Water Withdrawal Controversy
SUMMARY: Prairie Valley Water is installing a high-capacity well to support their expanded bottled water operations. But the company's permit is denied by the state because the state’s water withdrawal assessment tool predicted that their proposed well could adversely impact the nearby trout stream. The company argues that their well will pump deep groundwater which will have no impact on what happens at the surface. You be the judge…should Prairie Valley Water get a permit?
Background and Arguments
Prairie Valley Water is installing a high-capacity well to support their expanded bottled water operations. But the company's permit is denied by the state Department of Environmental Quality (DEQ) because the state’s water withdrawal assessment tool predicted that their proposed well could adversely impact the nearby trout stream.
The state DEQ requires that the reduction in baseflow due to pumping be limited to less than 5% of the index flow. Index flow is the statistically determined "extreme" dry season streamflow (very low discharge). For the stream of interest, index flow is estimated to be 5 cubic feet per second.
The company is frustrated and unconvinced. How did the DEQ calculate the impact? What even is adverse impact?!? They thought:
“our well is so deep, I can’t imagine this deep well would take water from this shallow little creek, even it if appears to be close by in the horizontal direction. We purposely designed the well to go deep into the confined aquifer !!”
So Prairie Valley Water complained to the local DEQ office:
“We happen to have consulted a hydrogeologist and groundwater modeler, and he told us that the area is geologically complex, with alternating layers of aquifers and aquitards. You cannot possibly predict the impact using such a simplistic, automated tool. He wouldn’t be surprised if our proposed well would take water from the big river a mile away.”
The DEQ countered:
“The tool is, by design, conservative. We always try to err on the safe side. If you are skeptical, you can actually challenge the result and request a site-specific analysis."
Aquifer-surface water system. Conceptual representation, plan view (top) and cross-section view (bottom).
Objectives and Deliverables:
You be the judge…should Prairie Valley Water get a permit?
Develop a model of groundwater flow at the site using the information provided to come to a conclusion regarding this situation. Specific questions to address as part of your analysis include:
- Where does the pumped water come from?
- How much water is coming from the shallow trout stream?
- How much water is coming from the big river?
- What is the percentage depletion of the streamflow because of pumping?
- Is the reduction in baseflow due to pumping limited to less than 5% of the index flow?
Prepare a 1-2 page report that summarizes your approach and findings. You should discuss your findings with regards to responsibility for the contamination. Include any detailed model results / graphics in support of your conclusions in an appendix.
Given Information:
Use the information provided in the graphic to build / parameterize a 3D groundwater model (shallow aquifer, clay layer, and confined aquifer). Note that for each layer, the following properties are given:
- Horizontal hydraulic conductivity (K)
- Layer thickness
- Anisotropy ratio (Kx/Kz)
Other useful information:
- Recharge to shallow aquifer: 12 in./year
- River depth: 25ft
- Trout stream depth: 3.3ft
- Riverbed leakance: 50 day-1
- Trout streambed leakance: 1 day-1
- Well screen interval (from -150ft to -160ft)
- Average land surface elevation: 15ft
MAGNET/Modeling Hints:
- Use 'Synthetic mode' in MAGNET to create a model domain with the same dimensions as described above.
- Go to: 'Other Tools' > 'Utilities' > and click "Go to Synthetic Case Area' to access Synthetic mode. (Click OK to prompts that appear)
- Once synthetic model domain appears, go to 'Utilities' > and click 'Geometry Locked' and then 'Geometry unlocked'. Then click anywhere inside the model domain. After answering OK to the prompts that appear, you will be able to click-drag any of the vertices to see the distance between vertices. NOTE: vertices are numbered and distances are indicated by d##, e.g., d21 is the distance from vertex 1 to vertex 2.
- Once you have the correct dimensions, you can click 'Geometry Locked' once more to lock-in the shape.
- Overlay the provided SiteMap image file included at the top of the problem description page.
- Go to: 'Other Tools' > 'Utilities' > 'Overlay myImage' and follow the instructions in the Help Page ('?' button)
- Click the 'Use Domain Extent' button to fix the image to the established domain size. (This should be after choosing the image file but before clicking 'Upload'.)
- Add a 2nd and 3rd 'GeoLayer' (geological layer, or conceptual aquifer layer)
- Go to: 'Conceptual Model Tools' > 'Layer' > and clickAdd a New Layer. The new layer is always added below the bottom-most layer
- Use the GeoLayer selector at the top of the MAGNET modeling environment to change between layers
- Use the Domain Attributes menu to assign aquifer attributes to each layer: aquifer elevatoins/thickness, hydraulic conductivity, and anisotropy
- NOTE: you can change layers within the Domain Attributes menu
- Conceptualize both the River and the Trout stream as a two-way head dependent Zone feature. Note that stages, depths and leakances are provided above.
- The River and Stream zone features should be placed in the first layer only
- The "break in the clay layer" can be represented using a zone feature that has the same hydraulic conductivity as the shallow aquifer.
- This zone can have a geometry identical to that of the River zone from the 1st layer.
- Note that the well is screened in the deep aquifer.
- To screen a well through only the 3nd layer, it should only exist in the 3nd layer (no well in the first layer or 2nd layer).
- Use the 'Screen Top' and 'Screen Bot' text fields to assign the well screen interval given above.
- Add "computational layers" (or sublayers) within each GeoLayer to resolve 3D head variability (impacts of anisotropy ratio)
- You can add layers by going to: 'Conceptual Model Tools' > 'DomainAttr' > 'Simulation Settings' tab > 'Number of Sublayers' under Grid & Layer Settings.
- First, simulate the model without using computational in any of your 3 Geolayers. This creates the initial water table.
- Then, check the boxes next to 'Number of SubLayers' and 'Water Table as Top' and re-simulate. The shallow aquifer layer and deep confined aquifer layer will use three (3) sublayers each. The clay layer can be assigned two (2) sublayers.
- Perform a water balance analysis before the pumping well is turned on to determine the baseflow to the stream under "natural conditions". Then turn the well on and perform a water balance analysis to determine the baseflow under "stressed conditions".
- Remember to follow the two-step process for representing 3D variability (first simulate with no sublayers, then simulate again after enabling sublayers).
- Use a large grid size (NX=80) to better capture the head dynamics at the well and to improve the water balance analysis.