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.