Southern Willamette Valley groundwater flow model.
In order to help us understand some of the issues associated with groundwater in the Southern Willamette Valley, a Master’s student at OSU, Jeremy Craner, developed a 3-D numerical groundwater flow model, using the GMS-MODFLOW program, to help answer questions about groundwater flow within the Southern Willamette Valley, including the GWMA area.
This presentation addresses how the model
works and describes the model’s predictions
in answer to some specific questions.
A model is a representation of real-world relationships, which can be used to make predictions about what will happen in the real world. Models can be used to help us understand complex problems, such as groundwater flow in the Southern Willamette Valley. Because we do not completely understand this problem, we must simplify it and make some assumptions about relationships between components of the problem.
What can a model NOT do?
Models cannot answer every question we might have. They can rarely incorporate every aspect of a problem. They may not provide completely accurate answers. In short, models can only make an approximation based on the information we give them.
South Santiam River
Long Tom River
Generalized Head Boundary
Generalized Head BoundarySome assumptions made in this model
As new data is collected, and we better understand the relationships described
in the model, we can improve, or refine, the model.
The MODFLOW groundwater flow model can be used to determine groundwater flow direction, groundwater velocities, and be used to help solve problems. We can use the model to help us to answer a wide variety of questions, though the answers depend on the assumptions we make in constructing the model. Here we ask three questions about groundwater in the Southern Willamette Valley.
What is the travel time of nitrate in the groundwater?
Simulated particles were tracked from their release at the water table to the time at which they reached either a model boundary or a river. The map shows the time that the particle took, or its travel time, at the particle’s starting location. This problem was modeled in both the middle and lower sedimentary hydrogeologic units.
Middle sedimentary hydrogeolgic unit
Lower sedimentary hydrogeolgic unit
Mean travel time = 29 years
Mean travel time = 100 years
WHAT DOES THIS MEAN?
How fast and where does nitrate in the groundwater around Coburg travel?
Particles representing nitrate were released in the model at the block dots shown here.
The blue arrows represent the model’s prediction of particles’ locations after 1000 days, if they had not already reached the Willamette River.
This cross section suggests that particles would travel just below the water table, in the middle sedimentary hydrogeologic unit.
Middle sedimentary hydrogeologic unit
Lower sedimentary hydrogeologic unit
WHAT DOES THIS MEAN?
x20 vertical exaggeration
A simulated well, pumping 750 gallons/minute, was placed West of the Willamette and Harrisburg.
The simulation was run twice, pumping at two different depths.
Middle Sedimentary Hydrogeologic Unit (MSHU)
The model predicts that a well pumping from the MSHU would draw from a narrow area and that travel times would be relatively fast. The water would travel from the MSHU, down to the LSHU, and back to the well in the MSHU. Blue arrows show water particle location every 100 days.
Lower Sedimentary Hydrogeologic Unit (LSHU)
The model predicts that a well pumping from the LSHU would draw from a wide area, and that water would pass beneath the Willamette River. The pumped water would originate at the water table, travel to the LSHU, and then move nearly horizontally to the pumping well. Blue arrows show water particle location every 1000 days.
WHAT DOES THIS MEAN?
There are a variety of practices which we may use to decrease the amount of nitrogen we add to groundwater. These may include:
To learn more about such recommendations, see the Southern Willamette Valley Groundwater Management Area Action Plan.