# Presentation: 364972

## Simulation of Short Circuit Flow Paths and Transient Conditions to Understand Vulnerability of Public Supply Wells to Contamination in the High Plains Aquifer, York, Nebraska

**Simulation of Short Circuit Flow Paths and Transient Conditions to Understand Vulnerability of Public Supply Wells to Contamination in the High Plains Aquifer, York, Nebraska**

- Brian Clark
- Matt Landon
- Leon Kauffman
- George Hornberger

**4/12/2005**

- TANC

## Take-Home Messages

**Wellbores drilled through confining units can act as “short circuits”**
**Transient stresses provide opportunities for pulses of flow**
**Combination of transient stresses and wellbores through confining units can allow contaminants to lower layers**

## Study Background

- USGS National Water-Quality Assessment (NAWQA) program

## Study Background

- USGS National Water-Quality Assessment (NAWQA) program

- NAWQA topical study: Transport of Anthropogenic and Natural Contaminants (**TANC**) to public supply wells

## Study Background

- USGS National Water-Quality Assessment (NAWQA) program

**TANC goal: determine controls on movement of contaminants to public supply wells**

## Slide 6

## Study area

**Study area**

## Study area

**Study area**

**Potential contaminant sources**

## Study area

**Study area**

**Recharge zones**

## Study area

**Study area**

**Recharge zones**

**Multi-node pumping wells**

## Study area

**Study area**

**Recharge zones**

**Observation wells**

**FP5**

**FP4**

**FP3**

**FP1**

## Study area

**Study area**

**Recharge zones**

**Transect**

**FP5**

**FP4**

**FP3**

**FP1**

## Hydrogeology

**Hydrogeology**

**unconfined**
**sand**

**upper confined**
**sand**

**Electrical Resistivity**
**logs**

**Well screens**

## Confined and unconfined water-levels

**many wells are commonly screened in unconfined and confined layers**

**Date**

## Model discretization

**Model discretization**

**unconfined**
**sand**

**upper confined**
**sand**

## Model discretization

**Model discretization**

## Model discretization

**Model discretization**

**Specific yield****0.2 to 0.25**

**upper confined**
**sand**

| HK values | 0.3 | 15 | m/d |
| --- | --- | --- | --- |
| Specific Storage | 1 x 10-4 | 7 x 10-7 | per meter |
|  | Fine coarse |  |  |

## Transient stresses

**Simulation time**
**Sept 1944 to Sept 2004**
**Seasonal stress periods**
**Irrigation pumpage off during winter**

## Transient stresses

**Multi-node wells**
**allow flow through wellbores**

- Halford, K.J. and Hanson, R.T., 2002

## Hydrograph comparison

**Hydrograph comparison**

**unconfined**

## Hydrograph comparison

**Hydrograph comparison**

**confined**

## Hydrograph comparison

**Hydrograph comparison**

**confined**

## Hydrograph comparison

**Hydrograph comparison**

**confined**

## Hydrograph comparison

**Hydrograph comparison**

**unconfined**

## Hydrograph comparison

**Hydrograph comparison**

**unconfined**

## Residual Statistics

| YEAR | MEAN | MIN | MAX | RMS | MEAN ABS | COUNT |
| --- | --- | --- | --- | --- | --- | --- |
| 1964 | 0.08 | -2.33 | 1.54 | 1.47 | 1.20 | 6 |
| 1974 | -0.07 | -2.88 | 1.42 | 1.29 | 0.95 | 9 |
| 1984 | 0.10 | -2.18 | 2.53 | 1.42 | 1.00 | 10 |
| 1994 | 0.15 | -2.60 | 1.75 | 1.31 | 1.07 | 12 |
| 2004 | -0.26 | -3.12 | 2.97 | 1.13 | 0.76 | 43 |

**Residual Statistics**

- all units in meters
- Residuals =
- observed minus simulated

## Transport model sub-grid

**13 layers****(2-14)**

**Version of****GWT to****support MNW**

**61 rows****122 columns**

**Age & CFC boundary by layer**

- observation wells

## Simulated age animation

**Age, in years**

**Cross-sectional view**

## Simulated age animation

**Age, in years**

**Map view cutaway**

## Age of water vs depth below water table 

- (unconfined wells only)

- (measured= tritium-helium ages)

## CFC concentration vs depth below water table 

- (confined wells only)

## Conclusions

**Wellbores drilled through confining units act as “short circuits” for flow**
**Simulated transient stresses provide pulses of flow**
**Transient stresses and wellbores through confining units allow flow and contamination to lower layers**

## Contact information

**Contact information**

- Brian Clark  - brclark@usgs.gov
- Matt Landon – landon@usgs.gov
- Leon Kauffman – lkauff@usgs.gov
- George Hornberger – gzhornbe@usgs.gov

**4/12/2005**

- TANC