From Rainwater to Mineral Water: a Chemical Journey

            You may think of rainwater as an ultra pure form of water, but this is not actually true.  While high in the atmosphere, the water droplets interact with numerous gases, which dissolve in the water droplets before they fall to the ground.  Carbon dioxide (CO₂) is the most important of these gases, as it combines with water to form carbonic acid (H₂CO₃), which is a small component of rainwater.  This small amount of carbonic acid causes natural rain to be slightly acidic.  But most other kinds of water such as seawater, river and lake water, and water in the ground is slightly basic, the opposite of acidic.  How does this transformation occur? 

The answer lies in the fact that as rainwater flows down through the soil and rocks on its way to the ocean, it interacts with rocks in a process called “chemical weathering”.  A major weathering reaction involves the breakdown of the mineral calcite (CaCO₃) by the acidic rainwater, releasing calcium (Ca⁺²) and bicarbonate (HCO₃^-) ions.  The bicarbonate ion is slightly basic, changing the water into a slight base.  This is a necessary process, as most aquatic and marine life could not survive if the water were acidic like rainwater.  You can picture the chemical weathering process using a simple analogy: think of a simple drip coffee maker.  The hot water (rainwater) drips through the coffee grounds (soil and rock) and comes out as coffee (groundwater and stream water).  Just like in the coffee maker, both the water and the rocks are changed in the process of chemical weathering.     

A lot of weathering reactions are poorly understood however, and while I was an undergraduate, I undertook research with a graduate student named Hyojin Kim to better understand how water changes as it flows from cloud to stream to ocean.  The site we studied was in a nature preserve in Northern California, the Angelo Coast Range Reserve, in the watershed of the Eel River. 

Rivendell, the site we studied, with the wells marked.  Each contour line represents the same elevation on the hillslope.

The Elder Creek, which sits at the bottom of our hill slope and has basic water

            We drilled wells into the hill slope and devised a suction system to sample the groundwater from these wells everyday for several years.  We then analyzed these water samples for many kinds of ions such as calcium (Ca⁺²), magnesium (Mg⁺²), sodium (Na⁺) and iron (Fe⁺²).  We plotted the concentrations of these ions in the various wells over time, to see how weathering changed seasonally.    

One of our autosampling machines on the steep hill slope

The data showed that there are multiple variation patterns in the concentrations of these ions, as they vary with rainfall, seasonally, and on longer timescales.  Different ions display different behavior, showing that weathering reactions of different rocks contribute to the concentrations of different ions.  Ion concentrations did not increase immediately post rainstorm, showing that weathering was a complex process, and it took different amounts of time post rainfall for different ions to appear in the groundwater.  All of these conclusions show that chemical weathering is a complex process that deserves further study. 

Clues in the Crust Help Resolve an Atmospheric Conundrum

Despite taking over 23,000 breaths per day, you probably don’t often wonder where the oxygen necessary for your survival comes from.  In fact, this question is still asked by scientists and many are currently working to better understand how and when oxygen arose in our atmosphere. The term Great Oxidation Event, or GOE, is used to describe the point (or period) when the Earth’s atmosphere began to accumulate free oxygen and is thought to have occurred about halfway through Earth’s lifetime, around 2.4 billion years ago.

One of the methods used to establish this date is to track where certain elements are distributed throughout Earth over time.  One of these elements is molybdenum (Mo). While uncommon, Mo is produced in the igneous rocks in the Earth’s crust as magmas rise towards the surface.  It has been thought that Mo bonds with the element sulfur (S) and forms minerals called sulfides. One of the most common sulfides in the crust is pyrite, FeS₂, as pictured below. 

Figure 1. A photomicrograph of a pyrite crystal in an igneous rock. The pyrite is the light gold blob. Chalcopyrite, another sulfide, forms a gold rim on the right side and magnetite, an iron-oxide, forms the silver rim on the left side.  The pyrite crystal is about 100 micrometers long (the width of your hair is about 10 micrometers for comparison).

The uniqueness of Mo in sulfides lies in its sensitivity to oxygen – when sulfides in the Earth’s crust are exposed at the surface to oxygen in the atmosphere they will break down through a process called weathering. This breakdown releases the Mo and erosion carries the Mo into rivers which lead to the oceans.  The Mo then settles into the sediments on the ocean floor.  Before oxygen was abundant in the atmosphere, Mo would have stayed locked up in the sulfides in the crust.

                Geologists can then measure Mo concentrations in shales, a sedimentary rock formed in the oceans, over time. Given this weathering process, geologists can determine the GOE to be the point in time where they start seeing Mo in ocean shales.

Figure 2. A schematic diagram showing how Mo forms in the continental crust and is weathered and deposited in ocean sediments in the presence of oxygen. Before oxygen arose in the atmosphere the Mo would have stayed locked in the sulfides.

While this method has timed the GOE in concordance with other methods, the use of Mo to track oxygen is a method that is still being refined. For example, this process is dependent on the assumption that Mo is primarily hosted in sulfides in the crust. However, it has been shown that Mo can also bond with other elements and therefore may be more abundant in other minerals.  If this is true then Mo may not weather out of the crust from the presence of oxygen in the atmosphere. Current research is being done to study Mo in the crust and validate its use to track oxygen. 

The Interior of the Earth is Balloons!

     The mid-ocean ridges are mountain chains that curve around the Earth like baseball seams. From these mountains, lava comes slowly pouring out from somewhere within the Earth. I think of the mountain chains as the line on the roof of a house where the two sides meet. If you let go of a bunch of tiny balloons from the inside they would end up at this region on the roof. The balloons can be thought of as lava inside the Earth. And the lava becomes solid rock as it pours out of the mid-ocean ridges (the seam of the roof) and creates new "crust" on the surface of the Earth. The "crust" is a term geologists use to refer to a specific portion of the Earth. It is a region of the Earth that is more rigid (like a pie crust) than the inside (which is still rock, but since it is hotter it can 'flow' in a similar manner to a bending spoon). The inside of the Earth is actually split into a few different parts, but the focus of this blog is on a part that I study, called the "mantle." It moves around slowly, similar to a boiling pot of water, heat from the bottom causes the mantle/water to move up, as it moves up (away from the heat source) it cools down, and when it reaches the top of the Earth/pot it returns downward. This is called a convection cell, and there are many of them inside of the Earth's mantle. 

     The mid-ocean ridges are where two of these convection cells are moving upwards together, and then move away (see this image for a better understanding of all this stuff).

Artists conception of the interior of the Earth: The motion of the Mantle (the middle, orange region), can really be seen nicely here. The Core (the red part, at the center) is the source of heat that causes the mantle to move up in this image. The Mid-ocean ridge is the thin yellow line on the surface of the Earth. The melting region is the triangle formed just below the ridge.              Image Source: National Geographic

     Now, there is a lot of discussion about what causes these movements, but the basics are that the heat inside of the Earth is escaping, and that the ridges are an area of the Earth where things are moving away from each other. Just below the surface, there is a region of the mantle which is just hot enough to cause some of the material to melt. This melted material is lighter than the rock, and so it moves upwards. Now as you can imagine, the region containing melt is quite large, because a lot of the mantle is hot here. One of the main problems when this is realized is that if everything is moving up in this large region, why do we only see a thin ridge where all the lava comes out. One idea that explains this, that I use, is that there is a layer in the mantle that funnels the lava to the ridges. Think of it as a roof, and picture yourself in an attic. In this attic you have a bunch of balloons, if you let go of them you would see them travel up, hit the bottom-side of the roof, and then get funneled to the center. This is the way that we think melt from a large area arrives at the much smaller mid-ocean ridge. In this analogy, the center of the roof would be just below the mid-ocean ridge. 

The 'Balloon Analogy": The crust is represented by the green panels, and the arrows show that the crust is moving away from the mid-ocean ridge. The Grey is the roof that focuses the balloons (melt) to the ridge. The red balloons turn into green crust. The red arrows show the direction of the convection cells in the mantle. 

     This "roof" is a somewhat complicated structure and the way we understand and explain it actually allows us to predict the way and the locations that it will cause the lava to come out. Now you'll have to imagine a roof with more than two sides, maybe a house with a corner in the middle. This corner will get more of those balloons than other parts of the ridge. And extending the analogy to the Earth, a larger amount of lava and hence a larger amount of rock will appear here. 

     The beauty of this relatively simple idea is that we can now start to add more complicated things to different parts of the attic, and see where, and how the roof structure will create changes that we can then see at the surface of the Earth (except this part of the surface is at the bottom of the ocean, but that just means we get to go down in submarines and explore!). And when we find the things that our complications predict, we then add this small piece of knowledge to the growing body of knowledge about the Universe. Which is really what it's all about.

The 'Balloon Analogy 2: Son of Balloon Analogy": The crust is represented by the green panels, and the arrows show that the crust is moving away from the mid-ocean ridge. The Grey is the roof that focuses the balloons (melt) to the ridge. Only in this cartoon we have a corner. There is more crust (the blippy lines at the top of the image) at this corner. And we've also added a different kind of rock (yellow balloons) to the idea. The red balloons turn into green crust, the yellow balloons turn into yellow-green crust. You can begin to imagine what adding more colors, in different regions can do!

See the Earth's interior through noise

For many of us, the first impression of earthquakes is hazard, disaster or catastrophe. However, on the other side, seismologists can make use of them when the seismic detectors record them. Like the sound wave could travel through air carrying the message; seismic wave propagates deep into the Earth and tells us how it looks like, is it cold or hot, liquid or solid. This even leads to the fact that for tens of years, seismologists struggles in studying most part of the Earth’s interior due to the lack to seismicity in those areas. Now there might be a new way of exploring the Earth that meets the interests of both the public and scientists -- seismic noise imaging.

Detecting the useful signal among the ocean of noise. Imaging you and your friend standing on the two sides of a noisy river. The sound of water flow is so loud that you can't  hear what your friend is talking. Seismologists may be able to help by measuring the similarity of what both of you have heard.

Like the definition of most noises, seismic noise is an unwanted component of the signal recorded by seismometers. The recent study is challenging this assumption. The main idea of seismic noise imaging is to detect and amplify the common feature between two noisy waveforms from different detectors. Specifically, seismologists take two noisy waveforms recorded at the same time but different locations; and then shift one of the record along time to measure the similarity of between them. If there is a signal propagating through the Earth between these seismometers, the similarity between the two records should be significantly higher than the other time. You may wonder what if this similarity is just by coincidence?  Luckily, this weak signal we detect happens all the time. By looking through tons of noise record and adding up the detected signal, seismologists are able to be more and more confident about the result. This detection is then translated into the time needed to travel through this path. Combined with the distance, the velocity of the Earth’s interior is obtained. This usually tells us if the Earth is hot or cold inside.

Seismicity of the United States The distribution of the earthquake in the US is uneven, which makes it hard to study the Earth's interior covering the whole area. The application of noise imaging will be a powerful addition because it requires no earthquake data.

While the analysis of one pair of stations only provides the velocity structure in a limited area, same work can be done between various pairs of seismometers within a dense seismic array. Then seismologists can use tomographic techniques to generate a map of seismic wave velocity. Recent studies have shown that the noise imaging result is consistent with geological structures in the studied area. As more and more dense seismometer arrays become available, such work can be carried out throughout the Earth with numerous possible applications.

Reference

Shapiro, N. M., Campillo, M., Stehly, L., & Ritzwoller, M. H. (2005). High-resolution surface-wave tomography from ambient seismic noise. Science,307(5715), 1615-1618.

Weaver, R. L. (2005). Information from seismic noise. Science, 307(5715), 1568-1569.

HELP!!! I THINK MY WATERSHED BROKE!

A watershed is the region of land within which rain and surface water flows into a common drainage.  We live in the Chesapeake Bay Watershed, a large and well-studied area that covers 64,000 km² and includes parts of Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia.  Currently, 17 million people and (185 million livestock) live here.  And thanks to urbanization and land use change needed to support all of us, the watershed has begun to break down.

Over the last few centuries, we have significantly changed the natural state of the land in this region.  Areas that were once forested are now farmland or are filled with roads, houses and parking lots.  All of that construction and farming brings with it substantial waste in the form of sediment, pollutants, and an overabundance of nutrients like nitrogen and phosphorus.  So where does a lot of it go?  You guessed it—our rivers, streams, and, eventually, the Bay.  Most of our urban waterways are not places I want to go for a swim or even walk next to!

In addition to the typical pollutants we tend to associate with populated areas—industrial chemicals, heavy metals, vehicle exhaust—we also use a lot of road salt in the winter in this region.  Gotta get to work on a snowy day, right?  You don’t want to slip and fall on an icy sidewalk either.  If you never thought about where the salt goes once everything melts…yikes!  The figure below shows what has happened to salt (chloride in this graphic) in Baltimore waterways recently. 

More salt accumulates in urban areas, and it has reached levels that are damaging to plants and animals. Salt has even impacted the drinking water in the Baltimore area and could be a few decades away from having a drinking water reservoir that, from a chemistry perspective, begins to resemble ocean water.  And as much as you may like to put salt on things, you can’t drink from the ocean.

So what are we doing to fix everything that is broken in Chesapeake Bay Watershed? A LOT!  Policies are in place to reduce nutrient loads entering our rivers from farms and cities.  Education efforts from elementary schools to town hall meetings teach people about these problems and some of the solutions.  Stormwater management is considered on any new building site. And the scientific community is part of a large, concerted effort to work toward solutions that will benefit future generations.

My personal interest is in what we can do to help out urban streams.  Many streams are candidates for “restoration” in order to stabilize their banks to reduce erosion, add engineered structures to slow down the water, or reconnect streams with their floodplains through the creation of wetlands.  These are all designed to be functional and aesthetically pleasing,   But restorations can be expensive, so we have to consider a lot of questions to determine if we are spending our dollars wisely:

·         Are restorations successfully changing the chemistry of the water?

·         How much improvement can we expect?

·         How long will it take?

·         What are the best practices?

·         Where are they most effective?

We take part in long-term monitoring and frequent sampling of water upstream and downstream of where a restoration strategy has been put it in place.  We analyze the water chemistry in a lab to measure any seasonal differences, changes between normal flow conditions and storm events, and any identify entry of pollutants and nutrients from sources we can’t see.  You might have been thinking that a watershed is just affected by what happens at the surface. We wish it was so simple!  We have to consider a complicated, dynamic system underground and all of the changes that occur through time.  This is especially true in urban areas.  All of the broken, leaky pipes and sewer systems are now part of the watershed and are potential problem areas.  Even if you don't live in a city, remember that everything is connected.

We have a long way to go to identify how much damage has been done and how we can make improvements.  This is not just a regional or local issue.  It is a global consideration on the level of climate change.  Water is a necessity for life.  People aren’t moving away from our nation’s waterways anytime soon, so we have to learn how to coexist and create sustainable communities.  Help solve the problems locally and surf your watershed to find out about efforts in your own area. A broken watershed can’t fix itself.

The important black carbon

      The isotopic compositions of total organic carbon (TOC) in lakes have been widely used to interpret paleoclimatic changes and the depositional environments. In the lacustrine environment, TOC generally represents mixture of lake (aquatic plants and microorganisms) and external sources, which is lack of indicator for a particular composition.

      Black carbon (BC) is uniquely produced from incomplete combustion processes, such as forest fires and the burning of fossil fuels. δ¹³C values of black carbon (δ¹³C_BC) is a potential indicator of terrestrial environmental changes, representing the isotope value of terrestrial vegetation. Because BC particles are chemically inert, resistant to oxidation and biodegradation in natural conditions, they can be preserved in soils, lake and marine sediments for as long as thousands to millions of years

      Terrestrial vegetation input play an important role for the variation in δ¹³C_org, so we can use the isotopic composition of BC to judge whether the sediments have been influenced by terrestrial plants input or not. It will help us to interpret carbon cycle in lacustrine system more directly.

Procedure:

1.      samples were acid treated sequentially with HCL(3M), HF(10M)/HCL(1M), and HCL (10M)

2.      the remaining samples were treated with an oxidizing solution, containing 0.1M K2CR2O7 and 2M H2SO4, at 55 for 60h.

3.      all samples washed with deionized water, centrifuged and freeze-dried. The remaining refractory carbon in the residues was defined as BC