Basic information
Potassium (K) is a metallic element. It has similar chemical properties to Na. It is highly reactive and reacts quickly in the air and water. In nature, K occurs mainly as potassium ion (K+). Its compounds are soluble. Unlike sodium, K is a biogenic element due to its utilization by growing vegetation.
Primary source of K in river water and groundwater come from silicate weathering e.g. potassium feldspar, mica, biotite. Among four principle cations among major rivers, K is the least abundant and the least variable one.
Measurement techniques
K+ can be measured using either ion chromatography (IC) or inductively coupled plasma optical emission spectroscopy (ICP-OES)
Applications
Initial concentrations of K on infiltration is closely linked to parent lithology and acts as a signature for the water which may then undergo transformation by secondary reactions (dissolution-precipitation, ion exchange or redox reaction) and mixing. It is generally less aboundant than Na with initial molar ratios of K/Na of the order 0.005-0.5. However where K-feldspar is dominant the groundwater may have higher ratios – around 0.1.
As water moves downgradient, a sequential process of geochemical reactions takes place in which K/Na ratios may be vary and be useful as indicators (for example of K-feldspar involvement in reactions) and may give relative indication of residence times, as in the arkosic East Midlands aquifer.
Potassium may also be used with other nutrients such as nitrate to indicate contamination from agrochemicals and possibly urban wastes.
References
- Edmunds, W.M. and Smedley, P.L., 2000. Residence time indicators in groundwater: the East Midlands Triassic sandstone aquifer. Applied Geochemistry, 15(6): 737-752.
- Moss P D and Edmunds W M., 1992. Processes controlling acid attenuation in the unsaturated zone of a Triassic Sandstone aquifer (U K), in the absence of carbonate minerals. Applied Geochemistry 7, 573-583.
- Appelo, C.A.J and Postma D., 2005. Geochemistry, groundwater and pollution. Balkema.