How Does Arsenic Get into the Groundwater?
The conditions that favour arsenic dissolution (becoming dissolved in the water) and mobilization (movement with the groundwater to your tap) depend on the circumstances. One thing that is certain is that it takes more than just high arsenic concentrations in the soil or rocks of a region. The soil in Bangladesh is much lower in arsenic than soils in many areas that do not share the problem of high arsenic in groundwater, so what is the difference? Well, for one thing, the arsenic needs to be in a soluble form to end up in drinking water.
Of the many known forms of arsenic, only a few are frequently detected in water.
- If the arsenic is not soluble, it will precipitate and remain in the solid phase of the groundwater system as part of the soil.
- If there is a binding site on the soil surface that is available, and the arsenic is in a form that binds strongly, it will also leave the water phase and attach to the soil.
So for most types of arsenic, they’ll just stay put. The deciding factor is probably some process that converts those stable, insoluble or stuck-on-surfaces forms of arsenic to a form that is soluble in the water
We have seen that the form of arsenic affects both its toxicity and its mobility. Organic forms of arsenic are rare in groundwater, so we will ignore them in this discussion. Most of the arsenic found in groundwater is either inorganic As(III) mainly as uncharged arsenious acid, or inorganic As(V) in the form of arsenic acid minus one or two of its protons (so with a charge of –1 or –2). That’s the case at neutral pH. As discussed in the “What is arsenic” section, the charge on As(V) allows it to bind to sites on the surface of soil particles, removing it from the water. As in that section we’ll assume that the interaction between arsenic compounds and surfaces is due to the charge, even though the real situation is more complicated than that.
As the pH is raised, the compounds will tend to become more and more negatively charged as the arsenic and arsenious acid lose H+ groups. So the charge of these arsenic compounds depends on the pH. You might (rightly) guess that as the pH goes up, the charge on the arsenic compounds becomes more negative and they should be better at binding to positively charged sites on the soil surface. The trouble is, the soil binding sites are also affected by the pH. As the pH goes up and the water becomes more basic, OH- groups from the water also associate with the adsorption or ion exchange sites on the soil, neutralizing them. Once they have been neutralized, they are not attractive to the arsenic compounds. The solubility of metals in water is also affected by pH, so if you get to a pH that dissolves the mineral phase, that will result in the release of anything bound to it. So instead of decreasing in concentration, the arsenic concentration in high pH water can actually go up!
In fact, in the southwestern part of the United States, the pH of the groundwater may be high due to the dry, arid conditions. Water evaporates under these conditions, raising the concentration of everything dissolved in it. These waters also tend to have a very long retention time, and older waters usually have higher pH values than water that is flushed through the system more quickly. So high arsenic concentrations in the arid southwest may be high because of (1) concentration by evaporation, and (2) desorption due to the high pH.
There is a general trend between pH and arsenic concentration in groundwater. As the pH increases, the arsenic concentration tends to go up. Unfortunately the correlation is not perfect: the real situation is much more complex than my explanation. Still, if the pH of your well water is high, you are at higher risk of also having high arsenic – so you should test the water.
The other major factor that affects the form of arsenic in solution is something called the redox state of the environment. The word redox is a contraction of “reduction” and “oxidation”. A redox reaction is any reaction in which electrons are passed from one atom to another. In a reaction like this, one chemical is reduced (that’s the chemical that gains the electron) and one chemical is oxidized (the one that loses the electron). Sometimes people use the saying “LEO the lion says GER” to remember which is which: Loss of Electron = Oxidation (LEO); Gain of Electron = Reduction (GER).
- Some chemicals, like O2, are really good at oxidizing other chemicals.
- Some chemicals like hydrogen sulfide (H2S) are great at reducing other chemicals.
- Many chemicals fall in between.
Since all chemicals are not equal in their ability to be reduced or oxidized, they have been tested and assigned values for their “electron transfer potential”. The lower the electron transfer potential, the less likely a compound will accept (gain) electrons or be reduced.
If you take all of the chemicals in a solution and add up the electron transfer potentials of each, you can find out if the environment will have a tendency to reduce chemicals that are added, or a tendency to oxidize them. In general, if the oxygen concentration in the environment is high, the redox potential will be positive and there will be a high likelihood that compounds in the system will be oxidized. This is an oxidizing environment. If, on the other hand, there is no oxygen present, and a high concentration of hydrogen sulfide is present, this is a reducing environment and chemicals that enter the system will tend to be reduced.
OK, so let’s get back to arsenic. Under reducing conditions, the most stable soluble form of inorganic arsenic is as arsenious acid (As(III)). Under oxidizing conditions, most of the arsenic will be in the As(V) form, arsenic acid, because it is more stable under oxidizing conditions. Again, remember that the mobility of arsenic may depend on its charge, so at neutral pH, arsenious acid is more mobile than the dissociated forms of arsenic acid. That means arsenic will probably be more mobile under reducing conditions because more of the arsenic will be present as arsenious acid.
Just as the pH of the system affects binding sites on soil particles, the redox potential also affects the binding sites. Many of the binding sites for arsenic are made of oxidized iron, aluminum or manganese species that form a coating on the soil particles or on the rock surface. Sometimes, the metals on the soil surface can also be reduced, releasing them into solution. That means the binding sites are no longer available on the surface and the arsenic that used to be bound is released into solution.
That makes two independent factors that are likely to increase the mobility of arsenic under reducing conditions:
- reduction of As(V) to As(III), which is more mobile
- reduction of binding sites, releasing bound arsenic
One more factor that can affect arsenic mobility under reducing conditions is sulfide. If there is sulfide present in water containing arsenic, the arsenic and sulfide may precipitate, removing both from the water phase. So you get increased mobility of arsenic under reducing conditions only as long as there is no sulfide in the water.
We’ve discussed all of this as if it were like chemicals in a jar of water with some soil in it. We need to add in another complicating factor. Living things make chemical reactions happen much faster than they would normally go in the absence of life by using proteins called enzymes. Enzymes bring together all the chemicals that are needed for a particular reaction so it can proceed more quickly. When the reaction is complete, the enzyme releases the products and starts over again.
The point of catalyzing reactions, from the organism’s point of view, is to trap the energy released from the reaction to fuel movement or growth, or to build new cellular material. Your cells use enzyme systems for respiration. In respiration, the food you eat and oxygen from the air you breathe react together releasing energy, carbon dioxide and water. You trap the energy so you can walk, run, grow, keep your heart beating, and much, much more!
There aren’t as many living things in groundwater as there are on the earth’s surface, but there are bacteria that survive and grow in this environment.
- Some of them can speed up the reduction of As(V) to As(III). That reaction would increase the mobility of arsenic in water.
- Other bacteria can reduce Fe(III) on soil surfaces to Fe(II), which is released into the water. Again, any arsenic that was attached to the Fe(III) binding site on the soil particle would also be released into the groundwater.
These two examples show how bacteria can affect arsenic mobility directly (by reducing the arsenic) or indirectly (by reducing the binding site). These activities in groundwater are usually limited by the amount of food available to the bacteria.
“Food” to the bacteria in this case is organic carbon – the same as your own food. Normally it is very low in groundwater because the degradable organic carbon is degraded near the surface of the soil column where there are plenty of organisms to use it, or becomes bound to particles near the surface. Less and less is available as the water moves downward toward the water table. In places where the organic carbon in the water is enriched – say by applying manure to soil just before a rain, or where a landfill releases organic carbon – more can reach depths in the groundwater to feed the bacteria that live there. Organic carbon in this system acts as a reductant, reducing the redox potential and fueling the reduction of As(V) to As(III) and Fe(III) to Fe(II). Again, both reactions increase the mobility of arsenic in water.
In groundwater environments with plenty of oxygen, oxidation reactions can also potentially release arsenic. In these situations, arsenic has to be associated with a reduced chemical, like sulfide. Many arsenic-containing minerals also contain sulfide.
In the presence of oxygen, bacteria can oxidize sulfides instead of organic carbon to generate energy (for these bacteria, reduced sulfur is “food”). Once the sulfide is oxidized to sulfate, it is soluble in water, and releases the arsenic. This is the same process as the one that releases acid and metals into water at mining sites, and it is the way that bacteria contribute to acid mine drainage.
Note that these reactions can occur without bacteria present, but they will proceed more slowly.
So to sum up, we have seen three ways that arsenic can get in to water.
- In situations where the pH is high, arsenic may be released from surface binding sites that lose their positive charge.
- When organic carbon is present in the groundwater, it can feed bacteria that release arsenic either by (a) directly reducing As(V) to As(III), which is more soluble, or (b) by reducing the element at the binding site which releases the arsenic that was attached there (for example, Fe(III) is converted to Fe(II) which dissolves in the water, freeing the arsenic).
- Finally, arsenic trapped in sulfide minerals can be released when the minerals are exposed to oxygen. This can happen when the water level drops and the minerals are exposed to air.