River pollution is a serious environmental issue.
River pollution is a serious environment issue.
These are not compound nouns, but syntactic constructions where the noun "issue" is modified by the noun "environment" and the adjective "environmental". The resultant nominals are in turn modified by the adjective "serious" to form the noun phrase "a serious environment(al) issue.
What is the molecular structure of water?
What is the molecule structure of water?
Similarly with this pair: the noun "structure" is modified by the adjective "molecular" or the noun "molecule" to form the noun phrases "molecular structure" and "molecule structure".
By contrast, compounds, or more precisely "morphological compounds" are written as single words such as "newspaper", "cowshed", "pillow-case", "life-guard", "chewing-gum", "pub-crawl", "record-player" and so on.
Note that compounds exclude modification of the first component, for example "a dark greenhouse" means a greenhouse that is dark", not a greenhouse that is painted dark green. Composites on the other hand can allow a wide range of modification, for example "a bright green car" is a car that is painted bright green.