USGS - science for a changing world

Scientific Investigations Report 2008–5201

Scientific Investigations Report 2008–5201

Back to Table of Contents

Appendix A. Quality Control and Quality-Assurance of Water Samples

Overall, 36 percent of dissolved nutrient samples, 33 percent of total phosphorus samples, 32 percent of total nitrogen samples, and 58 percent of chlorophyll a samples were collected for quality-assurance purposes. These samples included field blanks (the first sample collected every week) and either a split sample or a method replicate (each type every other week). Field blanks are samples of deionized water processed onsite through clean sampling equipment, before an environmental sample is collected. Analysis of blank samples determines if the processes of collection, handling, transport, and analysis cause measurable contamination. Split samples are environmental water samples collected once and divided into two samples that are used to determine the variability in the analytical methods. Replicate samples are environmental samples collected twice in rapid succession from the same location and analyzed to determine variability of the system and variability in the analytical methods.

The results of the quality-assurance sampling indicated that precision and accuracy were acceptable and the variability in sampling and processing was less than seasonal variability (table A1). Most field blank concentrations of orthophosphate and total phosphorus were less than the NWQL’s minimum reporting level. The median value of the blank samples greater than the minimum reporting level was 0.013 mg/L for orthophosphate and 0.006 mg/L for total phosphorus. Almost one-half of the field blank samples had concentrations of ammonia and total nitrogen greater than laboratory minimum reporting level. These concentrations, however, were minimal compared to the concentrations of the environmental samples. The number of nitrite-plus-nitrate blank samples measured that were greater than laboratory minimum reporting level was less than ammonia and total nitrogen, indicating that the contamination to total nitrogen was the ammonia-nitrogen species. Contamination of ammonia blanks in 2003 and 2004 was attributable to atmospheric deposition (Wood and others, 2006). This contamination subsequently has been reduced, but has not been eliminated by the use of capsule filters. Twice during the season, split samples of all constituents were sent to two other laboratories in addition to NWQL and Portland State University (table A2). Additionally, two interlaboratory split samples were sent to Portland State University and NWQL, for four interlaboratory chlorophyll a split samples between the two laboratories. The interlaboratory split samples were analyzed to determine variability in analytical methods between the laboratories. Some split samples also were collected to examine variability that might be expected due to different protocols used by USGS and the Klamath Tribes in the collection of their long-term biweekly data set. The coefficient of variation for each constituent measured on each sampling date ranges from 4 to 36 percent, where the greatest percentages of variance from the mean were in chlorophyll a and nitrite-plus-nitrate concentrations. Mean and standard deviations of split sample concentrations all of the split samples taken from the same churn splitter indicate that the variance in interlaboratory processing and analysis is generally low.

An extraordinary chlorophyll a concentration was measured at site HDB on July 24. The value of that sample, 9,305 µg/L, was the greatest ever measured for this project. As a result, the validity of this high value has been in question. The method used to determine chlorophyll a concentrations requires that values are diluted to fall within the upper limit for this method, 250 µg/L. The sample was diluted twice in order to reduce concentrations to fall within the upper limit, which allows for greater error in the process. While the error associated with this chlorophyll a value probably is higher than that associated with the rest of the 2006 chlorophyll a data, other data collected at the same site on the same day corroborate a high value. Total phosphorus and total nitrogen concentrations reached 1,210 and 14,700 µg/L, respectively at site HDB on June 24. Total phosphorus and total nitrogen concentrations measured at site HDB on June 24 were greater than any other measured that sampling season. These high values indicate that increased total nutrients was due to higher concentrations of algal cells required to take up the nutrients concentrations, thereby validating a greater concentration of chlorophyll a than measured at all other sites at any point in the field season. Additionally, results from dissolved oxygen production and consumption experiments show that on June 24 at site HDB, light and dark bottles on both racks measured net respiration rates that were greater than measurements from any other dark bottles measured during the sampling period. The depth of photic zone measured during June 24 at site HDB was less than 0.5 m, indicating a high concentration of AFA colonies near the surface of the water. The depth of the photic zone was shallower than the placement of the upper rack in the water column. The high respiration rates at site HDB on June 24 were measured from bottles that had a high algal concentration (the water was taken from the same churn splitter as the sample analyzed for chlorophyll a at that site) and the bottles were incubated on a rack that was below the photic zone. Therefore, the other measurements and samples collected on that day—total nutrient concentration, light penetration, and oxygen production and consumption—are all consistent with a high chlorophyll a measurement at site HDB on June 24.

Table A1. Quality-assurance results for the water quality collection program in Upper Klamath and Agency Lakes, Oregon, 2006.

[mg/L, milligram per liter; MRL, minimum reporting level]

Blank samples
Analyte Number of samples Percentage of blank samples MRL (mg/L) Number of blank samples greater than MRL Value of blank samples
greater than MRL (mg/L)
Blank Total Median Maximum
Orthophosphate-P 61 211 29 0.006 1 0.013 0.013
Ammonia-N 61 211 29 0.01 23 0.02 0.04
Nitrite plus nitrate-N 61 211 29 0.016 7 0.031 0.052
Total nitrogen 44 190 23 0.06 8 0.115 0.3
Total phosphorus 52 207 25 0.004 17 0.006 0.011
Chlorophyll a 61 130 47 0.004 0
Split samples
Analyte Number of samples Percentage of
split samples
Difference between split samples
Split Total Median
Orthophosphate-P 8 211 4 0.001 2.22
Ammonia-N 8 211 4 0.003 12.44
Nitrite plus nitrate-N 8 211 4 0.001 0.45
Total nitrogen 7 190 4 0.160 6.64
Total phosphorus 9 207 4 0.010 4.26
Chlorophyll a 8 148 5 0.006 11.38
Replicate samples
Analyte Number of samples Percentage of
replicate samples
Difference between replicate samples
Replicate Total Median
Orthophosphate-P 6 211 3 0.001 0.51
Ammonia-N 6 211 3 0.002 5.13
Nitrite plus nitrate-N 6 211 3 0.001 1.05
Total nitrogen 9 190 5 0.200 4.73
Total phosphorus 8 207 4 0.016 5.41
Chlorophyll a 9 148 6 0.021 15.38

Table A2. Distribution of differences from the median concentrations among split sample interlaboratory measurements of samples from the water quality collection program in Upper Klamath and Agency Lakes, Oregon, 2006.

Analyte Concentrations, in micrograms per liter
August 15, 2006 August 29, 2006
Mean Standard
of variation
Mean Standard
of variation
Orthophosphate-P 80.40 8.44 11 97.80 13.86 14
Ammonia-N 66.20 7.36 11 72.80 9.58 13
Nitrite plus nitrate-N 25.40 5.08 20 11.80 2.49 21
Total nitrogen 1,597.50 97.08 6 2,395.00 216.72 9
Total phosphorus 159.50 8.10 5 219.75 8.18 4
Chlorophyll a 27.78 9.88 36 86.33 14.28 17


Back to Table of Contents

AccessibilityFOIAPrivacyPolicies and Notices

Take Pride in America logoU.S. Department of the Interior | U.S. Geological Survey
URL: http:// /sir/2008/5201/section12.html
Page Contact Information: Contact USGS
Page Last Modified: Monday, 07-Mar-2016 12:19:50 EST