Presentation Transcript
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