Geologic Vulnerability - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 199

TABLE 6.1
Geologic Vuln erability Matrix and Scoring System

Parameter
Identification
Number

Rating
Strength Parameter Description

1 Depth to groundwater

<3 m (10 ft) below the ground surface

10

3-10 m (10-30 ft) 5

>10 m 1

2 Composition, areal extent, and thickness of soil units in the unsaturated zone

Thick and extensive sequence of sand and gravel

10

Interbedded sands and clay deposits 5

Thick and extensive sequence of clay 1

3 Composition, areal extent, and thickness of saturated zone

Thick and extensive sequence of sand and gravel 10

Interbedded sands and clay deposits 5

Thick and extensive sequence of clay 1

4 Occurrence and relative abundance of groundwater

25% or less likelihood before encountering an confining layer

10

25%-74% likelihood 5

>75% likelihood 1

5 Area of groundwater recharge

Signiicant area of recharge

10

Moderate area of recharge 5

Not a significant area of recharge 1

6 Areas of groundwater discharge

Signiicant area of recharge 10

Moderate area of recharge 5

Not a significant area of recharge 1

7 Travel time and distance to point of potential exposure

<10 years

10

10-25 years 5

>25 years 1

8 Source of potable water

Current source of potable water

10

Potential source of potable water 5

Not a potential source of potable water 1

A sequential description of the vulnerability matrix parameters presented in Table 6.1
follows:

• Depth to groundwater . The closer to the surface groundwater is encountered the
higher the geologic vulnerability because contaminants have a shorter distance to
migrate vertically before encountering saturated conditions.