What is the main topic of the talk?

Questions 18 to 20 are based on the following monologue about rainwater. You now have 20 seconds to read Questions 18 to 20,
    Last week we were discussing the way weather conditions wear away rock. At that time in talking about how rainwater affects rock, we concentrated on what happens to the rock. Today we’ll talk about what happens to the water.
    You know that rainwater usually alters rocks and soil chemically as it filters into the ground. This process results in ground water that contains dissolved irons and other materials from the plant and animal life on and in the soil. We use the terms"soft water"and "hard water" to indicate the extent to which water has minerals dissolved in it.
    Soft water has few impurities or none at all. Hard water contains calcium plus other dissolved minerals. Hard water is normally drinkable, but it may have a slight taste because of the various irons in solution. One place where hard ground water can be found is below the water table in the soil-covered terrain with an underlying calcium base of limestone. Rainwater dissolves materials as it filters down, and these contribute to the ground water’s taste and hardness. We can sometimes find ground water that is almost dissolved as rainwater under a bare sandy hill or sand dune that is made of pure quartz sand. Analysis of such ground water showed that it has about the same amount of carbon dioxide dissolved in it as rainwater does and little else. This is because quartz is so insoluble that for practical purposes it is inert and adds no dissolved substances to the water as it seeps into the ground.
    Now in view of the geological make-up of our own region, would you expect our ground water to be primarily hard or soft? Please look into the problem and give me your answers next Monday.