(1)
For purposes of this rule, the following definitions apply:
(a) "Lagoon area" means the surface area of the
lagoon within the design of the high-water mark.
(b) "Maximum day well demand" means the highest
volume of water discharged from a water well on any day in a year.
(c) "Sewage lagoon" means any holding or detention
pond that is used for treatment or storage of water-carried waste products from
residences, public buildings, institutions, or other buildings, including discharge
from human beings or animals, together with ground water infiltration and surface
water present. For purposes of this rule, the term includes concentrated animal
feeding operations but does not include storm water facilities or subsurface
wastewater treatment systems.
(d)
"Water well" has the same meaning as
75-5-103,
MCA.
(2) All new water wells
and new sewage lagoons must meet the setbacks in (3), unless the applicant
demonstrates that a shorter setback is allowed under (4) or (6). Water wells and
sewage lagoons that existed or were approved by the department before the effective
date of this rule must meet the setbacks under either of the following
circumstances:
(a) if the lagoon area is proposed
to be increased; or
(b) if the maximum
daily pumping rate of a water well is proposed to be increased.
(3) The following setbacks apply,
unless the applicant demonstrates that a lesser setback is allowed under (4) or (6):
(a) 1,000 feet between a water well and the design
high-water mark of a sewage lagoon;
(b)
200 feet between a well for a public water supply system with continuous
disinfection that meets the 4-log virus inactivation and the design high-water mark
of a sewage lagoon;
(c) 200 feet
between a water well and the design high-water mark of a sewage lagoon if the
geometric mean number of E. coli bacteria in the influent flow to
the sewage lagoon does not exceed 126 colony forming units per 100 milliliters and
10 percent of the total samples do not exceed 252 colony forming units per 100
milliliters during any 30-day period; and
(d) 100 feet between a water well and the design
high-water mark of a sewage lagoon if the applicant demonstrates there is no
hydraulic connection between the sewage lagoon and the water well as demonstrated by
groundwater gradients under the maximum day pumping rate or by confined conditions
that prevent lagoon discharges from impacting the water well.
(4) A setback less than the setbacks in (3)(a)
through (c) may be used if the applicant demonstrates that the distance needed to
achieve 4-log pathogen reduction of effluent migration from the sewage lagoon to the
water well is less than the setback distance in (3)(a) through (c). In no instance,
however, may the setback be less than 100 feet.
(5) To make the demonstration in (4), the pathogen
reduction between the sewage lagoon and the water well must be calculated according
to one of the following methods:
(a) METHOD 1 -
Travel Time Method - The vertical travel time in the vadose zone for the wastewater
to reach groundwater is calculated using the following equation:
t1 = (d)*([THETA]) ÷ ([ALPHA]) * 365
Where:
t1 = vertical travel time (days)
[ALPHA] is total effluent recharge - the maximum allowable leakage
rate or actual measured leakage rate if the measured rate is available
(in/yr)
[THETA] is volumetric soil moisture (percent)
d is the depth to groundwater (in)
The horizontal travel time in the saturated zone for the
wastewater to reach the water well is calculated using the following equations:
t2 = [ne(K*i)] * [x - {(Q÷(2*p*K*b*i)) * (ln(1+((2*p*
K*b*i*x)÷Q)))}]
Where:
t2 = horizontal travel time (days)
K is hydraulic conductivity of the saturated aquifer
(feet/day)
i is hydraulic gradient (feet/feet)
ne is effective porosity (dimensionless)
[PI] is pi, 3.14 (dimensionless)
ln is natural logarithm
Q is the maximum day well demand
(feet3/day)
x is the horizontal distance from the sewage lagoon to the water
well (feet). Value is positive when well is downgradient of sewage lagoon, negative
if well is upgradient of sewage lagoon.
The total log pathogen reduction from the bottom of the sewage
lagoon to the water well is calculated using the following equation:
Pt = (t1 + t2)*0.02
Where:
Pt = Log reduction of pathogens during vertical and horizontal
travel
0.02 = log 10 pathogen removal/day
(b) METHOD 2 - Travel time and VIRULO - The
horizontal travel time (t2) is calculated the same as for Method 1.
The horizontal log reduction is calculated using the following
equation:
Ph = (t2)*0.02
Where:
Ph = Log reduction of pathogens during horizontal travel
The pathogen reduction during vertical movement in the vadose zone
is calculated using VIRULO. The value of Ph is added to VIRULO results to provide
the total pathogen reduction from the bottom of the sewage lagoon to the water well.
(c) Other methods approved by
the department.
(6) In
calculating 4-log pathogen reduction under (4), the following requirements apply:
(a) Hydraulic conductivity must be based on the
aquifer material most likely to transmit lagoon discharges to the water well and be
determined by one of the following methods:
(i)
The maximum hydraulic conductivity value of the aquifer material shown in Table 1.
The hydraulic conductivity for aquifer materials not included in Table 1 may be
calculated by the applicant using other methods acceptable to the department. The
aquifer material must be the most permeable soil layer that is at least six inches
thick and is below the bottom of the sewage lagoon infiltrative surface, as
identified in any test pit or borehole. This method may only be used for facilities
that are not requesting a source-specific ground-water mixing zone, as defined in
ARM
17.30.518.
|
TABLE 1
|
|
MATERIAL
|
HYDRAULIC CONDUCTIVITY (ft/d)
|
|
Basalt (permeable/vesicular)
|
5,100
|
|
Clay
|
0.025
|
|
Clay (unweathered, marine)
|
0.00054
|
|
Coarse sand
|
2,950
|
|
Fine sand
|
51
|
|
Glacial Till
|
0.72
|
|
Glacial Till (fractured)
|
29.5
|
|
Gravel
|
13,500
|
|
Gravelly sand
|
1,020
|
|
Igneous/metamorphic rock (fractured)
|
76.5
|
|
Igneous/metamorphic rock (unfractured)
|
0.000054
|
|
Karst limestone
|
18,000
|
|
Limestone
|
1.5
|
|
Limestone (unjointed, crystalline)
|
0.30
|
|
Loess
|
0.27
|
|
Medium sand
|
569
|
|
Sandstone
|
1.5
|
|
Sandstone (friable)
|
3.0
|
|
Sandstone (well cemented, unfractured)
|
0.0036
|
|
Sandy clay loam
|
1.4
|
|
Sandy silt
|
0.27
|
|
Shale
|
0.00054
|
|
Silt
|
0.27
|
|
Siltstone
|
0.0036
|
|
Silty clay
|
0.013
|
|
Silty sand
|
45
|
|
Tuff
|
7.2
|
|
Very fine sand
|
21.4
|
(ii) A
pumping test at least 8 hours long, representative of the hydraulic conductivity of
the aquifer material, and conducted on a well(s) with complete lithology and
construction details. Results for pumping tests must be submitted electronically on
DNRC Form 633. Pumping tests must be conducted in accordance with the requirements
in ARM
36.12.121(2)(a) through
(f), (3)(a), (3)(c), (3)(g), (3)(i), (3)(j), and
(3)(k).
(b) Hydraulic
gradient must be based on the aquifer material most likely to transmit lagoon
discharges to the water well and must be determined by one of the following methods:
(i) The regional topographic slope in an area that
includes the water well and the sewage lagoon. The minimum hydraulic gradient that
may be used with this method is 0.005 feet/feet, and the maximum gradient that may
be used is 0.05 feet/feet. This method may not be used for facilities requesting a
source-specific ground-water mixing zone as defined in ARM
17.30.518.
(ii) Groundwater potentiometric maps of the
aquifer that accurately represent the local hydraulic gradient in the area of the
water well and sewage lagoon.
(iii)
Surveyed static water elevations in at least three wells that draw water from the
aquifer, accurately represent the local hydraulic gradient in the area of the water
well and sewage lagoon, and are measured on the same date to the nearest 0.01
foot.
(c) Soil type must be
determined by test pits or boreholes. The following requirements apply:
(i) Test pits or boreholes must be completed to a
minimum depth of 10 feet below the bottom of the sewage lagoon infiltrative surface
or until an impervious layer, as defined in Circular DEQ-4, is
encountered.
(ii) A minimum of two test
pits or boreholes must be completed for the first 0.5 acre of lagoon area that is
within 1,000 feet of a water well. A maximum of one additional test pit or borehole
for each additional acre of lagoon area within 1,000 feet of a water well may be
required if the department determines that additional test pits or boreholes are
necessary to adequately characterize the soils between the sewage lagoon and the
water well. The test pits or boreholes must be located to provide representative
information on the soils beneath the sewage lagoon that affect the vertical and
horizontal migration of pathogens from the sewage lagoon to the affected water
well.
(iii) If the test pit or borehole
locations are not within 50 feet of the toe of the sewage lagoon embankment, then
the locations must be approved by the department before they are completed. The
borehole method must provide a continuous soil sample that is representative of the
soil and lithology profile.
(iv) For
purposes of defining soil effective porosity and volumetric soil moisture that are
used in (5), soils must be described according to the Unified Soil Classification
System. The soil description must include information regarding the presence or
absence of seasonal saturated conditions. If there is no evidence of saturated
conditions from the test pit, borehole, or other evidence, then the depth to
groundwater must be estimated as the bottom of the test pit or
borehole.
(d) Soils with
greater than 35 percent retained on the No. 10 sieve and geologic materials with
fractures do not receive credit for virus reduction in the vadose zone.
(e) The well discharge rate used in calculations
must be based on the maximum day well demand, which must be determined by using
historic discharge rate records or other methods as approved by the department.
(7) The department may
determine the setback calculated in accordance with this rule should be
decreased-but in no instance shorter than 100 feet-if the applicant demonstrates
equivalent protection of the water source that supplies the water well.
Notes
Mont. Admin. r. 17.30.1702
NEW,
2019
MAR p. 836, Eff.
6/22/2019
AUTH:
75-5-411,
MCA; IMP:
75-5-411,
MCA
