Thursday, June 20, 2013

Research in the São Romão sandbox

Let's now introduce a group that hasn't featured much yet on this weblog. They are the guys who study the São Romão Catchment hydrology - Seife, Bart, Jelmer and Laurus). Where the other catchment groups have some geological variation in their area, this group basically works in a large sandbox (young and old dunes) with only small outcrops of Cretaceous clay at the surface. Nevertheless, although this seems a simple system, you can dedicate your life to such systems, as our Prof. Pieter Stuyfzand has proven.

With the chemistry done, nitrate pollution found and dirty smelling wells (indication of anoxic conditions in the clay) visited, this group is now focussed on determining the palaeo-topography of the clay surface. Between the Cretecous and the dune deposits, there is a time hiatus of some 60 million years! So there was ample time for erosion creating valleys in the clay deposits that are now covered by dune sand. You cannot see these valleys obviously, but they can influence the flow of water in the area. With our geophysics, you can find the depth of the clay below the sand, and if you do enough measurements, you can see a 3D pattern, or the old landscape that existed before the sand was deposited on it.

Seife, Jelmer and Laurus working on the terrameter in a Eucalypt plantation in search for clay
The group started out with electromagnetic measurements, and today did some more vertical electrical soundings to find the clay. First in a pleasant eucalypt forest where they found the clay at a few meters depth, and then to the south of their area where it was at even less depth. This means short measurements and quick results.

Bart looking on while Seife and Laurus calculate apparent resistivities, also discovered that Laurus name sounds better in French...
So we were done early and had some time to do a pumping test. These guys do really ahve an artesian well in their area and we decided to subject it to the pumping test.

Artesian (free flowing) piezometer in the São Romão catchment
This place has a pumping piezometer, a monitoring piezometer in the confined (artesian) aquifer, and a piezometer in the phreatic zone. If the clay layer causing the artesian conditions is closed, we should see no change in the phreatic level while pumping below the clay layer. 

Pumping test in the São Romão aquifer
Pump was brought out, water level data logger installed and programmed to do a pumping test, pump started and pump discharge measured. And indeed, while the water level in the confined piezometer went down like a pelican going for fish, the one in the phreatic zone did not care to respond at all to the pumping. Textbook case! Now they have a good indication of the hydraulic conductivity of their sandy area. We are nearing the end of our field course, next week the final measurements...

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