Wednesday, September 15, 2010

Urban Seep Soil Science

This week I decided to investigate the urban seeps of UVM in greater detail.  Since seeps come from underground, it makes sense to look at the soil!  I took out one of our trusty soil probes and took some samples!  I'm not sure exactly what I found means, but it adds another piece to the seep puzzle.

College Street Seep core

College Street Seep was quite soggy when I visited, since it was after a small recent rain.  The photo above shows the soil.  It seemed to be clay or silt for the first foot or so, then I hit sand.

The sand is particularly interesting:
College Street Seep closeup
It makes me think that this area may be on part of the old delta deposit that also covers most of Centennial Woods.  If this is true, bedrock is probably not close to the surface and the soil is probably rather well-drained.  It may not truly be a 'seep' with groundwater entering the site... it may be more similar to a vernal wet area fed by runoff from nearby areas.  Both sorts of wetland are worthy of note, but they are quite different in water source.  I think the best way to learn where the water is coming from is watching the area over the next few months.  If it is truly a seep the water will probably keep flowing even in midwinter (I have seen seeps in Maine that had liquid water when it was below 0F).  If it is fed by overland flow, it will probably eventually freeze.

There was some 'mottling' of red patches in the College Street Seep area.  This could potentially indicate wetland processes in the soil.  However, since I only saw this in a core away from the immediate seep, I'm not sure of that.

Centennial Seep had a soil profile that looked a lot more like natural wetlands I have seen.
Centennial Seep Core
This seep soil is full of clay.  The soil appeared to be saturated, and appeared to have been that way for a while, including during the driest time of the year.  A 'rotten eggs' smell told me that bacteria that do not tolerate oxygen live in this soil - a characteristic of many wetland soils.  This seep originates in one place and has surface water.  It is almost a spring.
Centennial Seep closeup
I also noticed that in the past, a small swale had been created for this seep; it simply fell into disrepair as of late.  Repairing this, and allowing some wetland plants to grow in this area rather than a sickly lawn, would improve the health of the Centennial Brook watershed and also reduce the amount of water flowing onto the nearby walkway, freezing, and causing a hazard during the cold season.

More on these urban 'wetlands' to follow as the seasons change!

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