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Message: <blockquote data-quote="Allen" data-source="post: 297176" data-attributes="member: 389">Most of central ND small lakes have a mercury problem.  I'd suggest that most of that mercury is naturally occurring in the local sediments that has been freed via oxidation of the organic carbons in the local shale rock.  Up around DL that would be either the Pierre or Niobrara shale (I forget offhand which daylights in that area).  In the many other small lakes, they are surrounded by soils and sands ground out of the sedimentary deposits of ND.  Again, much of the mercury is native to the local rocks and sands. Same goes for arsenic and selenium, both of which are known issues in some parts of the state.  At one point there was a push to declare a part of SE North Dakota as a Superfund site because of the known arsenic levels in the local groundwater.  They initially believed it was due to grasshopper baits/poisons used back in the dirty 30s.  However, a simple mass balance ruled out the poisons.  What does happen though is nitrate from farm fertilizers that escape the root zone and enter the groundwater react with the organic carbon and iron sulfides in the shale sediments. Both are sources of arsenic, mercury, and selenium. As the OC and sulfides (mostly pyrite) are consumed in the chemical reaction, the trace metals trapped within them are released.Yeah, I spent a little time on this topic at one point in life.Back in the 90s the state used to do lake by lake testing of fish for mercury.  Then they spent a few years trying to make more generalized assessments of the lakes in a given area.  At this point I am not exactly sure of the state of science for fish testing. In general though, unless you are talking Lake Sakakawea and Missouri River fish, the predators (walleye and pike in particular) get pretty heavily contaminated by mercury as they grow.  Hence the eating advisories are often listed via fish length where smaller fish, generally being younger, have less in the way of mercury per meal.  Some of the bigger fish could probably double as thermometers with the amount of mercury they have in them.</blockquote>

Verification: What is the most common fish caught on this site?

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