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Having quality groundwater means more than just having clean water. It takes into account the mineral content of the water as well. The quality of groundwater is controlled by many factors, including pollution, but there are three main factors that contribute to groundwater quality: climate, water depth, and soil and sediment.
The first factor is the climate of the area. In a hotter area that gets less precipitation, water evaporates faster as it gets near the surface of the ground. The water evaporates quickly, leaving the minerals in the ground. The next time it rains and water is soaked into the ground, the water absorbs the minerals and carries them into the water supply, giving it a salty taste. A cooler area that gets a lot of precipitation will have less of a mineral content in the water, improving the groundwater quality.
The second factor controlling groundwater quality is the depth of the groundwater from the surface. A deeper water level will see less pollution, because it is harder for the pollutants to reach the water. However, deeper water will have a higher mineral content. Water works to dissolve anything it comes in contact with, so as it sinks deeper into the ground, it is dissolving minerals and carrying them along toward the water supply.
Shallow water supplies will not be as mineralized, because the water doesn't go down as deep. It does, however, have higher levels of calcium, iron and magnesium because the soil doesn't have as long to remove them. This makes the water "hard." Shallow water also has a higher probability of being affected by contamination.
The third factor controlling groundwater quality is the type of soil and sediment in the area. Some types of sediment have chemicals and minerals, such as sulfur, that dissolve quickly, causing a higher mineral content in the water. Other types of sediment have fewer contents that can be easily dissolved and carried into the water supply.
Another aspect of groundwater quality controlled by soil type is the permeability of the soil. Sand, which has a high permeability, allows water to move through it easily, preventing the water from picking up a lot of minerals on its way through. Less permeable soil, such as clay, slows water down, giving it the chance to dissolve more minerals. However, when water moves slowly through the ground, there is a better chance for it to leave pollutants and other contaminants behind, creating a cleaner water supply.
@KoiwiGal - That's a good point. Even if they don't manage to change the way the land lies though, there are a lot of places where the groundwater is simply running out, whether because they are starting to pull up water from too deep (as it says in the article, deep water sources are often contaminated with minerals) or simply have used up too much of it and there's not enough to go around.
The most serious consequences of this are being seen in the agricultural sector. I know in Australia, fields in some places are beginning to suffer from being irrigated with water that contains too much salt.
I've also heard theories that the same thing happened to the Ancient Romans, and this is one of the reasons their civilization collapsed. They simply couldn't grow enough food on their fields anymore.
It's a bit scary to think about, but on the up side at least we understand what's happening, and in theory, have a chance to fix it.
Groundwater quality really depends on a lot of factors. For example, you can't get decent groundwater near the ocean a lot of the time, because you've got a combination of a nearby source of salty water, and likely very sandy soils.
If, however, you're lucky enough to get a reservoir of water that's protected from the ocean by clay or rock, you might be able to exploit that.
People should also bear in mind that groundwater is not a limitless resource. There are towns in the United States that have had to be abandoned because they used up too much groundwater and made the earth underneath the town unstable, leading to sinkholes.
It doesn't really occur to you when you're drinking from the tap, but that water has to come from somewhere and in some cases it's helping to hold up the earth beneath your feet.
One of the problems with pollution and groundwater is that some forms of groundwater move so slowly through the earth, they might be clean for years while pollutants are being used without qualms.
Then, decades later, the polluted groundwater begins to seep into the local lakes and springs and wells and people call for the pollution to stop.
Well, it's difficult to get active about some theoretical pollution until you see the real results of it, but by that point, even if you stop all the pollution immediately (I'm thinking specifically of pesticides and other chemicals) you'll still have decades worth of pollution working it's way through the groundwater.
This is why groundwater testing is so important, and why local people should act on the results. You might not have to deal with the consequences of using the land poorly, but your children will.
And unfortunately, there's no real way of cleaning up polluted groundwater once the damage has been done.