| Material | K (m/day) | i | ne | v (m/day) |
|---|---|---|---|---|
| Fine sand | 5.0 | 0.010 | 0.30 | 0.167 |
| Coarse sand | 30.0 | 0.005 | 0.28 | 0.536 |
| Silty sand | 1.2 | 0.020 | 0.25 | 0.096 |
| Gravel | 150.0 | 0.003 | 0.22 | 2.045 |
- i = Δh / L (hydraulic gradient; unitless).
- q = K · i (specific discharge, Darcy flux).
- v = q / nₑ (seepage or pore-water velocity).
- vₛ = v / R (solute velocity with retardation factor R ≥ 1).
- K = k · ρ · g / μ (compute conductivity from intrinsic permeability and fluid properties).
- Select an input mode: enter K, or compute K from k, ρ, g, μ.
- Provide a direct gradient i, or enter Δh and L.
- Enter effective porosity nₑ. Use field-effective values where possible.
- Optionally set retardation R and a distance to estimate travel time.
- Press Submit. Review results above, then download CSV or PDF.
Field Inputs That Control Velocity
Groundwater velocity depends on hydraulic conductivity, hydraulic gradient, and effective porosity. The calculator separates Darcy flux q from seepage velocity v, so you can see how low nₑ amplifies pore-water speed even when q is modest. Use measured Δh over a defensible flow length, not well-to-well spacing alone, and document each assumption. In layered media, consider directional K and note whether you used horizontal or vertical conductivity for the flow path.
Typical Ranges by Aquifer Material
Hydraulic conductivity commonly spans orders of magnitude across sediments. For planning, fine sand can be around 1–10 m/day, coarse sand 10–100 m/day, and clean gravel 100–1000 m/day. Silts and clays are far lower, often below 0.01 m/day. Pair these ranges with gradients such as 0.001–0.05 to bound expected velocities before sampling. Site-specific values should come from slug tests, pumping tests, or lab permeameters whenever possible.
Unit Consistency and Conversion Checks
Mixed units are a frequent source of error. This tool converts K, lengths, density, and viscosity to consistent SI bases internally, then returns results in your chosen output unit. When entering permeability, confirm the Darcy or millidarcy scale and keep viscosity in cP or Pa·s. A quick check is that K computed from k increases with density and gravity, and decreases with viscosity. Compare the reported K (m/day equivalent) with your expectation to catch unit slips early.
Interpreting Travel Time Outputs
Travel time estimates help screening and reporting. After you enter a distance, the calculator computes time for water using v and for a retarded solute using vₛ = v/R. Interpret these times as average advective indicators, not guarantees. Heterogeneity, dispersion, preferential pathways, and seasonal gradients can shorten or lengthen arrival relative to a single, steady-state estimate. Choose R conservatively when sorption data are limited.
Reporting and Quality Control Practices
To improve defensibility, run sensitivity cases. Hold K constant and vary i and nₑ within realistic limits to see how results shift. Save CSV outputs for peer review and attach the PDF report to field notes. If results look extreme, revisit data sources, verify gradient sign, and ensure porosity represents mobile pore space, not total porosity. Record fluid temperature, because viscosity changes can alter computed K in the permeability mode.
What is the difference between q and v?
q is the Darcy flux based on bulk area, while v is pore-water velocity. v divides q by effective porosity, so it is larger than q when nₑ is less than one.
Should I use total porosity or effective porosity?
Use effective porosity, representing connected pore space that actually transmits flow. Total porosity can include immobile pores and will often understate seepage velocity when substituted for nₑ.
When should I enter a direct hydraulic gradient?
Enter direct i when it was computed from a mapped potentiometric surface or a calibrated model. Use Δh and L when you have two heads and a defensible flow length along the suspected path.
How is K computed from intrinsic permeability?
In permeability mode, the calculator uses K = k·ρ·g/μ. Provide k, fluid density, viscosity, and gravity. Higher density or k increases K, while higher viscosity decreases it.
What does the retardation factor affect?
Retardation affects only solute transport. The calculator reports v for water and vₛ = v/R for a retarded solute, so larger R increases travel time without changing the water velocity.
Why can velocities seem unrealistically high?
High velocities usually come from large gradients, very low effective porosity, or a unit mismatch on K or length inputs. Check that i is unitless, K units are correct, and L is not accidentally entered too small.