# Presentation: 303615

## Groundwater Flux to the Upper Mississippi River – Approach and application to nutrient understanding

**Groundwater Flux to the Upper Mississippi River – Approach and application to nutrient understanding**

## 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 system
- Work presented here was team effort

## La Crosse Areas covered by Study

**La Crosse Areas covered by Study**

## 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 Approach

- Develop 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

_**La Crosse Area Projects**_

## Advantages

- Consistent base model helps ensure models agree with each other
- Simple regional model allows for investigation of regional flow system not possible with smaller model
- Leverage project funds because we aren’t reinventing the wheel

## How groundwater models work:

- Plumber’s Rule
- Scotty’s Rule
- Numerical equations representing real world entered into the computer
- Allows us to quantify system and forms the basis for prediction
- Data requirements can be large

## Conceptual model

## Regional Area Covered by the GFLOW AE Model

**Regional Area Covered by the GFLOW AE Model**

## Potential Problems

- Gaging 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 Q****50**** flows over time at Four Gaging Stations**

**Q****50**** of Kickapoo and La Crosse River (cfs)**

- Black nr Galesville

- Root nr Houston

- Kickapoo @ La Farge

- LaCrosse @ Sparta

**1938 to 2000**

**Q****50**** of Black and Root River (cfs)**

## Results 1999 Q50 baseflow

## R rates needed to match flux targets 

- 5

- 6

- 7

- 8

- 9

- 10

- 11

- 12

**Basin R rate (in/yr)**

- Optimized R rates
-  (1999 Q50 targets, base=500)

## Okay, complete results

- 5

- 6

- 7

- 8

- 9

- 10

- 11

- 12

**Basin R rate (in/yr)**

- Optimized R rates
-  (1999 Q50 targets, base=500)

## 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 7

- Stormflow

- Baseflow

## Geologic section

- Eau Claire 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)

## 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**

## 2D and 3D Regional Model Results

**2D AE Source of Water to Pool 8**

**3D FD  Source of Water to Pool 8**

## Distribution of Flux from FD model

**Distribution of Flux from FD model**

- **Preliminary Results

**River cell flux, ft****3****/d**

**2.7 – 0.05 **
**0.05 – 0.02**
**0.02 – 0.007**
**0.007 – 0.005**
**0.005-0.004**
**0.004 – 0.003**
**0.003 – 0.0**
**Recharge 0 – 2.6 **

**River Cell Flux (ft/d)**

## Nitrogen Data  from BRD

**Nitrogen Data  from BRD**

- **Preliminary Results

**River cell flux, ft****3****/d**

**2.7 – 0.05 **
**0.05 – 0.02**
**0.02 – 0.007**
**0.007 – 0.005**
**0.005-0.004**
**0.004 – 0.003**
**0.003 – 0.0**
**Recharge 0 – 2.6 **

**River Cell Flux (ft/d)**

## 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