Geology and Soils in Relation to Vadose Zone Hydrology

1. Geology and Soils in Relation to Vadose Zone Hydrology • Williams, 2002 http://www.its.uidaho.edu/AgE558 • Modified after Selker, 2000 http://bioe.orst.edu/vzp/

2. Open water surface • For gage pressure, p=0 at open water surface • In a porous medium, pressure is negative, or zero when saturated

3. Consider the following slides • Does the formation have: • Small pores (surface tension forces and capillarity • Large fractures, or tunnels, flowing like an open channel?

4. Typical Geologic Configurations: floodplains • Key points: • narrow continuous banding of alternating high and low permeability • not necessarily oriented “down stream” Floodplain deposits Point Bar Meandering direction Bedrock Bedload deposits

5. Typical Geologic Configurations: floodplains • Terraced stream channel with likely ephemeral perched water. Recent floodplain Bedrock

6. Typical Geologic Configurations: Karst Karst is water-eroded limestone. This creates subsurface channels, some large enough to survey by boat. Equivalent structures (macropores) are also critical in vadose environments. Local artesian pressure rises above this surface Sink Soil and clay Crevices saturated to this level

7. Geologic Configurations: beach deposits • Beach deposits, although similar to river deposits in texture, have unique structure • Generally (not always!) fining upward • Laterally extensive • Lower variation in energy (more uniform)

8. Typical Geologic Configurations: Basalt • Basalt flows may have alternating porous, fractured, • and low permeability regions with sedimentary • deposits between flows Vesicular structure near top of each flow Alluvial till btwn flows Open lava tube – high K Alluvial fill

9. Typical Geologic Configurations Fractures, Dikes, Fill Soil zone Undifferentiated glacial till Clays Fracture Unaltered bedrock Joint

10. Geologic Configurations: various “aquifers” • What’s an aquifer? Water that will flow into a well… Surficial unconfined aquifers Interfill blanket aquifer Interfill valley aquifer Bedrock valley aquifer Bedrock surface

11. Water Tables (continued) • Many aquifer systems have perched water tables that can be productive

12. A Primer on Properties and Description of Natural Media • Particle Size Distribution • Soil Classification • Clay mineralogy

13. Why do we care? • Transport through natural porous media cannot be understood from mathematical notation and boundary conditions alone. • The structure, setting, history and chemistry of the mineral system in the vz all play central roles in transport (e.g. in flow + transport)

14. The ultra-basics • Particle size distribution is plotted as the mass which is made up of particles smaller than a given size. • Very useful in estimating the soil’s hydraulic properties such as the water retention characteristics and the hydraulic conductivity.

15. Standard • sieve • sizes

16. Typical Particle Size Plot

17. Moisture Content Characteristic Curve • Grain size, or particle-size distribution is not enough. • We need to show how moisture content correlates to capillary pressures. • Also called matric pressures. • Negative pressures.

18. Capillary rise from water table into a porous soil

19. Particle size to Characteristic Curves • (a) Particles distributed between dmin and dmax • (b) Pore size distribution similar: • The ordinate goes from mass of particles, to volume of pores. • (c) Laplace’s eq. relate pore size filling pressure of each pore. • Plot becomes filling pressure vs. volume of pores. • (d) Finally note volume of pores = degree of saturation.

20. Summary statistics for particle size distribution • d50, d10, d80 etc. • Uniformity coefficient, U • U = d60 /d10 [1.1] • U between 2 and 10 for “well sorted” and “poorly sorted” materials

21. Dependence of bulk density on particle size distribution • Uniform particle size distribution gives uniform density • increasing the range of particle sizes gives rise to greater bulk density.

23. What are is the basis ofsize classes? • Clay: won’t settle (<2m: doesn’t feel gritty between your teeth). • Silt: settles freely, but cannot be discriminated by eye (isn’t slippery between your fingers; doesn’t make strong ribbons; goes through a number 300 sieve; 2m<silt<0.05mm). • Sand: you can see (>0.05 mm), but is smaller than pebbles (<2mm).

24. Systems of soil textural classification • (The USDA is standard in the US)

25. Sand, Silt, Clay – Textural Triangle • Standard textural triangle for mixed grain-size materials

26. What can we use from the soil type? • After soil size, what else can we get from the USDA soils map, or a soil investigation, to inform us re: flow and transport? • History (soil formation) • Chemistry and Mineralogy • Biology

27. Soil Classification • Based on present features and formative processes • Soil is geologic material which has been altered by weathering an biological activity. Typically extends 1-2 meters deep; below soil is “parent material” • Soil development makes sequence of bands, or horizons.

28. Eluvial processes • Clay is carried with water in eluviation and deposited in illuviation in sheets (lamellae) making an argillic horizon. • Soluble minerals may be carried upward through a soil profile driven by evaporation giving rise to concentrated bands of minerals at particular elevations.

29. Vertical Variations in Soils • Banding also arises from the depositional processes (parent material). • The scale of variation shorter in the vertical than horizontal. • Layers may be very distinct, or almost indistinguishable.

30. System of designations • Three symbol designatione.g. “Ap1” • “A” here is what is referred to as the designation of master horizon • There are six master horizon designations; O, A, E, B, C, and R.

31. Master Horizon Designations • O: dominated by organic matter • A: first mineral horizon in a soil with either enriched humic material or having properties altered by agricultural activities (e.g., plowing, grazing). • E: loss of a combination of clay, iron and aluminum; only resistant materials. Lighter in color than the A horizon above it (due to a paucity of coatings of organic matter and iron oxides)

32. Master Horizon Designations (cont.) • B: below A or E, enriched in colorants (iron and clays), or having significant block structure • C: soil material which is not bedrock, but shows little evidence of alteration from the parent material. • R: too tough to penetrate with hand operated equipment. • For complete definitions, see the SCS Soil Taxonomy (Soil Conservation Service, 1994).

33. Master Horizon Designations (cont.) • Major designations may be combined as either AB or A/B if the horizon has some properties of the second designation

34. Subordinate classifications • Lower case letter indicates master horizon features. • There are 22. e.g. • k = accumulation of carbonates • p = plowing • n = accumulation of sodium • May be used in multiple

35. Designations (last note) • Arabic numerals allow description of sequences with the same master, but with differing subordinate (e.g., Bk1 followed by Bn2). • Whenever a horizon is designated, its vertical extent must also be reported.

36. Color and Structure tell genetic and biogeochemical history • Dark colors are indicative of high organic content • Grayish coloration indicates reducing (oxygen stripping) conditions • Reddish color indicates oxidizing (oxygen supplying) conditions. • Relates closely to hydraulic conditions of site • VERY USEFUL !!

37. Munsell Color Chart • http://en.wikipedia.org/wiki/Munsell_color_system • “Munsell system is still widely used, by, among others, ANSI to define skin and hair colors for forensic pathology, the USGS for matching soil colors, and breweries for matching beer colors” • http://www.sbg.ac.at/ipk/avstudio/pierofun/protocol/soilchart.pdf • Question: • Any experience in lab, field, where color was important? • Distant students, feel free to send in any interesting examples, ag or environmental context

38. Quantification of Color • Munsell Color chart by hue, value and chroma; summarized in an alpha-numerical coding shorthand. • Pattern of coloration is informative. Mottling, where color varies between grayish to reddish over a few cm, most important. • Intermittent saturation; oxidizing then reducing • Precise terminology for mottle description (e.g., Vepraskas, M.J. 1992).

39. Structure • Must identify the smallest repeated element which makes up the “soil ped” Include details of the size, strength, shape, and distinctness of the constituent peds.

40. Climate • Six major climatic categories employed in soil classification; useful in groundwater recharge and vadose zone transport. • Aquic: precipitation always exceeds evapotransiration (ET), yielding continuous net percolation. • Xeric: recharge occurs during the wet cool season, while the soil profile is depleted of water in the hot season. • Identifying the seasonality of the local water balance is fundamental to understanding the vadose zone hydrology.

41. Six categories of climates

42. High Points of Clay Mineralogy • General • http://en.wikipedia.org/wiki/Clay • Clay constituents dominate hydraulic chemical behavior • They are REACTIVE • Two basic building blocks of clays • Silica-centered tetrahedra • Variously-centered octahedra

43. Clay: fn of size and mineralogy • Colloids • http://en.wikipedia.org/wiki/Colloid • Colloids* in soil water systems • Clay particles • Viruses and bacteria • Humic macromolecules • Other organics * All would classify as clay if size were only metric

44. Basic Formations • chain structures (e.g., asbestos) • amorphous structures (glasses) • sheet structure (phyllosilicates; includes clay)

45. Unit-cells octa- and tetrahedral units

46. Isomorphic Substitution • Silica tetrahedron: four oxygen surrounding one silica atom • Space filled by the silica can accommodate atoms up to 0.414 times O2 radius (5.8 x 10-9 m): includes silica and aluminum. • Balanced charge if the central atom has charge +4, negative charge if the central atom has a less positive charge (oxygen is shared by two tetrahedra in crystal so contributes -1 to each cell). • Same for the octahedra: 0.732 times O2 radius (1.02 x 10-8 m): iron, magnesium, aluminum, manganese, titanium, sodium or calcium, (sodium and calcium generate cubic lattice rather than octahedra)

47. Ion Ionic R : R x o radius. n m 2 - O 0.140 -- - F 0.133 -- - Cl 0.181 -- 4+ Si 0.039 0.278 3+ Al 0.051 0.364 3+ Fe 0.064 0.457 2+ Mg 0.066 0.471 4+ Ti 0.068 0.486 2+ Fe 0.074 0.529 2+ Mn 0.080 0.571 Ionic radii dictate isomorphic substitution Fit into Tetrahedron (radius <0.41 t imes that of oxygen Fit into Octahedron + (radius <0.732 Na 0.097 0.693 2+ Ca 0.099 0.707 t imes that of + K 0.133 0.950 oxygen) 2+ Ba 0.13 4 0.957 + Rb 0.147 1.050 Ca2+

48. Surface Functional Groups • Clay minerals surfaces made up of hexagonal rings of tetrahedra or octahedra. • The group of atoms in these rings act as a delocalized source of negative charge; surface functional group (a.k.a. SFG). • Cations attracted to center of SFG’s above surface of the sheet. • Some (e.g., K+ and NH4+) dehydrated and attached to the SFG: inner sphere complex with the SFG • Cations bound to the SFG by water: outer sphere complex • Inner and outer sphere ion/clay complexes are the Stern layer.

49. Details of Stearn Layer • Anions will be repelled from clay surfaces. • Zig-zag arrangement of negative and positively charged elements in the clay generates a dipole moment which attracts charged particles. • Diffuse attraction results in increased ionic concentration: Gouy layer (Gouy, 1910). • Dipole-dipole attraction also holds water to the clay surfaces, in addition to osmotic force from cation concentration near the clay surfaces.

50. Hydration of Cations