Transcript Investigation Of Flow And Seepage Conditions On A Critical

ABSTRACT
VERTICAL HYDRAULIC GRADIENTS
By using a highly instrumented 50-mile reach of the Rio Grande, we are able to see how
groundwater responds to changes in the river stage. This critical reach on the Rio Grande has
154 wells, 19 stream gages, and 70 pressure transducers that record hourly water levels,
providing a detailed look at flood events. This high-resolution data shows a vertical gradient
between the phreatic aquifer and the semi-confined aquifer that reverses during flood events.
Gradient changes during flooding confirm the presence of a low permeability zone, at a depth
varying between 20 and 50 feet, identified during well installation. A preliminary surface
water/groundwater interaction model depicts the low conductivity stratum as a continuous layer.
Flood data and a numerical model are being used to better characterize the low permeable
stratum.
STUDY AREA
CHANGES IN HYDRAULIC GRADIENTS DURING A FLOOD EVENT
• Graphs show the vertical gradient between well depths in response to a monsoon flood event
(September 2003) and a snowmelt/rainfall flood event (April 2004).
B/C Well Gradient
DOWNWARD
FLOW
•154 wells monitored monthly.
River Stage
B/C Well Gradient
DOWNWARD
FLOW
Socorro
A/B Well Gradient
60
Regional Model - ISC
Socorro
•Cells are 1000x1000 feet
San Antonio
I-25
Rio Grande
Regional model - ISC
•320 rows x 170 columns = 54400
grid cells, or
6 miles x 3 miles = 18 miles2
•Grid Cells are 100x100 feet
San Marcial
MODEL DOMAIN
Model Variables
•Evapotranspiration is estimated for
crops, riparian vegetation, and sandbars.
A/C Well Gradient
A/B Well Gradient
•Recharge from irrigation during the
farming season.
Figure 3b: Highway 380 Transect.
Figure 3a: Escondida Transect.
• Prior to the flood event vertical gradients at
the Highway 380 transect gradient are directed
upward.
•Constant head boundary is determined
from the regional model
Model Layers
• During the flood event the vertical gradient
becomes more pronounced, but does not
change direction.
•Current model has three layers, with
the middle layer being a continuous 2
foot thick low permeable layer (Figure
7a).
•High resolution data contradict this
initial assumption.
Texas
•New model will have a low permeable
layer, but thickness and converge will
more accurately reflect well logs and
observed data (Figure 7b).
A and B Well Water Elevation
A and C Well Water Elevation
DOWNWARD
FLOW
Telescopic model
-smaller domain
-refined grid
-constant head from
regional model
380
Figure 5: Map showing location of the Telescopic and Regional model
SNOWMELT/RAINFALL FLOOD EVENT
April 2004
Las Cruces
Mexico
River Stage
• The vertical gradient between the A and B
wells reverses during the flood event. The
vertical gradients between the B and C wells
and between the A and C wells do not reverse,
but increase in magnitude.
Study Area
San Acacia
UPWARD
FLOW
•19 staff gages, owned and maintained by the ISC.
NEW
MEXICO Albuquerque
Magdalena Mountains
•Smaller Domain (Figure 5)
UPWARD
FLOW
Rio Grande
N
Telescopic Model
MONSOON FLOOD EVENT
September 2003
•7 transects located along a 50 mile stretch of the Rio Grande from San Acacia to South of Fort
Craig (Figure 1).
•Staff Gages located in the Low Flow Conveyance Channel, the Rio Grande River, and in
various agricultural drains.
REGIONAL MODEL AND
TELESCOPIC MODEL
• The vertical gradient between the shallow and deep wells is influenced by the river stage.
A/C Well Gradient
•70 pressure transducers have been installed measuring the water level hourly.
MODEL SET UP
DOWNWARD
FLOW
UPWARD
FLOW
A/C Well Gradient
Figure 1: Map of Study Area
WELL CONFIGURATION
• Wells were installed at 3 depths
• B-wells are ~50 ft deep with a 5 foot
screen interval;
C
B
A
Rio Grande
LFCC
Low Permeability Zone
Figure 4a: Escondida Transect.
Figure 4b: Highway 380 Transect.
• The vertical gradient between the A and C
wells reverses during high flow conditions.
The vertical gradient reacts similarly for both
flood events.
• The vertical gradient becomes more negative
during high flow conditions.
• The flood pulse creates pressure at depth due to the presence of low-permeability clay-rich
sediments that are located between the A and B wells (Figure 3 and Figure 4).
Low Permeability Layer
• In areas where the normal gradient is upward, flood events result in an increase in magnitude of
the upward gradient.
Figure 2: Well Configuration
Figure 7a: Conceptual model with the continuous low
permeable layer.
Figure 7b: Conceptual model with a discontinuous
low permeable zone.
• This increase in pressure in the lower aquifer produces an upward gradient for both flood events.
• At locations where the normal vertical gradient is downward the gradient reverses during flood
events such that the flow is directed upward.
• C wells are ~100 ft deep with a 5
foot screen interval.
• A discontinuous low permeable zone
exists between the A and B wells.
UPWARD
FLOW
DATA SUMMARY
(Figure 2).
• A-wells are water table wells ~20 ft
deep with a 15 foot screen interval;
A/B Well Gradient
Figure 6: Map showing active cells, river and channel locations,
and well locations.
CONCLUSION
High resolution data shows that during flood events the
vertical gradient is upward due to high pressures below the
low permeability layer. This data cannot be explained with
a continuous low permeability layer. Therefore, this layer
must be revised.
Acknowledgments
•New Mexico Interstate Stream Commission
•S.S. Papodapulos &Associates Inc.
•Army Corp of Engineers, Albuquerque, NM
•Bureau of Reclamation
•New Mexico Tech Interns
•Field Volunteers
GROUNDWATER GRADIENT REVERSAL DURING FLOOD EVENTS
ALONG A CRITICAL REACH OF THE RIO GRANDE
Kate Richards, Robert Bowman, Laura Wilcox, and Nabil Shafike
Department of Earth and Environmental Science
New Mexico Institute of Mining and Technology
801 Leroy Place
Socorro NM, 87801
USA
New Mexico Water Research Symposium
Socorro, NM
16 August 2005