The measured levels of arsenic, strontium, and vanadium are below the limits for drinking water, even in California. And 4% of drinking water sources in California have higher hexavalent chromium content than the water in that ditch.[1] Besides sodium from salt, the only metal that was particularly high was lithium, at 0.0714mg/L or 71 micrograms per liter. A significant fraction of drinking water in the US has higher concentrations than that.[2]
The level of salt shouldn't affect much. Adding up the chloride and sodium content gets you 684mg/L, which is on the low end of brackish water (500-30,000mg/L). The limit for agricultural irrigation is 2,000mg/L, and photos of the pipe show plenty of grass growing around and in the water.
The phosphorous could come from fertilizers, as there's plenty of farm land in the area. That would also explain the higher ammonium levels, as both anhydrous ammonia and ammonium phosphate are common fertilizers.
The article is really about how sensitive our scientific instruments are, not how dangerous the water is. It reminds me of articles like Vice's American Honey Is Radioactive from Decades of Nuclear Bomb Testing[3], where the most radioactive honey they could find was 10 times less radioactive than a banana.
1. https://www.waterboards.ca.gov/drinking_water/certlic/drinki...
2. https://www.sciencedirect.com/science/article/abs/pii/S00489...
3. https://news.ycombinator.com/item?id=26906838