He can do it but the water will run out before the lake is completely purified. The shrine water will mysteriously refill at the end of the month, if Terry remembers to keep doing it. About three basins of water will completely purify the lake; 1 basin will decrease the severity of the sickness, and 2 basins will decrease the duration of the sickness.

The crows might flock around him and caw incessantly while he's at the shrine, though, because yo, this isn't what the water's supposed to be used for, don't take all the water, punk.