# Presentation: 831193

## 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

**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

**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

**Izbicki et al. (1999)**

**Notes:**
This figure shows conceptually how the dye-injector/well-bore sampler is used (the method has been called the tracer pulse method by Izbicki and others). 
A narrow diameter hose is lowered down the pumping well past the bowls and intake and the injection point can be lowered to different depths in the screen. 
An injection is made and the time of travel to an inline flourometer at the wellhead is measured. 
Differences in the time of travel between different depths indicates the velocity of the water in different screen intervals. 
In the case of the HPGW and SANJ, access tubes were installed in the public supply well to allow the dye-injector and sampling equipment to be put into the well while mimimizing the risk of getting equipment tangled in the pump works. 
While the tracer pulse apparatus can be used to collect samples, HPGW and SANJ used submersible pumps to collect samples for different depths in the screen based upon the results of the velocity profiling.

## 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 

**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, ********18****O, ********D, N****2**

- Preliminary – Subject to Revision

## Water18O and Cl 

**Water********18****O and Cl**** **

- Confined

- Supply
- well
- (includes
- depth-
- dependent)

- Urban confined
- mixed

- Ag unconfined

- Urban unconfined
- shallow

- Preliminary – Subject to Revision

## Unconfined-Confined Mixing from D 

**Unconfined-Confined Mixing from ********D**** **

- 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 D  

**Unconfined Mixing Fraction from ********D  **

**Confined Monitoring Wells**

**At Supply Well**

**Percent unconfined water**

**Percent unconfined water**

- Preliminary – Subject to Revision

## Large vertical head gradients across confining layers

Irrigation, commercial and old municipal wells are commonly screened in unconfined and confined sand 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 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 

**High****Plains ****TANC ****Conceptual ****Model **

**Denitrification**

**Denitrification**
**Reductive Dechlorination**

**Wellbore**
**leakage**

**Urban chemistry signature(s)**

**Agricultural chemistry signature(s)**

## 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	 

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

## 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**