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Matthew Landon Brian Clark Allen Christensen

Use of Depth-Dependent Sampling to Determine Source Areas and Short-Circuit Pathways for Contaminants to Reach Public-Supply Wells, High Plains Aquifer, York, Nebraska. Matthew Landon Brian Clark Allen Christensen. 2006 National Water Quality Monitoring Council Meeting, San Jose, California

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Matthew Landon Brian Clark Allen Christensen

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  1. Use of Depth-Dependent Sampling to Determine Source Areas and Short-Circuit Pathways for Contaminants to Reach Public-Supply Wells, High Plains Aquifer, York, Nebraska Matthew Landon Brian Clark Allen Christensen 2006 National Water Quality Monitoring Council Meeting, San Jose, California May 9, 2006

  2. Take-Home Messages • Supply-well vulnerability can be strongly influenced by wellbores that cross confining units • Sampling from different depths in supply wells, in addition to nearby monitoring wells, improves understanding of contaminant pathways to supply wells

  3. Objective of Depth-Dependent Measurements • Develop a refined understanding of where water and contaminants enter supply wells • Depth-Dependent Sampling: • Provides a more detailed vertical profile than is possible with a monitoring well cluster • Is potentially useful for identifying short-circuit contamination pathways to the supply well

  4. High Plains Aquifer TANC Study Area TANC Study Area

  5. Monitoring network Regional ground-water flow Sites with single wells screened near water table (8) Sites with nested wells (7) 1 km

  6. Geology and Well Construction – Supply Well Loess Unconfined sand & gravel Clayey till Confined fine sand Supply Well screen Silty clay Confined fine sand Carlile Shale

  7. Tracer Pulse Method Conceptual Diagram Izbicki et al. (1999)

  8. Well-Bore Flow Profile • 25% of flow from bottom half of screen Cumulative percent of total flow Loess Unconfined sand & gravel Pump Intake 39.6 m Clayey till 42.7 m Velocity Measurements Confined fine sand Supply Well screen Screen Silty clay 61.0 m Confined fine sand Carlile Shale Preliminary – Subject to Revision

  9. Depth-Dependent Sampling • Samples collected using a narrow-diameter pump lowered down an access tube that extends below the pump intake Loess Unconfined sand & gravel Clayey till Depth- Dependent Samples Confined fine sand Supply Well screen Silty clay Confined fine sand Carlile Shale

  10. Depth Dependent VOCs Loess Unconfined sand & gravel Clayey till Depth- Dependent Samples Confined fine sand Supply Well screen Silty clay Confined fine sand Carlile Shale • Similar results for: TCE, U, major ions, d18O, dD, N2 Preliminary – Subject to Revision

  11. Water d18O and Cl Urban unconfined shallow Supply well (includes depth- dependent) Urban confined mixed Ag unconfined Confined Preliminary – Subject to Revision

  12. Unconfined-Confined Mixing from dD Percent unconfined water Loess Unconfined sand & gravel Clayey till Depth- Dependent Samples Confined fine sand Supply Well screen Silty clay Confined fine sand Carlile Shale Preliminary – Subject to Revision

  13. Unconfined Mixing Fraction from dD At Supply Well Confined Monitoring Wells Percent unconfined water Percent unconfined water Preliminary – Subject to Revision

  14. Vertical Leakage Through Wellbores Unconfined Heads • Large vertical head gradients across confining layers • Irrigation, commercial and old municipal wells are commonly screened in unconfined and confined sand layers Confined Heads Preliminary – Subject to Revision

  15. Contaminant Pathway Hypotheses Supply well (SW) leakage or drawdown induced leakage Wellbore leakage in multi-layer wells Areally uniform leakage through confining unit SW SW SW LW MW MW MW MW MW MW MW – Monitoring Well LW – Leaky Well (Multi-Layer)

  16. Contaminant Pathway Hypotheses Supply well (SW) leakage or drawdown induced leakage Wellbore leakage in multi-layer wells Areally uniform leakage through confining unit SW SW SW LW MW MW MW MW MW MW Fits measured chemistry data MW – Monitoring Well LW – Leaky Well (Multi-Layer) Conceptual background confined concentration Conceptual unconfined concentration (increasing left to right)

  17. Model Simulations • Model simulations of ground-water age with MODFLOW-GWT (Ground Water Transport) and the MultiNode Well (MNW) package • Please see talk by Brian Clark at 3:55 today for more details • Simulations including wellbore leakage in multi-screen wells and known abandoned wells and test holes have the best match with observed age distribution

  18. HighPlains TANC Conceptual Model Agricultural chemistry signature(s) Urban chemistry signature(s) Denitrification Wellbore leakage Denitrification Reductive Dechlorination

  19. Conclusions • Supply well vulnerability can be strongly influenced by wellbores that cross confining units • Short-circuit pathways need to be considered in protecting ground-water supplies • Sampling from different depths in supply wells, in addition to nearby monitoring wells, improves understanding of contaminant pathways to supply wells

  20. Acknowlegements • USGS National Water Quality Assessment Program • Timothy Boyle • Benjamin Dietsch • Michael Turco • City of York, Nebraska

  21. Contact Information Matthew Landon U.S. Geological Survey 4165 Spruance Road, Suite 200 San Diego, CA 92101 Tele: 619-225-6109 Email: landon@usgs.gov TANC Website: http://oh.water.usgs.gov/TANCweb/NAWQATANC.htm

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