One of the most common ways any type of material can lose its ability to repel water is if they are not cleaned on a regular basis. The same concept applies to groundsheets but thankfully, they are pretty easy to clean. No matter the main fabric or materials, they can be washed generally in the same fashion. Again, doing so is necessary and can extend the lifespan of the tarp.
To begin, wash off a wide area in which you can lay out the tarp. Once done, shake the tarp to remove any loose dirt and mud and then lay it down on the surface you cleaned. From there, pour a solution of water and mild dish soap onto the tarp. Then, take a brush to scrub the surface area and once done, take a garden hose and thoroughly rinse it off. To finish, allow it to air dry.