By Hydrosimulatics INC  

Last updated: October 16, 2020

SUMMARY: Homeowners in a small community filed a lawsuit against a nearby food processing plant for the contamination of their water wells with heavy metals. They claimed that the source of the problem is the food processing company’s spray irrigation of its waste water at four large fields in the vicinity. But the food processing company denied the charges, arguing that even if the water from the spray irrigation is a problem, it cannot possibly reach most of the water wells because they are outside the impact area of the spray irrigation. You be the judge… is the company responsible?!? 

Background & Arguments

Homeowners in a small community recently found their wells contaminated with heavy metals, including arsenic, manganese and iron. They filed a lawsuit against a nearby food processing plant for the contamination. The homeowners claimed that the source of the problem is the food processing company’s spray irrigation of its waste water at four large fields in the vicinity. They further argued that the contamination is causing health issues (including cancer cases), depreciation of real-estate values, and plumbing problems. 

But the food processing company denied the charges, arguing that their operation is completely legal because it has a permit from the state Department of Environmental Quality (DEQ) for wastewater disposal. The wastewater from processing fruits and vegetables that was being sprayed contains mostly salts and sugars that present no particular issues. The company insisted that it never used any of the chemicals (heavy metals) found in the homeowners’ water wells, so they cannot possible be liable for the contamination. They pointed out that arsenic, manganese and iron occur naturally in the area

The citizens countered. They agreed that yes – arsenic, manganese and iron occur naturally in the area. But typically, they are locked up in the soil, or exist in a state that makes them immobile, presenting no particular risk. When you apply lots of wastewater to the subsurface containing heavy metals, the environmental conditions change. This changed condition can unlock heavy metals, causing the originally immobile metals to move, contaminating their wells downstream. More specifically, the experts hired by the community said that the interaction of wastewater with heavy metals on the soil depletes the subsurface oxygen, changing the heavy metals into an alternative form that are mobile.

The company stressed that even if the water from the spray irrigation is a problem, it cannot possibly reach most of the water wells because, according to their expert witness, they are outside the impact area of the spray irrigation; many of the plaintiff wells that are apparently "downstream" but screen below a low permeability clay layer and thus may not be affected; and almost half of the wells are located on the other side of a creek that functions as a hydraulic divide or boundary separating the wells from the infiltrated wastewater. But the experts representing the citizens argued that the creek – a small, 1st-order stream  cannot possible hydraulically separate groundwater on each side.

 

Top: Map of site, with spray irrigation fields and plaintiff wells (pink triangles). Also shown is the drain (solid blue line) and the approximate cutoff of the confining layer (yellow dashed line). Bottom: conceptual W-E cross-section of the subsurface beneath the site.

Objectives & Deliverable

You be the judge… is the company responsible?!?

Assume the role as an ‘expert witness’ to determine if the food processing plant is likely or unlikely to be responsible for the contamination of the plaintiffs' water wells. Develop a model of groundwater flow at the site and trace groundwater flow paths emanating from the irrigation fields to support your analysis. Specific questions to address as part of your analysis include:

  • Where does the contamination from from?
  • What is the impact area of spray irrigation?
  • Who / which wells may be affected by the sprayed waste water”
  • Can the small creek in the area function as a divide, preventing the waste water to reach the other wells on the other side?

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

You are provided with the following information that was collected during and after the lawsuit.

Site Hydrogeology

The topography at the site is relatively flat, with a gentle slope from west to east (from the food processing plant to the River). Directly under the spray fields, a shallow aquifer consisting of permeable sands with a very shallow water sits on top of a flat clay layer. Based on analysis of aquifer tests performed on side, this clay layer is nearly impervious and is truncated by high-energy river channel deposits in the northeast (these aquifer tests were also used to estimate the hydraulic conductivity of the different deposits). Beneath the clay layer are deep, inter-bedded glacial outwash deposits. The sandy channel deposits beneath the River are much thicker than the deposits underneath the spray fields. A drain exists between the spray fields, flowing predominantly west to east-northeast. In the winter time, the drain is covered with ice east of the road along which clusters 2,3, and 4 occur (but a freely flowing water surface is found west of this road).

Wells from Clusters 1-4 are screened in the shallow  sands and sandy channel deposits. Wells from Custer 5 are screened below the clay layer in the glacial outwash deposits.  Because the pumping rate is very low for these private domestic wells, there is virtually no impact on the groundwater flow patterns in the study area. 

Field Data

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

  • Average land surface elevation: 650 ft
  • Average clay layer top elevation: 600 ft
  • Average clay layer bottom elevation surface elevation: 500 ft
  • Average bottom elevation of deep glacial Average thickness of gravel aquifer: 390 ft
  • Hydraulic conductivity of shallow sands, thin part of aquifer: 30 ft/day
  • Hydraulic conductivity of  sandy channel deposits (thick part of aquifer): 150 ft/day
  • Hydraulic conductivity of  clay layer: 0.8 ft/day
  • Hydraulic conductivity of deep glacial outwash: 100 ft/day
  • Anisotropy ratio (Kx/Kz) of shallow sands and sandy channel deposits: 2
  • Anisotropy ratio (Kx/Kz) of clay layer:  10
  • Anisotropy ratio (Kx/Kz) of deep glacial outwash: 5  
  • Average recharge (outside of spray fields): 12 in./yr.
  • Spray field recharge: 25 in./yr. 
  • Average River stage: 644.7 ft
  • River leakance: 200 m/d
  • Average river depth: 2m
  • Drain stages (see figure below)
  • Drain leakance: 10 m/d
  • Drain depth: 0.8m
  • Average effective porosity in the aquifer system: 0.3




Further Hints and Suggestions (MAGNET-related)

    • 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 'SiteMap...10.16.20.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 Drain and River as two-way head-dependent boundary conditions. Note that the drain stages vary over space (interpolation will be done in between measurement locations).
      • Use Zone features to assign zone-specific recharge values in the spray field areas
      •  Develop a three-layer groundwater model to represent the aquifer system. Use zones to differentiate the aquifer areas to the left and right of the break in the clay 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 elevations/thickness, hydraulic conductivity, and anisotropy
      • 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 (1st) layer and deep (3rd) layer will use three (3) sublayers each. The middle (2nd) layer can be assigned five (5) sublayers.
      • Use particle tracking applications on your local flow patterns to i) determine source water contribution areas to well clusters; and ii) predict the movement of spray wastewater that has infiltrated into the Shallow aquifer.
      • Once you have the model setup properly, use a large grid size (NX=80 or 100) to improve particle tracking accuracy.
      • Utilize both plan view and cross-section views to analyze your model.