Chemical Properties

Chemical measurements include a wide range of chemicals and chemical properties. Most water chemistry tests measure concentration, defined as milligrams of chemical per liter of water (mg/l).

Even the purest water contains countless chemicals, and it would be impossible to measure all of them. Water quality studies therefore focus on the chemicals that are most important for the problem at hand. In agricultural areas, studies measure chemicals found in manure, fertilizers, and pesticides. In an industrial area studies focus on measuring chemicals used by the nearby industries.

The following basic parameters are measured in nearly all studies; petroleum products, metals, industrial chemicals, pesticides, and herbicides are added when needed.

Dissolved Oxygen (DO)

Dissolved Oxygen is the concentration of oxygen dissolved in the water, expressed as milligrams oxygen per liter water (mg/l). DO is an important measurement of aquatic health, since aquatic organisms must get all of their oxygen from water. Healthy water bodies usually have DO levels of 8 mg/l or higher.

Many fish and macroinvertebrate species are sensitive to low DO levels.  For example, salmonids suffer when the DO drops below 2 to 4 mg/L.  Larval and juvenile fish are even more susceptible to low DO levels (USEPA 1997).  

Low DO levels can have more negative effects than just suffocating fish.  Low DO can cause declines in macroinvertebrate populations which leads to the starvation of organisms higher in the food chain.  Also, low DO increase anaerobic bacteria that produce gases often associated with polluted streams.

Oxygen is introduced into water by two primary mechanisms; transfer from air or photosynthesis by aquatic macrophtyes.  
The transfer of oxygen from the air to water can occur by the following means:

Consumption of DO, on the other hand, occurs by several mechanisms:

Biological Oxygen Demand (BOD) measures how much oxygen is consumed by bacteria as they break down pollution and organic matter in the water. It is measured by observing how much dissolved oxygen levels decrease in a sealed sample over a 5-day period.

Nitrate (NO3) and Phosphate (PO4)

Nitrates and phosphates are nutrients that come from both natural sources and human activities (fertilizers, detergents, wastewater). These nutrients determine the productivity of a water body, and are needed at some level to provide good aquatic habitat. However, pollution from manure, fertilizer, and wastewater can cause excessive nutrient levels. Too much nitrate or phosphate causes algae to grow out of control, reducing light and oxygen for fish.

pH is a measure of the acidity of water, and is important in understanding the chemical balance of the water. pHs below 7 indicate acid conditions, while pHs above 7 indicate alkaline conditions. pH is a strong determinant of the solubility and availability of both nutrients and pollutants. Most natural water bodies will have pHs close to 7, depending on the local geochemistry. Very low pHs (less than about 6) can come from acid rain, industrial sources, or mine drainage.  Stream organisms cannot tolerate extreme pH values or large fluctuations in pH.  Deviations from close to neutral (7) increase stress levels and will ultimately decrease species diversity.