we do rely on research done before, because it would take immense effort to validate all things by ourselves. i dont have access to measure picogram quantities of gold in the whatever quantity of water. some folks had the right equipment and recieved the numbers we have. and there wasn´t one, there were many of them
i dont think that me or you can do better in analysis of such a low concentration of element. researchers didn´t have equivalent samples, and if you look to the numbers given here, you will see vast difference in concentration, depending on location. previously described (higher) numbers were questioned in terms of contamination. how do you ***** no contamination from the enviroment, transportation or instruments alone, regarding nearly everything inside the machines is made of metal ?
use search engine. most of the research you think of had been done before.
https://www.sciencedirect.com/science/article/abs/pii/0012821X9090060Bhttps://www.thoughtco.com/how-much-gold-is-in-the-ocean-4165904https://iopscience.iop.org/article/10.1088/1742-6596/908/1/012045/pdf
there is even a patent claiming process of extraction of gold from seawater, altough from 1901. and when you read first like 50 lines, it become obvious that this is complete nonsense
https://patents.google.com/patent/US679215A/en
more research reveal some older data:
https://www.sciencedirect.com/science/article/abs/pii/0375674288900519
recent advantage of usin MOF (metal-organic frameworks) with built polymeric substances inside the cavities, capable to adsorb, encage and chemicaly selectively reduce Au3+ to nanoparticles of gold. this may be the future of wastewater and seawater goldmining, if there would be some anywhere in the future:
https://pubs.acs.org/doi/abs/10.1021/jacs.8b09555