Sewage pollution alerts were in place at Jacksons Bay, Whitmore Bay, Watch House Bay, and Cold Knap in Barry following a sewage discharge which started at 2.46am on Monday, August 31.
A sewage discharge was also recorded at Penarth Beach on the same day at 12.59am.
The alerts, which were issued by Surfers Against Sewage, blamed Welsh Water for the discharges.
The alerts have now been lifted at the affected sites, according to Surfers Against Sewage.
Welsh Water explained that following the heavy rain and yellow weather warnings, storm overflows were used to stop flooding people’s homes, and that the spills were made up of approximately 95% rain water.
A Welsh Water spokesperson said: “Over the weekend, during a yellow weather warning for thunderstorms, parts of Wales saw heavy downpours.
“As a result, some storm overflows operated as designed and spilled to prevent local networks from becoming overwhelmed and flooding people’s homes.
“These spills were relatively short in duration – for example Cold Knap for three minutes.
“Due to the volumes of water in our network during downpours the spills are heavily dilute, around 95% rainwater.
“However, we know how important water quality is to our customers, which is why we are investing £2.5bn on environmental projects between now and 2030. This includes £889m on storm overflows.
“It is important to note that until the weekend, a lot of our storm overflows in the area had not spilled for months due to the dry weather.
“The total removal of storm overflows is unaffordable and would take decades, so we target investment on storm overflows that have the biggest environmental impact.
“River pollution and marine water quality is not just an issue for water companies. We need to work together with other sectors, including industry and agriculture, to ensure improvement”.
An alert remains in place at Watch House Bay due to a poor annual classification, which means swimming in the sea is not advised.
