I notice piles of news sources are latching onto the 'Alexander the Great did not found Alexandria' research which is based on lead levels in the harbour. Here's the abstract of the actual study:

It is generally accepted that Alexandria ad Aegyptum was founded ex nihilo in 331 BC by Alexander the Great, rapidly growing into one of antiquity's most opulent economic and intellectual centers. However, ancient texts by Strabo (17.1.6) and Pliny (NH 5.11.62) suggest the existence of a pre-Hellenistic settlement named Rhakotis. This literary evidence has fuelled contentious scholarly debate for decades. Here we present new geochemical data from Alexandria's ancient bay sediments, elucidating unequivocal proof for pollutant lead (Pb) input into the harbor during the Egyptian Old Kingdom (2686–2181 BC). A second contamination peak is detected during the Iron Age (1000–800 BC), at the end of the prosperous Ramesses reigns. These findings evidence thriving pre-Hellenistic settlements in Alexandria. During the Greek and Roman periods, we expound the largest Pb pollution ever encountered in ancient city sediments with Pb levels twice as high as those measured in contemporary industrialized estuaries.

Now I know that no serious Classicist will suggest Alexandria was created ex nihilo (and the arguments will continue over the pre-Alexander existence of a library there), but can someone tell me how these researchers can distinguish lead from 'pre-Alexandria Alexandria' and lead from places further up the Nile?