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 sitespecific 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 subfactors 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 lowpermeability 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 fourmile 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.