Final answer:
The reason why calcium concentrations in groundwater are usually higher than in surface freshwater is due to longer contact time with bedrock in groundwater. Option b.
Step-by-step explanation:
As rainwater percolates through the soil and into the groundwater system, it has more time to dissolve minerals, such as calcium carbonate (CaCO3), from the surrounding bedrock. This dissolution process increases the concentration of calcium ions in the water. The persistent nature of groundwater movement allows for more extensive interaction with rock formations, leading to a greater dissolution of calcium-rich minerals. Furthermore, the naturally occurring carbon dioxide in the water forms carbonic acid, which assists in dissolving calcium carbonate, thus releasing calcium ions into the solution.
On the other hand, surface water has a shorter contact time with rocks and soils, limiting the amount of mineral dissolution that can occur. Moreover, other processes such as dilution from rainwater or the uptake of minerals by aquatic organisms can further reduce mineral concentrations in surface waters. Therefore, groundwater typically has higher levels of dissolved calcium compared to surface freshwater, which might lead to hard water conditions in areas where groundwater is the primary water source. Option b.