2.e. Concentration-Flow Relationships
Concentration exceedency curves often indicate violations of water quality standards relative to certain water uses. As the adjacent graph shows, nitrate in Honey Creek exceeds the drinking water standard of 10 mg/L about 9% of the time. Honey Creek serves as a drinking water source for two public water supplies.
To determine appropriate measures to reduce nitrate concentrations, it is useful to know whether point sources, nonpoint sources or both contribute to the elevated nitrate concentrations. One way to determine the major source is to look at the relationship of nitrate concentrations to stream flow. In general, where pollutants are derived from nonpoint sources, their concentrations increase with increasing flow. The same rainfall runoff that increases stream flow also carries the pollutants from land surfaces and tile flow into streams.
Where pollutants are derived from point sources, their concentrations increase as stream flow decreases. Generally, point sources, such as municipal sewage treatment plants discharge pollutant at relatively constant rates. As stream flow decreases, there is less stream water to dilute the pollutants so pollutant concentrations increase.
In the Honey Creek example to the right, it is evident that, for the higher concentrations, nitrate increases as stream flows increase. The violations of drinking water standards (10 mg/L) result from nonpoint sources.
It is also evident that nitrate concentrations increase within the low range of stream flows. This suggests that point sources also contribute nitrate to the stream. However, these concentrations do not exceed the drinking water standard at the monitoring station. (See section on nutrient processing within streams (under development).)
Nitrate Concentration, mg/L
Water Quality Laboratory, Heidelberg College http://wql-data.heidelberg.edu/index2.html 9/27/05
Honey Creek Nitrate Exceedency , WY 2000-2004
10% 20% 30% 40% 50% 60% 70% 80% 90% 100
% Percent of time nitrate concentration is exceeded
10/1/1999 - 9/30/2004
10.0 100.0 Flow (cfs)