The salinity $S$ (in parts per thousand) of an estuary varies periodically with the tide and is modeled by $S(t) = 4 \sin\left(\frac{\pi}{6}t\right) + 18$, where $t$ is the number of hours after high tide. What is the minimum salinity value of the estuary, and at what time $t$ does it first occur within the first 12 hours?