The Stibnite mining area was intermittently mined for most of the 20th

The Stibnite mining area was intermittently mined for most of the 20th century, and decades after mining ceased, water quality in the area continues to be impaired. Concentrations of dissolved arsenic and antimony generally increased from upstream to downstream in the East Fork of South Fork of the Salmon River. Based on the majority of arsenic and antimony occurring in the dissolved phase, and inverse relationships with streamflow at most sites, the primary route of arsenic and antimony to surface waters is likely from groundwater. Mercury, in contrast, was directly related to streamflow and was associated with particulates, suggesting erosion of surface materials to be the primary route to surface waters.

From the bold parts we can conclude the following.

1. Dissolved concentration of arsenic and antimony increase from upstream (mouth of the river) to downstream.
So source upstream near river mouth. This pollutant gets more and more dissolved as river flows.

2. Inverse relationships with streamflow for arsenic and antimony :
Faster the flow -> Less dissolution -> Less arsenic concentration

3. For mercury directly related to streamflow :
Faster the flow more erosion -> More mercury concentration

4. Mercury erosion from surface (sediments)

The source of Arsenic likely be near upstream.
Headwater chemical infiltration (C)

The source of Mercury likely be downstream.
Downstream sedimentary erosion(A)

These two options explain streamflow and other characteristics.