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: 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


Take-Home Messages: 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


Objective of Depth-Dependent Measurements: 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


High Plains Aquifer TANC Study Area: High Plains Aquifer TANC Study Area TANC Study Area


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


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


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


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


Depth-Dependent Sampling : 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 Confined fine sand Silty clay Confined fine sand Carlile Shale Supply Well screen Depth- Dependent Samples


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


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


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


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


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


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


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


Model Simulations : 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


HighPlains TANC Conceptual Model : High Plains TANC Conceptual Model Denitrification Denitrification Reductive Dechlorination Wellbore leakage Urban chemistry signature(s) Agricultural chemistry signature(s)


Conclusions: 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


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


Contact Information : 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