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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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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
monitoring network
Monitoring network

Regional ground-water flow

Sites with single wells screened near water table (8)

Sites with nested wells (7)

1 km

geology and well construction supply well
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

well bore flow profile
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

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

Depth-

Dependent

Samples

Confined

fine sand

Supply

Well

screen

Silty clay

Confined fine sand

Carlile Shale

depth dependent vocs
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

water d 18 o and cl
Water d18O and Cl

Urban unconfined

shallow

Supply

well

(includes

depth-

dependent)

Urban confined

mixed

Ag unconfined

Confined

Preliminary – Subject to Revision

unconfined confined mixing from d d
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

unconfined mixing fraction from d d
Unconfined Mixing Fraction from dD

At Supply Well

Confined Monitoring Wells

Percent unconfined water

Percent unconfined water

Preliminary – Subject to Revision

vertical leakage through wellbores
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

contaminant pathway hypotheses
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)

contaminant pathway hypotheses16
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)

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
high plains tanc conceptual model
HighPlains TANC Conceptual Model

Agricultural chemistry signature(s)

Urban chemistry signature(s)

Denitrification

Wellbore

leakage

Denitrification

Reductive Dechlorination

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