According to recent reports, the concentration of gadolinium in the San Francisco Bay has risen over 700% between 1993 and 2013 to 171.4 picomoles per kilogram of seawater. The likely source of this anomalous concentration is the prevalent use of gadolinium based MRI contrast agents by the area's healthcare system. Once injected and the imaging is completed and assuming the patient has healthy kidneys (if not then they are at risk for nephrogenic systemic fibrosis, but that is another story) the gadolinium containing contrast agent is urinated out over the next several hours (usually not directly into the bay mind you although surely that happens at certain times and places, like off of Pier 39 after the bars close down) with most of it making it's way through the sewer system to the treatment plants before being discharged into the bay.
Well now, it's been almost 3 decades since I thought about Avogadro's number and such calculations but if I got it right (and please my engineering and science professor friends and colleagues, chime in if not) and assuming a 100% extraction rate, one would have to process about 300 million gallons (or about 454 olympic sized swimming pools worth) of sea water from the San Francisco Bay to obtain one troy ounce of gadolinium. Alternatively you can buy ours (pee-free gadolinium ingots that is) in about a month along with our ytterbium ingots which are also close to their nonbiologic release. Own what's rare.