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Evaluation of Nutrient, Alkalinity, and Acid-Neutralizing Capacity Stabilities in Water Samples Analyzed by the U.S. Geological Survey National Water Quality Laboratory, 2023–24

Scientific Investigations Report 2026-5014
By: , , and 
https://doi.org/10.3133/sir20265014

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Abstract

The U.S. Geological Survey evaluated the stability of water-sample chemical analysis of nutrient, alkalinity, and acid-neutralizing capacity constituents with respect to the duration between sample collection and laboratory analysis, also known as the sample holding time. A study began in the spring of 2023 to evaluate the sample stability, between 2 and 180 days after sample collection, of the chemical properties and chemical constituents of alkalinity as calcium carbonate, filtered; acid-neutralizing capacity as calcium carbonate, unfiltered; total ammonia as nitrogen, filtered; total ammonia plus organic nitrogen as nitrogen, filtered and unfiltered; nitrite as nitrogen, filtered; nitrate plus nitrite as nitrogen, filtered; total nitrogen, filtered and unfiltered; orthophosphate as phosphorous, filtered; and total phosphorus as phosphorus (filtered and unfiltered) in water. Both surface water and groundwater matrices were represented.

Sample instability varied by observed property and matrix; therefore, providing general guidance for sample holding time is not possible based on matrices alone. No correlations between field measurements of sample characteristics and sample instability were observed. Although observations for some properties indicate sample stability that exceeds the recognized U.S. Geological Survey National Water Quality Laboratory method holding times, this is not necessarily the case for matrices and seasonal characteristics that were not investigated.

Based on the limited number of six sample sources used in this study, some patterns emerge for the 12 observed properties studied. Five observed properties generally indicate stability for as many as 180 days after sampling (total nitrogen as nitrogen, both filtered and unfiltered; orthophosphate as phosphorus, filtered; and phosphorus as phosphorus, both filtered and unfiltered). Other observed properties indicate stability for as many as 180 days for some matrices, but not for others. Finally, some observed properties indicate instability well before 180 days.

Plain Language Summary

In 2023–24, the U.S. Geological Survey (USGS) studied how long different chemical measurements in water samples remain reliable if analysis is delayed after collection. This question became important after the National Water Quality Laboratory (NWQL) experienced a large backlog of sample analyses causing many samples to be analyzed outside the timeframe required by the NWQL, which affected many USGS studies, some of which were being done for regulatory purposes (such as to meet requirements set by the Environmental Protection Agency). There are specific time frames in place to ensure that analyte concentrations do not change significantly between sampling and the time of analysis. The study focused on those water‑quality measurements that tend to be most affected by delayed analysis including forms of nitrogen and phosphorus (nutrients), alkalinity, and acid‑neutralizing capacity. Because the type of water can affect sample stability, samples used in the study were collected from six separate locations (five streams and one well) from across the United States. Samples were analyzed repeatedly for periods ranging from two days to 180 days after collection. The study found that sample stability depends on what is being measured and on the type of water the measurement comes from. Overall, the study found that some analyses performed after the required timeframe can still be useful, but data quality depends on the specific chemical measurement, the water type, and the intended use of the data.

Suggested Citation

Struzeski, T.M., Wetherbee, G.A., and Morrison, J., 2026, Evaluation of nutrient, alkalinity, and acid-neutralizing capacity stabilities in water samples analyzed by the U.S. Geological Survey National Water Quality Laboratory, 2023–24: U.S. Geological Survey Scientific Investigations Report 2026–5014, 36 p., https://doi.org/10.3133/sir20265014.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Plain Language Summary
  • Introduction
  • Purpose and Scope
  • Holding-Time Study Design
  • Sample Collection, Processing, and Handling
  • Approaches to Evaluating Sample Stability
  • Interpretation of Analytical Results
  • Determination of Observed Property Stability
  • Summary
  • Acknowledgments
  • References Cited
  • Appendix 1. Time Series Charts Showing Sample Stability Through Time
Publication type Report
Publication Subtype USGS Numbered Series
Title Evaluation of nutrient, alkalinity, and acid-neutralizing capacity stabilities in water samples analyzed by the U.S. Geological Survey National Water Quality Laboratory, 2023–24
Series title Scientific Investigations Report
Series number 2026-5014
DOI 10.3133/sir20265014
Publication Date June 08, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston VA
Contributing office(s) WMA - Laboratory & Analytical Services Division
Description Report: vi, 36 p.; Data Release
Country United States
Other Geospatial conterminous United States
Online Only (Y/N) Y
Additional publication details