# 40 CFR § 146.6 - Area of review.

§ 146.6 Area of review.

The area of review for each injection well or each field, project or area of the State shall be determined according to either paragraph (a) or (b) of this section. The Director may solicit input from the owners or operators of injection wells within the State as to which method is most appropriate for each geographic area or field.

(a) Zone of endangering influence.

(1) The zone of endangering influence shall be:

(i) In the case of application(s) for well permit(s) under § 122.38 that area the radius of which is the lateral distance in which the pressures in the injection zone may cause the migration of the injection and/or formation fluid into an underground source of drinking water; or

(ii) In the case of an application for an area permit under § 122.39, the project area plus a circumscribing area the width of which is the lateral distance from the perimeter of the project area, in which the pressures in the injection zone may cause the migration of the injection and/or formation fluid into an underground source of drinking water.

(2) Computation of the zone of endangering influence may be based upon the parameters listed below and should be calculated for an injection time period equal to the expected life of the injection well or pattern. The following modified Theis equation illustrates one form which the mathematical model may take.

$r={\left(\frac{2.25\mathrm{KHt}}{{S10}^{x}}\right)}^{1}{2}}$

where:

$X=\frac{4\mathrm{\pi KH}\left({h}_{w}-{h}_{\mathrm{bo}}×{S}_{b}{G}_{b}\right)}{2.3\phantom{\rule{0ex}{0ex}}Q}$

r = Radius of endangering influence from injection well (length)
k = Hydraulic conductivity of the injection zone (length/time)
H = Thickness of the injection zone (length)
t = Time of injection (time)
S = Storage coefficient (dimensionless)
Q = Injection rate (volume/time)
hbo = Observed original hydrostatic head of injection zone (length) measured from the base of the lowermost underground source of drinking water
hw = Hydrostatic head of underground source of drinking water (length) measured from the base of the lowest underground source of drinking water
Sp Gb = Specific gravity of fluid in the injection zone (dimensionless)
π = 3.142 (dimensionless)
The above equation is based on the following assumptions:

(i) The injection zone is homogenous and isotropic;

(ii) The injection zone has infinite area extent;

(iii) The injection well penetrates the entire thickness of the injection zone;

(iv) The well diameter is infinitesimal compared to “r” when injection time is longer than a few minutes; and

(v) The emplacement of fluid into the injection zone creates instantaneous increase in pressure.