True confessions….
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Groundwater Flux Groundwater Flux to the Upper to the Upper Mississippi River – Mississippi River – Approach and Approach and application to application to nutrient nutrient understandingunderstanding
True confessions….The “Upper Mississippi River” has a mix of
geologic terrains and associated groundwater flow systems
This talk focuses on the unglaciated areas However, approaches/insights gained are
applicable to larger systemWork presented here was team effort
La Crosse Areas covered by Study
Coon Valley Watershed
Pool 8 of Mississippi River(Rick Hooper $)
City of La Crosse
La Crosse County
Present objectives Characterize regional groundwater flow system Quantify groundwater flow into Pool 8 and
estimate importance for nitrate loading Identify the area that contributes water to each
municipal well Investigate virus occurrence in municipal wells Evaluate the effects of changing land use
(primarily ag practices) on groundwater resources
Project ApproachDevelop a simple regional groundwater
flow model for greater La Crosse area Cut out smaller models from regional
model (Pool 8, La Crosse County, Coon Valley)
Perform project objectives on smaller models
La Crosse Area Projects
Regional ModelPool 8 ModelLa Crosse County ModelCoon Valley ModelLa Crosse Virus study
How groundwater models work:
Plumber’s RuleScotty’s RuleNumerical equations representing real
world entered into the computerAllows us to quantify system and forms
the basis for predictionData requirements can be large
Conceptual modelM
issi
ssip
pi R
iver
Lem
on W
eir
Riv
er
La
Cro
sse
Riv
er
Miss. River
Regional Area Covered by the GFLOW AE Model
La Crosse
Black River
La Crosse
Kick-apooCoonRoot
175 miles
Black River @ Neilsville
simulated areas outside of watershed of interest
Potential ProblemsGaging station data sets may not
overlap in time Large basins have long-term gaging
stations, but also may have confounding factors (e.g., dams)
Definition of baseflow = ?Average baseflow may not be stationary
in time
“Average baseflow” may not be stationary in time…
Changes in Q50 flows over time at Four Gaging Stations
0
200
400
600
800
1000
1200
Q50
of K
icka
poo
and
La C
ross
e R
iver
(cfs
)
0
20
40
60
80
100
120
140
160
180
Black nr GalesvilleRoot nr HoustonKickapoo @ La FargeLaCrosse @ Sparta
1938 to 1976 1992 to 2000 1997 to 20001938 to 2000
Q50
of B
lack
and
Roo
t Riv
er (c
fs)
Results 1999 Q50 baseflow
0
200
400
600
800
1000
1200Q
50 B
asef
low
(cfs
)
Root River @Houston
Black @Galesville
La X @ Sparta Kickapoo @ LaFarge
Coon @ CoonValley
Measured Simulated
R rates needed to match flux targets
5
6
7
8
9
10
11
12
Bas
in R
rate
(in/
yr)
9.1
8.0 8.2
6.4
R_Black R_Lax R_Kickapoo R_Coon
Optimized R rates (1999 Q50 targets, base=500)
NE to SW
Okay, complete results
5
6
7
8
9
10
11
12
Bas
in R
rate
(in/
yr)
11.7
R_Root
9.1
8.0 8.2
6.4
R_Black R_Lax R_Kickapoo R_Coon
Optimized R rates (1999 Q50 targets, base=500)
NE to SW
Digression: What is going on in the Root River? Root River (using 1999 target) is higher than
expected, with R rate higher than we’d expect to see in the NE parts of Wisconsin!
If we used the entire record (from 1909 until the present) the Root River has the lowest R rate of all basins
Upstream gage knocked out and not replaced Suggests lack in our understanding,and a
problem with our handling of the 1999 measured target
Need to monitor the tributaries if you want to simulate the larger system
Looking at the Pool as a whole under baseflow conditions…
Source of Water to Pool 8
93.1%
6.4%
0.5%
Pool 7 inflow
Tributary inflow
Direct GW discharge
Being that simulated streams at baseflow = “indirect gw dischg”…
93%
0%
7% Pool 7 inflow
Surface water
Groundwater
(indirect and direct)
Not to say there is no event water…Estimated Total Flow using Sartz (1977)
91%
2%7%
Pool 7StormflowBaseflow
Geologic sectionM
issi
ssip
pi R
iver
Lem
on W
eir
Riv
er
La
Cro
sse
Riv
er
Eau Claire Confining Unit
Hole in Confining Unit
Well Logs compiled by WGNHS
Top of Mt. Simon
Top of Eau Claire(blue=sh/slt or ss w/ much sh; green=ss w/ some sh)
Northern part of Pool 8
Top of Bedrock or Land Surface
Add Fluvial Sediments
Add Surface Water features
Note how similar model is to conceptual model in cross section
Plan view Pool 8 model domain extracted from AE 2D model
Pool 7
Pool 8
City of La Crosse
Area of strong upward gradients (artesian wells)
E-W cross section near Goose Island
E-W cross section near Goose Island
2D and 3D Regional Model Results
2D AE Source of Water to Pool 8
93.1%
6.4%
0.5%
Pool 7 inflow
Tributary inflow
Groundwater
93.1%
6.5%
0.4%
3D FD Source of Water to Pool 8
Distribution of Flux from FD model
**Preliminary Results
River cell flux, ft3/d
2.7 – 0.05 0.05 – 0.020.02 – 0.0070.007 – 0.0050.005-0.0040.004 – 0.0030.003 – 0.0Recharge 0 – 2.6
River Cell Flux (ft/d)
Nitrogen Data from BRD
**Preliminary Results
River cell flux, ft3/d
2.7 – 0.05 0.05 – 0.020.02 – 0.0070.007 – 0.0050.005-0.0040.004 – 0.0030.003 – 0.0Recharge 0 – 2.6
River Cell Flux (ft/d)
Max NO3= 16 mg/l as NMax NH4= 0.4 mg/l as N
Max NO3= 13 mg/l as NMax NH4= 0.3 mg/l as N
Max NO3= 6 mg/l as NMax NH4= 7 mg/l as N
Max NO3= 3 mg/l as NMax NH4= 15 mg/l as N
Take Home Points We have gw-sw modeling tools that can cover
large areas (Mississippi Pool scale) 2D models performed well for:
Calculating boundaries for 3D inset models Pool 8 water balance information
3D model performed well for: representing best available knowledge Time of travel, vertical gradients, distribution of flow
into pool Direct GW discharge to Pool 8 was small %
Distribution important for other research questions Would be dominant if gw included discharge to tribs
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