Genesis and function of pore structures formed in saprolite by Mike Vepraskas, NCSU, Soil Science

Veins in saprolite

Focus of past research Identify materials suitable for on-site wastewater treatment and disposal –suitable means k sat > 0.4 cm day -1 Determine if quartz veins conduct water quickly Identify horizon properties that allow prediction of restrictive k sat

Objectives Review typical k sat changes with depth Discuss water movement through saprolite matrix Review research on quartz veins and water movement

No BC or CB and shallow Cr K sat Depth Cr

No BC or CB and sandy C K sat Depth Sandy C Cr

BC or CB with no mixed mineralogy K sat Depth BC

BC or CB with mixed mineralogy K sat Depth BC Bt ???

How does water move through the saprolite matrix?

K sat profile – C2 horizon K sat (cm h -1 ) Depth (cm) C2 CB C2 C1 Bt C3

Veins and structure in C

Water conducting fractures in mineral grains

Fracturing of mineral grains due to biotite weathering

Water movement between mineral grains in C2 High K sat

Photo of dye in C2

K sat profile – CB horizon K sat (cm h -1 ) Depth (cm) C2 CB C2 C1 Bt C3

Veins and structure in CB

Clay plugging in CB

Clay plugging of pores in CB or BC Clay in pores Low K sat

K sat profile – Bt Horizon K sat (cm h -1 ) Depth (cm) C2 CB C2 C1 Bt C3

Clay skin on ped face in Bt horizon

Illuvial clay in pores in Bt horizon

Water movement in Bt horizon Low to high K sat Peds

Quart veins and water movement

Photo Quartz vein

Questions about quartz veins 1.Do quartz veins and fractures in saprolite conduct water 2. How can non-conducting veins be separated from conducting veins in the field

Schematic of field experiment

Procedure Pond water over saprolite that has a vein to measure K Apply dye to stain flow paths Apply Br- salt for a specific time period and measure depth of penetration to estimate K

Illustration of Br concentrators with depth across ponded area

Estimated K K e = -D T ( H/Lw) K e = Estimated K D = Depth of Br penetration = Water content T = Time Br applied H/Lw = Hydraulic gradient

Results KeKe DrainfieldVeinMatrix cm/day

How to field identify conducting veins

Look for clay or Fe/Mn in voids and between fragments

Observe infillings

Pore fillings and coatings Spaces between gravels in veins are filled with clay or Fe/Mn minerals These materials restrict water flow through veins All veins and fractures studied had materials in pores or gravels

If pore fillings are hydrothermal then veins are probably plugged very deeply

Conclusions Water conducting voids in saprolite are: –Spaces between mineral grains –Channels made by organisms Veins and fractures conduct water at rates similar to the saprolite matrix Veins do not impede flow Clay and oxide minerals plug pores in fractures and veins No unplugged features were observed

Conclusions The saprolite (C horizon) usually had a higher k sat than the BC horizons BC horizons formed under well developed Bt horizons Water flow through C horizon occurs between mineral grains (like sand) BC horizons have a low k sat values when spaces between grains are filled in.

Where does the water go and how fast?