Thank you very much.
So I'm going to start off today talking about water.
So here we have our magnificent blue planet.
Abundant water or so it seems until we add to this picture 7 billion people.
And now we're increasingly hearing the phrase freshwater security.
What I'm going to do today is zoom in and talk about freshwater security in the Monterey
bay region.
Where freshwater is challenging us to think about how we can ensure that there is adequate
freshwater for people while not damaging the natural systems that provide that water.
So here we are we've arrived in the Monterey bay region.
Now where does the freshwater come from that is the supply of water for this region.
Well there's very little surface water.
So most of the water comes from groundwater.
Groundwater is that water that's held below the surface meters to hundreds of meters below
the surface in regions referred to as groundwater aquifers.
Groundwater aquifers are a great place to store water.
So many people think of groundwater aquifers like we think about savings accounts - It's
there for rainy days.
In the case of groundwater aquifers it's there for those days when it doesn't rain.
But it's a great place to store water.
In this area we've become very dependent upon groundwater.
When we look at the percent of freshwater used that his groundwater the national average
is 44% statewide it's 30% Santa Cruz County 79% Monterey County 99% of your water of your
freshwater is coming from this groundwater.
Much of the demand for water in this area is driven by agriculture.
Agriculture from this area is an important part of this food supply of the country an
essential part of the local economy.
So we can't just stop providing water for agriculture.
So freshwater security in this region challenges us to think how can we continue to use our
groundwater but in a way that is not threatening the long-term viability of our groundwater
Akal first.
So what's the concern along this coast the concern is salt water intrusion.
All along this Coast you're getting your groundwater from groundwater aquifers that are present
in various depths below the surface.
In this simple schematic I have two groundwater aquifers.
When we pump water from the upper aquifer we can potentially start pulling salt water
into the aquifer.
When I then turn on the pump water from the lower aquifer you can start pulling salt water
into the lower aquifer and that's what's happening all along this coast.
When we look at aquifers the different depths there's evidence of salt water intrusion at
various depths into a number of these aquifers here's an example of an aquifer.
Along this coast and this map of salt water intrusion put together by measurements of
the salinity of water in water wells shows that saltwater intrusion in this upward aquifer
has moved up to 10 miles inland.
Now salt water intrusion in this aquifer first started in the 1930s.
But people continued to pump the groundwater and now this region in red can no longer be
used as a supply of freshwater.
So what do you do when you have salt water intrusion in this upward aquifer from where
you're getting your water?
Well,you drill deeper.
And so in this area the groundwater is now pumped from the lower aquifer but when we
look at the lower aquifer we see that salt water intrusion is starting into this lower
aquifer.
To the north and south to the north there's evidence of saltwater but the informations
very limited.
To the south, interestingly, there has been no evidence of saltwater intrusion into the
ground aquifers that are being used there for water supply.
Well something that's very obvious is that there's this close connection between fresh
water security and pro-active ground water management.
So the moment there's evidence of saltwater in an aquifer, proactive groundwater management
would take actions to prevent the further movement of saltwater into your aquifer.
The kind of actions that can be taken - first of all, you can reduce the amount of water
coming out of the aquifer.
You can also think about engineered systems that help get more fresh water into the aquifer.
Because this salt water moving in means we're out of balance - the amount of water coming
out is greater than the amount of water going in.
But any kind of proactive ground water management - there's an essential part of it.
We have to know where the salt water is.
We have to know where the saltwater is and ideally we have to be able to detect that
saltwater the moment it arises.
Well in the seaside basin, we now have four sentinel wells open along the coast.
These sentinel wells are on the lookout for saltwater.
We have four sentinel wells along the coast.
How are these sentinel wells looking out for water?
Measurement are being made in those wells at electrical resistivity.
How do you measure electrical resistivity?
You have your electrodes, you inject current, you measure voltage, you calculate your electrical
resistivity and the electrical resistivity that you measure is sensitive to the amount
of clay in the material, the amount of water in the material but it's very sensitive to
the amount of saltwater - the salinity of the water.
As the salinity of the water increases - shown in this plot - increase in concentration of
sodium chloride, your electrical resistivity decreases.
So back to our sentinel wells, great idea.
To be measuring electrical resistivity to detect saltwater intrusion.
But there's a problem with wells.
You're getting a measurement at one specific location.
You're sampling a very small volume of the sub-surface.
So you're not getting the complete picture - not capturing the spatial complexity of
saltwater intrusion.
And what if a tongue of saltwater makes its way right between your wells.
So what we're suggesting is that yes, you use electrical resistivity measurements to
watch out for saltwater along the coast.
But we're suggesting you don't make measurements at one location but you make millions of measurements
all along the coast using a geophysical map called electrical resistivity imaging.
So instead of a measurement at one location, we want to cover the entire coastline making
electrical resistivity measurements.
This is a type of geophysical imaging method.
Geophysical imaging is very much like medical imaging but instead of getting a slice through
a human, you get a slice into the ground.
So what we'll get with our electrical resistivity imaging along this coast is an image all along
that blue slice - that blue section into the ground from Monterey to Santa Cruz, we're
going to be able to see three hundred meters into the ground watching out for saltwater
intrusion.
This is what we call sentinel geophysics.
We're going to be on the lookout for saltwater through this electrical resistivity measurement.
Today I'm going to talk about the results of a pilot survey we've completed over the
last year.
In this stretch of coast highlighted here in yellow.
So how exactly does this work?
We're making electrical resistivity measurements by moving a kilometer long cable along the
coast.
This cable has ninety six electrodes in it that go into the beach.
It's a very minimally invasive method so it's not like drilling a well.
You just put these shallow steaks into the beach.
We made one hundred and thirty thousand measurements along this stretch.
Total coverage was 6.8 kilometers it took six people ten days and cost us $60,000.
And what did we get?
Well the next image is going to look at the electrical resistivity image of this section
of coast.
This yellow part of the coast.
So we're going to be looking from the bay at a very colorful geophysical section through
this stretch of coast.
And here we are.
Now I'm a bit biased, but this is incredible.
This is an amazing success!
What you're seeing - the color scale goes from blue which is freshwater, red is saltwater
- we are mapping out the distribution of freshwater and saltwater along this coast to a depth
of 150 meters.
Isn't that amazing?
Yes, stunning.
You can see into the earth.
What we're very pleased with - there were the four sentinel wells along this stretch
and we see excellent agreement with the sentinel wells.
The color in the wells is the salinity of the water measured in the wells.
We get the same color, we get the same salinity of the water in our geophysical image.
But we're covering a much larger section.
And another point - the cost estimate for those four wells was $900,000.
We did the entire stretch of coast for $60,000.
But most importantly we're capturing the complex spatial distribution of the saltwater.
We can really start understanding what's happing along this coast.
In the south, there's saltwater very near the surface and we've long known that there's
saltwater in that upper region.
The key is the underlying region - that's the source of freshwater.
That's the freshwater aquifer for this area.
What do we see?
Great - blue.
Freshwater.
We have freshwater punching out into Monterey Bay.
Another important point we see - very good delineation between this upper red saltwater
section and the underlying blue sections so we don't have communication.
The saltwater's not getting down into our freshwater aquifer.
To the north what do we see?
A different story.
There's nothing separating the saltwater at the top from getting into the area from below.
So we, unfortunately, see saltwater throughout this entire section, although higher salinity
towards the top than at the bottom.
So what are we planning to do?
Join us on the beach this October.
We are covering the entire stretch from Santa Cruz to Monterey - a 50 kilometer line - because
it's a longer line we'll be looking three hundred meters below the surface.
Eight people, fifteen days, $250,000.
So before I go to my closing remarks, I do want to acknowledge people who have made this
project the success it's been.
First of all - Adam Pidilsecky and Tara Moran - an integral part of the science project.
Along with my graduate students and post doc's, this has just been a real team effort.
Also have to thank the people who gave us all the logistical support including 85 individual
land owners, some of whom could be out here who are letting us walk along there beach
this fall, and then financial support which makes it possible at Stanford to do cutting
edge research while solving real world problems.
This real world problem, being an important local problem.
So I'll conclude by asking you as you leave today to go out of here thinking about freshwater
security.
How can people get the freshwater they need without damaging those natural systems that
are the source of our freshwater.
And of course I also hope you'll leave here thinking about sentinel geophysics, because
I'm convinced that sentinel geophysics is truly the type of advanced technology we need
for proactive effective groundwater management which is the base for building freshwater
security.
Thanks very much.