When we used the term “sand” we are referring to sand-sized grains (0.25 to 2.0 mm) composed of the mineral Silicon dioxide {SiO2), aka silica crystals, as being typical found in (sourced from) granite and quartz rock.

Criteria 1:

That the sand used “drains well” … and nearly completely.  “Drains well” basically means going from a fully saturated (completely flooded) condition to when observable (unrestricted) drainage ceases within approximately 10 to 20 minutes from a 1/3 meter deep sand profile – once input pumping has stopped.

Drainage rate (aka hydraulic conductivity) is dominantly influenced by the average particle size but also by the specific particle size distributions (proportions).  Smaller particles (the more of them there are and the smaller they are on average) will produce increased drainage times (slow the drainage rate).  Also, the smaller the average particle size is the more water will be retained (not freely drain),  which would then occupy more of the pore space (blocking O2 and therefore inhibit aerobic organisms which includes plant roots), especially in the lower portion of the depth profile.

Criteria 2:

That the sand is chemically inert, meaning that it does not significantly change the pH of water coming into contact with it.

In other words, it does not contain water soluble compounds (material), such as Calcium carbonate (CaCO3) or Calcium Oxide (CaO), as two common examples.

With a  source water at pH 7 or below, a pH change of the water with each flooding/return of 0.0 would be ideal,  but pH +0.1 to +0.3 under initial conditions may be acceptable.  Remember than pH is a logarithmic scale, a change of one pH unit (+/-1.0) corresponds to a ten-fold change in the Hydrogen ion concentration.

NOTE: Use of source water above pH 7 is NOT advised.  If your ground/source water is, for example, pH 8, then lower the pH (with an acid, i.e Phosphoric or Sulphuric) to below pH 7, BEFORE adding it to the system.  In such a case, a separate (independent) storage tank expressly for this purpose is advised.  Harvesting and use of rainwater is strongly recommended for virtually all locations.

Criteria 3:

That the sand contains NO contaminants such as heavy metals, radio-nucleotides, water-soluble salt(s), plant pathogens, or man-made chemical pollutants to include all pharmaceuticals.

Example metals and nucleotides include:  Antimony, Arsenic, Cadmium, Cesium, Chromium, Lead, Mercury, Palladium, Platinum, Plutonium, Radium, Thorium, and Uranium isotopes.

These elements are found only EXTREMELY rarely (not a legitimate concern) in manufactured sand (crushed from ‘virgin’ granite or quartz) but MAY be found in certain natural sand deposits, particularly along river banks and lakes downstream or downwind of human/industrial activities.

Beach sand is almost always dominantly Calcium carbonate and therefor is not suitable (exception being volcanic glasses).  Limestone is obviously not viable and neither are most sandstones and other sedimentary composites, unless exceptionally ‘pure’ (99% SiO2).

That’s it, folks:  1) drains well, 2) chemically inert, and 3) no toxic contaminates.


A final note of CAUTION:  Do NOT accept any vendor’s claim of anything as being true (factual).  Always test the sand source being considered (drainage rate and pH effects) for yourself.  Minimize the risk of potential contaminants by sourcing ‘virgin’ crushed stone.


Further information can be found at:

Sand Selection Guide


Sand versus Gravel as a Biofilter Media

Sand Bio-filter Construction and Operation – Part 1