Hydraulic Conductivity & Darcy's Law Calculator (Groundwater)
Pick a soil type or enter K, add the gradient from two wells, and get Darcy velocity, seepage velocity, and how far the plume front moves in a year.
Darcy velocity spreads flow over the whole cross-section, pores and grains alike. Water only travels through the pores, so the real speed is Darcy divided by effective porosity, typically 4x faster. A plume estimate built on Darcy velocity underpredicts how soon contamination reaches the property line.
Related calculators
Hydraulic conductivity by soil type
The dropdown's reference values, with the ranges behind them. Site-specific slug or pump test data always beats a table value; the table is for scoping before that data exists.
| Material | Typical K (ft/day) | Common range (ft/day) | Typical effective porosity |
|---|---|---|---|
| Gravel | 1000 | 100 to 10,000+ | 0.25 |
| Coarse sand | 100 | 10 to 1,000 | 0.27 |
| Fine sand | 5 | 0.5 to 50 | 0.25 |
| Silt | 0.05 | 0.001 to 1 | 0.2 |
| Clay | 0.0001 | under 0.001 | 0.1 |
Layered reality beats any single row. A UST site with 30 feet of clay over a sand seam does not behave like the average of the two; the sand seam carries essentially all the flow.
Darcy's law, in plain language
The equation is q = K times i. Flow per unit area (q, the Darcy velocity) equals how permeable the ground is (K) times how steeply the water table slopes (i, the hydraulic gradient). Henry Darcy worked it out testing sand filters for Dijon's water supply in 1856, and every groundwater model since is elaboration on his one-liner.
Groundwater is lazy. It moves downhill, slowly, through whatever lets it.
For contaminated sites the law does three jobs. It converts a measured gradient and a tested K into a flow estimate. It predicts which way and how fast a dissolved plume migrates from a release. And it sizes remediation hydraulics, because a pump-and-treat well has to outcompete the natural flow Darcy describes. The law holds for the slow, laminar flow of real aquifers; it breaks down in karst conduits and open fractures, where water behaves more like a pipe than a sponge.
Getting the hydraulic gradient from two wells
Gradient is water-level drop divided by distance. Measure depth to water in two wells, convert to elevations using the surveyed top-of-casing datums, subtract, and divide by the distance between the wells. Two wells 200 feet apart with a 2 foot head difference give i = 0.01, a common magnitude at UST sites (flat terrain runs 0.001, hillside sites a few hundredths). The calculator's two-well mode does this division for you.
Two wells define a slope only along the line between them. The true flow direction needs three or more wells and a triangulated water table map, which is exactly why site investigations install monitoring well networks instead of pairs. A two-well gradient pointed the wrong way across a plume can be off by half or more, so treat pair math as reconnaissance.
Darcy velocity vs seepage velocity: why porosity matters
Darcy velocity imagines flow smeared across the whole aquifer face, solid grains included. Actual water threads through the connected pores, which are only a fraction of the volume (the effective porosity, around 0.25 for sands, down toward 0.1 for clays whose pores barely connect). Divide Darcy by that fraction and you get seepage velocity, the speed at which a molecule of dissolved benzene actually advances. Same aquifer, same data, a 4x difference in the answer.
Seepage velocity still is not the whole plume story. Sorption retards most petroleum compounds below water speed, dispersion smears the front, and degradation eats the edges. Those refinements need site data and a modeler; the seepage number from this page is the honest first bound, and the travel line under the result states it as such.
Field glossary: the terms that show up in the report
Slug test
Single-well K measurement: displace the water level suddenly, record the recovery, fit the curve. Cheap, fast, and local to the well screen; expect scatter across a site and treat any single result as one data point.
Transmissivity
K times saturated thickness: what the whole aquifer can move, not just a unit cube of it. Two aquifers with identical K differ enormously if one is 5 feet thick and the other 50. Pump-test analyses report transmissivity first and back out K second.
Storativity
How much water an aquifer yields per unit head drop, the companion number transmissivity needs for pumping predictions. Water-table aquifers store in the pores themselves (values around 0.1 to 0.3); confined aquifers squeeze water from compression and run thousands of times lower.
Perched water table
A lens of groundwater sitting on a shallow low-K layer above the regional water table, with dry soil between them. UST releases love perched zones: the first "groundwater" a tank pull encounters may be a perched lens whose flow direction has nothing to do with the regional aquifer, a distinction worth catching before anyone draws plume arrows.
Frequently Asked Questions
What is hydraulic conductivity?
A measure of how easily water moves through soil or rock, written K and expressed here in feet per day. Gravel passes water at hundreds to thousands of feet per day; clay passes it at a fraction of an inch per year. That million-fold range is why K dominates every contaminated-site decision: the same gasoline release sits nearly still in clay for decades but rides a sand aquifer toward the neighbor's supply well in months. K is a property of the material, not the water, and at real sites it varies layer by layer, which is why single-number answers deserve suspicion.
How fast does groundwater move?
Usually inches to a few feet per day, and sometimes far less. Take a typical UST-site sand with K of 10 ft/day, a gradient of 0.005, and porosity of 0.25: Darcy velocity is 0.05 ft/day, and seepage velocity is 0.2 ft/day, roughly 73 feet per year. Silts and clays move orders of magnitude slower; clean gravels can genuinely flow feet per day. Dissolved plumes lag the water itself because contaminants adsorb to soil grains (retardation), so the water speed is the ceiling on plume speed, not the guarantee.
What is the difference between Darcy velocity and seepage velocity?
Darcy velocity is discharge spread over the entire cross-section of aquifer, solids included, which makes it an accounting number rather than a physical speed. Seepage velocity divides Darcy by effective porosity to reflect that water only moves through the connected pore space, and it is the speed a tracer, or a benzene molecule, actually travels. With porosity at 0.25, seepage runs four times Darcy. Using Darcy velocity in a plume travel estimate is the classic error, and it always errs in the comfortable direction, which is the wrong direction to err.
What is a slug test?
A quick field method for measuring K in a single monitoring well. Drop or remove a solid cylinder (the slug) to suddenly change the water level, record the recovery with a pressure transducer, and fit the recovery curve (Bouwer-Rice and Hvorslev are the standard analyses) to get K near the well screen. Slug tests are cheap and disturb nothing, which makes them the default at UST sites. Their limit: they sample a small volume around one well, so a site's slug-test K values commonly spread across an order of magnitude, and pump tests remain the tool when a defensible site-wide K matters.
Screening tool, not for regulatory submittals. Real sites are heterogeneous and anisotropic; table K values span orders of magnitude; and plume behavior involves retardation, dispersion, and degradation this page does not model. Numbers destined for a corrective action plan need site-measured K, a proper gradient map, and a qualified professional's stamp.
Soil K ranges and the 0.25 default effective porosity were compiled by USTContractors.com from standard hydrogeology references. Last verified July 2026.
Authoritative references:
When the scoping math says the plume is moving, the next call is a remediation contractor or environmental drilling crew for real site data. You can also request quotes for site investigation work.
Spot an error? Report it and we will check the math against the sources.