Almost none
It is not hard to figure out where the salt is coming from. Here is a google earth shot of three coal bed methane pivots. You can still see the salt on the top of the ground close to 20 years later.
View attachment 411173
Based on the study around 2012, the salt flush in the spring into the Tongue, is coming from a different source.
"CBNG produced water is being
ruled out as the source because it
has a distinct water chemistry and
isotopic signature. CBNG produced
waters have water compositions that
are strongly sodium bicarbonate
and have a unique carbon isotopic
signature. The isotopic compositions
of most CBNG water samples are
much higher than normal surface
water. This isotopic fingerprint is a
by-product of the microbes that have
generated the gas that resides within
the coal beds. The scientists tested
for CBNG produced water, the
composition that is high in sodium
bicarbonate with a unique carbon
isotopic signature; neither of which
was observed in the surface water
of the Tongue River. As a result, the
preliminary findings indicate that
the elevated TDS in the water in the
Tongue River was not caused from discharged CBNG
produced waters.
Conclusion
The preliminary results of this study show that CBNG
produced water is not the cause of the elevated salinity in
the Tongue River in early spring. The scientists indicate
that the elevated TDS measured at the Wyoming/Montana
border in February and March is a natural process, which
was likely caused by the underlying geology of the Powder
River Basin and watershed, and related to the earlier
snowmelt in the basin interior.
The Wyoming DEQ and Montana DEQ can use this
research as a basis for future inquiry on the water quality of
the Tongue River. Geologists can also apply this research
and water sampling methods to better characterize other
watersheds in the state and study whether rivers in other
basins are also affected by the underlying geology of the
region."
Now if land owners are using CBNG well water to feed their pivots for the past 20 years, a new study should be done to see if it can now be detected.