Basics of Environmental Sampling and Analysis - Fundamentals of Environmental Sampling and Analysis - page 31

2.3.2 Groundwater Flow in Aquifers

Groundwater flow measurement is extremely important. Imagine if your measure-
ments result in the groundwater flowing north when it is actually flowing south,
sampling expenses will be wasted and the whole remediation effort will be in vain.
Unlike surface water, groundwater flows very slowly (several feet to several hundred
feet per year) and the flow direction cannot be visualized but has to be determined by
other means, such as using wells (Chapter 3). To understand the groundwater flow,
which is critical to groundwater sampling and monitoring, some basic concept and
terminologies are introduced below.
Groundwater is stored in an aquifer, which typically is soil or rock that has a
high porosity and permeability. Here a rock refers to two or more minerals with
defined chemical composition (e.g., SiO 2 for quartz), and soil is the product of the
physical breakdown of rock. Soils that produce pumpable groundwater include
gravel, sand, and silt. Groundwater in clay can be abundant but it is often not
possible to pump water from a clay formation. In bedrock, abundant water can
generally be found in highly fractured rock. A few unfractured rock types can also
support abundant water, for example, sandstone, limestone, and dolomite.
The grain size dictates where and how fast water and contaminant will flow. In
large-sized gravels, the permeability is high, so water and contaminants will flow
quickly through gravel. Of the four sizes of soil, gravel is the largest and clay is the
smallest. For example, gravel measures less than 76.2 mm (3-inches) and greater
than 4.74 mm (0.19 inches). Sand measures between 4.74 mm (0.19 inches) and
75

m m (0.003 inches). Silt is between 0.075 mm (75 m m) and 0.002 mm (2 m m),

whereas clay particles measure less than 2 m (Bodger, 2003). In soil sample

preparation, these sizes are measured by sieves for gravel and sand, and by a
hydrometer for silt and clay. Table 2.6 is a representation of common sieves used in
soil sample preparation.
Most groundwater is present within 300 ft of the surface, but it can be as deep as
2000 ft (Moore, 2002). The vertical distribution of subsurface water can be
illustrated in Fig. 2.4. The groundwater table divides the subsurface into an
unsaturated zone (vadose zone) and a saturated zone. The unsaturated zone extends
from the surface to the water table through soil water zone to the capillary fringe.
The soil water zone is usually a few feet in thickness, and the capillary rise ranges

m

Table 2.6 Soil sieving (nominal dimensions of standard sieves)

Tyler Equivalent USA standard sieve British standard Sieve opening

(mesh/in) size (ASTM 11-61) (mesh/in.) (mm)

250 230 240 0.063

100 100 - 0.149

60 60 60 0.250

20 20 - 0.841

10 10 12 1.68