How does water move from the ground surface into an aquifer?

Not always. Some precipitation infiltrates—enters the soil or rock—but it can remain in shallow soil, move downward, move horizontally through subsurface material, or continue deeper and recharge groundwater in an aquifer. Some infiltrated water can also return to the atmosphere. So “rain goes straight down” is a useful first sketch, not a guaranteed route for every drop. [S1]

Here’s a labeled, simplified cross-section. The arrows show one possible path, not a map of every location:

                     precipitation
                    ↓    ↓    ↓
GROUND SURFACE  ─────────────────────
                  ↓ infiltration
UNSATURATED       soil and rock spaces
MATERIAL          are not filled with water
                  ↓       ↘ sideways movement
                  ↓
SATURATED ZONE    spaces between soil and
                  rock particles filled with water
                  ↓ possible recharge
AQUIFER           groundwater-storage area

Below the unsaturated zone, water fills spaces between soil and rock particles; this is the saturated zone. [S1] The sketch shows a general pathway, not a particular site or a precise boundary depth.

The unsaturated material is above the saturated zone. In the saturated zone, water fills the voids between soil and rock particles. [S1] In this simplified diagram, water that continues down and recharges groundwater in an aquifer has reached groundwater storage. But infiltration alone does not mean water has reached an aquifer: some stays shallow, moves sideways, or returns to the atmosphere. [S1]

The material matters. Sandy soils tend to absorb water more readily than clay soils, which tend to absorb it more slowly and in smaller amounts. Saturation, slope, vegetation, and precipitation also affect infiltration. [S1] Farther underground, permeability matters: interconnected spaces or cracks allow water to move through material, while movement through low-permeability materials such as clay and shale is very slow. Movement can vary greatly from place to place. [S2]

For instance, imagine two nearby patches after precipitation. You might sketch one with an arrow continuing down through more permeable material and another with a shorter or slower arrow. This illustrates how material can affect movement; it does not establish what happens at a particular site or how quickly water travels there.

Use labels and a note that makes the variability clear:

PRECIPITATION
      ↓
GROUND SURFACE
      ↓ infiltration
UNSATURATED SOIL AND ROCK
      ↓ possible downward movement
SATURATED ZONE: spaces filled with water
      ↓ possible recharge
AQUIFER: groundwater-storage area

Note: Some infiltrated water stays shallow, moves sideways, or returns
 to the atmosphere. Routes and rates vary with local conditions.

The general pathway and alternatives in the note are described by the USGS. [S1] Permeability affects groundwater movement, so the downward arrows show possible movement, not a promise that every kind of soil or rock transmits water equally. [S2] These sources explain general processes; they do not establish the geology or water movement at a specific location.