Geoscience Reference
In-Depth Information
consistent with neutron and X-ray diffraction data and confirm the theoretical
model of Weinhold (
1998
); this model considers an asymmetric H-bonding pattern
in agreement with general quantum mechanical principles underlying H-bonding.
The subsurface liquid phase generally is an open system, and its composition is
a result of dynamic transformation of dissolved constituents in various chemical
species over a range of reaction time scales. At any particular time, the liquid
phase is an electrolyte solution, potentially containing a broad spectrum of inor-
ganic and organic ions and nonionized molecules. The presently accepted
description of the energy characteristics of the liquid phase is based on the concept
of matrix and osmotic potentials. The matrix potential is due to the attraction of
water to the solid matrix, while the osmotic potential is due to the presence of
solute in the subsurface water.
The composition of the subsurface liquid phase varies over time, mainly due to
recharge with rainwater, irrigation water, or the fluctuation of the water table
(groundwater) level. In addition, disposal and discharge of wastes and effluents
and the application of agrochemicals are key sources of pollution into the sub-
surface, contributing significant changes to the composition of the liquid phase.
1.2.1 Near Solid Phase Water
Water molecules that are oriented preferentially with the polar axis perpendicular
to the solid surface, in the vicinity of a solid surface, are considered near solid
phase water. When the net surface charge of the polar phase is negative, the
hydration occurs through one hydrogen of water forming a hydrogen bond with
specific atoms at the boundary of the polar surface, in such a way that the second
hydrogen still can form a hydrogen bond with another water molecule outside the
primary hydration level. In contact with a nonpolar solid, water molecules are
oriented such that the positive hydrogen points into the bulk solution (Yariv and
Cross
1979
).
A water molecule exhibits a series of spatial arrangements with great irregu-
larity as it moves through a bulk liquid phase. Unlike a solid, which has a well-
defined structure, the liquid phase has instantaneous structures (I type) comprising
molecules in highly irregular arrangements. Sposito (
1984
) showed that, with
lengthening of the time scale, two additional type structures are defined: vibra-
tionally averaged (V type) and diffusionally averaged (D type). These structures
highlight the fact that the concept of molecular structure in liquid water is a
dynamic one.
Stillinger (
1980
) conceives of liquid water as consisting of macroscopically
connected random networks of hydrogen bonds with frequent strained and broken
bonds that continually undergo topological reformation. The water properties arise
from competition among relatively bulky means of connecting molecules into
local patterns, characterized by strong bonds and nearly tetrahedral angles and
more compact arrangements characterized by more strain and bond linkage. The
