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Michael J. Brayton MD/DE/DC Water Science Center

Hydrologic Controls on Nutrient and Pesticide Transport through a Small Agricultural Watershed, Morgan Creek, Maryland, USA. Michael J. Brayton MD/DE/DC Water Science Center. U.S. Department of the Interior U.S. Geological Survey.

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Michael J. Brayton MD/DE/DC Water Science Center

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  1. Hydrologic Controls on Nutrient and Pesticide Transport through a Small Agricultural Watershed, Morgan Creek, Maryland, USA Michael J. Brayton MD/DE/DC Water Science Center U.S. Department of the Interior U.S. Geological Survey

  2. Morgan Creek watershed is located in the northern part of the Delmarva Peninsula • ACT is a water-budget approach to sampling nutrients and pesticides • Define the conceptual hydrology and identify transport pathways • Sample stream water chemistry, which represents an integration of all source water inputs • Part of the National Water Quality Assessment (NAWQA) trends network • Part of the NAWQA topical team study of Agricultural Chemical Transport (ACT)

  3. Physical Setting: • 31 square kilometer area (12.7 square miles) • 90% agricultural • Soils moderately well-drained, porous sandy loams • Underlying aquifers are marine sand, silt, and clay • Minimal irrigation with no inter-basin transfer • Humid sub-tropical climate • Elevation range is sea level to 25 meters • Existing stream gage with long-term record Morgan Creek Watershed

  4. Morgan Creek Watershed- Annual Water Budget • PPT = Precipitation • = 112 cm/yr • 8-10 cm/month, evenly distributed • ET = Evapotranspiration • = 78 cm/yr • Calculated from budget equation • SW = Stream flow • = 32 cm/yr • Estimated from long-term stream flow record • GW= ground-water loss to adjacent basin • = 2 cm/yr • Estimated from ground-water model Budget Equation: PPT = ET + GW + SW 112cm = 78cm + 2cm + 32cm

  5. Sampling Approach How does the rainfall-runoff process work in the watershed? How is stream chemistry affected by changes in season or stream flow? Use a combined approach: • Fixed interval sampling at regular flows throughout the year (2002-04) • Targeted storm sampling during growing season (2003, 2004) • Analyzed for: pesticides, nutrients, sediment, TOC, DOC, major ions 9 storms were sampled in 2003 MAJOR APPLICATION 7 5 6 8 4 9 1 MISSING RECORD Stream Discharge (cfs) and Specific Conductance (uS/cm) 2,3 TRIED TO SAMPLE EVERY EVENT, EARLY ON PRE-APPLICATION April May June July August

  6. Shallow Ground Water Stream Deep Ground Water Precipitation Contributing Sources of Water • Stream water is a variable mix of different sources • Sampled other hydrologic compartments for chemical signatures

  7. At what flow does runoff become important? Runoff begins around 12cfs Base flow accounts for 60% of the annual flow at Morgan Creek Suspended Sediment Concentration (mg/L) Storm Related Flow Base Flow Stream Discharge (CFS) WINTERSPRING SUMMER AUTUMN

  8. Less soluble compounds move in runoff For Morgan Creek, these include: ammonia, organic nitrogen, phosphorus, suspended organic carbon, pesticide parent compounds Concentrations can also vary by season Concentrations vary by stream discharge Runoff begins Summer Ammonia + Organic Nitrogen (mg/L as N) Spring Stream Discharge (CFS) WINTERSPRING SUMMER AUTUMN

  9. More soluble compounds move in base flow For Morgan Creek, these include: nitrate and other major ions, iron, bicarbonate, pesticide degradates Nitrate is primarily from groundwater Runoff begins Nitrate is present all year and at all flows Nitrate + Nitrite, filtered (mg/L as N) Stream Discharge (CFS) WINTERSPRING SUMMER AUTUMN

  10. Dynamic chemistry during storm events Potassium Concentration (mg/L) Nitrate + Nitrite Particulate Nitrogen 3:00 15:00 3:00 15:00 3:00 15:00 3:00 May 25 May 26 May 27 May 28

  11. Metolachlor-ESA Metolachlor Metolachlor-OA July 12-14, 2004 at Morgan Creek Atrazine DEA Variable transport behavior during storm events • Parent pesticides are transported primarily in runoff • Metabolites of metolachlor are transported in ground water However • DEA, a metabolite of atrazine, is transported in runoff similar to its parent compound Concentration (ug/L) Concentration (ug/L) 4:00 16:00 4:00 16:00 4:00 16:00 4:00

  12. Summary • Stream chemistry reflects flow-dependent contributions from many source waters • Insoluble compounds tend to travel in runoff, at stream flows greater than 12 cfs • Soluble compounds can travel in both runoff and base flow (primarily ground-water discharge) • Pesticide and nutrient loads are greatest in storms after application, usually in May and June • Pesticides and their metabolites are present year round in streams and can show contrasting transport behavior

  13. Watershed Management Implications • Develop a conceptual hydrologic understanding to focus monitoring strategies • Build a robust data set by sampling all seasons at normal flow and during storm events • Use an iterative process of monitoring to identify hydrologic transport pathways • Target specific management strategies based on primary transport pathways • Remember: Hydrologic residence times may strongly affect when you begin to see changes! THE END Contact: Mike Brayton (mbrayton@usgs.gov), 410-238-4315

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