Common Water Quality Limits 

When not affected by stormwater runoff, water temperature, pH, and dissolved-oxygen concentration can change in daily (diurnal or diel) cycles with values for all three typically lowest in the early morning and highest in the late afternoon in many freshwater systems. Turbidity typically shows 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. During the September 18 through October 18 monitoring period, dissolved-oxygen concentrations at saturation typically would range from about 8 (at the warmest temperatures) to about 11 (at coolest temperatures) milligrams per liter (mg/L). If your stream is affected by appreciable biological uptake concentrations would decrease 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.