the key parameters used to determine water quality is dissolved oxygen (DO).

Dissolved oxygen is the amount of oxygen that is dissolved in a water body. Oxygen levels in water are crucial for aquatic life, as they affect the ability of organisms to resist pollutants and parasites. DO levels are influenced by various factors, including temperature, pressure, salinity, and altitude. As temperature increases, particle motion increases, leading to a decrease in DO concentration. Similarly, as altitude increases, the concentration of DO decreases due to reduced pressure. Salinity also impacts DO levels, as oxygen is less attracted to water molecules in the presence of salt, resulting in lower DO concentrations.

To measure dissolved oxygen levels, modern techniques employ either electrochemical or optical sensors. Electrochemical sensors can be further categorized into polarographic, pulsed polarographic, and galvanic sensors. These sensors are attached to meters or data loggers to obtain measurements. The data obtained can be recorded using a dissolved oxygen meter, water quality sonde, or data logging system.

Optical DO sensors, also known as luminescent DO sensors, consist of a semi-permeable membrane, sensing element, light-emitting diode (LED), and photodetector. The sensing element contains a luminescent dye that reacts to the blue light emitted by the LED. The intensity and lifetime of the luminescence are dependent on the amount of dissolved oxygen in the water sample.

In addition to modern techniques, there are other methods for measuring dissolved oxygen, such as colorimetry and titration. Colorimetry provides a basic approximation of DO concentrations, while the traditional Winkler titration method was once considered the most accurate but is subject to human error and is less practical for field testing.

Water temperature

The solubility of oxygen and other gases in water is inversely proportional to temperature. Colder lakes and streams can hold more dissolved oxygen than warmer waters. If the water temperature increases due to thermal pollution or other factors, the dissolved oxygen levels decrease, which can have detrimental effects on aquatic life. Therefore, maintaining optimal water temperature is crucial for preserving the health and diversity of aquatic ecosystems.

Acidity (pH)

Acidity, or pH, is a crucial parameter in determining water quality. The pH scale measures the acidity or alkalinity of a liquid, with a pH of 7 being neutral. Values below 7 indicate acidity, while those above 7 are basic or alkaline. The pH scale typically ranges from 0 to 14, with each unit change representing a ten-fold difference in acidity or basicness. For example, water with a pH of 5 is ten times more acidic than water with a pH of 6.

The pH of water is influenced by various factors, including natural and human processes. Natural factors include lightning, volcanic ash, bacteria in wetlands, and decomposing organic matter such as pine needles. Human activities, particularly those related to pollution, can significantly impact the pH of water. Acid rain, resulting from the emission of nitrogen oxides, sulfur oxides, and other acidic compounds, is a well-known example of human influence on water pH. These emissions often originate from industrial sources, power plants, and the combustion of fossil fuels.

Pollution can also introduce chemicals into water bodies, causing an increase or decrease in pH levels. Agricultural runoff, wastewater discharge, and industrial activities can all contribute to these changes. For instance, detergents and soap-based products in wastewater can make a water source more basic, while mining operations can lead to acid runoff and acidic groundwater seepage.

The pH of water has significant ecological implications. It can affect the solubility and bioavailability of chemical constituents, including nutrients and heavy metals. Changes in water pH can harm aquatic plants and animals, as it influences the availability of toxins and nutrients. For instance, at low pH, metals tend to have higher solubility, increasing the toxicity of heavy metals in water.

The World Health Organization (WHO) has established a standard drinking water guideline for pH, recommending a range of 6.5 to 8.5. Municipal water suppliers often voluntarily test the pH of their water to monitor for pollutants, as pH can serve as a sensitive indicator of contamination.

Electrical conductivity

Conductivity is measured by applying a voltage between two electrodes in a probe immersed in the water sample. The drop in voltage caused by the resistance of the water is used to calculate the conductivity per centimetre. The probe measurement is then converted by a meter into micromhos per centimetre (µmhos/cm) or microsiemens per centimetre (µs/cm), which is the basic unit of measurement for conductivity. Distilled water, for example, has a conductivity range of 0.5 to 3 µmhos/cm, while US rivers tend to range from 50 to 1500 µmhos/cm.

Conductivity is influenced by several factors, including temperature and salinity. Warmer water tends to have higher conductivity, and this relationship between temperature and conductivity is why conductivity is reported at 25°C. Salinity also affects conductivity, with increasing salinity leading to higher conductivity. This strong correlation between conductivity and salinity is utilised in algorithms to estimate salinity and total dissolved solids (TDS), both of which are important for aquatic life and water quality.

TDS refers to the sum of all ion particles smaller than 2 microns in water. In clean water, TDS is approximately equal to salinity. However, in polluted areas, TDS can include organic solutes like hydrocarbons and urea, in addition to salt ions. While TDS measurements are derived from conductivity, some regions set a TDS maximum instead of a conductivity limit for water quality.

Conductivity is useful for monitoring pollution in shallow lake waters, as well as in streams and rivers. Each water body tends to have a relatively constant range of conductivity that serves as a baseline for comparison. Significant deviations from this baseline can indicate the presence of pollution or other disturbances, such as natural flooding or evaporation. For instance, an increase in conductivity could suggest a discharge of pollutants, while a decrease may indicate the presence of oil or hydrocarbons, which reduce the conductivity of water.

Turbidity

The material that causes water to be turbid includes clay, silt, very tiny inorganic and organic matter, algae, dissolved coloured organic compounds, and plankton and other microscopic organisms. Turbidity makes water cloudy or opaque. The higher the turbidity level, the higher the risk that people may develop gastrointestinal diseases. This is especially problematic for immunocompromised people, because contaminants like viruses or bacteria can become attached to the suspended solids.

There are two standard units for reporting turbidity: Formazin Nephelometric Units (FNU) from ISO 7027 and Nephelometric Turbidity Units (NTU) from USEPA Method 180.1. ISO 7027 and FNU is mostly widely used in Europe, whereas NTU is mostly used in the US. The ISO 7027 provides the method in water quality for the determination of turbidity. It is used to determine the concentration of suspended particles in a sample of water by measuring the incident light scattered at right angles from the sample. The scattered light is captured by a photodiode, which produces an electronic signal that is converted to a turbidity reading.

A turbidimeter is a device used to measure turbidity by shining a light through a water sample and measuring how much light is scattered by the particles. State-of-the-art turbidity meters are beginning to be installed in rivers to provide an instantaneous turbidity reading. A long device is lowered into the water and at the end is a turbidity sensor. It reads turbidity in the river by shining a light into the water and reading how much light is reflected back to the sensor.

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