A well pumps a confined or unconfined aquifer at a constant rate Q. Drawdown around it follows the Cooper–Jacob straight-line approximation of the Theis well equation, valid once pumping has run long enough for the well function's early-time terms to become negligible:
s(r,t) = 2.30·Q / (4πT) · log10( 2.25·T·t / (r²·S) )
R(t) = √( 2.25·T·t / S ) (radius of influence)
T is transmissivity, S is storativity, r is distance from the well and t is pumping time. A confined aquifer has a much higher T and a far lower S than an unconfined one, so for the same Q it draws down less at the well but the cone of depression spreads faster and farther.
As the water table (or piezometric head) at the well drops, the Ghyben–Herzberg relation says the freshwater–saltwater interface below it rises roughly 40× as fast as the head falls: z = 40·h, where h is the freshwater head above sea level. Once that interface reaches the well screen, saline water is drawn in and the wellhead TDS jumps toward seawater levels — the classic saltwater-intrusion failure mode of coastal groundwater desalination.
- Cone of depression — the drawdown curve s(r,t) plotted across distance from the well, redrawn every frame from the live head.
- Saltwater wedge — rises from below as the freshwater lens margin (screen depth − interface depth) shrinks toward zero.
- Artificial recharge — injecting water back into the aquifer effectively halves the stress driving drawdown, letting a wellfield sustain a higher Q without breaching the lens.