Roger Bould, of Waikanae, asks :-

When I walk along the beach at low tide, each time I put my foot down, the sand around it goes drier than it was before my foot landed. Logic makes me believe my weight would compress the sand particles. This would force the water out from between them. So why don’t I see water around my foot? I noticed that when I lifted my foot again, the footprint I left behind was watery.

Peter King, a petroleum geologist with GNS Sciences, responded.

Your are correct that your weight has initially compressed the sand grains and displaced the water sideways, by virtue of a local pressure gradient that you have created. Initially, the pores (gaps) between the grains would have been filled with a mix of air and sea water (residual from when the tide had been in and then retreated).

The water was able to be displaced into surrounding gaps to take the place of air between those gaps, primarily because the air in its turn would have been partly compressed (within the pores) and partly pushed out at the surface. The latter effect occurs because there is no seal or cap to hold the air in, the beach surface is simply exposed to the atmosphere.

When you step away, the pressure from your weight is released. This allows the local pressure gradient to re-equilibrate, and the water to reverse its lateral (sideways) movement back towards your footstep. However, in the meantime you have compacted the sand, which means that there is no longer the same air space available between the grains. As a result, the water flowing back momentarily ponds in the compacted footstep.

Nevertheless, the sand is not completely consolidated, which means that it still remains relatively permeable (to the flow of water from pore to pore). So soon afterwards the ponded water seeps back into the surrounding sand, in part simply through gravity.