LAND AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 66

Soil and Subsurface Conditions
The creation of the surface layer of soil from weathered rock is largely due to
rainfall, with freezing in cold climates and dissolution of minerals in all climates
gradually allowing vegetation to get a foothold and begin the process of making
earth. The newest “dirt” is situated at the rock interface, and the oldest earth is
at the land surface. It requires thousands of years to form a soil mantle, and the
surface is constantly altered by wind and flood, redepositing the weathered soil
in distant locations.
The role of the soil mantle as the primary medium for rainfall infiltration was
discussed previously, as well as the physical, chemical, and biological processes
that remove and break down most of the pollutants that decompose on the surface
and are gradually incorporated into the organic surface layer. If the decomposition
products or land-applied chemicals can be solubilized, they will move with the
infiltrating rainfall and percolate through the soil mantle to the groundwater. Thus,
chemicals such as salts, nitrates, and various synthetic solvents and dielectric
solutions can become pollutants in the aquifer system. The greatest issue, of
course, is when these aquifers provide potable supply sources, and the risk of
ingesting these chemicals, even in minute quantities, is a cause for concern.
A natural soil with a vegetated surface responds slowly to precipitation. Initial
rainfall collects on vegetation and drips slowly onto the underlying soil surface.
Once rainfall has wetted the surface of soil particles (or any porous surface), it
begins to drain vertically under the inexorable pull of gravity. This movement is
altered by the size of particle spaces, the type of soil, and the variation in particle
density or compaction as the rain is pulled downward, with lateral movement
comprising a significant component in many complex soils. With the pore spaces
open to the atmosphere, the movement is constant, but can vary in duration. The
process can take weeks to pass through the soil mantle and become a part of the
groundwater system, but it forms a significant component of the hydrologic cycle.
Soil “design” in the land development process seldom considers what lies
beneath the surface, other than to attempt a “balance” of required earthwork
(excavating and filling) to implement the development desired. This is strictly
an economic consideration and reflects the mind set that regards the Earth as a
resource to be exploited rather than sustained. It assumes that our perceived needs
warrant any alteration deemed necessary to build successfully. In effect, we treat
the soil like dirt. One of the greatest obstacles to be overcome in the sustainable
design process is to enlighten the development community as to the importance
of what lies beneath the ground surface and the Earth itself. This water reservoir
storage medium, water purification, system, and growth medium, and its relevance
to what we build on the surface, is critical to sustaining the hydrologic cycle.
The best guidance that can be applied to make a development program sustain-
able is to fit the building program to the land surface, minimizing land disturbance
by carefully sculpting the land rather than destroying it. Numerous projects that
followed this guidance are discussed or referenced in later sections, but the princi-
ple is uncomplicated, and while the solutions will vary in different physiographic