There are three options you can use as a solution:
Extrude the bottom of your sand plane and add a boolean
Select the outside Edge Loop of your plane by
Left Mouse Button on the outside vertex or edge. (If it doesn't select the whole loop do this for all four sides by pressing
Shift to keep the previous loop selected). Then Extrude down on the Z-Axis by pressing
Z (to lock to Z-Axis) on your keyboard. Then once you have extruded to the bottom plane press
0 to flatten it. Depending on which object you don't want to intersect to (if you don't want the sand to intersect with the bottom plane add the boolean modifier on the sand and vice versa if you don't want the bottom object to intersect with the sand) add a boolean modifier on the specific object of your choice (see my comment in the brackets) and set the specific object of your choice (see my comment in the brackets) as the Target Object.
The Soldify Modifier
You can add a solidify modifier to your sand plane and play with the Thickness and Offset to get your desired result.
If you don't want the objects to intersect, follow the same boolean procedure as above. Also, if you want to get the same solidify result, place the solidify modifer on the top in all of the modifier stack.
The Boolean Modifier
You can add a boolean modifier on the bottom object, select the sand plane as the Target Object, and set the boolean to Intersect.