Common Water Quality Limits 

Water has many measurable characteristics that reflect the quality of the water. Several of these characteristics commonly fluctuate in daily (diurnal or diel) cycles. For example, water temperature, pH, and dissolved-oxygen values typically are lowest in the early morning and highest in the late afternoon in many freshwater systems. In contrast, other characteristics, such as turbidity, typically show limited change in such daily cycles.

Unless affected by a factor other than daily heating and cooling, water temperature typically will be between the morning low and afternoon high in air temperature for the day you are testing your stream. Water-temperature values will not be negative; zero degrees Celsius is freezing.

The pH of water typically is near 7.0 in waters not affected by other factors. In streams known to be affected by sources of acid such as acid-mine drainage, pH can be 5.0 or lower; aquatic life becomes limited at a low pH. The pH of streams affected by photosynthesis of algae and other aquatic plants will change in daily cycles, commonly ranging from 7.0 to 8.5. Although not common, late afternoon values as great as 9.5 have been measured in Virginia.

Dissolved-oxygen concentrations are affected by water temperature, photosynthetic contribution, and biological uptake. For example, cold waters normally hold more dissolved oxygen at saturation than do warm waters. During the monitoring period, dissolved-oxygen concentrations at saturation typically would range from about 7 (at the warmest temperatures) to less than 15 milligrams per liter (mg/L)(at the coldest temperatures). If a stream is affected by appreciable biological uptake, concentrations would decrease below saturation but commonly would remain above 5 mg/L. If your stream is affected by photosynthesis, dissolved-oxygen concentration could be greater than saturation but would not commonly exceed 15 mg/L.

Turbidity which is a measure of the clarity of the water will commonly range from 0 to 10 units when the water is clear. If you test the water during or immediately after a period of rainfall that produces stormwater runoff, turbidity could increase to several hundred units. The water would appear muddy at such high values.