A time-series analysis approach developed by the U.S. Geological Survey was used to analyze trends in total phosphorus and evaluate optimal sampling designs for future trend detection, using long-term data for two water-quality monitoring stations on the Quinebaug River in eastern Connecticut. Trend-analysis results for selected periods of record during 1971–2001 indicate that concentrations of total phosphorus in the Quinebaug River have varied over time, but have decreased significantly since the 1970s and 1980s. Total phosphorus concentrations at both stations increased in the late 1990s and early 2000s, but were still substantially lower than historical levels. Drainage areas for both stations are primarily forested, but water quality at both stations is affected by point discharges from municipal wastewater-treatment facilities.
Various designs with sampling frequencies ranging from 4 to 11 samples per year were compared to the trend-detection power of the monthly (12-sample) design to determine the most efficient configuration of months to sample for a given annual sampling frequency. Results from this evaluation indicate that the current (2004) 8-sample schedule for the two Quinebaug stations, with monthly sampling from May to September and bimonthly sampling for the remainder of the year, is not the most efficient 8-sample design for future detection of trends in total phosphorus. Optimal sampling schedules for the two stations differ, but in both cases, trend-detection power generally is greater among 8-sample designs that include monthly sampling in fall and winter. Sampling designs with fewer than 8 samples per year generally provide a low level of probability for detection of trends in total phosphorus.
Managers may determine an acceptable level of probability for trend detection within the context of the multiple objectives of the state’s water-quality management program and the scientific understanding of the watersheds in question. Managers may identify a threshold of probability for trend detection that is high enough to justify the agency’s investment in the water-quality sampling program. Results from an analysis of optimal sampling designs can provide an important component of information for the decision-making process in which sampling schedules are periodically reviewed and revised.
Results from the study described in this report and previous studies indicate that optimal sampling schedules for trend detection may differ substantially for different stations and constituents. A more comprehensive statewide evaluation of sampling schedules for key stations and constituents could provide useful information for any redesign of the schedule for water-quality monitoring in the Quinebaug River Basin and elsewhere in the state.
Purpose and Scope
Description of the Study Area
Phosphorus and Water Quality in the Quinebaug River Basin
Sources of Phosphorus
Trends in Phosphorus Concentrations
Current Challenges Related to Phosphorus
Assessing and Managing Water Quality
Methods for Time-Series Analysis of Trends and Evaluation of Sampling Designs
Selection of Data for Trend Analysis
Effects of Method Changes on Historical Data
Discharge-Related Variability in Concentration
Differentiating Trends from Other Sources of Concentration Variability
Application of the Time-Series Model
Evaluation of Sampling Designs
Sensitivity and Efficiency of Sampling Designs
Acceptable Levels of Probability for Trend Detection
Variability and Correlation in the Daily Concentration Anomaly
Long-Term Phosphorus Trends in the Quinebaug River
Evaluation of Sampling Designs for Detecting Trends in Total Phosphorus
Size of the Characteristic Trend
Low-Frequency Sampling Designs
Variability in Optimal Designs Among Stations and Constituents
Effects of Seasonal Changes in the Daily Concentration Anomaly
Summary and Conclusions
This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.
Download the report (PDF, 1.6 MB)
Download the front cover (PDF, 1.48 MB)
Document Accessibility: Adobe Systems Incorporated has information about PDFs and the visually impaired. This information provides tools to help make PDF files accessible. These tools convert Adobe PDF documents into HTML or ASCII text, which then can be read by a number of common screen-reading programs that synthesize text as audible speech. In addition, an accessible version of Acrobat Reader 5.0 for Windows (English only), which contains support for screen readers, is available. These tools and the accessible reader may be obtained free from Adobe at Adobe Access.
For more information about USGS activities in Connecticut, visit the USGS Connecticut District home page.
|AccessibilityFOIAPrivacyPolicies and Notices|
|U.S. Department of the Interior, U.S. Geological Survey
Page Contact Information: Contact USGS
Last modified: Thursday, December 01 2016, 08:05:34 PM