By Karen Argenti

City Comptroller William Thompson provided many of us with a fresh drink of water with his audits of the Croton Water Filtration Plant, which is one of Mayor Michael Bloomberg’s biggest boondoggles and now confirmed as one of the biggest frauds in the city’s history.

Many following this scandal closely were not the least bit surprised at Mr. Thompson’s finding that the timing and planning have been bungled, nor did we blink an eye at unjustifiable cost overruns into the billions. We’ve known this and have said it repeatedly all along.

But there’s a missing piece in the comptroller’s report and that is to answer the fundamental questions of why? Why were the Department of Environmental Protection’s calculations so far off? Though DEP incompetence is legendary, former Commissioner Chris Ward’s misguided insistence that the plant should be put in the Bronx because it was the cheapest place to build by hundreds of millions of dollars cannot be ignored.

There was a fascinating — and largely overlooked — revelation in the report. That is the creation of a new Cost Estimating Division of the Bureau of Engineering and Construction that has one function: conduct regular meetings between the DEP, other city agencies and the project engineers and contractors.

The comptroller needs to go the next step and fully investigate the relationship between Mr. Bloomberg’s DEP and those who gained most from the experimental underground design in Van Cortlandt Park — the diggers and dirt haulers and the contractors, consultants and engineers who would not have been needed had the plant been built on top of the ground in Westchester.