Porosity and Permeability and Hardpan

In some of the Diamond Hunting videos (and likewise some Gold Hunting videos), you may hear references to "Hardpan" encountered when digging for subsurface Gravel Deposits (which - depending on the locality - hold the "Heavy Minerals" with gold, diamonds, and other "goodies" of Specific Gravities higher than Quartz).  (Quartz has a Specific Gravity of 2.66-2.72 - from different internet sources.  In other words, it is approximately 2.7 times the density of water.  As quartz is the most common mineral, it is used as the comparative standard.)

With the (overall) vertical downward Percolation of rainwater through the various Soil Horizons (to be discussed another time) the "Micro" and "Meso" pathways are influenced by the Porosity and Permeability of the soil and its constituents (which likely change vertically due to variations in grain composition and "packing").

As for the terms "Porosity" and "Permeability", "Porosity" is the percentage of open space within a defined medium.  "Permeability" is a measurement of the "connectivity" between the spaces.  IOW, you can have a rock that "looks like Swiss cheese", but if the spaces are not connected, there can be no fluid movement.

Within the Unsaturated Zone, water can move upward by Capillary Action for short distances depending on the dominant mineral types.  Below, in the Saturated Zone, water moves by gravity.  In deeper settings, groundwater can also be driven by Lithostatic and Hydrostatic Pressure (from the weight of overlying rock and groundwater, respectively).

In the previous video at 0:23, the pathway variations are described.  Aside from the Granular, Platy, Prismatic, and Sub-angular Soil Textures, this resource has a few more textures, as well as other bits of important information. 

Back to the issue of "Hardpan", Groundwater Chemistry is influenced initially by ambient Atmospheric Chemistry and then downward by interactions with decaying organic matter and soluble minerals.  It is normal for rainwater to be slightly acidic due to mixing with Atmospheric Carbon Dioxide and with local human-concentrated gases such as NiOx gases (largely a byproduct of auto catalytic converters) and Sulfur gases (related to the combustion of coal and lower-quality diesel fuels and emissions from petroleum refineries and mineral smelters).  

With the initially-acidic groundwater, available cations can be added and when there is a change in chemistry and/or percolation rates.  Essentially, it becomes a site of chemical deposition developing an impervious "crust" usually cemented by iron oxides, silica, or, in arid and semi-arid climates, calcium carbonates (aka "caliche").  This mineral "cement" bonds together whatever else is available, in some cases locally creating a sandstone, breccia, or if buried river gravels are available, then you can get a conglomerate such as in Figure 1, with clasts of jasper and quartz in the Crater of Diamonds.

Figure 1.

Figure 2.  Iron-cemented sandstone (left) vs. Silica-cemented sandstone (right).

Figures 3 & 4 show exposed Caliche-cemented Hardpan, in the Thunderbird Hills, north of Glendale, Arizona.  The clasts are predominantly Tertiary or Quaternary basalt.  The available calcium carbonate probably came from the dissolution of regional Paleozoic and Mesozoic (?) carbonates and freshwater Limestones in the Verde Formation.

Figure 3.

Figure 4.

In past cycles of erosion and deposition, hardpan can sometimes serve as a "temporary" base for deposits of overlying alluvium, colluvium, or "lag" residuum left behind with the chemical weathering and erosion of the original local diamond host-rock (Lamproite).  In the case of Crater of Diamonds mining, the hardpan, and overlying gravel is hoped to hold diamonds (as it seems to do so).

More posts here and here and here about Heavy Minerals.