Plant operator Tokyo Electric Power Company, or TEPCO, reported the results of the operation so far at a meeting of experts at the industry ministry on Friday.
TEPCO began the so-called groundwater bypass operation in May. It involves draining water from wells and releasing it into the sea to keep it from flowing into reactor buildings and becoming contaminated.
The utility had said the operation would reduce the amount of highly radioactive water created this way by up to 100 tons per day, down from roughly 400.
But TEPCO officials told the meeting that in the past 2 months, water levels at 3 wells located 70 to 150 meters from the reactor buildings dropped by only around 10 centimeters at most.
Experts voiced concerns about the limited effects of the operation. Others pressed TEPCO to come up with concrete data, saying that fishermen in Fukushima accepted the plan because they expected results.
The utility says rain is partly to blame for the limited effects of the operation. It says it plans to cover soil near the wells with asphalt by the end of next March, to keep rain from seeping into the ground.