FACTORS WHICH AFFECT CONCENTRATION OF IONS IN WATER
The Pathway from Rain to Water Body: Rain contains just a few ions - just small concentration of carbonate (from dissolved CO2) and some Na+, Cl- and SO42- ions from sea spray that winds carry into clouds. When rain runs off bushland into streams, it picks up small amounts of nitrates and phosphates from natural nutrients on the surface and perhaps small amounts of Ca2+ and Mg2+ from decomposing materials.
If, however, rain water soaks into the ground and flows through underground aquifers and then into a stream, it will contain increased amounts of these (and more) ions by dissolving them from the soils and rocks it flows through.
If water perculates down to deep underground aquifers, heavy metals and other metal ions may be absorbed.
The pH of Rain: Water from acidic rain is better able to leach certain cations such as calcium, magnesium and iron from the soil it passes over.
The Nature and Amount of Human Activity in the Catchment: Land clearing generally leads to more water rapidly running across the land into streams. This results in more dissolved Na+, K+, Ca2+, and more.
Aguicultural pursuits often lead to fertiliser run-off. This increases the concentration of nitrates and sulfates.
Discharges into the Water: Discharges of raw and/or treated sewage increases the concentration of many ions, particularly nitrates and sulphates. Even good sewage treatment can increase the TDS of the water by 200 ppms or more.
Storm-water runoff in urban areas can similarly increase the concentration of a variety of ions.
Industrial effluents, if carefully monitored and controlled, can discharge heavy metal ions into water bodies, such as lead, mercury and cadmium.
Leaching from Rubbish Dumps: When rain and storm water flows over and seeps through poorly designed rubbish dumps, it dissolves many harmful substances such as cadmium from nickel-cadmium batteries, mercury and lead from other batteries, zinc from old galvanised iron and anions such as nitrate and phosphate from decaying organic wastes are carried into streams.
