A material is considered permeable if it contains interconnected pores, cracks, or other passageways through which water or gas can flow. Hydraulic conductivity (synonymous with permeability as used in this text) is a measure of the ability of liquids and gases to pass through soil. A preliminary estimate of permeability can be found in most Natural Resources Conservation Service (NRCS) soil surveys. The final site and process selection and design should be based on appropriate field and laboratory tests to confirm the initial estimates. The permeability classes as defined by the NRCS are presented in Table 2.17. Natural soils at the low end of the permeability range are best suited for ponds, wetlands, OF, and treatment of industrial wastewaters and sludges that might have metals. Soils in the midrange are well suited for SR and for land application of biosolids; these soils can be rendered suitable for the former uses via amendments or special

TABLE 2.18 | ||

Typical Vertical Hydraulic Conductivity Values | ||

Soil or Aquifer Material |
K (m/d) |
K (ft/d) |

Clay soils (surface) |
0.01-0.02 |
0.03-0.06 |

Deep clay beds |
1 x 10-8-0.01 |
3 x 10-8-0.03 |

Clay, sand, gravel mixes (till) |
0.001-0.1 |
0.003-0.3 |

Loam soils (surface) |
0.1-1 |
0.3-3.0 |

Fine sand |
1-5 |
3-16 |

Medium sand |
5-20 |
16-66 |

Coarse sand |
20-90 |
66-300 |

Sand and gravel mixes |
5-100 |
16-330 |

Gravel |
100-1000 |
330-3300 |

treatment. The soils at the upper end of the range are suited only for SAT systems in their natural state but can also be suitable for ponds, wetlands, or OF with construction of a proper liner.

The movement of water through soils can be defined using Darcy's equation:

where | |

q = |
Flux of water (the flow per unit cross-sectional area (in./hr; cm/hr) |

Q = |
Volume of flow per unit time (in.3/hr; cm3/hr). |

A= |
Unit cross-sectional area (in.2; cm2). |

K= |
Permeability (hydraulic conductivity) (in./hr; cm/hr). |

H= |
Total head (ft; m). |

L= |
Hydraulic flow path (ft; m). |

AH/AL = |
Hydraulic gradient (ft/ft; m/m). |

The total head can be assumed to be the sum of the soil water pressure head (h) and the head due to gravity (Z); that is, H = h + Z. When the flow path is essentially vertical, the hydraulic gradient is equal to 1 and the vertical permeability (Kv) is used in Equation 2.12. Typical values of vertical permeability are presented in Table 2.18.

When the flow path is essentially horizontal, then the horizontal permeability (Kh) should be used. The permeability coefficient (K) is not a true constant but is a changing function of soil-water content. Even under saturated conditions, the K value may change due to swelling of clay particles and other factors, but for general engineering design purposes it can be considered a constant. The Kv will not necessarily be equal to the Kh for most soils. In general, the lateral Kh will be higher, because the interbedding of fine- and coarse-grained layers tends to restrict vertical flow. Typical values are given in Table 2.19.

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