The qualities of groundwater and surface water are quite different. When compared together, groundwater is known to have the following qualities: Groundwater has higher salinity：As groundwater flows farther and slower, it has longer contact with the rocks and soil, so the soluble minerals can be dissolved more easily. Thus, groundwater contains more salt than surface water. Groundwater has higher density：With higher salinity, groundwater has higher density as a result. Groundwater has smaller temperature differences：Groundwater exists and flows in the soil and under the rocks, and is less affected by changes in temperature, and the temperature difference throughout the year is also small. Thus, groundwater is cool in summer and warm in winter. Groundwater is less susceptible to pollution. However, once polluted, it is more difficult to recover.