Over the next couple of days, we managed to double the number of flow sensors in the core systems along the transect that Trish was studying as part of a biology project headed up by Dr. Fernando Alvarez from UNAM in Mexico City. We revisited our test rig at the coastal site, leaving a set of three flow units there along with our 2 Bar pressure sensor. All had been tested showing good sleep currents, so hopefully we would not loose our data this time. Those dives went smoothly, and we ended up with a rare day on our fieldwork schedule that was not already booked.
We knew there was a system in the area were some biologist friends had a Lowel Instruments flow meter (the first time I have seen a commercial unit use the tilt/drag principal…) installed beside their monitoring equipment. But the meter was scheduled for removal in January of 2015 – so this opportunity was brief. If we could make it out, a co-deployment would give us a chance to compare point velocity numbers from a commercial unit with our DIY project data. On the down side, it was quite a haul to get out there, on a road that our little rental car was not really capable of handling. But, thanks to my marathon rebuild session, we still had three flow sensors that we could deploy…
We decided to go for it, and just bring everything along in case the system looked interesting. Thirty minutes of slowly bumping, clunking and screeeeeeeeeetttcccching past the machete stumps, and we made it to the walking trail. From there we still had a good march out to the cenote, so we donned our gear and set out with doubles on our backs; placing each foot as carefully as we could on the uneven trail, while also trying to move fast enough to outpace the mosquitoes. By the time we reached the water, it was getting pretty hot in those wetsuits!
After we had cooled off , we did the pre-dive checks, secured our mesh bag full of sensors, and set off along the line. And the water was…. brownish green…(?) Much more than the other cenotes we had been diving in. But then I noticed that some of those bits of perc were actually moving around, under their own steam, and I understood why a group of biologists would select a site that was so darned awkward to get to. It was heaving with little critters! When we found the site they had chosen for their monitoring equipment it was at the top of a trapped dome, probably perfect for monitoring things like nutrients, but almost certainly with too little flow for my Pearls to register. I set up a sensor anyway, thinking that if it was a zero flow location, we were not going to have much data to work with for that calibration. Who knows, perhaps we would get lucky and catch a big rain event flushing out the system during the overlap of the two meters?
Then we explored further into the cave. The line soon descended below the limits of our humble point and shoot camera (so we had to leave it behind), but the visibility opened up below the tannic water showing a spectacularly wide passage, with the fresh/saline transition smack-dab in the center. Trish immediately became very excited, pointing out that there were ripples in the sediment. We swam further into the passage, and she hand signaled her intent to find a location for a dual installation with our last two units. I waited on the line while she searched, watching the slowly undulating halocline as it scattered our dive lights across the walls of the passage like a fun-house mirror. Once a site was selected, we configured one unit as a float, for installation on the floor of the passage, and directly above that we hung our last unit from the ceiling; up in the fresh water zone. After an inspection swim to check connectors & compass bearings, we shook hands in an exaggerated ceremony before following the jump reel back to the main line. I don’t think we could have picked a better place for our last installation of the trip if we tried. Worth the hike, and the bugs, and all the scratches & dents we put on that rental car…which now all seemed quite minor…really…no problem at all 😉