Free Online Well Capture Zone Calculator: Stagnation Point and Width

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The stagnation point sits downgradient of the well; nothing beyond it is ever captured. The zone widens upgradient toward its maximum width. These three lengths, from four inputs, are the skeleton of every wellhead protection area.

The envelope a well actually drinks from

Drop a pumping well into an aquifer with regional flow and a precise envelope forms around it: water inside will eventually arrive at the well, water outside slides past forever. The tool above computes that envelope from four numbers, pumping rate, hydraulic conductivity, aquifer thickness, and regional gradient, and reports its three defining lengths: the stagnation point downgradient, the capture width at the well, and the maximum width far upgradient. These are the analytical capture zone equations used to draft wellhead protection areas the world over.

Plan view of a pumping well in regional groundwater flow showing the capture zone envelope, stagnation point downgradient, and maximum capture width upgradient
The capture zone: three lengths from four inputs draw the envelope of every drop that will ever reach the well.

This tool is part of the Hydrogeology and Water Suite in the bar above. Transmissivity from the pumping test tool divided by thickness gives the conductivity this calculator needs.

Reading the three lengths

  • The stagnation point is the downgradient limit: a spill beyond it cannot reach the well no matter how long you wait.
  • The maximum width is the upgradient limit: sources outside that corridor belong to some other well or discharge zone.
  • Everything scales linearly with pumping rate, which is why cutting Q is the cheapest remediation lever there is.
  • The equations assume a uniform confined aquifer at steady state; strong heterogeneity or partial penetration calls for a numerical model, with these numbers as its sanity check.

What this tool is for

Drafting first-pass wellhead protection zones, screening whether a contaminant source threatens a supply well, and teaching the elegant superposition of a sink on uniform flow. Time-of-travel capture zones, the 1-year and 10-year contours regulators often want, add the travel time arithmetic from the Darcy flow tool in this suite.

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