Drinking Water Contamination Dispute
(Intermediate – Multi-layer aquifer analysis; source water tracking; contaminant tracking)
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.
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 irrgation 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 compeltely legal because it has a permit from the state Department of Enviromental Quality (DEQ) for wastewater disposal. The wastewater from processing fruits and vegtables 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 aresenic, manganese and iron occur naturally in the area
The company further stressed that even if the water from the spray irrigatoin 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 wels are located upgraidient of the irrigation fields, and almost half of the wells are located on the other side of a creek that functions as a hydraulic divide or boundary seperating the wells from the infiltrated wastewater. Some of the plaintiff wells are below an impervious confining layer.
The citizens countered. They agreed that yes – aresenic, 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 particulary 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 orginally 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 subsuface oxygen, changing the heavy metals into an alternative form that are mobile. The experts also stressed that the creek – a small, 1st-order stream, cannot possible hydraulically separate groundwater on each side.
Objectives & Deliverable
You be the judge… who is 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 plainfeff 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. This clay layer is nearly imperivous and was truncated by high-energy river channel deposits in the northeast. The sandy aquifer beneath the River is 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).
Field Data
The following information/data are available from a preliminary study:
- Average aquifer top elevation: 650 ft
- Average clay top elevation: 600 ft
- Average bedrock top surface elevation: 480 ft
- Hydraulic conductivity of permeable sands, thin part of aquifer: 30 ft/day
- Hydraulic conductivity of permeable sands, thick part of aquifer: 50 ft/day
- Average recharge: 22 in./yr.
- Average effective porosity in the aquifer system: 0.3
- Average drain stage: 646 ft
- Average drain bottom: 643 ft
- Drain leakance … 5 d-1
- Average river stage: 645 ft
- Average river bottom: 640 ft
- River leakance:50 d-1
Further Hints and Suggestions (MAGNET-related)
- A site map is available for overlaying in the MAGNET modeling environment. It is included in the problem posting on the MAGNET Curriculum Network.
- Choose ‘Use Domain Extent’ to fit the image to the established domain size.
- Develop a single-layer groundwater model to represent the aquifer system underneath the spray fields. Use zones to differentiate the aquifer areas to the left and right of the break in the clay layer.
- Conceptualize the Drain and River as head-dependent boundary conditions.Treat the domain boundaries as ‘no-flow’ boundaries.
- 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.
- Utilize both plan view and cross-section views to analyze your model.