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I have a Mayfly sensor station on a small perennial trib to Marsh Creek. I was noticing step changes in the EC of over 100µS in the 5 minute sampling interval. Suspecting that this was a defective sensor so I co-located another sensor and got a similar result and the two EC readings were uncorrelated. Therefor I put both sensors in a filled bucket and observed no erratic behavior. I was told by a tech at Meter Group that moving water can cause that large a step change in the EC reading. I have looked at a number of stations in some of the local streams and have not seen such large step changes in EC. I attached some photos for reference.
I wonder if any one else is observing step changes of EC of this magnitude?
I have seen similar fluctuations in some of the streams where I have Hydros21 sensors. Typically, step changes > 100 µS in 5 or 15 minutes only occur during runoff/snowmelt events. In the winter, after a snow event with road salt application and subsequent melt I have seen > 2,000 µS fluctuations in 15 minutes as the salt pulses work their way down the streams. In the non-snow season a rain event can quickly dilute the streams that have base flows with high conductivity, dropping very quickly in 15 minutes.
If there is no active runoff/snowmelt I tend to see more stable conditions without such rapid fluctuations.
Thanks for the input, Dan@wachusett. I think I figured out my problem – it was the bubbles in the stream as you can see in the attached photo when I was co-locating two sensors! I moved the sensor probe about a foot to calmer waters and now the results look normal.