11.4 Contaminant Transport and Dewatering Decisions
Key Takeaways
- Groundwater contaminant travel time starts from seepage velocity v = K i / n_e, not Darcy flux alone, and travel time is roughly L / v.
- Advection moves the plume with groundwater, dispersion spreads it, sorption retards it, and decay or reaction reduces concentration.
- A sorbing contaminant moves at v / R, where the retardation factor R is greater than 1, so it lags conservative tracers.
- Dewatering method choice depends on soil permeability, depth, inflow, and risk: sumps, wellpoints, deep wells, eductors, cutoff walls, and recharge systems.
- Lowering the water table increases effective stress and can settle compressible soils, while discharging untreated pumped water can violate water-quality permits.
Transport and Dewatering Decisions
The PE Civil WRE specification separates Groundwater and Wells from water-quality topics, but real exam scenarios connect them. A leaking tank, landfill cell, construction excavation, utility trench, or pump-station site can require both a groundwater calculation and engineering judgment about capture, discharge, treatment, settlement, or receptor protection.
Transport Mechanisms
A first screening calculation begins with seepage velocity v = K i / n_e, where 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 calibrated fate-and-transport model, and the exam expects you to label it as such.
| Process | Effect on plume | WRE implication |
|---|---|---|
| Advection | Moves mass with groundwater flow | Use seepage velocity for travel time |
| Mechanical dispersion | Spreads plume along flow paths | Arrival spread over a range of times |
| Diffusion | High-to-low concentration migration | Matters in low-flow zones and clays |
| Sorption | Slows dissolved movement | Retarded mass lags the groundwater |
| Decay or reaction | Reduces or transforms mass | Concentration drops with time/distance |
For a sorbing contaminant, the retarded velocity is groundwater seepage velocity divided by the retardation factor R, which exceeds 1. With no retardation, decay, or dispersion data given, do not invent them; use the stated assumptions and treat the result as conservative-tracer travel time.
Dewatering Objectives
Dewatering is not merely removing water. It must deliver a stable, workable excavation while controlling impacts. A shallow trench in clean sand may need only sumps if minor seepage and erosion are manageable. A deep excavation below the water table in permeable sand may need wellpoints or deep wells. Fine silts drain slowly and may need eductors, close spacing, or cutoff methods. 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, permeable soils | Limited lift (about 15-20 ft), many points |
| Deep wells | Larger drawdown or deeper cuts | Settlement, off-site drawdown |
| Eductors (ejectors) | Low-permeability silts, fine sands | Higher energy, setup complexity |
| Cutoff wall / sheet piles | Limit inflow and drawdown footprint | Leakage, basal heave, cost |
| Recharge wells / trenches | Protect nearby wells, wetlands, structures | Mounding, water-quality controls |
Decision Workflow
- Define the objective: lower the water table, capture a plume, reduce inflow, protect a receptor, or stabilize the excavation.
- Estimate flow direction and magnitude from 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 depth.
- Plan water handling: sediment control, sampling, treatment, discharge permit, or sanitary-sewer approval.
- Monitor water levels, turbidity, settlement, and receptor response during pumping.
- Adjust the system if drawdown is insufficient or off-site impacts appear.
Engineering Judgment
A purely hydraulic answer is incomplete when contamination, adjacent structures, or sensitive receptors appear. Pumping can spread a plume if extraction wells lack hydraulic capture. Dewatering can settle compressible soils by increasing effective stress, since lowering the water table removes buoyant support. Discharging untreated pumped water to a storm drain can violate water-quality rules. In construction sitework, pumping can also drive erosion, piping, and slope instability where water exits the excavation face.
The exam-level move is to pair the calculation with the controlling constraint. Clean water in stable sand: rate and drawdown control. Contaminated groundwater: capture and treatment or permitted discharge control. Near buildings or utilities: settlement monitoring and a limited drawdown footprint become central. The defensible choice is rarely the simplest pump method by convenience.
Capture Zones and Permitting
A single extraction well captures groundwater within a limiting flow envelope. The maximum width of the capture zone far upgradient is Q / (K i b) and the down-gradient stagnation point sits at x = Q / (2 pi K i b), where b is saturated thickness. If the plume is wider than the capture width, one well cannot contain it and additional wells or a higher rate are required. This is a common judgment item: the calculation shows the plume escaping the capture envelope, and the right answer adds capacity rather than accepting incomplete capture.
Discharge of pumped groundwater is also regulated. Clean construction water may go to a storm drain only under a construction general permit with sediment controls; contaminated water typically needs treatment, a National Pollutant Discharge Elimination System (NPDES) permit for surface discharge, or a local sanitary-sewer discharge authorization. The exam rewards recognizing that the discharge pathway, not just the pump, governs a contaminated-site plan.
Worked Example
A capture well pumps 20,000 ft^3/day from an aquifer with K = 50 ft/day, i = 0.004, and b = 25 ft. The far-field capture width is Q / (K i b) = 20,000 / (50 x 0.004 x 25) = 20,000 / 5.0 = 4,000 ft, and the stagnation point lies x = 20,000 / (2 pi x 5.0) = 637 ft down-gradient. A 600 ft wide plume is captured; a 5,000 ft wide plume is not, so a single well would be undersized.
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?