 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued February 20, 2018               Decided May 18, 2018

                        No. 16-1416

                 GENUINE PARTS COMPANY,
                       PETITIONER

                              v.

          ENVIRONMENTAL PROTECTION AGENCY,
                    RESPONDENT


                 Consolidated with 16-1418


   On Petitions for Review of Final Agency Action of the
     United States Environmental Protection Agency


    Catherine E. Stetson argued the cause for petitioner. With
her on the briefs were Douglas E. Cloud, C. Max Zygmont,
Jennifer A. Simon, and Scott H. Reisch.

    Paul Cirino, Trial Attorney, U.S. Department of Justice,
argued the cause for respondent. With him on the brief was
Jeffrey H. Wood, Acting Assistant Attorney General.

   Before: HENDERSON, Circuit Judge, and EDWARDS and
GINSBURG, Senior Circuit Judges.
                                2
  Opinion for the Court filed by Senior Circuit Judge
EDWARDS.

    EDWARDS, Senior Circuit Judge: Pursuant to the
Comprehensive Environmental Response, Compensation, and
Liability Act of 1980 (“CERCLA”), 42 U.S.C. § 9601 et seq.,
Environmental Protection Agency (“EPA”) maintains a
National Priorities List (“NPL”) of hazardous waste sites that
are high priorities for remedial action due to their “relative risk
or danger to public health or welfare or the environment.” Id. §
9605(a)(8)(A). The primary method EPA uses to determine
which sites to add to the NPL is the Hazard Ranking System
(“HRS”), see 40 C.F.R. Pt. 300, App. A, § 1.0, a
comprehensive scientific methodology that quantifies site-
specific risk-based criteria. In 2015, EPA conducted an HRS
analysis of the West Vermont Drinking Water Contamination
Site (“Site”), a site of ground water contamination beneath
Indianapolis, Indiana. Because the final HRS score exceeded
the threshold required for listing a site, EPA added the Site to
the NPL by regulation in 2016. National Priorities List, 81 Fed.
Reg. 62,397 (Sept. 9, 2016). This case presents petitions for
review of that final rule.

    The Site includes a contaminated ground water plume
located beneath a commercial and residential area. EPA
believes that the Site’s contamination emanates from polluted
soil sources at two facilities: the Genuine Parts Company
(“Genuine Parts”) facility, an area associated with auto parts
degreasing operations and waste burial activities, and Aimco
Michigan Meadows Holdings, LLC (“Aimco”)’s Michigan
Plaza facility, a former shopping center where discharges of
solvents from a dry cleaning business entered a leaky sewer
line. In scoring the Site, EPA assessed the Site’s aquifers –
bodies of permeable rock, sediment, or soil that can contain or
transmit ground water. EPA determined that a porous upper
                                3
aquifer, consisting of sand and gravel, rested directly on top of
a limestone bedrock aquifer within two miles of the Site.
Because the two aquifers were considered to be interconnected
such that contamination could migrate from one aquifer to the
other, EPA treated both aquifers as a single hydrologic unit.
Had EPA treated the aquifers separately, the final HRS score
would not have qualified the Site for listing.

     The studies that EPA relied on to support its conclusion that
the aquifers interconnect included three different diagrams, as
explained below. The problem is that the diagrams appear to
contradict the agency’s position. The cross sections on the
diagrams show independent layers of sediment dividing the
upper and lower aquifers throughout the relevant area.
Petitioners pointed this out to EPA in their comments on the
proposed rule. EPA, however, never addressed the cross
sections in the rule making record. Because EPA “entirely
failed to consider an important aspect of the problem” by
failing to address evidence that runs counter to the agency’s
decision, we hold that the listing of the Site is arbitrary and
capricious. Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto.
Ins. Co., 463 U.S. 29, 43 (1983).

    EPA may be able to offer convincing scientific evidence to
support a conclusion that the aquifers are hydraulically
interconnected. That evidence has yet to be shown, however.
Regarding the action before us, we are constrained to grant the
petitions for review because EPA has failed to offer substantial
evidence to support its finding of an interconnection, it has
ignored evidence undercutting its conclusion, and it has failed
to state a reasoned basis for overcoming the regulatory
presumption of non-interconnection. See 40 C.F.R. Pt. 300,
App. A, § 3.0.1.2.1.
                               4
    Petitioners also claim that the rule should be vacated
because EPA failed to take into account the direction of ground
water flow beneath the Site when calculating the target
population potentially subject to contamination. We reject this
claim. EPA relied on established HRS instructions and
reasonably took into account the distance between the sources
of contamination and drinking water wells in computing the
Site’s “targets” score. We have no grounds to second guess
EPA’s decision on this point.

                      I.   BACKGROUND

A. Statutory and Regulatory Background

    Under CERCLA, EPA is authorized to establish and revise
annually a National Priorities List of known hazardous waste
sites considered high priorities for environmental remediation.
See 42 U.S.C. § 9605(a)(8)(A). Sites listed on the NPL are
eligible for CERCLA-funded remedial action through EPA’s
“Superfund program.” CTS Corp. v. EPA, 759 F.3d 52, 56
(D.C. Cir. 2014). While placement on the NPL does not
automatically render any party liable for cleanup costs, it “can
have significant adverse consequences for the owner of a listed
property” by, for example, damaging the business’s reputation
or property values. Carus Chem. Co. v. EPA, 395 F.3d 434, 437
(D.C. Cir. 2005); see also US Magnesium, LLC v. EPA, 630
F.3d 188, 190 (D.C. Cir. 2011).

    Under the statute, listing criteria are based “upon relative
risk or danger to public health or welfare or the environment”
of actual or threatened releases of hazardous substances. 42
U.S.C. § 9605(a)(8)(A). Pursuant to this mandate, EPA
promulgated the Hazard Ranking System, 40 C.F.R. Pt. 300,
App. A, “a comprehensive methodology and mathematical
model the agency uses to . . . quantify the environmental risks
                               5
a site poses.” Carus Chem., 395 F.3d at 437. EPA is required
by the statute to “assure, to the maximum extent feasible, that
the hazard ranking system accurately assesses the relative
degree of risk to human health and the environment posed by
sites and facilities subject to review.” 42 U.S.C. § 9605(c)(1).

    The HRS measures the risk posed by migration of
hazardous substances through four possible pathways: air, soil,
surface water, and, relevant here, ground water. See 40 C.F.R.
Pt. 300, App. A, § 2.1. Each potentially affected pathway
receives a numerical score based on the “[l]ikelihood of
release, waste characteristics, and targets” of the contamination
associated with that pathway. Id. § 2.1.2. EPA must then “plug
the resulting individual pathway scores into a formula to obtain
the site score,” US Magnesium, 630 F.3d at 189–90, which
ranges from 0 to 100, 40 C.F.R. Pt. 300, App. A, § 2.1.1. Sites
with scores at or above 28.50 are eligible for inclusion on the
NPL. National Priorities List, 77 Fed. Reg. 15,276, 15,278
(Mar. 15, 2012).

    In analyzing ground water migration pathways, EPA
computes an individual pathway score for each aquifer located
within a four-mile radius of a site’s sources. See 40 C.F.R. Pt.
300, App. A, §§ 3.0, 3.0.1.1. EPA must “[a]ssign the
highest . . . score that results for any aquifer as the ground
water migration pathway score for the site.” Id. § 3.0. However,
if two or more aquifers are interconnected “within 2 miles of
the sources at the site,” EPA must “combine the aquifers
having interconnections in scoring the site.” Id. § 3.0.1.2.1.
EPA may not assume interconnection within the two-mile
radius. “If data are not adequate to establish aquifer
interconnections,” the HRS instructs EPA to “evaluate the
aquifers as separate aquifers.” Id.
                               6
    EPA considers aquifers to be interconnected if their
boundaries do not “impede the flow of ground water and
hazardous substances between the aquifers.” Hazard Ranking
System Guidance Manual, EPA, at 116 (Nov. 1992), available
at       Joint     Appendix       (“J.A.”)      434,       444,
https://semspub.epa.gov/work/HQ/189159.pdf         (hereinafter,
“HRS Guidance”). Aquifers are not interconnected if they are
separated by a “confining layer” of materials that serves as an
aquifer boundary that water cannot easily move through. Id. A
“confining layer” serves as such a boundary if it has lower
“hydraulic conductivity” – a measure of the permeability of a
geologic material – than adjacent geologic materials by “at
least two orders of magnitude.” Id.

B. Factual Background

    In September 2015, EPA performed an HRS analysis of the
West Vermont Drinking Water Contamination Site. See HRS
Documentation Record, West Vermont Drinking Water
Contamination (Sept. 2015), available at J.A. 14–29,
https://semspub.epa.gov/work/05/921199.pdf.        The    Site
consists of a plume of ground water contamination, primarily
vinyl chloride, extending more than twenty acres beneath
Indianapolis, Indiana. As noted above, EPA believes the
hazardous waste emanates from two sources: a Genuine Parts
property associated with auto parts remanufacturing,
degreasing, and industrial waste burial, and a former shopping
center owned by Aimco where discharges of solvents from a
dry cleaning business entered a leaky sewer line.

    Because EPA scored the Site on the basis of the ground
water pathway, the agency considered each aquifer layer
located beneath the Site. It is undisputed that the shallowest
aquifer, the “Glacial Outwash Aquifer,” consists of
unconsolidated sand, gravel, and clay immediately below
                                7
ground and stretching throughout the entire two-mile radius of
the Site’s sources. The deepest aquifer, the “Limestone
Bedrock Aquifer,” is made of crystalline limestone and also
extends two miles in every direction beneath the Site. The
parties also agree that the aquifers are separated by a middle
layer of New Albany Shale that begins west of the two-mile
radius and ends before reaching its eastern edge.

    EPA determined that where the shale is absent, the Glacial
Outwash Aquifer sits directly on top of the Limestone Bedrock
Aquifer for a portion of the two-mile radius. HRS
Documentation Record, West Vermont Drinking Water
Contamination, at 44 (modified Sept. 2016), available at J.A.
73, https://semspub.epa.gov/work/05/929575.pdf. As support
for that conclusion, EPA cited three studies. First, EPA
referenced an Indiana University geological survey showing
the New Albany Shale layer ending within two miles of the
Site’s sources. See id. (citing Ref. 120, p. 10, available at J.A.
593).

    Second, EPA pointed to well log data – obtained by boring
vertically down through wells into the earth – showing layers
of sand, clay, and “sandy clay” resting on top of the limestone
bedrock within two miles east of the Site. See id. (citing Ref.
116, pp. 51–55, available at J.A. 578–82).

     Third, EPA cited an Indianapolis Water Company report
containing two cross sections of the study area. See id. (citing
Ref. 129, pp. 10, 23, available at J.A. 612, 625). Cross sections
are visual depictions of subsurface geological features derived
from well log data. Cross section B-B’, one of the diagrams on
page 23 of Reference 129, shows a thin white layer labeled
“till” – a term meaning a mixture of clay, sand, and gravel –
running from the shale in the west to the eastern boundary of
the two-mile radius. The diagram displays the till layer
                                8
separately from both the outwash and limestone layers.
Nonetheless, EPA’s Documentation Record does not explain
why it considered this diagram to support, rather than
contradict, its contention that the Glacial Outwash Aquifer
rests on top of the Limestone Bedrock Aquifer, undivided by a
confining layer of till meeting shale.

    Having concluded that the two aquifers are in physical
contact, EPA further found that the hydraulic conductivities of
the upper and lower aquifers are within two orders of
magnitude of each other. Id. Therefore, EPA concluded that the
aquifers were interconnected, and accordingly combined the
two aquifers in its calculation of the ground water migration
pathway score. As a result, for the purposes of the HRS
calculations, EPA treated the Glacial Outwash Aquifer and
Limestone Bedrock Aquifer as one aquifer, receiving one
ground water migration pathway score under the HRS, rather
than separate, individualized pathway scores.

    In calculating the ground water migration pathway score
for the interconnected aquifers, EPA considered the “targets”
of the contamination at issue. 40 C.F.R. Pt. 300, App. A,
§ 2.1.2. This included the human and environmental
populations at risk of exposure to the hazardous waste. Id.
§ 2.5. EPA arrived at a “targets” value after following the
process established in the HRS: it assigned values to sub-
factors that included the size and location of the relevant
populations, id. § 3.3, the location of the nearest wells, id., and
“whether the target is subject to actual or potential
contamination,” id. § 2.5. Following the HRS formula, EPA
“distance weighted” those subfactors: wells more than four
miles from the Site’s sources were not assigned any value in
the equation, while those within four miles received greater
weight the closer they were to the Site’s sources. See id. § 3.3.1
& Table 3–12. Within four miles of the Site, EPA identified
                               9
three wellfields serving a population of over 96,000 people. See
HRS Documentation Record 2016 at 81–83, J.A. 110–12.
Those facts, once plugged into the formula from Table 3–12 of
the HRS, produced a “targets” value of 929. Id. at 3, 36, J.A.
32, 115.

    Finally, EPA calculated scores for the “likelihood of
release” and “waste characteristics” factors, and entered those
figures, along with the “targets” value, into the ground water
migration pathway equation. The result was the highest
possible ground water migration pathway score of 100 for the
interconnected aquifers and, ultimately, a final HRS Site Score
of 50. See id. at 2–3, J.A. 31–32. Because the HRS Site Score
exceeded the listing threshold of 28.50, EPA issued a proposed
rule listing the Site on the NPL and invited public comment.
See National Priorities List, 80 Fed. Reg. 58,658, 58,662 (Sept.
30, 2015).

    In response, Petitioners and their consultants submitted
comments raising two objections that are relevant here. First,
they argued that EPA lacked substantial evidence of hydraulic
interconnection between the Glacial Outwash Aquifer and
Limestone Bedrock Aquifer. They noted language in a
geological survey report that described the upper aquifer as,
“for the most part, distinct from deeper sand and gravel or
bedrock aquifers,” and that referenced “a thick, persistent
sequence of pre-Wisconsin till units [that] create[] a low-
permeability confining unit.” Anthony H. Fleming et al., The
Hydrogeologic Framework of Marion County, Indiana, at 37
(2000), available at J.A. 266, 310. EPA separately relied on a
chart from the same geological survey in support of its
interconnectivity conclusion. See HRS Documentation Record
2016, at 44, J.A. 73 (citing Ref. 120, p. 10, J.A. 593).
Petitioners also submitted a cross section derived from well log
                                10
data that showed a layer of “clay/till” dividing the Glacial
Outwash Aquifer from the Limestone Bedrock Aquifer.

    In addition, Petitioners argued that EPA overstated the
population affected by potential contamination of drinking
water wells by failing to take into account ground water flow
direction. According to Petitioners, the “targets” factor score
should not have assigned any weight to the fact that three well
fields are located within four miles of the Site’s sources
because record evidence and the agency’s own statements
suggested that the ground water contaminated by the Site flows
away from the wells.

    On September 9, 2016, EPA published a final rule adding
the Site to the NPL, see National Priorities List, 81 Fed. Reg.
62,397, 62,401 (Sept. 9, 2016), and a support document
responding to comments, Support Document for the Revised
National Priorities List Final Rule – West Vermont Drinking
Water Contamination, U.S. EPA, at 24 (Sept. 2016), available
at J.A. 126, https://semspub.epa.gov/work/05/633727.pdf.
With respect to the interconnection issue, EPA did not directly
address the cross sections in the diagrams showing a separate
layer of clay or till beginning where the shale ends and
stretching across the rest of the two-mile radius. Instead, EPA
cited data from two well logs that it claimed show “no clay
layer being present above the limestone aquifer.” Support
Document at 24, J.A. 157. Further, EPA maintained that well
log data “identified clay layers throughout the [Glacial
Outwash] aquifer but did not identify a clay layer that was
consistently present to separate the [Glacial Outwash] aquifer
from the limestone aquifer.” Id. Reiterating that, in its view, the
upper aquifer physically touches the lower aquifer within two
miles of the Site’s sources, EPA reaffirmed its conclusion that
the two aquifers are interconnected. Id. at 22–24, J.A. 155–57.
                                 11
    As for its calculation of the “targets” factor, EPA explained
that it had not evaluated ground water flow direction because it
lacked “sufficient data . . . at this stage in the listing process to
accurately assess the ground water flow directions” near the
Site. Id. at 29, J.A. 162. Therefore, the agency found that “it
cannot be determined that contaminated ground water cannot
reach the municipal well fields.” Id.

     Genuine Parts and Aimco petitioned this court for review
of the final rule.

                          II. ANALYSIS

A. Standard of Review

     CERCLA does not specify a standard of review applicable
to EPA’s NPL listing decisions. See Carus Chem., 395 F.3d at
441. We have accordingly reviewed the agency’s decisions
under the Administrative Procedure Act (“APA”)’s substantial
evidence and arbitrary and capricious standards. See CTS
Corp., 759 F.3d at 59 n.1, 63; Nat’l Gypsum Co. v. EPA, 968
F.2d 40, 41, 44 (D.C. Cir. 1992). Applying these standards, we
will vacate an EPA listing action if the agency has failed to
“examine[] [the] the relevant data” or failed to “articulate[] a
rational explanation for its actions.” Carus Chem., 395 F.3d at
441.

     When reviewing for substantial evidence, we must
consider the whole record upon which an agency’s factual
findings are based, including “whatever in the record fairly
detracts” from the evidence supporting the agency’s decision.
Universal Camera Corp. v. NLRB, 340 U.S. 474, 487–88
(1951); see also 5 U.S.C. § 706(2)(E). “[E]vidence that is
substantial viewed in isolation may become insubstantial when
contradictory evidence is taken into account.” Landry v. Fed.
                               12
Deposit Ins. Corp., 204 F.3d 1125, 1140 (D.C. Cir. 2000).
Therefore, an agency cannot ignore evidence that undercuts its
judgment; and it may not minimize such evidence without
adequate explanation. See Morall v. DEA, 412 F.3d 165, 179–
80 (D.C. Cir. 2005); see also, e.g., Bellagio, LLC v. NLRB, 863
F.3d 839, 849–52 (D.C. Cir. 2017); Lakeland Bus Lines, Inc. v.
NLRB, 347 F.3d 955, 963 (D.C. Cir. 2003) (holding that the
agency could not rely on a “clipped view of the record” to
support its conclusion).

      We also review EPA’s action to determine whether it
passes muster under the APA’s arbitrary and capricious
standard of review. See 5 U.S.C. § 706(2)(A); Carus Chem.,
395 F.3d at 441. An agency action is arbitrary and capricious
when, inter alia, the agency has “entirely failed to consider an
important aspect of the problem [or] offered an explanation for
its decision that runs counter to the evidence before the
agency.” State Farm, 463 U.S. at 43. “Given the highly
technical issues involved,” the EPA’s listing decisions are
entitled to “significant deference.” Bradley Mining Co. v. EPA,
972 F.2d 1356, 1359 (D.C. Cir. 1992). But “our reviews of
listing decisions” are not “of the rubber-stamp variety.” Bd. of
Regents of the Univ. of Wash. v. EPA, 86 F.3d 1214, 1218 (D.C.
Cir. 1996). The agency still “must examine the relevant data.”
State Farm, 463 U.S. at 43. “Conclusory explanations for
matters involving a central factual dispute where there is
considerable evidence in conflict do not suffice to meet the
deferential standards of our review.” Int’l Union, United Mine
Workers v. Mine Safety & Health Admin., 626 F.3d 84, 94
(D.C. Cir. 2010).

     Ultimately, “in their application to the requirement of
factual support[,] the substantial evidence test and the arbitrary
or capricious test are one and the same.” Butte Cty. v. Hogen,
613 F.3d 190, 194 (D.C. Cir. 2010); see also Ctr. for Auto
                               13
Safety v. Fed. Highway Admin., 956 F.2d 309, 314 (D.C. Cir.
1992) (“An agency action is arbitrary and capricious if it rests
upon a factual premise that is unsupported by substantial
evidence.”). Under both standards, “an agency cannot ignore
evidence contradicting its position.” Butte Cty., 613 F.3d at
194.

B. Record Evidence of a Confining Layer

     Petitioners claim that the cross section diagrams and
geological survey statements in the record demonstrate that a
layer of clay and till stretches from the New Albany Shale
throughout the rest of the two-mile radius, forming an
uninterrupted confining layer that divides the Glacial Outwash
Aquifer from the Limestone Bedrock Aquifer for two miles
surrounding the Site. This evidence, which Petitioners raised in
their comments on EPA’s proposed rule, appears in the very
reports relied on by EPA. And tellingly, it appears to refute the
agency’s conclusion that the aquifers are interconnected. While
EPA may be able to explain how the evidence is consistent with
its original conclusion, it has not done so in the record under
review.

    1. EPA Ignored Evidence That Is At Odds With Its
       Conclusion

     As noted above, three cross sections of the Site’s
subsurface geology depict a thin, continuous layer of
geological material in approximately the same location,
running from the shale in the west to the end of the two-mile
radius. See J.A. 625 (cross section B-B’); id. at 649 (cross
section 8J-8J’); id. at 378 (cross section Plate R-5). Each cross
section appears in the record. Petitioners’ consultant cited one
of these cross sections in commenting on the proposed rule, and
EPA cited to the sources containing each of the diagrams in its
                                14
Documentation Record. Each diagram visually represents the
layer in question as an independent stratum, separate from and
dividing the Glacial Outwash Aquifer and Limestone Bedrock
Aquifer. These three cross sections clearly call into question
EPA’s conclusion that, within two miles of the Site, the Glacial
Outwash Aquifer sits on top of the Limestone Bedrock Aquifer
without any dividing layer separating the two aquifers.

     Furthermore, the dividing layer does not appear to merely
represent a constituent part of the Glacial Outwash Aquifer, as
EPA now argues. Not only do the cross sections portray the
dividing layer as visually distinct from the aquifers in question,
but the diagrams also indicate that the layer consists of different
materials from the aquifers: Plate R-5 labels the dividing layer
“clay/till,” id. at 378; cross section B-B’ likewise terms this
layer “till,” id. at 625; and in cross section 8J-8J’, the layer is
called “unconsolidated nonaquifer material,” id. at 649.
Meanwhile, the upper aquifer is labeled “outwash,” id. at 625,
“sand & gravel (aquifer),” id. at 378, and “sand and gravel,” id.
at 645. Thus, three independent cross sections all appear to
indicate that the Glacial Outwash Aquifer consists of sand and
gravel, but sits on top of an independent layer of “nonaquifer
material” consisting of clay or till.

     There is no dispute that clay and till would serve as a
confining layer if their hydraulic conductivities are more than
two orders of magnitude lower than that of the upper aquifer’s
sand and gravel. See HRS Guidance at 116, J.A. 444. The HRS
also makes clear that certain types of clay and till have
hydraulic conductivities as low as 10-8 cm/sec while some
types of gravel and sand are assigned conductivities as high as
10-2 cm/sec, a difference of six orders of magnitude. See 40
C.F.R. Pt. 300, App. A, Table 3-6. In the record below,
however, EPA does not suggest that the hydraulic conductivity
of the separate clay or till layer is within two orders of
                                15
magnitude of the aquifers that surround it. EPA simply ignores
the presence of the clay/till dividing layer in the three cross
sections in the record.

     It was arbitrary and capricious for EPA to rely on portions
of studies in the record that support its position, while ignoring
cross sections in those studies that do not. See Butte Cty., 613
F.3d at 194. Furthermore, if Petitioners are correct in claiming
that a confining layer of clay or till separates the aquifers, then
EPA may not combine the upper and lower aquifers in
computing the Site’s HRS score. See 40 C.F.R. Pt. 300, App.
A, § 3.0.1.2.1. Although EPA “is not required to discuss every
item of fact or opinion included in the submissions it receives
in response to a Notice of Proposed Rulemaking, it must
respond to those comments which, if true, would require a
change in the proposed rule.” La. Fed. Land Bank Ass’n v.
Farm Credit Admin., 336 F.3d 1075, 1080 (D.C. Cir. 2003).

     In response to these petitions for review, EPA’s counsel
offered arguments as to why the diagrams are consistent with
the agency’s conclusion that there is no continuously confining
layer of sediment between the aquifers. These arguments come
too late. We may only uphold a rule “on the basis articulated
by the agency” in the rule making record. State Farm, 463 U.S.
at 50 (citing SEC v. Chenery Corp., 332 U.S. 194, 196–97
(1947)). Counsel’s “post hoc rationalizations for agency
action” carry no weight with the court. Id.

    2. EPA Lacked Substantial Evidence of Interconnection

     In its brief to this court, EPA offers several arguments why
its interconnection conclusion is supported by substantial
evidence despite Petitioners’ confining layer evidence. First, it
argues that Petitioners err by treating “till,” “nonaquifer
material,” and “clay” – the three separate labels assigned to the
                               16
layer at issue by the three different cross sections – as if each
term had one meaning, that of “pure clay.” Respondent’s Br.
34–36. According to EPA, “till” may “refer[] to a mixture of
clay, sand, and loam,” id. at 34, or “silty sand, sandy clay,
gravelly clay,” id. at 35–36. Since the Glacial Outwash Aquifer
consists of an unconsolidated mix of the same or similar
materials, EPA argues that the “till” indicated in two of the
cross sections is just a component layer of the upper aquifer.
Nevertheless, the fact remains that three different cross
sections portrayed this particular layer of till (or clay)
individually, separate from their portrayal of the Glacial
Outwash Aquifer. EPA offered no explanation for this, either
in its brief to the court or during oral argument.

     “Till” may consist of enough clay mixed in with sand to
produce a substantially lower hydraulic conductivity than that
of the geology that surrounds it. See 40 C.F.R. Pt. 300, App. A,
Table 3-6. Further, cross section 8J-8J’ terms the layer
“unconsolidated nonaquifer material.” J.A. 645. The plain
meaning of the “nonaquifer” designation is that this layer is not
part of the upper aquifer, and therefore it may very well be a
confining layer dividing the two aquifers. Given these facts,
EPA counsel’s belated definition of “till” is not substantial
evidence of interconnection.

     Second, in its argument to the court, EPA also downplays
the reliability of cross sections in “mapping discrete geological
units or areas that are not uniform along multiple well
locations.” Respondent’s Br. 33. Because cross sections
extrapolate data from well log to well log, EPA claims that they
are prone to showing uniformity in otherwise discrete
geological units. In EPA’s view, well log data more accurately
indicate the presence or absence of a confining layer separating
the two aquifers. Indeed, data from two well logs within two
miles of the Site show a mix of sand, gravel, and clay – the
                                17
component materials of the Glacial Outwash Aquifer – from
the surface down to the Limestone Bedrock Aquifer. See J.A.
578–81.

     The problem with EPA’s reliance on the well log data is
that it does not respond to Petitioners’ argument. Petitioners
allege the existence of a horizontal layer of sediment. Well log
data, however, which is obtained by boring straight down into
the earth through a well, necessarily reflect a narrow, vertical
measurement of the makeup of subsurface geology. To be sure,
well logs would cast doubt on the presence of a continuously
present confining layer of clay or till if they failed to find those
materials in the approximate locations that Petitioners claim
they exist. But, to the contrary, the well logs cited by EPA
indicate the presence of clay and till at the approximate
elevations indicated on the cross sections. And these layers of
clay line up fairly closely between the wells, which are more
than a mile apart from each other.

     Third, EPA found that there was no confining layer in part
because it observed that vinyl chloride contamination had
migrated “through the fine-grained sediments . . . throughout
the [upper] aquifer to a depth of at least 70 feet.” HRS
Documentation Record 2016 at 43–44, J.A. 72–73. This, too,
is unresponsive to Petitioners’ comments that a confining layer
of clay or till exists more than 70 feet below ground.
Petitioners’ Br. 38–40.

     Finally, EPA has failed to explain how the other sources it
relied on provide substantial evidence of interconnection. EPA
cites a geological survey prepared by Indiana University,
which shows that the confining layer of shale is not present in
eastern portions of the Site. HRS Documentation Record 2016
at 44, J.A. 73 (citing Ref. 120, p. 10, available at J.A. 593). As
far as we can tell, this survey does not show layering of
                               18
sediment in sufficient detail to be able to determine whether a
confining layer of clay or till rests between the upper and lower
aquifers where the shale ends.

      EPA also cites a ground water modeling report that states:
“Based on available hydrogeologic data in the area (including
DNR well log records) and the bedrock topography, it is likely
that the limestone aquifer is hydraulically connected to the
outwash sand and gravel aquifer.” J.A. 629. But the very next
sentence of that report reads: “Further from the streams in the
till deposits, water levels recorded in residential wells indicate
that the limestone aquifer is hydraulically isolated from the
upper intertill aquifer system.” Id. It is not clear which
statement, if either, refers to the two miles at issue here. These
equivocal reports do not relieve EPA of its obligation “to
address significant comments raised during the rulemaking.”
Ass’n of Private Sector Colls. & Univs. v. Duncan, 681 F.3d
427, 441 (D.C. Cir. 2012).

    In scoring the Site based on interconnected aquifers, EPA
“ignore[d] evidence contradicting its position,” Butte Cty., 613
F.3d at 194, and failed to support its conclusion with substantial
evidence. Therefore the rule adding the Site to the NPL cannot
stand and the case must be remanded for further consideration.

C. Ground Water Flow Direction

     Petitioners raise a second, independent objection to the
HRS scoring of the Site. They claim that, in computing the
ground water migration pathway score, EPA should not have
enhanced the “targets” value based on the existence of three
municipal wellfields less than four miles away. Evidence in the
record suggests that ground water beneath the Site area
generally flows to the south, away from the wells, which are to
the north of the Site. On the record here, Petitioners claim that
                                19
EPA acted beyond its statutory authority in treating the wells
as potentially subject to ground water contamination from the
Site’s sources.

     As a threshold matter, EPA argues that this challenge is
procedurally barred. CERCLA requires any petition for review
of an implementing regulation to “be made within ninety days
from the date of promulgation of such regulation[].” 42 U.S.C.
§ 9613(a). According to the government, Petitioners’ claim
amounts to a substantive challenge to the HRS regulation,
which does not require EPA to consider ground water flow
direction in determining the target population. Because the
HRS was promulgated in 1990, EPA maintains that this
objection is untimely. See Carus Chem., 395 F.3d at 441. We
disagree.

       Petitioners’ ground-water-flow-direction argument falls
within an established, narrow exception to the statutory time
limit. See US Magnesium, 630 F.3d at 194 (explaining that
“even under § 9613 there may be some room to challenge a
regulation when litigating its application”). Our case law
makes it clear that “[a]n agency’s regulations may be attacked
. . . once the statutory limitations period has expired . . . on the
ground that the issuing agency acted in excess of its statutory
authority in promulgating them.” NLRB Union v. Fed. Labor
Relations Auth., 834 F.2d 191, 195 (D.C. Cir. 1987); see also,
e.g., Nat’l Air Transp. Ass’n v. McArtor, 866 F.2d 483, 487
(D.C. Cir. 1989) (“NLRB Union allows, outside statutory time
limits, substantive claims that the rule ‘conflicts with the statute
from which its authority derives’” (quoting NLRB Union, 834
F.2d at 196)). Petitioners bring just such a claim here; their
argument, at bottom, is that the HRS conflicts with CERCLA
because it is “feasible” to examine ground water flow direction
and 42 U.S.C. § 9605(c) requires that the HRS, “to the
maximum extent feasible, . . . accurately assess[] the relative
                              20
degree of risk to human health and the environment.”
Therefore, Petitioners’ claim is not procedurally barred.

     On the merits, however, Petitioners’ argument fails. As
noted above, EPA calculated the “targets” factor value
pursuant to the HRS’s instructions. The HRS requires EPA to
assign values to wells located within four miles of the Site’s
sources, without requiring EPA to directly consider the
direction of ground water flow. See 40 C.F.R. Pt. 300, App. A,
§ 3.0.1.1; id. § 3.3.1 & Table 3–12. EPA followed those
instructions to a tee. “Our case law endorses the Hazard
Ranking System’s preference for using formulas,” even when
application of the formula results in a degree of “Agency
imprecision in calculating the target population.” B&B Tritech,
Inc. v. EPA, 957 F.2d 882, 884–85 (D.C. Cir. 1992).

      There is no basis in this record for concluding that the
“targets” calculation was so imprecise as to violate the APA or
CERCLA. “Congress intended that the [NPL] would serve
simply as a tool for identifying quickly and inexpensively those
sites meriting closer environmental scrutiny.” CTS Corp., 759
F.3d at 56. In EPA’s view, the cost required to accurately
measure ground water flow direction would outweigh the
utility achieved by improving the accuracy of the listing
decision, “because this level of accuracy is not required for a
screening tool that is intended to assess relative risk.” Hazard
Ranking System, 55 Fed. Reg. 51,532, 51,553 (Dec. 14, 1990).
The decision to instead “distance weight[]” populations subject
to potential contamination, id., reflects a reasonable
interpretation of CERCLA’s mandate that EPA “accurately”
assess risk “to the maximum extent feasible” given the NPL’s
purposes, 42 U.S.C. § 9605(c)(1).

    The fact that EPA acknowledged a southerly ground water
flow direction for other purposes did not render its
                              21
decisionmaking arbitrary and capricious. Ground water flow
direction “may be different in each aquifer at the site” and “is
not always the same as the direction of” the flow of
contaminants. 55 Fed. Reg. at 51,553. While EPA evaluated
flow direction in the immediate area of the Genuine Parts and
Michigan Plaza facilities, it lacked data “to accurately assess
the ground water flow directions throughout the” entire four-
mile area surrounding the Site. Support Document for Final
Rule at 29, J.A. 162. The HRS regulation does not foreclose
EPA from evaluating ground water flow direction at future
stages of the administrative process, when the evidence may be
firmer. For now, however, EPA’s decision to distance weight
the wells without consideration of ground water flow direction
was rational under the APA and a reasonable interpretation of
CERCLA.

                      III. CONCLUSION
     For the foregoing reasons, we grant the petitions for
review, vacate the rule to the extent that it places the West
Vermont Drinking Water Contamination Site on the NPL, and
remand to EPA for further proceedings consistent with this
opinion.
