Step 1: Understanding groundwater hardness.
Groundwater hardness is determined by the concentration of calcium and magnesium ions dissolved in the water. These ions are the primary contributors to the hardness of water.
Step 2: Analyzing the options.
(A) Mohs' scale of hardness: This scale is used to measure the hardness of minerals, not groundwater.
(B) concentrations of calcium and magnesium: Correct — The hardness of groundwater is typically due to the concentration of these two minerals.
(C) Bernoulli equation: The Bernoulli equation is used to describe the flow of fluids and is not related to water hardness.
(D) Darcy's law: Darcy's law is used to describe the flow of water through porous media, not the hardness of water.
Step 3: Conclusion.
The correct answer is (B) concentrations of calcium and magnesium, as they directly contribute to the hardness of groundwater.