Translation:This land is being used to make a polder.
"There are a few of these in the Netherlands" LOL…only some 3000 or 4000.
Keep in mind that polders like the 3 that make up the province of Flevoland (Oostelijk Flevoland, Zuidelijk Flevoland and the Noordoostpolder) are really big ones, in the range of 500 square kilometres. Other big and well-known ones are the Haarlemmermeer (a former lake now containing Airport Schiphol) of around 180 km2, the Wieringermeer, the Schermer and the Beemster (the definite article de is always put in front of polder names). But there are lots and lots of small ones, with sizes like 300 by 800 metres. Especially the smaller ones usually have very irregular shapes and curvy dijken (levees) around them (with usually roads on the levees)…the curves are often caused by former breeches in the levee and rebuilding the new levee around it.
That is accepted. Problem is that the dutch sentence is "bad". It is not clear what is happening, usually one makes a polder out of water, or swampy area, but if it is already land, what are they planning to do? It would make more sense to make a sentence like: "This lake is changed into a polder". Or this water is partly used to make a polder.