Final answer:
Moisture among sediment grains reduces compaction by acting as a lubricant that helps maintain space between the grains, preventing them from being packed closely together and preserving porosity during the lithification process.
Step-by-step explanation:
The question pertains to the effects of moisture on sediment grains within the geological context of groundwater and sedimentary rock formation. Specifically, it asks what happens when moisture is present amongst sediment grains. In this case, moisture among sediment grains reduces compaction.
This occurs because the presence of water within the sediments can act as a lubricant, allowing the grains to maintain their space apart from each other, thus reducing the effectiveness of compaction which would otherwise bring grains closer together and decrease the porosity. This concept aligns well with the lithification process, wherein sediments are compacted and water is expelled from pore spaces, with minerals precipitating into the voids, forming 'cement' that binds the grains into sedimentary rock.