Following the Tide

Science has continued and a fast pace since I last wrote.  We only have a bout 1.5 weeks left to complete our fieldwork, but I think we are on track to complete most of the tasks we came down to do.

 

One of those tasks was to try and evaluate how groundwater discharge along the coast changes with tides.  The plan...we want to measure all the chemical tracers (mentioned in earlier blogs) in the surface water and groundwater every two hours over a 36 hour time period, while imaging the subsurface using resistivity.

 

Lets start with the tide…crazy thing down here.  The tides change from diurnal (1 high tide and 1 low tide in a lunar day, 24 hours and 50n minutes) to semi-diurnal (2 high tides and 2 low tides in a lunar day)…within the same month…not many places switch between the two cycles.  As the tide changes, so does the interaction of the surface water and groundwaters.  As the tide rises, surface water tends to move into the sediments.  In contrast, as the tide drops, groundwater is more likely to discharge into surface water.  This back and forth movement of the water is often referred to as tidal pumping.   We wanted to evaluate whether this process was occurring here and the possible changes in water chemistry during the process.

 

So, the site we chose is one of the few areas around here that actually has a sandy beach.  Most of the coast is very rocky.  This area was recently exposed due to the retreating glacier, so it offers a great opportunity to study a type of landform that is likely to be exposed more and more as the glaciers continue to retreat.  The area is know as Point 8 around here, named by the Navy during early surveys of the area (see the first picture, setting up the site).

 

Again, we collected surface water directly with a pump and groundwater using a drive point sampler (a pvc pipe we worked into the sediment about 30 cm). From these we can evaluate changes in the tracers and nutrients as the tide changes.   We also measured the resistivity of the subsurface.  This is a method that allows us to evaluate the difference between saltwater and freshwater  beneath the ground.  Doing this over the course of the tidal cycle provides direct evidence of subsurface mixing and discharge of water.  Included is a single image of one of the resistivity measurements.  The light blue colors are saltier water and the warm colors are fresher water.  The left side of the image in the water…the last 6 meters of the cable is submerged.  The right side of the image (when this was collected) is not submerged at all.  Again, just to give you a sense of the sort of data we are reviewing to provide information of the subsurface.

 

Time-series work can be quite exhausting.  Our crew was awake for about 48 hours when it was all said and done, preparing, running experiments, breaking down and cleaning up.  But the data looks really great.  Being out on the "ice" and away from station for that long was also quite interesting.  The glacier was calving (large ice chunks breaking off) all around us.  On large calving event actually created a small local tsunami about 7 ft high.  Impressive in its power…creating havoc on some of our equipment.  Also, there were times when we had to deal with the locals…penguins and seals.  That was exciting at times as well.  But, some of the images that mother nature provided us are better than any data we could have ever collected.  The last image is a picture I took at about 5am.  The sky never gets dark, but this is equivalent to a sunrise over the glacier…magnificent.

 

What a great experience…all around!