By M. Zreda, University of Arizona, 10/5/2020  

Trouble is a  brewin' in the valley.  Lil' Pete is darn tooting mad 'cause he thinks his well water is being contaminated by Phil's-not-so-fabulous pigs.  He is getting ready to sue Phil to cover the losses he faced on last year's bumper crop of mutant tomatoes and he wants to get the facts right before he hires Johnnie Cochran and turnsvthis thing into a national scandal.  He hires you to model the groundwater hydrology of the basin to corroborate his theory.  

Don't forget the other characters in this story.  Could the culprit really be Megalo Vegetable Farms (you've heard that they are not organic farmers as they claim)? Maybe it is dear sweet Henry and his Fertilizer & Co. (you've heard that he dumps assorted fertilizers and effluent into the river)!  Perhaps it is a government conspiracy perpetrated by the County Landfill?

 

You are to prepare a short/concise but professional report (expectation: 1-2 pages in length; but it can be more detailed if necessary), including supporting figures/screenshots in an Appendix section.  You should discuss  your findings with regards to responsibility for the contamination. As part of your analysis, you should determine:

  1. source water area of the well (groundwater capture zone of the well)
  2. the water budget of the aquifer 
  3. the groundwater flow paths emanating from potential sources of contamination

You have the following information about the basin:

  • The area you should model is 2km x 2km.
  • The lake is fully connected with the aquifer. The head in the lake is 500m. 
  • The river is partially connected with the aquifer, with a head of 500m, a leakance of 0.05 /day; and river bed elevation of 497m. The river gently flows from west to east
  • The head in the lake is 500m. The lake is fully connected with the aquifer
  • The head in the south-west corner of the grid is 505m
  • The basin has an uniform hydraulic conductivity of 2m/day.
  • The average land surface elevation is 510m;
  • The average aquifer bottom elevation is approximately 450m.
  • The aquifer abruptly pinches out along the northwest boundaries and along the eastern boundary (no-flow condition)
  • The head in the southeast corner was measured at 505m. Head is known to vary linearly along the southwest boundaries (between the southwest corner and the river). 
  • Lil Pete's irrigation well pumps at an average rate of 75 GPM
  • Recharge in the area is 13 inches per year. But recharge is reduced in two areas of the model: the capped (but leaky) County Landfill and the Megalo Vegetable Farms. Use a recharge of  2 in./yr for the County Landfill and 7 in./yr. for the Vegetable Farms. 

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 (use the .jpg file).
    • 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 a 1-layer aquifer system. Use the given transmissivity and assume an aquifer thickness to get the hydraulic conductivity of the aquifer
  • Conceptualize the lake as a prescribed (constant) head boundaries using Zone features added to the domain.
    • Make sure any added zones are completely within the model domain, otherwise they will "disappear" when the model is submitted for simulation.
  • Conceptualize the river as a two-way head dependent Zone feature.
    • Assume a riverbed leakance of 0.05 1/day and a riverbed elevation of 497
  • Use Zone features to add zone-specific recharges for the Countywide Landfill and the Megalo Vegetable Farms.
  • Use Line features of prescribed (spatially-variable) head to set boundary conditions along southwest boundaries of the model. 
    • Use the 'Prescribed Head' > 'Variable' > '>>more' option to assign the head for each placed line vertex.
    • Use 2 vertices for all line segments to capture the linear trend in head between two points of known head. First give a beginning head and then ending head. MAGNET will linearly assign heads on the line.
    • Make sure to uncheck the 'Head=TopE mins' box in the Edit Polyline Attributes menu.
    •  Make sure any added lines are completely within the model domain, otherwise they will "disappear" when the model is submitted for simulation. 
  • Consider using particle tracking techniques to determine:
    • i) the groundwater flow paths emanating from potential sources of contamination;
    • ii) source water area of the well (groundwater capture zone)
  • Use the default grid setting (NX=40) when setting up and testing the model. Consider using a finer simulation mesh (e.g., NX= 60) when performing particle tracking.