Geologic Vulnerability - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 200

• Depth to groundwater varies widely across the United States depending on
geology, climate, and anthropogenic influence. For example, groundwater
is routinely present at depths very near the surface along the east coast and
southeast; from a few meters to tens of meters in the Midwest; to perhaps sev-
eral hundred meters in the southwest. With respect to anthropogenic influ-
ence, the landscape in urban areas is characterized by extensive modifications.
The natural hydrology is altered by the impervious surface, the construction
of stormwater retention basins, and the use of more permeable areas for arti-
ficial recharge or groundwater storage. In addition, leaks from sanitary sew-
ers, storm sewers, potable water lines, and surface watering (especially golf
courses) may profoundly impact the depth to groundwater and the direction
of flow locally or over larger areas.
• Composition, areal extent, and thickness of soil units in the unsaturated zone . Composition
of soil or sediment above the water table is important to evaluate because it helps
to determine whether the soil or sediments above the water table will impede
the vertical migration of contaminants through the soil or sediment column.
For instance, clay soils are fine grained and generally impede or slow migra-
tion, whereas sand and gravel deposits tend to facilitate contaminant migration.
Interbedding of geologic units is also considered.
• Other significant factors include the presence of unconformities, primary
and secondary porosity and the potential for unsaturated soils located above
aquifers to be subject to anthropogenic disturbance. For instance, a clay layer
that is 6 m thick may appear to significantly impede the vertical migration
of contaminants. However, if portions are excavated, or if the integrity of the
clay has been compromised by vertical fractures, root fragments, or anthropo-
genic activities, these events increase the potential for contaminant migration
through the overlying clay into groundwater (Murray et al. 1997).
• Scoring this value therefore involves a consideration of multiple factors. With
the 10-point scoring system, sands and gravels typically receive 10 points. Clay
deposits of adequate integrity, thickness (generally more than 3-6 m thick),
and areal extent receive a score of 3 or less.
• Composition, areal extent, and thickness of saturated zone . Composition, areal extent,
and thickness of aquifer materials are important factors because they influence
the rate and direction of groundwater flow. Risk may be higher in a scenario
where the saturated thickness is 23 m (75 ft) and the material is composed of sand,
as opposed to a saturated thickness of 1 m composed of silt. The areal extent and
thickness of the aquifer must also be evaluated to establish whether multiple aqui-
fers exist and are hydraulically connected. For instance, using the example above,
if the 1 m thick saturated silt layer was hydraulically connected to the 23 m thick
saturated sand layer; this would change the risk evaluation.
• In an urban environment, perched saturated zones often result from anthropo-
genic influences. Construction activities such as roads, building foundations,
and utility corridors are backfilled with porous materials, and then become
more porous than natural soils and sediments. Over time these porous back-
filled materials become saturated with water. Therefore, investigative activities
must be very detailed in nature to evaluate the significance of potential anthro-
pogenic influences on the hydrology of the area being investigated and mapped.