Sampling Waters in the Naukluft

For an arid environment, there sure was a lot of water around. Apparently there has been a lot more rain than usual; that is why the area was greener and buggier than anyone had seen in recent memory.

Water resource characterization is actually the main motivator behind this whole effort; my nominal (and periferal) role is to determine what role (if any) ancient or recent faults and fractures are playing in water exchange between the surface and the subsurface.

Most of the water resource extraction in the Naukluft Mountains, and pretty much all of it on the surrounding farms, is accomplished by "borehole". Many have a real working windmills - These usually pump directly into troughs for herd animals (and wild animals) to access.

Although these are often the easiest source for ground water samples, there are a few issues with them. For one, the boreholes are usually lined with some kind of pipe - usually metal - this will affect the geochemistry of the water if it sits in the well a while. Second, they are drawing all the time (whenever the wind blows). This helps with the first concern, as it means water doesn't reside in the borehole too long. However, it means the water level in the borehole might not be in equillibrium with the water level in the surrounding formation - and we want to know the water level of the regional/local water table. If the windmill pumps water out of the well, it takes some time for the water to flow from the rock formations back into the bore holeand during this time, the water level we measure will be too low. This ranger at Zais helped us open the cap under the windmill so we could drop the water level meter down.
We found a few boreholes, such as this one, which had solar panels apparently to drive pumps. This one looked like it was installed to keep a shower ready to go at this outpost on Die Walle farm. Alet is checking the shower tank for water... empty. Kate is getting ready to check the borehole for water... sealed.
The coolest samples come from the rock springs and seeps. Here's Kate suspended by her toes on a slippery algae-covered cliff wall, catching tiny drops of water as they emerge from the rock. We also sampled the surface water such as the stream Kate is trying not to fall into - this will all give a picture of what underground water looks like, what surface water looks like, and how the streams are fed during the dry season... Due to the high visibility of travelling thunder storms, Jodie and the students were able to chase down a few and collect several samples of rain water as it fell out of the sky... this is a great data point to have because it will tell us what the water looks like when it enters the Naukluft. I wish I had a picture of Jodie in a sudden deluge, holding a giant green bucket to shield her head from the huge rain droplets... so pleased with her cold rain sample... but as I was watching from the warmth of the dry bakkie the photos look like a rainy sheet of wet window. hm.

I have to say, Kate was really the workhorse of the water sampling team. She climbed walls and dove to the bottom of clear spring pools. Here she is after sampling the spring-fed creek in Waterkloof. You can see the wall of tufa behind her - more on the tufa deposits in a future post. Anyway it suggests that both surface and groundwater flow through Waterkloof were much greater in the past than today. How far back is the past? We don't know.
Here's Ben in the gorge at Die Walle, entertained by the struggling students climbing the walls to capture tiny drips of water! Behind him in the cliff you can see the reason for this canyon's name, Die Walle: the falls. Standing under that wall, you could see shoots of droplets literally spraying a meter or more out from the wall in the blackened area. A big pool at the bottom feeds the creek running out of the gorge.

We tested the water onsite for dissolved oxygen, pH, temperature, conductivity, eH, and total dissolved solids... Kate and Chris Harris have already started isolating the CO2 dissolved in the water to look at the carbon and oxygen isotopes. She will also analyse the water molecules themselves for hydrogen isotopes. She will also look at the isotope signatures of the rocks in the area, and the carbonate rocks that precipitate in these waters...These will hopefully be able to tell her about the chemical interaction between rocks and water, and possibly help trace the pathways by determining which rocks have more contact with water before it flows to seeps or springs.

Shane and Alet at U. Stellenbosch will find out about those total dissolved solids - what are they? What cations and anions are present, and do they come from rocks, from the rain itself, from biological processes, or human pollution? Does the water get saltier as it flows underneath the ground from the mountains out toward the plane, and the farmers there with their bigger ranches and multiple boreholes?

Pride from U.Nam will be putting it all together, as they say, he will be using MODFLOW to build a 3D picture of the Naukluft groundwater in space and in chemistry.

The big questions we all will work on together:
- If the farmers say some springs are drying up, is that due to over use of a static reservior? Or is there something fundamentally changing in the water system of the Naukluft?
- From raindrop to tap, how does the water evolve and change in chemistry? Is it always safe for humans and animals to use? if the water regime of the area is changing, will this affect the chemistry and safety of the water, as well as quantity?
- How much new water demand can the system support?
- Why are the rocks so damn cool?
Thank you.

1 comment:

Erin K. said...

Awesome Crowe! That is so cool. I can't believe how big that tufa deposit is. There are some cool tufa deposits in the Blue ridge, but not as big as that - and most are actually related to knickpoint/stream power changes which induce cavitation (and therefore ptt), not 'spring' deposits.

Are these your PhD students?