The Charles River has a long his­tory of pol­lu­tion — Boston’s anthem, “Dirty Water,” was first recorded more than forty years ago. But taking a dip was common prac­tice until the 1950s, when people started to realize that indus­trial waste and sewage runoff were making swim­ming hazardous.

Since joining the North­eastern fac­ulty in 2004, envi­ron­mental engi­neering asso­ciate pro­fessor Ferdi Hell­weger has been working to make the river swim­mable again. Ear­lier this month, the New Eng­land Region of the Envi­ron­mental Pro­tec­tion Agency hon­ored him with an Envi­ron­mental Merit Award for his effort.

Today, after decades of work by many people to clean up the Charles, the river is swim­mable approx­i­mately 70 per­cent of the time.  But, Hell­weger explained, the problem is pin­pointing that time.

I can go out now and col­lect a water sample, but I have to filter it, incu­bate it and wait 24 hours before I know it’s good. If I’m inter­ested in swim­ming, I want to know if it’s good now,” he said. “We’re trying to find a way to pre­dict where and when it is safe to go swimming.”

Hell­weger said two main cul­prits pre­vent swim­ming in the river the remaining 30 per­cent of the time: fecal bac­teria like E. coli and toxic algae.

Fecal bac­teria, he said, orig­i­nate in sewage, which can enter the river because of the way Boston’s sewer system was orig­i­nally designed. Unlike in modern sys­tems, there is one set of sewer pipes that move both rain­water and san­i­tary sewage to a treat­ment facility. On rainy days, those pipes would over­flow into the river. Today many pipes have been sep­a­rated, but some remain com­bined and many illicit sewage con­nec­tions endure.

Toxic algae, specif­i­cally cyanobac­teria, grow in the river and pro­lif­erate when the tem­per­a­ture and nutrient con­di­tions are just right. On very hot days, algae take over.

Hell­weger is working to model the Charles’ water quality with the even­tual goal of devel­oping a com­puter pro­gram or appli­ca­tion that can pre­dict how swim­mable the river will be from one day to the next.

The vision is to have a weather fore­cast for the river,” he explained. “The idea is you go to a web­site for a weather fore­cast, and you see not only the tem­per­a­ture for tomorrow but also whether you can swim at the beaches.”

In other cities such as Zurich and Bern, swim­ming in urban lakes and rivers is part of the cul­ture. Hell­weger believes that the Charles River is an under­uti­lized but impor­tant resource with great poten­tial, noting, “If we could swim in the river, it would tremen­dously increase the quality of life in the city.”

Gov­ernor Patrick has ini­ti­ated a com­mis­sion to study the water quality with the hope of opening the river to swim­ming within five years. If suc­cessful, the Charles would be the first urban river in the country to wel­come swim­mers within the last sev­eral decades.