|
The U.S. Geological Survey (USGS)
reports these data, which were collected by various groups for
World Water Monitoring Day 2006 in Virginia. The Water
Environment Federation (WEF), in coordinating World Water
Monitoring Day, receives the data from those who collected it.
After processing the data, WEF provides the data to the USGS for
posting on this web page. When such water-quality data are
collected, reported, and processed, errors can be introduced
such that reported values do not represent the quality of the
water at the time the water was collected. Because the data
reported for World Water Monitoring Day 2006 in Virginia are not
collected by the USGS, the data have not been verified or
evaluated by USGS quality assurance procedures. Thus, individual
users are responsible for determining if any reported values
might be incorrect. A brief discussion provides general
guidelines to assist users who might not be fully knowledgeable
of water quality in this determination.
When not affected by stormwater runoff and point sources of
contamination, water temperature, pH, and dissolved-oxygen
concentration can change in daily (diurnal or diel) cycles with
values for all three of these parameters 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 temperatures typically have a range between the
morning low and afternoon high of the air temperature for the
day the water is tested. Water-temperature values in freshwater
systems will not be negative; zero degrees Celsius is freezing
for fresh water.
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 this low of a 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.
Water temperature, production by photosynthesis, and biological
uptake affect dissolved-oxygen concentrations. 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 a stream
is affected by appreciable biological uptake concentrations
decrease below saturation but commonly would remain above 5
mg/L. If photosynthesis affects a stream, dissolved-oxygen
concentration could be greater than saturation but commonly
would not exceed 15 mg/L.
Turbidity, which is a measure of the clarity of the water,
commonly ranges from 0 to 10 units when the water is clear. If
water is tested during or immediately after a period of rainfall
that produces stormwater runoff, turbidity could increase to
several hundred units. The water would appear cloudy at such
high values.
|
