He visited the site off the River South Esk just south of Newbattle Abbey College recently to see the issues for himself.
The debris which has made its way to the burn includes objects such as wet wipes and sanitary items, which have polluted and affected wildlife as well as put at risk the health of both people and animals.
Recently the Advertiser reported on similar pollution nearby on the River South Esk at Newbattle Viaduct.
The Scottish Environment Protection Agency (SEPA) is aware of this latest issue at the Mary Burn and is monitoring the clean-up by Scottish Water.
Mr Beattie (SNP) said: “To have the beauty of this rural gem polluted by sewage sediment and sanitary waste is of real cause of concern to the environment and to the effect it has on public health.
“Based on the number of people who have contacted me in regards to this issue, it is clear that the burn is in great usage from children or ramblers. It’s a great concern that the burn itself has put the entire ecosystem in danger, and it’s important to ensure these issues do not happen again.
“I understand from Scottish Water that a clean-up is now in progress. I’m seeking solid reassurances that this situation won’t reoccur.”
A Scottish Water spokesman said; “Since being made aware of the issue, we have had teams on site regularly to clean the local burns and have been inspecting our infrastructure to ensure the network is operating as it should be.
“We are now pro-actively carrying out inspections of the local area and carry out cleaning as necessary.
We will also be continuing to engage with the community via the MSP and local community council.”
Iain Cruickshank, area manager at SEPA, said: “We received a report from a member of the public on September 16. We have subsequently been on site with Scottish Water and witnessed the clean-up in action. This is expected to be ongoing into early October, alongside work to undertake sewer network surveying and cleaning.
“We will re-visit on completion to ensure the work is completed to the required standard. We are clear that compliance with environmental regulations is non-negotiable.
“We are continuing to investigate the circumstances surrounding this incident and the impact it has caused. We encourage the public to contact our Pollution Hotline on 0800 80 70 60, or use our online reporting form at www.sepa.org.uk/report, to report their concerns.”