**1.5 Surface water quality categories**

In general, the quality of surface water is determined by its loading with mineral or organic substances, suspended particles, living organisms, and dissolved gases, and in terms of water quality characteristics, the following terms are generally used [23]:


Water quality indicators provide basic information and help identify the trends of the changes in water quality over time. The most important indicators considered in the monitoring strategies and plants are dissolved oxygen, biological oxygen demand, water temperature, pH, nitrate (NO3), nitrite (NO2), and ammonia (NH3), metals, transparency, turbidity, coliforms, etc. [24, 25].

*Dissolved oxygen* (DO) is essential for plants and animals, but at high levels, in water, it can be harmful to fish and other aquatic organisms. Dissolved oxygen is measured in milligrams per liter (mg/L). Expected levels range between 4.0 and 12.0 mg/L [24].

*Biological oxygen demand* (BOD) is the amount of oxygen consumed by bacteria in the breakdown of organic matter. It also includes the oxygen needed to oxidize various chemicals in water, such as ammonia. BOD is determined by measuring the level of dissolved oxygen in a freshly collected sample and comparing it with the level of dissolved oxygen in a sample that was collected at the same time but incubated under specific conditions for several days. The difference in oxygen reading between the two samples is recorded in units of mg/L. Unpolluted, natural water should have a BOD level of 5 mg/L or even less, and wastewater may have BOD levels varying from 150 to 300 mg/L [25].

*Water temperature* affects many other parameters of water, including the amount of available dissolved oxygen, the types of plants and animals present, and the susceptibility of organisms to parasites, pollution, and diseases. The temperature is measured in degrees Celsius (°C). Seasonal trends: May–October: 22–35°C, from November to April: 2–27°C [24].

The *pH* test measures the alkalinity or acidity of the water. A pH of 7 is neutral, below 7 is acidic, and above 7 it is basic or alkaline. Acid rain, car leaks, or coalfired power plants cause a drop in water pH [24]. Pollution from accidental spills, agricultural spills, and sewage spills can also change the pH. While juvenile fish and insect larvae are sensitive to low pH (acid), extreme values at each end of the scale can be lethal to most organisms. Expected levels are 6.5–9.0 [25].

*Nitrogen* from the atmosphere or soil can undergo many complex chemical and biological changes, being a necessary nutrient for the growth of all living organisms. Nitrogen is found in natural waters in various forms: *nitrate* (NO3), *nitrite* (NO2), and *ammonia* (NH3). The test results are usually expressed as nitratenitrogen. Ammonia is the least stable form of nitrogen and thus it is difficult to

measure it accurately. In large quantities, nitrates lead to excess growth of algae. Under certain conditions, high levels of nitrates (10 mg/L or more) in drinking water can be toxic to humans and have also been linked to serious illness and even death in infants. Nitrates are expressed in milligrams per liter (mg/L) [25]. *Ammonia* is one of the most important pollutants in the aquatic environment due to its extremely toxic nature. It is discharged in large quantities into industrial, municipal, and agricultural wastewater [24]. *Nitrite* (NO2) is extremely toxic to the aquatic environment but is usually present in very small amounts in most natural water systems because it is rapidly oxidized to nitrate. Organic nitrogen and ammonia can be determined together and have been referred to as "Kjeldahl nitrogen, or TKN", a term that reflects the technique used in their determination (edition 19, Standard Methods, 1995).

*Transparency* is the quality of water to let the light energy through. Sunlight provides photosynthetic energy and determines the depth at which algae and other plants can grow. A change in the clarity of the water can be observed after heavy rains when mud and other debris cause decreased visibility [24].

The *turbidity* of water represents its lack of transparency being caused by suspended solid particles and plankton (microscopic plants and animals), which cannot be individualized with the naked eye. A low level of turbidity indicates a healthy ecosystem with moderate amounts of plankton, but a higher level of turbidity raises many problems for flow systems, surface water temperature can rise above normal, as particles suspended close to the water surface facilitate absorption heat from the sun. Suspended particles can carry nutrients, pesticides, and other pollutants and cloudy waters may have low levels of dissolved oxygen [25].

Water quality standards are based exclusively on scientific data on the relationship between pollutant concentrations and their effects on the environment and human health. The quality criteria provide a framework for the control of discharges and pollutant emissions [23]. Water quality standards define the water quality objectives in a body of water by designating beneficial uses and establishing criteria to protect those uses [14]. Beneficial uses may include fishing, swimming, aquatic habitat, navigation, agriculture, etc.
