Identifying Changes in Background Water-Quality Conditions, Arkansas River and Fountain Creek, near Pueblo, Colorado

Explanation of tolerance limit plots

Tolerance limit is the daily threshold defined by the confidence in the test and the fixed proportion of the population covered by the test. As defined for the use here, a 95 percent confidence and a 97.5 percent population coverage were selected. An exceedance of the tolerance limit occurs when the current value exceeds the threshold. Seven exceedances within a 10-day period has been determined to constitute a change in water quality.

Current value represents the estimated concentration or load for the time frame displayed. The current values are estimated using daily specific conductance and/or daily streamflow data obtained from real-time monitors at each site location.

Mean value represents the daily average for each day of the calendar year calculated from as many as 13 to 18 years of background data. As an example, the mean value for 18 years of data collected on January 1 was 280.

Plot example of flow adjusted load with daily load exceeding tolerance limit.

Y-axis (vertical) format will show the estimated concentration in milligrams per liter or the estimated load in tons per day. Plotting intervals will vary depending on the range of data values.

X-axis (horizontal) format will show a series of date stamps that are dependent on the duration of the plot(s) selected.