Here's why:

* Contact with Rocks and Minerals: Groundwater travels through underground rock formations. As it does, it dissolves minerals and other inorganic compounds from the rocks, increasing its mineral content.

* Filtration and Chemical Reactions: Groundwater undergoes natural filtration through porous rock and soil, which can remove some organic matter but not all inorganic material. In fact, the filtration process can also lead to chemical reactions that further dissolve minerals.

* Longer Residence Time: Groundwater spends significantly more time underground than surface water. This longer residence time allows for greater mineral dissolution and potential contamination from underground sources.

Surface water, on the other hand, is more exposed to the atmosphere and interacts with various sources of organic material. This can lead to higher levels of dissolved organic matter but generally lower levels of dissolved inorganic materials.

However, it's important to note that:

* The specific mineral content of both surface and groundwater can vary greatly depending on the surrounding geology and human activities.

* Some surface water sources, like deep lakes or ancient aquifers, can have higher mineral content than some shallow groundwater sources.

Therefore, while groundwater generally contains more inorganic material, there are exceptions and specific examples may vary.