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Converting
EMI Conductivity Values into
Salinity Units

To relate conductivity measures to salinity units, EMI sampling was paired with direct measures of salinity in the field. This work served to refine and strengthen site-specific calibration curve for Great Marsh. In this case, direct measure of salinity is defined as extraction of pore water using stainless steel sippers. Direct measures were collected approximately every 10-15 EMI samples, or where abrupt habitat shifts warrant additional context. The paired EMI and salinity data were used to develop a regression equation that we use to convert apparent conductivity (ECa) to salinity (ppt). The two are fairly well correlated (r2 = 0.7, p<0.001), supporting a fairly strong and predictable relationship between ECa and pore water salinity. This work is based on our recent peer reviewed paper in the journal Wetlands (Moore et al., 2011).
 
The equation we used to interpret the Great Marsh data is as follows:
 
Conductivity = 127.75764 + 16.85357 * Salinity

Plot of measured salinity against apparent conductivity obtained using electromagnetic induction (EMI). This plot forms the basis of the equation that is able to predict salinity from conductivity measures so that we can use EMI to confidently map salinity in the Great Marsh.

Salinity Mapping
with Dr Gregg Moore

Dr. Moore explains the cutting edge technology used for salinity mapping on the Great Marsh and it's value in helping reduce the cost of restoration.


Session Date Rough Edit Transcript Tight Edit Youtube Posting
8/15/13 9/21/13 2/24 2/27 2/27

Salinity mapping near Phragmites stands in the Great Marsh
Dr Moore documenting salinity near small stands of Phragmites in central areas of the Great Marsh.
Transcript
We’ve been monitoring salinity in the Great Marsh for the last three years, intensely in the last three years, and the reason is primarily that we know salinity is among the most important factors that determine the presence of Phragmites or the areas that Phragmites can exploit in the marsh. We look at it both in terms both of surface water salinity and salinity at various depths within the soil. The difference between Phragmites root depth compared to the native species, Phragmites can root a lot deeper and that means if we’re going to monitor salinity in the marsh we need to look at various depths. When we’re sampling salinity there can be a dramatic difference between the salinity at low tide and the salinity at high tide. Or the salinity in July verses the salinity in June.

It’s important that when salinity is measured in the salt marsh it be done as quickly as possible so that data from one portion of the marsh relates to data in another. This is one of ways, reasons using electromagnetic induction is of such value. The EMI allows us to take conductivity measurements, gather tremendous amounts of data very quickly, keep those data in check by having some real time salinity measurements as well. So I refer to conductivity as apparent conductivity, it’s inferred, as apposed to the real time salinity that we collect using the sipper.

When we combine these two approaches together we can get an extraordinary amount of data and resolution regarding the salinity patterns on the marsh at any giving day and compare them over time and see how those patterns change.

This is really what has made EMI such a great tool for guiding management out at the site.

EMI is based upon the simple principle of Faraday’s Law. Sending an electric charge through the ground and if the matrix that it’s passing through is conductive then the instrument receives a strong signal and if it’s not very conductive the instruments finds little to no signal, and everything in between. The conductive of the soil is very closely tied to salinity so we’re not just mapping conductive where Phragmites grows. Instead, we’re mapping it throughout the marsh to understand where Phragmites could expand, or is likely to expand, or, conversely, where the environmental conditions are such that it probably wouldn’t.

The data are collected on the spot, on a handheld computer, and we can then take all the data back to the lab and project them in a GIS-based mapping program that allows us to generate conductivity maps that look a lot like topographic maps but the different lines are salinity. When we then take our mapped areas of Phragmites and overlay those onto the salinity contour maps we get a really effective tool to make predictions and to prioritize management. All in all, it makes the project much more efficient and more effective.

We still use salinity wells, still use the sipper technique, those are our standbys. It’s absolutely necessary that we maintain salinity wells throughout the marsh. And the sipper technique that we also use is similar, but it doesn’t require that the well be installed, so we can go with that stainless steel sipper anywhere we want and just place it into the marsh at what ever depth we want and get data.

All these are kind of calculated redundancies in the data we collect. And the EMI allows us to gather data in all the places in between, at what ever frequency we want, with great ease. So all these approaches together really constitute a solid monitoring program.

Through the process of data collection we’ve had a lot of different folks collecting salinity data with us and for us. We’ve had many volunteers in some cases collecting salinity [data]. There in lies the real value of having sipper techniques, or per haps more so, the fixed monitoring wells, that allows anybody at any skill level to be able to come and collect data throughout the year.

When we combine all these different approaches we got a really confident look at how salinity is playing out in the marsh and again how that’s going to influence our success in management of a plant that thrives in a lower salinity environment.

 
 

Danger in the Reeds is being produced by Staddles Productions
with the help of Dr Gregg Moore, Peter Phippen and Geoff Walker.

We would all like to express our graditude to our many Kickstarter supporters
without whom this project would not be possible.


Rick Hydren  ~ Office: 978-948-3346, Cell: 508-954-1298
PO Box 715, Rowley MA 01969

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