A water company is facing legal action, after a stomach bug outbreak hit parts of Gwynedd and Anglesey last year.
The reservoir is the "probable source" of the outbreak
The cryptosporidium bug affected more than 230 people and led to thousands of households having to boil their drinking water for two months.
Lawyers acting for 31 people are starting legal proceedings against water company Dwr Cymru Welsh Water.
The company has said it will consider an official report into the outbreak due this month before responding.
A total of 70,000 households in Anglesey and Gwynedd were told to boil their drinking water or take bottled supplies when the outbreak first emerged last November.
Water restrictions were not lifted for some customers until two months later.
The source of the outbreak has never been identified, although investigations pointed to the source being the Cwellyn reservoir in Snowdonia.
Lawyers acting on behalf of the 31 people said a year on from the outbreak four of them were still ill.
Solicitor Mike Neale said he had written to both Welsh Water and the Drinking Water Inspectorate almost a year ago and because his clients' questions have been answered, they have issued legal proceedings.
He said: "They want some answers. The clients were quite ill as result of the cryptosporidium. We are acting on behalf of some elderly people and some very young children."
The compensation could range from £1,000 to £9,000 plus possible additional claims for loss of earnings, Mr Neale said.
Daniel Evans from Deiniolen is one of those taking legal action. A year on from the outbreak he is still ill with depression and irritable bowel syndrome and cannot work.
"We've been waiting for this report six months, " said Mr Evans.
"I can't understand why the inspectorate won't issue any information on this matter
"If Welsh water are to blame, why cover it up, you know. Come clean.
"Enough's enough, I've been through a nightmare year. I feel like I've lost a year out of my life and I've lost just over £17,000 in wages alone as well as all the distress and heartache it's caused."
Caernarfon MP Hywel Williams said: "Obviously this is a serious outbreak and in the backs of many people's minds, I'm sure, is what would happen should there be something like this again.
"I hope there will not be, of course. And I know Welsh Water have taken substantial and expensive steps to make sure it doesn't happen again.
"But we need to get to the bottom of this as soon as possible."
In response to the action, Welsh Water said they expected a full report to be published by the outbreak control team in coming weeks.
"We will be considering it and will make a response on this matter shortly after," a statement read.
A spokesperson for the National Public Health Service for Wales said the report into the outbreak was a "substantial piece of work".
"It's been important to get it right and it has involved a number of different organisations working together. We're coming up to publishing it imminently."