11.4 Contaminant Transport and Dewatering Decisions
Key Takeaways
- Groundwater contaminant travel time begins with seepage velocity v = K i / n_e, not Darcy flux alone.
- Advection moves the plume with groundwater, dispersion spreads the plume, sorption can retard movement, and decay or reaction can reduce concentration.
- A capture or dewatering decision should consider hydraulic control, discharge quality, treatment, settlement risk, nearby receptors, and permitting.
- Dewatering methods include sumps, wellpoints, deep wells, eductors, cutoff walls, and recharge systems; the right choice depends on soil, depth, flow, and risk.
- PE WRE sitework questions may combine groundwater flow with erosion control, excavation stability, adjacent structures, contaminated water handling, and construction safety.
Transport and Dewatering Decisions
The PE Civil WRE specification separates Groundwater and Wells from Surface Water and Groundwater Quality, but real exam scenarios can connect them. A leaking tank, landfill cell, construction excavation, utility trench, or pump station site may require both a groundwater calculation and an engineering judgment about capture, discharge, treatment, settlement, or receptor protection.
Transport Mechanisms
For a first screening calculation, contaminant movement often starts with seepage velocity:
v = K i / n_e
where K is hydraulic conductivity, i is hydraulic gradient, and n_e is effective porosity. Travel time is approximately L / v for a conservative dissolved constituent moving with groundwater. This is a screening estimate, not a full fate-and-transport model.
| Process | Effect on plume | PE WRE implication |
|---|---|---|
| Advection | Moves mass with groundwater flow | Use seepage velocity for travel time |
| Mechanical dispersion | Spreads the plume along flow paths | Concentrations arrive over a range of times |
| Diffusion | Moves mass from high to low concentration | Important in low-flow zones and clays |
| Sorption | Slows dissolved movement | Retarded contaminants move slower than groundwater |
| Decay or reaction | Reduces mass or changes form | May lower concentration with time or distance |
For sorbing contaminants, the retarded velocity is lower than groundwater velocity. A common conceptual form is contaminant velocity = groundwater seepage velocity / R, where R is a retardation factor greater than 1. If no retardation, decay, or dispersion data are given, do not invent them. Use the assumptions stated in the problem.
Dewatering Objectives
Dewatering is not just removing water. It must provide a stable, workable excavation while controlling impacts. A shallow trench in clean sand may be handled with sumps if minor seepage and erosion are manageable. A deep excavation below the water table in permeable sand may require wellpoints or deep wells. Fine silts may need eductors, close well spacing, or cutoff methods because they drain slowly. Contaminated groundwater may require collection, storage, treatment, and permitted discharge.
Method Selection
| Method | Best fit | Main risk |
|---|---|---|
| Sump pumping | Shallow, low inflow, stable soils | Piping, erosion, turbid discharge |
| Wellpoints | Shallow to moderate drawdown in permeable soils | Limited lift, many points needed |
| Deep wells | Larger drawdown or deeper excavations | Settlement and off-site drawdown |
| Eductors | Low-permeability silts or fine sands | Higher energy and setup complexity |
| Cutoff wall or sheet piles | Limit inflow and drawdown footprint | Leakage, basal heave, construction cost |
| Recharge wells or trenches | Protect nearby wells, wetlands, or structures | Mounding and water quality controls |
Decision Workflow
- Define the objective: lower water table, capture plume, reduce inflow, protect a receptor, or stabilize excavation.
- Estimate flow direction and magnitude using heads, K, gradient, and aquifer geometry.
- Identify receptors: wells, streams, wetlands, basements, utilities, slopes, and contaminated zones.
- Select a dewatering or containment method matched to soil permeability and excavation depth.
- Plan water handling: sediment control, sampling, treatment, discharge permit, or sanitary sewer approval.
- Monitor water levels, turbidity, settlement, and nearby receptor response during pumping.
- Adjust the system if drawdown is insufficient or off-site impacts appear.
PE WRE Judgment
A purely hydraulic answer can be incomplete when the problem mentions contamination, adjacent structures, or sensitive receptors. Pumping can spread a plume if extraction wells are placed without hydraulic capture. Dewatering can induce settlement in compressible soils by increasing effective stress. Discharging untreated pumped water to a storm drain can violate water-quality requirements. For construction sitework, pumping can also cause erosion, piping, and slope instability if water exits through the excavation face.
The exam-level approach is to pair the calculation with the controlling constraint. If the scenario is clean water in stable sand, rate and drawdown may control. If the scenario includes contaminated groundwater, discharge treatment and capture control may control. If the site is near buildings or utilities, settlement monitoring and limiting the drawdown footprint become central.
A conservative dissolved contaminant is 900 ft upgradient of a receptor. The aquifer has K = 30 ft/day, hydraulic gradient = 0.0015, and effective porosity = 0.25. Ignoring dispersion, decay, and retardation, what is the approximate travel time?
A deep excavation in silty sand will extend below the water table near an occupied building and a known dissolved-solvent plume. Which dewatering planning response is most appropriate?