Background & Constraints

A university is planning to build an extension campus near a city. Capitalizing on their prestigious academic programs and worldwide reputation, and satisfying the educational needs for a rapidly growing urban and suburban population, the campus is designed for 45,000 students, professors, and supporting staff.

Planners have identified a potential site approximately 1 by 2 miles in size with very useable land that works well in terms of stormwater drainage, geotechnical stability, and transportation and infrastructure. But there may be a “drinking water supply issue”. Although there is a large river going through the site area, consistent water availability is a concern because the river freezes over in the winter.

Planners identified groundwater as the best or most cost-effective source because a very permeable aquifer exists beneath the campus site.  There are, however, a number of delicate, potentially limiting constraints:

1. To the north of the site, a short stretch of a trout stream gets close (2.5 miles) to the proposed site. The state Department of Environmental Quality (DEQ) will deny any application for pumping which would cause significant streamflow:

   (pre-pumping flux – post-pumping flux)/pre-pumping flux > 5% …is not allowed

2. To the northeast, about 3.5 miles away, is a special, highly valuable groundwater-fed wetland: the prairie fen. These types of wetlands harbor an extreme amount of biodiversity, including endangered species like the Mitchel Satyr butterfly and many other rare or valuable animals, insects and plants. No significant influence is allowed on the wetland and on the groundwater hydrology in the fen’s source water area. Again,

   (pre-pumping flux – post-pumping flux)/pre-pumping flux > 5% …is not allowed

3. Farm lands of assorted crops exist to the west of the site about 3.5 miles away. These areas are known nitrate contamination hotspots.
4. Southeast of the campus, about 2 miles away, is a residential area. Homeowners rely on their private water wells for drinking water supply. Any well interference (lowering of water levels) more than 1m due to new large-supply wells is prohibited.

5. Groundwater in the aquifer – which is part of the master discharge area of the entire basin – sits on a pool of brine that is naturally inching upwards. To avoid “pulling up” the salty/mineralized groundwater, the available drawdown needs to be limited to less than 20m based on a prior study in the area. (In other words, pumping “a ton” of water from a single well may cause an acceptable drawdown.)

Objective and Tasks:

Perform a system-based water supply feasibility analysis to present to the campus planners. As part of your overall analysis, you should:

• Under “natural conditions” (before adding water supply wells):
  • Map flow patterns across the groundwater system
  • Analyze aquifer balance and fluxes to or from sinks and sources of groundwater.
  • Identify the source water area for the prairie fen
  • Delineate potential impact areas downstream from the farms (nitrate sources)
  • “Sample” groundwater heads in key areas (e.g., residential area, proposed pumping locations, etc.)

• Determine the total water demand and number of pumping wells needed to meet demand

• Design an optimal pumping pattern within the given limiting factors:
  • Map flow patterns during “operational mode” (pumping wells turned on)
  • Identify source water area for the prairie fens, and compare to natural conditions (and changes to source water area extent/shape)
  • Identify potential impact areas downstream from the farms (any capture of pollution by water supply wells?)
  • Analyze aquifer balance and source/sink terms, and compare to natural conditions
  • “Sample” groundwater heads in key areas, and compare to natural conditions
  • Determine the source of water for the pumping wells:
    • Delineate the Wellhead Protection Areas (WHPAs), or 10-year source water areas for the pumping wells
    • Use water balance analysis to analyze potential contributions from sources/sinks (surface water)

Deliverables

Prepare a report that summarizes your approach and findings. You should discuss your findings with regards to the ecological, hydrological, and legal constraints. Include any detailed model results / graphics in support of your conclusions in an appendix. 

Given Information

The aquifer consists of relatively uniform fine-grained sands extending from the bedrock to the land surface, resulting in unconfined conditions.

Near the River, channel deposits of relatively high permeability exist from the land surface to the underlying bedrock. 

Both Lost Lake and Long Lake are deep and assumed to be fully connected to the aquifer.

The wetland functions as a “drain”, with the head at or above the land elevation year-round.

The Trout Stream is partially connected to the aquifer and will exchange water to/from the aquifer depending on the local hydrogeologic conditions.

The River is large and well connected to the aquifer and will readily exchange water to/from the aquifer

Both the proposed campus site and the residential areas have significantly reduced (or even negligible) infiltration of precipitation due to extensive impervious surfaces and infrastructure.

Field Data

The following information/data are available from a preliminary study:

• Average Trout Stream stage: 39.3 ft
• Trout Stream leakance: 5 m/day
• Average Trout Stream depth: 1m

• Average River stage: 10 ft
• River leakance: 50 m/day
• Average River bottom elevation: 0ft
• Aquifer hydraulic conductivity of channel deposits: 200 ft/day

• Average water level from monitoring well on east edge of prairie fens: 40.5 ft
• Leakance of wetland sediments: 1 day^-1

• Average Long Lake water level: 41.5ft
• Average Long Lake depth: 38ft
• Long Lake bed leakance: 150 day^-1

• Average Lost Lake water elevation: 41ft
• Average Lost Lake depth: 34ft
• Lost Lake bed leakance: 150 day^-1

• Average aquifer thickness: 150 ft

• Average annual recharge in non-residential areas and outside campus: 6.8 in./yr.
• Average annual recharge in residential areas and inside campus: 1 in./yr.

• Aquifer hydraulic conductivity: 50 ft/day
• Aquifer effective porosity: 0.16

• Typical per capita water use in the region: 165 gallons per day
• Typical pumping rate for a high-capacity drinking water supply well: 1000GPM

Land surface elevations were measured at the points shown in the graphic below. Also included in the graphic are the elevation measurements at each location.

For one of the farms in the northwest portion of the study area, near the Trout stream, monitoring of nitrate concentrations in groundwater is typically 25mg/L. The data comes from the small farm furthest east in this cluster of farms. 

MAGNET/Modeling hints

• (Optional, but recommended): Navigate to the ‘Synthetic Model’ Quick Tutorial on the magnet4water website and follow the instructions to reproduce the example. This will help with the following steps.

• 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'.)

• Conceptualize the model as 1-layer, unconfined aquifer.

• Conceptualize both the River and the Trout stream as two-way head dependent line features. Note that stages, depths/bed elevations, and leakances are provided above.  

• Conceptualize the lakes as two-way head dependent flux boundaries using Zone features added to the domain.

• Conceptualize the Prairie Fens as a one-way head-dependent zone feature. Leakance and seepage (drain) elevation are given above.

• Use Zone features to assign zone-specific recharge values in the residential area and campus area (all other areas will use the recharge assigned in the Domain Attributes menu; local zone values "override" the domain-assigned values).

• Use a Zone feature to assign zone-specific hydraulic conductivity to the channel deposits.  (Again, all other areas will use the hydraulic conductivity assigned in the Domain Attributes menu; local zone values "override" the domain-assigned values.). Note that all of the zone feature must be INSIDE of the model domain; if any portion is outside of the model domain, the entire zone will be removed from the model next time you SIMULATE. 

• Use a Zone feature to input the land elevation data (scatter points) to be spatially interpolated as the aquifer Top Elevation.
  • First, overlay the provided image file of the measurement locations (see top of problem description), following the same steps as you did when you overlaid the site map.
  • Then add a zone that stretches across the entire domain (you can use the Zone=DM option under Zones to do this). Again, make sure that all of the zone feature is INSIDE of the model domain; if any portion is outside of the model domain, the entire zone will be removed from the model next time you SIMULATE. 
  • Go to the Elevation tab, check the box next to ‘Top Elevation’, select ‘Scattered Points’, and click the ‘…’ options button. A new submenu will appear.
  • Use the ‘Click to Add Data’ button to interactively add scatter points to the model domain. (You need to click this button each time you want to add a data point)
    • Each time you add a point, a row entry is added to the table. The second to last value in that (comma-separated) row is the observed elevation.
    • By default, the observed value is 0.0.Change this (edit the text) to the appropriate value for each entry (units: meters) using the information in the graphic provided above.
    • You can see where your input data are located on the map by clicking the ‘Add Markers’ button after adding a data entry to the table. The ‘Show Points/’Hide Points’ button allows you to toggle back and forth between displaying and not displaying the points.

• Use a large grid size (NX= 100) to better capture the head dynamics at the well and to improve the water balance analysis.

• Note that the lateral and bottom domain boundaries are treated as ‘no-flow’ boundaries.

• To analyze the aquifer water balance and fluxes to/from sources and sinks:
  • During simulation or after, go to: ‘Analysis Tools’ > ‘Analysis’ >’Display Charts’
  • This will automatically open a number of charts, include the Mass Balance Chart. Use the ‘?’ button to access the Help Page which explains what is shown in the interface.
  
  
• Use particle tracking applications on your simulated flow patterns to track the movement of groundwater flow or potential contaminants.
  • For source water area delineation of the prairie fen, draw a particle zone and perform backward particle tracking
  • For identifying potential impact areas downstream from the farms, draw particle zones and perform forward tracking. For one of the farms where nitrate concentration data is available, use a zone feature to add a "plume source" to the model and visualize plume transport downstream - see next major bullet point. 
  • For WHPA delineation, place particles around the pumping wells at a radius of 1000ft and perform backward particle tracking
    • WHPA delineation is done for 10yr of travel time. Go to Domain Attributes (‘Conceptual Model Tools’ > ‘DomainAttr’) > ‘Simulation Settings tab’ and change the simulation length to 3650 days or 10 years (note: just change the simulation length, don’t check the box next to ‘Modeling Transient Flow’, your flow is steady state).
    • Consider reducing the time-step (e.g., to half a year or a quarter-or-a-year) to improve the accuracy of particle tracking.

• To add a "plume" source to the small farm in the northwest portion of the model:
  • Draw a zone where the small farm is located. Click 'SaveShape' to finalize the geometry
  • In the Zone attributes interface that appears, go to the Sources and Sinks Prescribed 'tab' 
  • Check the box next to 'Source Concentration' and make sure the 'Continuous' option is selected
  • Enter a typical concentration in the box provided (see Field Data above for information on typical concentrations), then click 'Save'
  • The next time you simulate, you will see the plume transport results in the model. 

• To analyze the head at a particular location in the model: 

• Go to: 'Analysis Tools' > 'Analysis' > and click 'NodalValues'.  Then, when you use the cursor to click anywhere on the map, a "flag" marker will appear. 
• When you click on the flag, model results for that location will be shown in a pop-up window, including the simulated head.