In another slum, we meet a middleman who is openly collecting money as our cameras roll. When we ask where the water comes from, he says, "Can't say that. You have to ask the boss," although he wouldn't tell us who his boss was.
But Parvin and the families in her slum have been told their illegal water comes direct from the municipal water supply, known as WASA -- in their case, from a local WASA substation just a few hundred yards down the road.
The middleman from Parvin's slum admits the same thing in an interview: "The water commissioner is giving us these two lines, and from these lines everyone gets water. It comes from WASA."
But when we interview the manager of the WASA substation, he professes total ignorance.
"People are going in and putting in the pipes themselves," he protests. "The office isn't doing it. It is 'system loss.'"
"System loss" is the term WASA officials use for the water that gets siphoned off by illegal means. But critics say "system loss" is lining the pockets of corrupt officials all over Dhaka.
Editorials in Dhaka newspapers periodically call for investigations, but according to Carel de Rooy, not much changes.
"The solutions are there -- the question now is to obtain the political will from the government to actually address the slum areas seriously," he said.
Non-government organizations (NGO's) like UNICEF, WaterAid, and DSK have been able to persuade WASA to install legal metered water lines in a small number of Dhaka's 5,000 slums -- cutting out the possibility of corruption, and putting the middlemen in those slums out of business. But widespread implementation of this policy is still a distant dream, according to Shafiqur Rahman of WaterAid. The sale of illegal water is just too lucrative.
According to a 2010 Bangladeshi government report, system loss in Dhaka now stands at a staggering 38 percent of total daily capacity -- in a city that already faces acute water shortages. Last April, government troops were called out to protect WASA stations from public mobs protesting corruption and contaminated water.
And the results of slums without safe water can be deadly. "20/20" interviewed six women who have recently lost children to diarrhea and other waterborne diseases. Five had no choice but to buy illegal water from middlemen and in each case, samples taken from their water sources by "20/20" tested positive for E. coli.
Dr. Steve Luby, a research scientist at the International Center for Diarrheal Disease Research, says that the slums of Bangladesh represent a window into our own future; a future where water becomes a scarce and precious commodity.
"We need to learn as a species," says Dr. Luby, "how we're gonna manage this scarce resource. And in some ways, we ought to think about our groundwater like we think about our oil, because it's more like a nonrenewable resource."
The "Be the Change: Save a Life" initiative is supported in part by the Bill & Melinda Gates Foundation.