Reactions between infiltrating rainwater and rocks over timescales of months, years, even millennia, give groundwater its essential and unique mineral character. An example of water-rock interaction is provided by the red sandstones of the East Midlands.
In this example, K-feldspar, a constituent mineral of the sandstone (Figure 1), reacts with carbonic acid (derived from the soil and atmosphere) to form secondary minerals including kaolinite and illite/smectite. At the same time silicon (Si), potassium (K+) and some trace elements (e.g. Ba2+) are released to the groundwater.
The left hand side of this photograph shows an outcrop of red Triassic sandstone.
A photomicrograph of sandstone is shown on the upper right hand side.
The lower right hand side shows a scanning electron microscope image of a K-feldspar grain from Triassic sandstone; the highly corroded framework of the mineral is still visible and is marked by feather-like overgrowths of clay minerals.
In simple form this reaction looks like this:
4K(Ba)AlSi3O8 + 4H2CO3 +
18H2O = 4K+(Ba) + 4HCO3- +
K-feldspar carbonic acid bicarbonate kaolinite silic acid
This is one of the ways that major ions (potassium, bicarbonate) and trace elements may enter the groundwater.
Ba (barium) is present in water primarily from natural
sources. It is a potentially toxic element (Figure 2) and a
guideline value of 0.1 mg/L has been given by EC regulators.
In most groundwater, its solubility is limited to values lower
than the guideline concentration by the presence of sulphate.