on this link, the institudo estadual do ambiente - inea, provides the suitability of several beaches in the sepetiba bay area, including praia do cardo, which is close by to praia da ponta grossa and can be used to guess water quality there.

constantly updated, a red flag means that swimming is not advisable, a green flag means waters are clean, and a yellow flag suggests using own judgment.  

any of rio's beaches may have polluted waters within 24 to 48 hours following heavy rains.