                              In the

    United States Court of Appeals
                For the Seventh Circuit
                    ____________________
Nos. 18-1918 & 18-2598
WILLIAM LIEBHART & NANCY LIEBHART,
                                               Plaintiffs-Appellants,
                                v.

SPX CORPORATION, TRC ENVIRONMENTAL CORPORATION, &
APOLLO DISMANTLING SERVICES, INC.
                                  Defendants-Appellees.
                    ____________________

        Appeal from the United States District Court for the
                  Western District of Wisconsin.
         No. 16-cv-700 — James D. Peterson, Chief Judge.
                    ____________________

    ARGUED DECEMBER 4, 2018 — DECIDED MARCH 6, 2019
                ____________________

   Before BAUER, KANNE, and BRENNAN, Circuit Judges.
   KANNE, Circuit Judge. William and Nancy Liebhart to-
gether own three houses on the same block in Watertown,
Wisconsin. Besides a few other houses, the rest of the block
was previously occupied by an abandoned transformer fac-
tory, last owned by SPX Corporation. In 2014, SPX demol-
ished the building with the assistance of TRC Environmental
Corporation and Apollo Dismantling Services (collectively,
2                                       Nos. 18-1918 & 18-2598

“the defendants”). The Liebharts allege that dust and debris
containing toxic chemicals migrated onto their properties,
contaminating their yards and jeopardizing their health and
the health of their tenants.
    The Liebharts sued under federal statutes authorizing pri-
vate rights of action for environmental contamination. They
also brought various state-law claims. Following discovery
and the submission of expert witness reports, the district
court denied the Liebharts’ motion for partial summary judg-
ment and granted summary judgment to the defendants with
costs. Although the district court adequately evaluated the ex-
pert witnesses and did not abuse its discretion in its proce-
dural decisions, the court set the bar unnecessarily high for
the plaintiﬀs to show a violation of the applicable federal stat-
utes. For that reason, we vacate the district court’s judgment
and remand for reconsideration.
                       I. BACKGROUND
    The factory dates to the 1920s and was used to manufac-
ture various industrial equipment. Most relevant to our pur-
pose, the factory manufactured power transformers contain-
ing polychlorinated biphenyls (PCBs), a carcinogenic chemi-
cal banned by the Environmental Protection Agency in 1979.
See Polychlorinated Biphenyls (PCBs) Manufacturing, Pro-
cessing, Distribution in Commerce, and Use Prohibitions, 44
Fed. Reg. 31,514 (May 31, 1979) (codiﬁed at 40 C.F.R. pt. 761).
The parties all agree that the facility has not manufactured or
handled any items containing PCBs since 1971, and that it
shut down completely in 2005.
  But even with all the products and manufacturing equip-
ment removed, PCB contaminants remained in the facility. In
Nos. 18-1918 & 18-2598                                       3

2009, SPX retained TRC and another company (not named as
a defendant in this suit) to study the property and determine
the extent and precise location of any PCB contamination.
Those studies revealed that the concrete ﬂoor of the factory
was generally contaminated, with concentrated amounts lo-
cated in speciﬁc areas throughout the site.
   Several years went by. In 2014, SPX decided to move ahead
with the demolition, retaining Apollo to conduct the work
and TRC to supervise the project. That November, the defend-
ants timely proposed a self-implementing cleanup plan to the
EPA, as required by 40 C.F.R. § 761.61(a). The trouble began
as demolition commenced in January 2015. The Liebharts al-
lege that Apollo demolished the building recklessly, failing to
use appropriate safety methods to control the dust generated
by demolition equipment. They assert that their properties
were covered in dust, and they submitted hundreds of photos
and videos of dust from the facility blowing toward their
homes to support their allegation.
    The Liebharts ﬁled a complaint with the local government
in February. They also collected a dust-covered sample of
snow from their yard and placed it in a mason jar. Soon there-
after, a representative of the Wisconsin Department of Natu-
ral Resources (“DNR”) contacted the defendants about the
Liebharts’ concerns. In April, TRC collected samples of the
surface soil (roughly down to eight inches below ground) on
both the industrial and residential properties. Sure enough,
the properties tested positive for the presence of PCBs. In Au-
gust, the Liebharts vacated the property on advice of their
physician to avoid further exposure to the chemicals. Frus-
trated with the lack of action, William Liebhart had the snow
4                                       Nos. 18-1918 & 18-2598

sample tested for PCBs. Although the sample did, in fact, con-
tain PCBs, the irregular manner of collection and storage
spoiled the sample, and the laboratory declined to endorse the
results with any conﬁdence.
    In September 2015, SPX submitted a plan to remediate the
contamination to the DNR. In turn, the Liebharts sued in fed-
eral district court in October. The complaint sought injunctive
relief under both the Resource Conservation and Recovery
Act (RCRA), 42 U.S.C. § 6901 et seq., and the Toxic Substances
Control Act (TSCA), 15 U.S.C. § 2601 et seq. In addition, the
Liebharts brought a host of state-law claims under supple-
mental jurisdiction, including strict liability, negligence, tres-
pass, nuisance, and negligent inﬂiction of emotional distress.
    During discovery, the Liebharts submitted to further test-
ing for PCBs both in their yard and inside their home. They
also underwent blood testing. Although the external surveys
revealed a more comprehensive picture of the extent to which
the soil on the Liebharts’ properties contained PCBs, the in-
ternal sampling and blood tests were both negative for con-
tamination. The Liebharts later learned in October 2017 that
the defendants allegedly buried some of the concrete remains
on-site rather than removing them to a toxic waste dump as
required by the EPA-approved clean-up plan.
    The parties prepared and submitted the reports of the ex-
pert witnesses who were to testify at trial. Among those, three
are pertinent to our discussion. First, the Liebharts submitted
a report by John Woodyard, a licensed professional engineer.
His report included a description of standard methods used
when demolishing PCB-contaminated buildings and an anal-
ysis of the purported ways in which the defendants deviated
from those practices, thereby causing the contamination of the
Nos. 18-1918 & 18-2598                                        5

residences. The Liebharts’ second expert was Dr. David Car-
penter, a public health physician who opined on ways in
which the Liebharts might have been exposed to PCBs and the
potential health eﬀects of continuing exposure. He concluded
that “there is no ‘safe’ level of exposure to PCBs that does not
increase the risk of disease.” Finally, the defendants submit-
ted a report prepared by Dr. Russell Keenan, a toxicologist
who analyzed the survey data and determined that it was im-
possible to determine whether the presence of PCBs on the
Liebharts’ property was due to the recent demolition or to
runoﬀ that occurred over the last several decades.
    In December 2017, the Liebharts moved for partial sum-
mary judgment on the issue of causation, reserving the issue
of damages for a jury trial. The defendants cross-moved for
summary judgment; they also ﬁled a motion to exclude the
testimony of plaintiﬀs’ experts Woodyard and Carpenter un-
der Fed. R. Evid. 702. In February 2018, after those motions
were fully briefed to the court, the Liebharts sought leave to
amend their complaint. In light of the information they ob-
tained through discovery regarding the burial of concrete on
the property, they intended to allege separate RCRA and
TSCA violations and seek an enlargement of the proposed in-
junction to include removal of that material.
    The district court issued its decisions on March 30. First,
the court granted in part the defendants’ motion to exclude
the plaintiﬀs’ expert witness reports. The court explained that
Woodyard’s report was “equivocal” as to the issue of causa-
tion; it hedged on whether the contaminants came from dem-
olition or from runoﬀ during the preceding decades. See
Liebhart v. SPX Corp., No. 16-cv-700-jdp, 2018 WL 1583296 at
6                                       Nos. 18-1918 & 18-2598

*3–4 (W.D. Wis. Mar. 30, 2018). Second, the report “over-
look[ed] too much signiﬁcant evidence” and failed to account
for “obvious alternative explanations” in determining the
cause of the contamination. Id. at *4 (quoting Fed. R. Evid. 702
advisory committee’s note to 2000 amendments). Finally, the
court noted that Woodyard’s report depended on “unreliable
or uninformative” evidence, including the Liebharts’ defec-
tive snow sample, while omitting other evidence such as grass
clippings and a chromatogram that allegedly detected PCBs
on the property. Id. at *4–5. For those reasons, the court ex-
cluded Woodyard’s report in its entirety. Id. at *5.
    Although the court admitted the vast majority of Dr. Car-
penter’s report, it struck Carpenter’s conclusion that “there is
no ‘safe’ level of exposure to PCBs that does not increase the
risk of disease” as unsupported by the medical studies he
cited. Id. at *5. In doing so, the court pointed to the absence of
PCBs inside the Liebharts’ home and in their blood, suggest-
ing that the Liebharts had not actually been exposed to PCBs
and so could not have suﬀered harm. Id. at 5–6.
    With that expert testimony oﬀ the table, the district court
concluded that the Liebharts failed to present any admissible
evidence to support their RCRA and TSCA claims. The re-
maining photos and videos certainly showed dust migrating
onto the Liebharts’ property, but there was no reliable evi-
dence proving that the dust contained PCBs. Given that any
PCBs detected in the soil may have been there prior to the
demolition, the lack of evidence doomed the Liebharts’ case.
The court denied partial summary judgment to the plaintiﬀs,
granted summary judgment to the defendants on the federal
claims, and dismissed the state-law claims without prejudice.
Nos. 18-1918 & 18-2598                                         7

    The district court also denied the Liebharts’ petition for
leave to amend their complaint on two grounds. First, alt-
hough the court acknowledged that the plaintiﬀs discovered
the factual basis for the new counts over a year after ﬁling the
initial complaint, it also noted that they took an additional
four months thereafter to seek leave to amend. During that
intervening period, the parties had submitted and briefed
their motions for summary judgment, and the trial date was
fast approaching. It found their petition untimely. Second, the
district court determined that the new claims were futile un-
der both statutes' advance-notice requirements.
   The plaintiﬀs immediately appealed. Several months after
the judgment, the clerk of the district court imposed costs on
the Liebharts in the amount of $46,320.02. The Liebharts sep-
arately appealed that decision. We consolidated the two ap-
peals and now consider them together.
                         II. ANALYSIS
    The Liebharts raise ﬁve issues for our consideration. First,
they argue that the district court erred in excluding the opin-
ions of expert witnesses Woodyard in full and Carpenter in
part. As part of that argument, they allege that the district
court failed to apply the same stringent standard to the de-
fendants’ expert Keenan. Second, they contend that summary
judgment was inappropriate because, even absent the testi-
mony of their expert witnesses, they submitted adequate pho-
tographic and scientiﬁc evidence to state a valid claim under
RCRA and TSCA. Third, they challenge the district court’s de-
nial of injunctive relief in light of deﬁciencies they have iden-
tiﬁed in SPX’s state-approved clean-up plan. Fourth, they ar-
gue that the district court abused its discretion in denying
8                                       Nos. 18-1918 & 18-2598

leave to amend their complaint. Finally, they challenge the
imposition of costs.
    But there’s a larger issue looming in the background. The
district court’s opinion operates under the assumption that
RCRA plaintiﬀs must demonstrate “an imminent and sub-
stantial danger with evidence of health problems they have
already suﬀered.” Liebhart, 2018 WL 1583296 at *6. Relying
primarily on Foster v. United States, 922 F. Supp. 642 (D.D.C.
1996), the district court set a high bar for RCRA plaintiﬀs,
holding that “it doesn’t follow necessarily that there is an im-
minent and substantial risk of harm simply because there is
some amount of PCBs on the property.” Liebhart 2018 WL
1583296 at *5. We review that question of law, which was the
underlying basis for the award of summary judgment, de
novo. Daugherty v. Harrington, 906 F.3d 606, 609 (7th Cir. 2018).
    A. RCRA Requires Only that Harm “May” Be Imminent
    The question of how much harm a plaintiﬀ must prove to
make out a prima facie violation of RCRA has led to much dis-
cussion among the circuits. We have yet to tackle that issue
head on, so the district court necessarily had to look else-
where for guidance. Passed in 1976, “RCRA is a comprehen-
sive environmental statute that empowers [the] EPA to regu-
late hazardous wastes from cradle to grave.” City of Chicago v.
Envtl. Def. Fund, 511 U.S. 328, 331 (1994). The statute’s “pri-
mary purpose … is to reduce the generation of hazardous
waste and to ensure the proper treatment, storage, and dis-
posal of that waste which is nonetheless generated, ‘so as to
minimize the present and future threat to human health and
the environment.’” Meghrig v. KFC Western, Inc., 516 U.S. 479,
483 (1996) (quoting 42 U.S.C. § 6902(b)).
Nos. 18-1918 & 18-2598                                           9

    Although the EPA has the primary responsibility for en-
forcing the law, the statute, “like other environmental laws,
… contains a citizen suit provision, § 6972, which permits pri-
vate citizens to enforce its provisions in some circumstances.”
Id. at 484. As relevant to this case, the statute provides that
       any person may commence a civil action on his own
       behalf … against any person, … including … any
       past or present owner or operator of a treatment,
       storage, or disposal facility, who has contributed or
       who is contributing to the past or present handling,
       storage, treatment, transportation, or disposal of any
       solid or hazardous waste which may present an im-
       minent and substantial endangerment to health or
       the environment.
42 U.S.C. § 6972(a)(1)(B) (emphasis added). Notably, Con-
gress amended the language in 1980 by substituting the
phrase “may present” for the original 1976 wording “is pre-
senting.” Maine People’s All. and Nat. Res. Def. Council v.
Mallinckrodt, Inc., 471 F.3d 277, 287 (1st Cir. 2006) (citing Solid
Waste Disposal Act of 1980, Pub. L. 96–482, § 25, 94 Stat. 2334,
2348). The section authorizes district courts to grant injunctive
relief and “order [a violator] to take such other action as may
be necessary” to remediate the endangerment. Id.; see also Me-
ghrig, 516 U.S. at 484 (“Under a plain reading of this remedial
scheme, a private citizen suing under § 6972(a)(1)(B) could
seek a mandatory injunction, i.e., one that orders a responsible
party to ‘take action’ by attending to the cleanup and proper
disposal of toxic waste, or a prohibitory injunction, i.e., one
that ‘restrains’ a responsible party from further violating
RCRA.”); Avondale Fed. Sav. Bank v. Amoco Oil Co., 170 F.3d
692, 694 (7th Cir. 1999).
10                                     Nos. 18-1918 & 18-2598

    The critical question in this case is how to determine
whether alleged contamination “may present an imminent
and substantial endangerment to health.” § 6972(a)(1)(b). We
touched on this question only brieﬂy in Albany Bank & Trust
Co. v. Exxon Mobil Corp., 310 F.3d 969, 973 (7th Cir. 2002).
There, we ﬁrst outlined the elements of a prima facie RCRA
claim: “a plaintiﬀ must allege (1) that the defendant has gen-
erated solid or hazardous waste, (2) that the defendant is con-
tributing to or has contributed to the handling of this waste,
and (3) that this waste may present an imminent and substan-
tial danger to health or the environment.” Id. (citing Cox v.
City of Dallas, 256 F.3d 281, 282 (5th Cir. 2001)). We then ob-
served, interpreting the ﬁnal element, that “[i]mminence does
not require an existing harm, only an ongoing threat of future
harm.” Id. (citing Cox, 256 F.3d at 299). That is the extent of
our circuit precedent on the “may present” language con-
tained in § 6972(a)(1)(B).
    But our sister circuits have engaged with this question in
great depth. In the ﬁrst major case interpreting the provision,
the Third Circuit emphasized that the statute “enhanced the
courts’ traditional equitable powers by authorizing the issu-
ance of injunctions when there is but a risk of harm, a more
lenient standard than the traditional requirement of threat-
ened irreparable harm.” United States v. Price, 688 F.2d 204,
211 (3d Cir. 1982). The court found the statutory language
“unequivocal,” demonstrating that Congress “intended to
confer upon the courts the authority to grant aﬃrmative eq-
uitable relief to the extent necessary to eliminate” the risks
posed by toxic waste. Id. at 213–14.
   In the subsequent decades, several other circuits adopted
and further developed the same interpretation of § 6972. See
Nos. 18-1918 & 18-2598                                         11

Mallinckrodt, 471 F.3d at 277; Dague v. City of Burlington, 935
F.2d 1343, 1355–56 (2d Cir. 1991), rev’d in part on other grounds,
502 U.S. 1071 (1992); Interfaith Cmty. Org. v. Honeywell Int’l,
Inc., 399 F.3d 248 (3d Cir. 2005); United States v. Waste Indus.,
Inc., 734 F.2d 159, 165 (4th Cir. 1984) (rejecting the argument
that “[§ 6972] was designed to control pollution only in emer-
gency situations”); Cox, 256 F.3d at 299–301; Price v. United
States Navy, 39 F.3d 1011, 1019 (9th Cir. 1994); Burlington N.
and Santa Fe Ry. Co. v. Grant, 505 F.3d 1013, 1019–22 (10th Cir.
2007); Parker v. Scrap Metal Processors, Inc., 386 F.3d 993, 1014–
15 (11th Cir. 2004). We now join those courts and explicitly
embrace the core content of their interpretation.
    The district court therefore used an incorrect legal stand-
ard to evaluate the Liebharts’ argument that PCBs from the
demolition may present an imminent and substantial danger
to their health and that of their tenants. It did not cite any of
the cases we listed above, but instead relied on a single district
court decision from a circuit that has not yet addressed the
issue. See Liebhart, 2018 WL 1583296 at *5 (quoting Foster, 922
F. Supp. at 662 (“While there can be no question that the levels
of contamination present at the Site may warrant future re-
sponse action, the plaintiﬀ cannot establish either a current
risk of substantial or serious threatened harm, or some neces-
sity for action.”)).
    That error undercuts several of the reasons the district
court gave for rejecting the Liebharts’ RCRA claim. For exam-
ple, the district court cursorily adopted a passing statement
we made over twenty-ﬁve years ago that “[t]he [EPA’s] ac-
cepted safe level of PCBs in the environment is ﬁfty parts per
million,” id. (quoting Cincinnati Ins. Co. v. Flanders Elec. Motor
12                                         Nos. 18-1918 & 18-2598

Serv., Inc., 40 F.3d 146, 148 (7th Cir. 1994)), to create a require-
ment that the Liebharts show that PCB contamination on their
property met that threshold. But there is no requirement in
RCRA for a plaintiﬀ to make “a particular quantitative show-
ing as a sine qua non for liability.” Interfaith, 399 F.3d at 260; see
also Dague, 935 F.2d at 1356. Only one circuit has come to an
arguably contrary conclusion. In Price v. United States Navy,
818 F. Supp. 1323, 1325 (S.D. Cal. 1992), aﬀ’d 39 F.3d 1011 (9th
Cir. 1994), the district court required RCRA plaintiﬀs to show
that “the level of contaminants [was] above levels that are con-
sidered acceptable by the State.” But as the Interfaith court
noted, the Ninth Circuit’s opinion aﬃrming the district
court’s judgment did not discuss that requirement. RCRA
does not require that plaintiﬀs demonstrate contamination
above some agency-derived threshold level of concentration.
399 F.3d at 260–61. It merely requires that they show that con-
taminants on the property are seriously dangerous to human
health (or will be, given prolonged exposure over time). See
Cox, 256 F.3d at 299–300 (“an endangerment is ‘substantial’ if
it is ‘serious’” (quoting United States Navy, 39 F.3d at 1019)).
    That’s especially true when the standard is taken out of
context. The district court relied on Cincinnati Ins. Co. to sup-
port its contention that contamination below the threshold
level of ﬁfty parts per million (“ppm”) does not qualify as a
regulatory violation, thereby contradicting Woodyard’s opin-
ion regarding safe levels of PCBs and providing the grounds
for excluding it. In that case, we evaluated whether an insur-
ance company was required to indemnify its insured for dam-
ages arising out of PCB contamination at an industrial site.
The EPA had identiﬁed the insured as potentially liable under
the Comprehensive Environmental Response, Compensation,
and Liability Act of 1980 (CERCLA), 42 U.S.C. § 9601 et seq. In
Nos. 18-1918 & 18-2598                                       13

that context, we noted that the contaminated site contained
PCBs concentrated at levels up to 58,000 ppm, well above the
EPA’s general regulatory threshold of ﬁfty ppm. 40 F.3d at 148
(citing 40 C.F.R. § 761.60).
    But that regulation, still in eﬀect today, identiﬁes indus-
trial equipment and other wastes contaminated at levels
above ﬁfty ppm and requires special procedures for their stor-
age and disposal. It does not unequivocally state that PCBs
concentrated at levels lower than ﬁfty ppm are safe for human
exposure. Neither Cincinnati Ins. Co. nor the EPA regulation
stand for the proposition that PCBs concentrated at forty-nine
ppm on residential property do not present a substantial endan-
germent to the health of the residents. That is especially ap-
parent when we consider that the same set of regulations dis-
tinguishes between low and high-occupancy areas and re-
quires that, in high-occupancy areas, bulk PCB remediation
waste, such as soil, be cleaned up to levels of concentration at
or below one ppm. § 761.61(a)(4)(i)(A). Section 761.3 conﬁrms
that the term “high-occupancy area” includes residences. See
also § 761.123 (regulating spills of PCBs concentrated at ﬁfty
ppm or greater but emphasizing that “[t]he concentration of
PCBs spilled is determined by the PCB concentration in the
material spilled as opposed to the concentration of PCBs in
the material onto which the PCBs were spilled”). “Proof of
contamination in excess of [agency] standards may support a
ﬁnding of liability, and may alone suﬃce for liability in some
cases, but its required use is without justiﬁcation in the stat-
ute.” Interfaith, 399 F.3d at 261.
   This is not to say that all the plaintiﬀs must do is to show
some bit of soil on their property that tests positive for PCBs
14                                               Nos. 18-1918 & 18-2598

above one ppm (or even above the purported lower Wiscon-
sin DNR standard, which the defendants seem to have
acknowledged in their state-approved clean-up plan). Of
course, there must be accompanying evidence that establishes
some connection between the existing contaminants and
some imminent and substantial endangerment to health. 1 But
the Liebharts attempted to provide such evidence, and the
district court held the bar higher than necessary under
RCRA’s standard. In criticizing expert witness Carpenter’s
statement that “there is no ‘safe’ level of exposure to PCBs that
does not increase the risk of disease,” the district court re-
jected Carpenter’s citation to studies showing the general
risks of PCB exposure because “the Liebharts do not cite any
evidence that they have been exposed to PCBs.” Liebhart, 2018
WL 1583296 at *5. Moreover, the district court concluded that
“[t]he Liebharts do not contend that they can prove an immi-
nent and substantial danger with evidence of health problems
they have already suﬀered.” Id. at *6.
   As we noted in Albany Bank, “[i]mminence does not re-
quire an existing harm, only an ongoing threat of future


     1 On appeal, the Liebharts contend that there is no separate require-
ment for expert witness testimony because the photos and videos they
have submitted into evidence suffice to prove their RCRA claim. The de-
fendants push back on that argument, insisting that many of the issues are
beyond the competence of lay fact-finders and require the assistance of
experts to address. See, e.g., C.W. ex rel. Wood v. Textron, Inc., 807 F.3d 827,
838 (7th Cir. 2015) (“Given the complex nature of this case, … [and w]ith
no experts to prove causation … the appellants cannot prove their toxic-
tort case under [state] law.”) Because the district court did not make a fac-
tual finding as to whether this case is sufficient complex to require expert
testimony, and because we remand for reconsideration of a separate legal
issue, we need not reach that question today.
Nos. 18-1918 & 18-2598                                        15

harm.” 310 F.3d at 973. Although the Supreme Court has in-
structed that “[a]n endangerment can only be ‘imminent’ if it
‘threaten[s] to occur immediately,’” Meghrig, 516 U.S. at 485
(quoting Webster’s New Int’l Dictionary of English Language
1245 (2d ed. 1934)), it qualiﬁed that statement when it approv-
ingly quoted the Ninth Circuit’s interpretation that the statu-
tory term “implies that there must be a threat which is present
now, although the impact of the threat may not be felt until
later.” Id. at 486 (quoting United States Navy, 39 F.3d at 1019).
We take that to mean that the Liebharts must show that there
are PCBs currently on the property that have the potential to
substantially threaten their health at some point in the future
if they continue to occupy the premises and prolong their ex-
posure. On remand, the district court should reevaluate its ex-
clusion of Dr. Carpenter’s assertion regarding PCB safety un-
der the standards we have outlined above and determine
whether, if admissible, the report demonstrates that a sub-
stantial and imminent threat to the Liebharts’ health may be
present.
   B. TSCA Likewise Requires No Heightened Showing
    TSCA authorizes a separate, private cause of action allow-
ing “any person [to] commence a civil action … against any
person … who is alleged to be in violation of … any rule
promulgated under” the Act. 15 U.S.C. § 2619(a)(1). The EPA
promulgated the PCB regulations in question pursuant to
statutory authority under TSCA, designating PCBs as “haz-
ardous waste” and controlling their manufacture, use, stor-
age, and disposal. See 40 C.F.R. pt. 761. The Liebharts’ TSCA
claim alleged that the defendants violated those regulations,
thereby authorizing the district court to “restrain [the] viola-
tion.” § 2619(a). Unlike RCRA, which authorizes mandatory
16                                             Nos. 18-1918 & 18-2598

injunctions to compel remediation of previous violations,
TSCA authorizes only prohibitory injunctions to stop ongoing
violations. Id. The defendants argued that there was no longer
an ongoing violation, as demolition was complete by the time
the Liebharts ﬁled their claim. 2
    The purported violation was based on 40 C.F.R. § 761.123,
which, as we noted above, regulates “spills” of PCB materials
concentrated at levels above ﬁfty ppm. The district court cor-
rectly noted that the Liebharts had not shown any evidence
that any PCBs had been “spilled” onto their property (by
means of dust migration) at that level of concentration, and it
granted summary judgment to the defendants on that basis.
    But the district court seems to have overlooked the follow-
ing sentence in the regulation, which directs that “[t]he con-
centration of PCBs spilled is determined by the PCB concen-
tration in the material spilled as opposed to the concentration
of the material onto which the PCBs were spilled.” Id. The
Liebharts’ complaint seems to have alleged that the concrete
ﬂoor of the factory, which contained at least some contami-
nated areas above the ﬁfty-ppm threshold, was demolished
and “spilled” onto the Liebharts’ property. By the plain text
of the regulation, there is no need to show that contaminants
on the Liebharts’ property are themselves concentrated above
ﬁfty ppm so long as the original material that was the source
of that spill met the criteria. On remand, the district court may
again determine that there is no ongoing violation, but it must

     2 The district court determined that the Liebharts waived this claim
by failing to engage the defendants’ argument that there was no “ongoing
violation.” But the court chose to analyze the merits of the claim regardless
of waiver and reached its conclusion on an alternative basis. We likewise
set aside the issue of waiver to reach the district court’s merits analysis.
Nos. 18-1918 & 18-2598                                        17

ﬁrst determine whether the alleged violation comes within
the meaning of § 761.123 in the ﬁrst place.
   C. The District Court Should Reconsider the Remaining Issues
on Remand
   Now that we have clariﬁed the standard the plaintiﬀs
must meet, we turn to the remaining procedural issues the
Liebharts have asked to us to address.
  1. The District Court did not Abuse Its Discretion in Excluding
Woodyard’s Testimony
    “The summary-judgment decision here turned [in large
part] on the district court’s conclusion that [the] expert testi-
mony was inadmissible. Whether the district court applied
the appropriate legal framework for evaluating expert testi-
mony is reviewed de novo, but the court’s choice of relevant
factors within that framework and its ultimate conclusion as
to admissibility are reviewed for abuse of discretion.” Lees v.
Carthage College, 714 F.3d 516, 520 (7th Cir. 2013). When a dis-
trict court is “[f]aced with a proﬀer of expert scientiﬁc testi-
mony … the trial judge must determine at the outset …
whether the expert is proposing to testify to (1) scientiﬁc
knowledge that (2) will assist the trier of fact to understand or
determine a fact in issue.” Daubert v. Merrell Dow Pharm., Inc.,
509 U.S. 579, 592 (1993). “The trial court must have … latitude
in deciding how to test an expert’s reliability.” Kumho Tire Co.,
Ltd. v. Carmichael, 526 U.S. 137, 152 (1999).
   The district court was well within its discretion to exclude
the proﬀered testimony of expert witness Woodyard. Beyond
the problem that the plaintiﬀs seemed to have waived this is-
sue in the district court by failing to engage with the defend-
ants’ arguments in their motion to strike the testimony,
18                                     Nos. 18-1918 & 18-2598

Woodyard’s analysis suﬀers from a critical ﬂaw that stems
from the way in which the parties have framed this lawsuit.
The district court brieﬂy noted in its opinion that “both sides
assume that none of the defendants can be held liable for any
PCB contamination on the Liebharts’ property that occurred
before the demolition began, so the court will make the same
assumption.” Liebhart, 2018 WL 1583296 at *1. That’s a curious
choice. RCRA permits the Liebharts to obtain injunctive relief
from the owner of the facility for any cognizable contamina-
tion, regardless of whether the PCBs migrated onto their
property before or after the demolition occurred. See 42 U.S.C.
§ 6972(a)(1)(B) (authorizing suit against “any person … who
has contributed … to the past or present handling … or dis-
posal” of hazardous waste); see also Meghrig, 516 U.S. at 486
(“RCRA contains no statute of limitations.”).
   But because the Liebharts restricted their claims to PCBs
that allegedly migrated onto their property via the dust from
the 2015 demolition, they necessarily excluded claims result-
ing from PCBs that may have gotten onto their property dur-
ing the previous century. The district court correctly noted
that Woodyard’s expert opinion could not distinguish be-
tween these two alternatives, as both sets of contaminants
originated from the same source. There was even evidence
showing that PCBs existed in the soil beneath the Liebharts’
asphalt driveway—Woodyard could not explain how that
might have occurred as the result of dust blowing in the wind
as opposed to seepage over the course of years before any
demolition occurred.
  Perhaps the Liebharts decided to frame their claims in this
way because they wanted to go after defendants TRC and
Apollo, who would not be liable for what happened before
Nos. 18-1918 & 18-2598                                           19

demolition. Or perhaps some element of their state-law
claims, which we have not addressed, required them to con-
strict the scope. They did not explain their reasoning to us (or
it seems, to the district court). But the distinction makes it dif-
ﬁcult for Woodyard to make his case, given that he did not
“adequately account[] for obvious alternative explanations.”
Gopalratnam v. Hewlett-Packard Co., 877 F.3d 771, 787 (7th Cir.
2017). Viewed in this light, the district court was within its
discretion to exclude the testimony as unreliable and unhelp-
ful to the trier of fact. The district court may choose to rethink
that evaluation on remand under the legal standards we out-
lined above, but it may well reach the same conclusion.
    2. The District Court Should Reconsider Its Denial of Injunctive
Relief
   The district court denied injunctive relief to the Liebharts
because the defendants had already agreed to a clean-up plan
that had been approved by the Wisconsin DNR. We review
that decision for an abuse of discretion. See AMI Int’l, Inc. v.
Datacard Corp. 106 F.3d 1342, 1352 (7th Cir. 1997) (remanding
grant of RCRA injunction).
    As we mentioned above, RCRA authorizes district courts
to issue either mandatory or prohibitory injunctions, while
TSCA authorizes only prohibitory injunctions. “[I]t is appro-
priate ‘to give great deference to the district court’s decision
as to the precise equitable relief necessary in a particular
case.’” Bowes v. Ind. Sec. of State, 837 F.3d 813, 817 (7th Cir.
2016) (quoting Gjersten v. Bd. Of Election Com’rs for City of Chi-
cago, 791 F.2d 472, 479 (7th Cir. 1986)). That is especially true
when a plaintiﬀ requests a mandatory injunction, which “im-
poses signiﬁcant burdens on the defendant and requires care-
ful consideration of the intrusiveness of the ordered act, as
20                                      Nos. 18-1918 & 18-2598

well as the diﬃculties that may be encountered in supervising
the enjoined party’s compliance with the court’s order.” Kart-
man v. State Farm Mut. Auto. Ins. Co., 634 F.3d 883, 892 (7th Cir.
2011).
    Considering SPX’s decision to formulate a clean-up plan
and obtain approval from the state agency, the district court
determined that there was no need for separate, federally-su-
pervised remediation. It reached that conclusion even while
assuming that the defendants were in violation of either
RCRA or TSCA, despite the fact it found no such violation.
Other courts have determined that the existence of a parallel
plan of remediation supervised by the state does not neces-
sarily prevent a federal district court from granting an injunc-
tion if it ﬁnds the state agency’s actions to be insuﬃcient to
remedy the violation of federal law. See, e.g., Interfaith, 399
F.3d at 264–68. On the other hand, the same courts have also
upheld denials of injunctions in similar situations when the
district court ﬁnds that the parallel plan is adequate to the
task. See Trinity Indus., Inc. v. Chicago Bridge & Iron Co., 735
F.3d 131, 139–40 (3d Cir. 2013) (distinguishing Interfaith). We
recently reached the same conclusion. See LAJIM, LLC v. Gen.
Elec. Co., No. 18-1522, 2019 WL 1011021, at *4–11 (7th Cir. Mar.
4, 2019) (“[T]he district court correctly held that it has discre-
tion to award injunctive relief under the RCRA and is not re-
quired to order relief after a ﬁnding of liability,” particularly
in light of ongoing relief supervised by a state environmental
agency).
     Here, the district court determined that SPX’s existing
plan was suﬃcient and that the Liebharts had not “iden-
tif[ied] any way that SPX’s plan [was] deﬁcient or violate[d]
Nos. 18-1918 & 18-2598                                             21

federal law.” Liebhart, 2018 WL 1583296 at *7. We agree. Alt-
hough the Liebharts point to certain statistical sampling re-
quirements outlined in 40 C.F.R. § 761.125 and contend that
SPX’s existing plan fails to meet those requirements, the dis-
trict court noted that SPX’s plan includes further sampling of
the soil on the Liebharts’ property. It emphasized that “the
Liebharts do not otherwise explain why SPX’s proposed sam-
pling scheme is substantively inadequate.” Liebhart, 2018 WL
1583296 at *7. We see no evidence that compels us to contra-
dict that factual ﬁnding.
    However, in light of the legal standards we outlined
above, we ask the district court to reconsider its decision to
deny injunctive relief on remand. Because the bar for estab-
lishing an imminent and substantial danger is lower than the
district court believed when evaluating the request for an in-
junction, it would be prudent to reassess whether the DNR-
approved plan adequately remedies harms that may come
within RCRA’s scope. See LAJIM, 2019 WL 1011021 at *6
(“[O]nce a court ﬁnds a defendant liable for creating a risk of
imminent and substantial danger, it will usually be the case
that injunctive relief is warranted. But that is not always the
case.”) (internal quotations and citations omitted).
   3. The District Court did not Abuse Its Discretion in Denying
Leave to Amend the Complaint.
    We review a denial of leave to amend a complaint for
abuse of discretion. Life Plans, Inc. v. Sec. Life of Denver Ins. Co.,
800 F.3d 343, 357 (7th Cir. 2015). District courts are to “freely
give leave [to amend] when justice so requires.” Fed. R. Civ.
P. 15(a)(2). “The Supreme Court has interpreted this rule to
require a district court to allow amendment unless there is a
good reason—futility, undue delay, undue prejudice, or bad
22                                     Nos. 18-1918 & 18-2598

faith—for denying leave to amend.” Life Plans, 800 F.3d at 357
(citing Foman v. Davis, 371 U.S. 178, 182 (1962)). The district
court in this case identiﬁed two of those grounds as bases for
denial: futility and undue delay. While we disagree with the
futility analysis, we concur with the ﬁnding of undue delay
and decline to reverse.
    We begin with the question of futility. Both statutes con-
tain advance-notice requirements that citizen plaintiﬀs must
observe prior to ﬁling RCRA or TSCA suits. They require
those plaintiﬀs to notify the EPA 60–90 days in advance of ﬁl-
ing the complaint. 42 U.S.C. § 6972(b)(2); 15 U.S.C.
§ 2619(b)(1)(A). But the district court chose to apply the re-
quirements not only to the ﬁling of the complaint, but also to
any subsequent amendments that sought to add new RCRA
or TSCA counts. Although the Liebharts complied with the
requirements prior to ﬁling their original complaint in April
2016, they gave no new notice to the EPA before ﬁling for
leave to amend in February 2018. The district court held that,
because the prior notice did not contain any mention of bur-
ied PCBs on the SPX property, it was insuﬃcient to alert the
EPA to the particular violation the Liebharts sought to include
in the amended complaint.
    The notice provisions serve two purposes. “First, notice al-
lows Government agencies to take responsibility for enforc-
ing environmental regulations, thus obviating the need for
citizen suits. … Second, notice gives the alleged violator ‘an
opportunity to bring itself into complete compliance with the
Act[s] and thus likewise render unnecessary a citizen suit.’”
Hallstrom v. Tillamook Cty., 493 U.S. 20, 29 (1989) (quoting
Gwaltney of Smithﬁeld, Inc. v. Chesapeake Bay Found., Inc., 484
Nos. 18-1918 & 18-2598                                         23

U.S. 49, 60 (1987)). The district court’s decision to require no-
tice for an amendment to a complaint fulﬁlled neither pur-
pose. By the time the Liebharts attempted to amend their com-
plaint, the EPA had been on notice of alleged PCB contamina-
tion at the SPX property for over two years—and it had de-
clined to get involved. Moreover, it was unlikely that the de-
fendants would resolve the alleged violation to avoid being
sued when they were already in the midst of an extended law-
suit with the same plaintiﬀs. See AM Int’l, 106 F.3d at 1351
(“[T]he delay period is designed to allow a [defendant] a pe-
riod in which to clean up its act and avoid litigation. [This de-
fendant] was clearly not interested in using the delay period
to resolve the dispute without going to court.”); see also Dague,
935 F.2d at 1351 (“There is no need to maintain a window of
opportunity for the government to take the lead enforcement
role … when a citizen, acting as a private attorney general, has
already lawfully assumed the lead role in bringing a … claim
against the same facility.”) (quoting Dague v. City of Burling-
ton, 733 F. Supp. 23, 26 (D. Vt. 1990)). When the original notice
is “suﬃciently speciﬁc to inform the alleged violator [and the
agency] about what it is doing wrong, so that it will know
what corrective actions will avert a lawsuit,” Atl. States Legal
Found., Inc. v. Stroh Die Casting Co., 116 F.3d 814, 819 (7th Cir.
1994), there is no need to require a second round of advance
notice for an amended complaint in the same litigation.
    But an error in the futility analysis alone does not neces-
sarily constitute an abuse of discretion on the whole. In addi-
tion to futility, the district court cited undue delay and preju-
dice in its decision to deny leave to amend. “[D]elay by itself
is normally an insuﬃcient reason to deny a motion for leave
to amend. Delay must be coupled with some other reason.
Typically, that reason … is prejudice to the non-moving
24                                      Nos. 18-1918 & 18-2598

party.” Dubicz v. Commonwealth Edison Co., 377 F.3d 787, 793
(7th Cir. 2004). The district court cited the Liebharts’ admis-
sion that they learned of the possible burial of contaminated
concrete on the SPX property on October 11, 2017 but failed
to seek leave to amend their complaint until February 22,
2018. During that more than four-month period, the parties
submitted and briefed their cross-motions for summary judg-
ment, and by February, trial was only three months away. The
Liebharts provided no justiﬁcation for their delay.
    In response, the Liebharts contend that there was no un-
due prejudice to the defendants: by the time they ﬁled their
motion, ﬁve weeks of discovery still remained. We disagree.
While the district court’s explanation of prejudice was fairly
short, even if the district court had failed to articulate a ﬁnd-
ing of prejudice altogether, we might still “aﬃrm providing
that ‘the prejudice that would result from such amendment
was apparent.’” Park v. City of Chicago, 297 F.3d 606, 613 (7th
Cir. 2002) (quoting Feldman v. Am. Mem’l Life Ins. Co., 196 F.3d
783, 793 (7th Cir. 1999)). In Park, the plaintiﬀ waited six
months to move for leave to amend a complaint after she
knew (or should have known) of the additional alleged viola-
tion. Id. Because trial was approaching, we concluded that the
district court was well within its discretion to deny leave. Id.;
see also Perrian v. O’Grady, 958 F.2d 192, 194–95 (7th Cir. 1992)
(“Because substantive amendments shortly before trial serve
to defeat the public’s interest in speedy resolution of legal dis-
putes, ‘[a] district court judge is entitled, in such circum-
stances, to refuse to allow a plaintiﬀ’s amendment.’” (quoting
Campbell v. Ingersoll Mill. Mach. Co., 893 F.2d 925, 927 (7th Cir.
1990)) (internal citation omitted)).
Nos. 18-1918 & 18-2598                                                      25

    On the contrary, in Life Plans, we found no undue delay
when a plaintiﬀ promptly moved to amend its complaint just
ten days after learning of the factual basis for its new count
during discovery. 800 F.3d at 358. The Liebharts delayed more
than four months in the same circumstances. Although dis-
covery was ongoing at the time of the Liebharts’ motion, the
parties had already briefed their dispositive motions, and the
district court was preparing its summary-judgment order that
would ultimately dispose of the case. We therefore cannot say
that denial of leave for undue delay and prejudice was an
abuse of the district court’s discretion. Of course, the district
court may permit an amendment on remand if the interests of
justice so require.
                            III. CONCLUSION
    Although the district court properly exercised its discre-
tion on the various evidentiary and procedural issues the
plaintiﬀs have raised, its analysis on the merits was narrower
than the statutes demand. It may reach the same conclusions
on reconsideration, but the parties should have another op-
portunity to litigate whether a substantial and imminent en-
dangerment to health exists in this case under the standards
we have outlined. We therefore VACATE the order of the dis-
trict court and REMAND for further proceedings consistent
with this opinion. 3




    3 The Liebharts also challenge the taxation of costs against them by the

clerk of the district court. Because we vacate the district court’s judgment,
we assume that the imposition of costs is automatically vacated and so do
not reach that issue. The clerk of the district court is free to reassess costs
on remand as the case proceeds.
