The ongoing problem with groundwater contamination at the Indian Point nuclear power plants is apparently heading to court. The environmental group Riverkeeper will announce plans to file a federal lawsuit Tuesday claiming Entergy is illegally polluting the Hudson River. A small leak of radioactive water from Indian Point Two's spent fuel pool led to the discovery of radioactive contamination in groundwater under the Indian Point complex. Most of the contamination is radioactive tritium and considered one of the least dangerous radioactive substances. Tests of the groundwater, however, have also detected the more dangerous element strontium-90. Public health officials say the amount of contamination is relatively small and does not appear to pose an immediate threat to the public. A spokesman for Entergy declined to comment, saying the company first wants to see what specific claims Riverkeeper makes in its federal lawsuit.