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.