**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 R. Clark^1^, Matthew K. Landon^2^, Leon J. Kauffman^3^, George
Z. Hornberger^4^**

1 U.S. Geological Survey, 401 Hardin Road, Little Rock, AR 72211

2 U.S. Geological Survey, 5735 Kearny Villa Road, Suite O, San Diego, CA

3 U.S. Geological Survey, 810 Bear Tavern Road, Trenton, NJ 08628

4 U.S. Geological Survey, 12201 Sunrise Valley Drive Mail Stop 431,
Reston, VA 20192

*2006 National Monitoring Conference: Concurrent Session G: G3 Assessing
Ground Water Vulnerability Through Mechanistic Methods III, May 7-11,
2006, San Jose, CA.*

**Biographical Sketches of Authors**

Brian Clark currently serves as the primary modeler for the High Plains
Ground Water study unit of the NAWQA Transport of Anthropogenic and
Natural Contaminants (TANC) topical study.

Matt Landon has been a hydrologist with the USGS since 1990. He is
currently working on studies of transport of anthropogenic and natural
contaminants for the National Water-Quality Assessment Program and the
California Ground water Ambient Monitoring and Assessment program.

Leon Kauffman has been a hydrologist with the USGS since 1995. Currently
he is working on the

Hydrologic System Team of NAWQA, providing technical support for
ground-water modeling activities.

George Z. Hornberger graduated from the University of Vermont in 1993.
Since then he has been developing ground water solute transport models
as part of the National Research Program of the USGS.

**Abstract**

The High Plains aquifer is a primary source for drinking-water supply in
many areas of the High Plains. Several wells in the area are known to be
screened in both the unconfined and confined portions of the aquifer
which may provide a preferential pathway, or "short circuit", for water
moving from the unconfined to the confined units of the aquifer. As part
of the U.S. Geological Survey National Water- Quality Assessment
Program, a ground-water flow and transport model was developed to
simulate processes controlling movement of contaminants to public supply
wells in the layered unconsolidated deposits of the High Plains aquifer
near York, Nebraska.

Water-quality samples were collected from wells screened through
unconfined and confined water-bearing units. Samples also were collected
from a public supply well, screened in the confined portion of the
aquifer. Analytical results show that samples collected from the public
supply well have chemistry and age-tracer concentrations consistent with
young water derived from unconfined recharge areas mixed with relatively
older waters from the confined part of the aquifer. This implies the
existence of preferential flow paths that permit shallow recharge water
and contaminants to move through the confining layer.

To test the conceptual model developed from the measured water-quality
data, a transient ground-water flow and transport model was constructed
assuming seasonal ground-water withdrawals over a 60-year period
simulated using the multi-node well package, which allows water to flow
between different layers through the simulated well. A new version of
MODFLOW-GWT (Ground-Water Transport) was used to simulate particle
movement through the system, while also assigning ages to the particles.
The simulations indicate that the transient conditions and wells
screened through multiple layers can introduce shallow recharge waters
and contaminants to lower layers, increasing the vulnerability of public
supply wells completed in the confined system to contamination from
unconfined waters.