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Linda Debrewer Shenandoah Valley Water Conference October 28, 2008
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  1. Linda Debrewer Shenandoah Valley Water Conference October 28, 2008 Trends in Water QualityGreat Valley Carbonate Region

  2. Water-Quality Trend StudiesNAWQA Program Long-term goal: to describe changes in the quality of the Nation’s water resources over time and to provide scientific understanding of the primary natural and human factors contributing to these changes Decadal sampling in networks representing major aquifers and river basins and different land-use settings Biennial sampling at a subset of wells Local-scale flow system studies

  3. Factors Affecting Trends in Ground-Water Quality • Hydrogeology • Geochemical environment • Land use and land-use change • Change in chemical applications • Climate

  4. Great Valley Carbonate Region Hydrogeologic Setting Carbonate bedrock Well-drained soils Ground water flows through regolith and along fractures, bedding planes, and solution channels (well-mixed) Ground water is susceptible to contamination

  5. Great Valley Carbonate Region • Median nitrate: 5.1 mg/L • National median: 3.4 mg/L in agricultural settings • Potomac River Basin median: 1.8 mg/L • Oxygen-rich system throughout

  6. Great Valley Carbonate Region1993 to 2002 • No significant change in nitrate • Reflects relatively steady nitrogen fertilizer use over past several decades • Also may reflect differences in climate during sampling periods

  7. Great Valley Carbonate Region1993 to 2002

  8. Great Valley Carbonate Region1993 to 2002 • Wide range of ages and large overlap in recharge dates relative to the 9 year sampling interval • Age is mixture of pre-1940s and recently recharged. Young fraction ranges from 16% to 96% (median 68%). Typically less than 30 years. • Drought conditions during later sampling period: larger influence of older water in 2002 sampling

  9. Great Valley Carbonate Region1993 to 2002 • Pesticides detected in 89% of wells (frequencies among the highest in the Nation) • Mixtures commonly detected • Decreasing atrazine, deethylatrazine, and prometon concentrations reflect changes in use despite higher percentage of older water sampled in 2002 • Limited use data shows decrease in atrazine and prometon between sampling periods

  10. Great Valley Carbonate Region1993 to 2002 • Detecting trends in ground water is complicated by long open boreholes which facilitate ground-water mixing • Trends may be more apparent over multiple decades or with controlled tracking of change along specific flow paths

  11. Muddy Creek at Mt. Clinton, Virginia Watershed is 68% agriculture, with no apparent point sources Poultry, cattle, corn, soybeans Routine and storm samples: 1993 to 1997 Fixed interval samples: 1997 to 2005 (long term trends approach) Currently on 4 year cycle: 30 samples ( monthly and more frequently during growing season)

  12. Muddy Creek at Mt. Clinton, Virginia Nitrate • Detected throughout the year • Little seasonal pattern • Exceeds ecoregion criteria of 0.01 mg/L

  13. Muddy Creek at Mt. Clinton, Virginia Phosphorus highest during base flow, April-Sept; exceeds criteria of 0.01 mg/L.

  14. Pesticides A variety of pesticides detected Present in surface water year-round Concentration generally increases with flow during application period and diluted by runoff during remainder of year Pattern typical of most NAWQA sampling Issues: fate of compounds (parent and degradate showing up in streams) transport (in streams from ground water at low flow and from runoff) persistence (all year, not just during application) Muddy Creek at Mt. Clinton, Virginia

  15. Atrazine Detected in every sample to date Higher concentration during application period Muddy Creek at Mt. Clinton, Virginia

  16. Deethylatrazine Detected in every sample to date Concentrations increasing over time Pesticide degradates are transported primarily by ground water Muddy Creek at Mt. Clinton, Virginia

  17. Metolachlor Detected in most samples to date Higher concentration during application period Muddy Creek at Mt. Clinton, Virginia

  18. Future NAWQA Study of GW Trends • Status and Trends • Continue study of temporal changes (trends) in water quality in response to changing land use, chemical use, or agricultural-management practices. • Understanding • Continue study to identify natural and human-related factors that govern the degree, direction, periodicity and amplitude of changes observed in water quality. • NEW: Simulation and Forecasting • Reproduce observed water-quality measurements, and forecast future conditions based on anticipated changes in land or chemical use.

  19. Simulation and Forecasting Studies

  20. For more information: Recent publication: JEQ Supplement, Special Submission, Sept-Oct 2008, Temporal Trends in Nitrate and Selected Pesticides in Mid-Atlantic Ground Water http://jeq.scijournals.org/cgi/reprint/37/5_Supplement/S-296 Contacts: • Linda Debrewer, Hydrologist lmdebrew@usgs.gov, (443) 498-5562 • Judith Denver, Potomac River Basin and Delmarva Peninsula NAWQA Chief jmdenver@usgs.gov, (302) 734-2506 x229 • Bruce Lindsey, NAWQA Ground Water trends coordinator blindsey@usgs.gov,(717) 730-6964