Most freshwater on earth is not that highly saturated with dissolved metals or minerals. However, there are exceptions to be found all over the world from natural acid rock drainage to the alkali springs of Jordan. If the concentrations of dissolved metals are high enough the water can be toxic. For example, water draining from gold mines is often very high in arsenic and must be contained and cleaned. It is incredibly important to understand what will happen to these dissolved ions because they have profound implications on the health of the environment and people. Water like this can occur naturally or due to mining, deforestation, or other human industrial activities.
One tool that we can use to understand water and what is happening to the dissolved metals and minerals is the geochemical modelling program called PHREEQC a.k.a PHREEQ (pronounced freak) to those in the business. PHREEQC is pretty much the industry standard amongst geochemists for modelling the composition and behaviour of dissolved ions and minerals in water and every aspiring geochemist has to be familiar with the basics of the program and what the information it provides is telling us. PHREEQC is a quick, easy and free way to do a huge number of tedious calculations really, really quickly. Yep, that’s right, it’s free on the USGS website, which is another great thing about it.
PHREEQC works by taking the concentration of ions in water such as calcium, sodium, sulphate, etc and calculating the concentration of these ions that actually participate in geochemical reactions at certain temperatures, pressure, salinities, pH’s and redox conditions. Once we know these values PHREEQC then calculates how much of these ions and the minerals that they combine to form are dissolved in the water and if they will precipitate out of solution to form actual minerals. It can do a lot more than this as well such as incorporate isotopes, model ion-ion interactions, ion-surface interactions, etc.
For this post I thought it might be interesting to show the PHREEQC output from one of the creeks that I sample called Red Creek and it is a bit of a weird one. Red Creek is located in the central Yukon and the most notable thing about it is the colour of the water and the rocks around it.
As you can see the rocks around Red Creek are red and black. They are shale and are loaded with all sorts of interesting elements, particularly iron. In fact the iron concentration in this water is about 3 ppm, the nickel and zinc values are 0.3 and 0.9 ppm respectively and the sulphur concentration is a whopping 340 ppm. These numbers are all way out of the ordinary for the rest of the creeks I sampled throughout the Yukon. In fact, the Fe, Ni, and Zn values are at least 10 times higher than anywhere else! WOW…(did I just find a new mine?…I wish)
—————————-Description of solution—————————-
pH = 6.790
pe = 4.000
Activity of water = 1.000
Ionic strength = 1.170e-002
Mass of water (kg) = 1.000e+000
Total carbon (mol/kg) = 2.435e-004
Total CO2 (mol/kg) = 2.435e-004
Temperature (deg C) = 25.000
Electrical balance (eq) = 2.563e-004
Percent error, 100*(Cat-|An|)/(Cat+|An|) = 2.15
Iterations = 10
Total H = 1.110128e+002
Total O = 5.552116e+001
I have included some of the highlights for what is called the saturation index. Basically this number tells us if a mineral is under-saturated in the water, meaning it will stay in solution or over saturated, meaning it will precipitate. If the number is negative the mineral is undersaturated and will not precipitate and if it is positive it is over saturated and will. In Red Creek there are hundreds of mineral species that are undersaturated and only a handful that are oversaturated. I have listed the oversaturated ones below. Some of these numbers are super high such as magnetite and hematite, which are clearly the ones precipitating on the rocks.
Barite — 0.47 — BaSO4
Fe(OH)2.7Cl.3 — 6.54
Fe(OH)3(a) — 1.89
Fe3(OH)8 — 2.35
Goethite — 7.78 — FeOOH
Hematite — 17.57 — Fe2O3
Maghemite — 7.17 — Fe2O3
Magnetite — 18.83 — Fe3O4
ZnSiO3 — 1.12
Red Creek is obviously a pretty wild place geochemically and the PHREEQC modelling opens the door for us to interpret it. There is a lot going on and one has to ask, where did all of the high concentrations of these metals come from? Well, in this case the question is a fairly easy one to answer. All you have to do is look around at the bedrock.
The local bedrock is black shale, a rock that is notoriously full of metals due to is high organic content. Red Creek is fed by springs issuing from the shale and the groundwater, which has had moved from its recharge point to discharging in the creek, has had time to leach metals from the rock. The water gets so loaded with metals from the bedrock that it carries them along as minerals in suspension as well as dissolved, which is why the water is that weak coffee colour. Actually, when the spring emerges from the shale the water is not white/red. It is, in fact, black!!! And I mean jet black. This is because it is loaded with reduced iron in suspension. Once the iron oxidizes at the surface it turns red. Futhermore, there is so much sulphur in the water that elemental sulphur often precipitates around the springs and the reduced, and highly toxic form of sulphur, hydrogen sulphide gas is bubbling out of the water as well because of the massive partial pressure difference in H2S in the atmopshere versus the water. What a wonderful place for a geochemist!!
Places like Red Creek have interesting geochemical stories to tell. In this case the dissolved metals are naturally occurring and no one lives in the area so no remedial action is necessary to make the water drinkable. However, water like this has major impacts on the life that can survive in the region and in the creek. Indeed, natural places like this are home to a wide variety of life that has adapted to survive and flourish in these harsh conditions that are found in very few places on Earth and we can learn a lot about life on our planet and potential life on others from places like Red Creek. However, if such a water body was the result of mining operations it is absolutely necessary that it be treated lest is thoroughly contaminate the local environment with heavy metals such as arsenic or mercury. It is the geochemists responsibility to ensure that places like this are understood so that when remedial actions are necessary the lessons learned from natural places can be applied.
Hope you enjoyed this geochemical adventure to Red Creek!