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Analyzing Ground-Water Quality Trends: USGS NAWQA Program

This presentation highlights the analysis of ground-water trends in the USGS National Water Quality Assessment Program (NAWQA). It showcases improvements in trend analysis, quarterly sampling analysis, and a comparison with New Zealand's groundwater monitoring program. The talk discusses the importance of understanding trends in ground-water quality for managing practices, land-use changes, and preventing future contamination. Key components of the NAWQA GW Trends Program, such as well networks, major aquifer surveys, and land-use surveys, are outlined. The challenges of quantifying seasonal and random variations in water quality are discussed, along with the significance of quarterly sampling in determining intra-annual variations. Lessons learned from various national sampling programs and recommendations for further improvements to the NAWQA Trends Assessment are also presented.

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Analyzing Ground-Water Quality Trends: USGS NAWQA Program

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  1. Evaluation of Ground-Water-Quality Trends Design as Part of the USGS National Water Quality Assessment Program Michael Rosen U.S. Geological Survey Carson City, NV National Water Quality Monitoring Conference, San Jose, May 2006

  2. Objectives of Talk • Describe How Ground-Water Trends are Analyzed in NAWQA • Background for Talks in Session • Illustrate Improvements in Trend Analysis • Quarterly sampling analysis • Comparison with New Zealand National Groundwater Monitoring Program

  3. Why Care about Trends in Ground-Water Quality? • Performance of Management Practices (Land, Water, Chemical Use) • Effects of Land-Use Changes (Land Conversions) • Scientific Basis for Preventing Future Contamination • Prediction of Trends

  4. Components of the NAWQA GW Trends Program • Well Networks (generally 30 wells per study) • Major Aquifer Surveys (deep)large area • Land Use Surveys (shallow)large area • Agriculture • Urban • Sampling • Decadal (all wells) • Biennial (5 wells) • Quarterly (5 wells) • Flow System Studies(transect)small area

  5. NAWQA Trend Networks Red outlined areas - current trend reports produced

  6. Trends Reporting • National Reports • Nutrients • Pesticides (presented here) • Study Area Reports • 6 study areas (three presented here) All Articles in J. Environ. Qual. in 2007 VOCs Reported Separately

  7. Improvements to Trend ProgramPurpose of Quarterly Sampling • Determine intra-annual variation not related to trends • Allows confidence limits and error bars • Assess magnitude of long-term changes relative to seasonal and/or random changes

  8. Quantifying seasonal and random variations in water quality is not easy! Variations due to: • Application and degradation rates of contaminants • Seasonality of the area (i.e. precipitation or river flows) not related to seasonal chemicals inputs • Travel times of contaminants • Variation in samplers • Other random variations? Does a Subset of Wells Represent the Variation in the Entire Aquifer?

  9. Comparison of Concentration and Percent Change in Nevada What'sthe Problem? Axis break Variations may be larger or smaller than actual changes in a network Wide range of concentrations may indicate larger variations than actually occurs

  10. What's the Problem? • Distinguishing between non-random and random variation can be difficult • Seasonal inputs of fertilizers and other chemicals may make natural variation difficult to determine • How long a record is needed before "noise" can be determined? • What happens if intra-annual variation is superimposed on non-random variation?

  11. Conundrum of Sampling for Intra-annual Variation in Trend Wells • You don’t want a trend because it confounds determining the range of noise • What do you do with unidirectional variation? • Samples are taken from wells that are likely to have land use inputs that may engender a trend response • Average of 130 wells per analyte examined

  12. What Was Expected? 60% < 3 detects

  13. Percent Change in Nitrate Concentrations – Agricultural Wells 13% < 3 detects

  14. Percent Change in Atrazine Concentrations – All Ag wells - Detects 43% < 3 detects

  15. Percent Change in Atrazine Concentrations – Ag Wells in Corn

  16. Continuous Water Levels and Nitrate Concentrations in San Joaquin

  17. Lessons Learned – quarterly sampling analysis • Quarterly data for one year does not answer the intended questions • Water-level data helps, but analysis must be done well by well • Ag land use does not explain observed seasonal variations • Quarterly sampling is expensive, funds could be better used elsewhere in the program

  18. New Zealand example of intra-annual variation and changing land use patterns

  19. Large Intra-annual Variation (at Times) But No Trends

  20. Lessons Learned – from Other National Sampling Programs • Quarterly sampling is needed for a minimum of 3 years to be successful • Not likely that a subset of wells will be representative of an entire network or aquifer

  21. Lessons Learned – Further improvements to NAWQA Trends Assessment • Better Nesting of Large and Small Networks • Incorporation of More Ground-Water Age Dating • Use of Ground-Water Flow and Contaminant Transport Models

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