Current through Register Vol. 63, No. 3, March 1, 2024
The site-specific parameters are to be scored as specified in
this section. If any of the parameters in sections (1)-(5) of this rule is
unknown, that parameter must be given a score of ten:
(1) Depth to Groundwater: This is the
vertical distance (rounded to the nearest foot) from the surface of the ground
to the highest seasonal elevation of the saturated zone. The score for this
parameter is:
(a) > 100 feet, 1;
(b) 51-100 feet, 4;
(c) 25-50 feet, 7;
(d) < 25 feet, 10.
(2) Mean Annual Precipitation: This
measurement may be obtained from the nearest appropriate weather station. The
score for this parameter is:
(a) < 20
inches, 1;
(b) 20-45 inches,
4;
(c) > 45 inches,
10.
(3) Native Soil or
Rock Type: The score for this parameter is:
(a) Low permeability materials such as clays,
silty clays, compact tills, shales, and unfractured metamorphic and igneous
rocks, 1;
(b) Moderate permeability
materials such as fine and silty sands, sandy loams, loamy sands, and clay
loams; moderately permeable limestones, dolomites and sandstones; and
moderately fractured igneous and metamorphic rocks, 5;
(c) High permeability materials such as sands
and gravels, highly fractured igneous and metamorphic rocks, permeable basalts
and lavas, and karst limestones and dolomites, 10.
(4) Sensitivity of the Uppermost Aquifer: Due
to the uncertainties involved in the Matrix evaluation process, this factor is
included to add an extra margin of safety in situations where critical aquifers
have the potential to be affected. The score for this parameter is:
(a) Unusable aquifer, either due to water
quality conditions such as salinity, etc.; or due to hydrologic conditions such
as extremely low yield, 1;
(b)
Potable aquifer not currently used for drinking water, but the quality is such
that it could be used for drinking water, 4;
(c) Potable aquifer currently used for
drinking water; alternate unthreatened sources of water readily available,
7;
(d) Sole source aquifer
currently used for drinking water; there are no alternate unthreatened sources
of water readily available, 10.
(5) Potential Receptors: The score for
potential receptors is based on both the distance to the nearest well and also
the number of people at risk. Each of these two components is to be evaluated
using the descriptors defined in this section:
(a) The distance to the nearest well is
measured from the area of contamination to the nearest well that draws water
from the aquifer of concern. If a closer well exists which is known to draw
water from a deeper aquifer, but there is no evidence that the deeper aquifer
is completely isolated from the contaminated aquifer, then the distance must be
measured to the closer, deeper well. The distance descriptors are:
(A) Near, < 1/2 mile;
(B) Medium, 1/2-2 miles;
(C) Far, > 2 miles.
(b) The number of people at risk is to
include all people served by drinking water wells which are located within two
miles of the contaminated area. For public wells, count the number of users
listed with the Oregon Health Division, Drinking Water Systems Section. For
private wells, assume three residents per well. In lieu of a door-to-door
survey of private wells, it may be assumed that there is one well per
residence. The number descriptors are:
(A)
Many, > 3000;
(B) Medium,
100-3000;
(C) Few, <
100.
(c) The score for
this parameter is taken from the combination of the two descriptors using the
following grid: [NOTE: View a PDF of the Grid by clicking on "Tables" link
below.]
(6) The Matrix
Score for a site is the sum of the five parameter scores in sections (1)-(5) of
this rule.
To view tables referenced in rule text,
click here to view
rule.
Statutory/Other Authority: ORS
465.400
& 466.746
Statutes/Other Implemented: ORS
465.200
- 465.455 & 466.706 - 466.835
