One is an amount of arsenic that is a quarter of what's allowed in drinking water. So technically, someone dumping drinking water in the ditch could contaminate this measurement.
The other is hexavalent chromium, which is 4% higher than allowed. According to wikipedia that is "indeed one of the more widely used heavy metals in various sectors and industries (metallurgy, chemicals, textiles, etc.) with particular involvement in the metal coating sector" and used in the production of all kinds of dyes, paints, plastics, etc. It can also be formed by welding stainless steel, and is found in drinking water ... that doesn't sound very specific to me.
I don't know where that ditch is, but on google maps the Tesla lithum plant is right next to a place storing drilling equipment outdoors. Runoff from any kind of industry nearby could end up in that ditch. After all, collecting runoff is what ditches are there for