Geologic Vulnerability - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 201

• Scoring this parameter is similar to scoring the composition of the unsaturated
zone (parameter #2). In practice, however, the factors determining the score are
the areal extent of the saturated zone and whether the aquifer is hydraulically
connected to additional or larger aquifers. If a saturated zone is encountered
and evaluated to be perched or discontinuous and therefore isolated, the scoring
value of 1 may be assigned. However, if the saturated thickness is 23 m and lies
within a mappable geologic formation, the scoring value of 10 may be assigned.
• Occurrence and relative abundance of groundwater . This parameter is similar to the
depth of groundwater (parameter #1) but focuses on whether groundwater is pres-
ent at relatively shallow depths, and of a sufficient quantity to sustain a rate of
withdrawal. A key aspect of this measure is whether any groundwater is encoun-
tered before a confining layer. The abundance of groundwater relates to the trans-
missivity of the formation and is not intended to focus on whether groundwater
is encountered.
• With this parameter, there is high scoring variability depending on the type of
materials in the deposit. Consider an extensive deposit composed of saturated
sand and gravel. This deposit would tend to have a much higher transmis-
sivity value, and would likely receive a high vulnerability score of 8 or more.
When this deposit is compared to a similarly extensive and saturated silty clay
deposit with a low relative transmissivity, the silty clay deposit would receive
a score of 5 or less.
• Area of groundwater recharge . This parameter focuses on the source of groundwater.
If the area being evaluated is a source for groundwater recharge, its risk will be
higher than an area not considered a significant source of groundwater discharge.
• Special care should be taken in urban areas to evaluate the anthropogenic
sources of groundwater recharge. Significant anthropogenic influences
include storm water infiltration, sanitary sewer leakage, and water supply
leakage. Other human impacts to consider are detention basins and wetland
modification.
• Areas of groundwater discharge . This parameter requires an understanding of
groundwater flow at different geographic scales; including specific sites, off-site
areas of larger extent, and the entire watershed.
• The migration and final discharge locations for groundwater includes an
understanding of the relationship between groundwater and surface water,
and permits an evaluation of the potential for interconnecting aquifer systems
and discharge to deeper aquifers. It is also essential to understand the geology
of a region and the potential influence of unconformities associated with the
depositional units within the watershed.
• Travel time and distance to point of potential exposure . In general, the longer the time
and distance required for a contaminant to reach its potential point of exposure,
the lower the risk. This is true for most contaminants that degrade naturally in the
environment.
• Evaluating anthropogenic influence with this parameter is also important. For
instance, if groundwater is used as a source of potable water and pumped
from the ground using extraction wells, the residence time of potential con-
taminants in the aquifer may be greatly reduced.