United States Court of Appeals
                           FOR THE EIGHTH CIRCUIT
___________

No. 05-1906
___________

Carol Marmo,                          *
                                      *
            Appellant,                *
                                      *
v.                                    *
                                      *
Tyson Fresh Meats, Inc.,              *
f/k/a IBP, Inc.,                      *
                                      *
          Appellee.                   *
___________
                                          Appeals from the United States
No. 05-3649                               District Court for the District of
___________                               Nebraska

Carol Marmo,                          *
                                      *
            Appellant,                *
                                      *
v.                                    *
                                      *
Tyson Fresh Meats, Inc.,              *
f/k/a IBP, Inc.,                      *
                                      *
            Appellee.                 *
                                 ___________

                           Submitted: March 17, 2006
                               Filed: August 3, 2006
                                 ___________
Before ARNOLD and SMITH, Circuit Judges, and MAGNUSON,1 District Judge.
                            ___________

MAGNUSON, District Judge.

      Carol Marmo appeals from the final judgment entered on a jury verdict
awarding her $17,500.00 on a nuisance claim against Tyson Fresh Meats, Inc., f/k/a
IBP, Inc. (“IBP”). She also appeals the subsequent award of costs. We affirm.


BACKGROUND


       Marmo commenced this action in the District Court of Dakota County,
Nebraska, in September 2000. IBP timely removed the case to the United States
District Court for the District of Nebraska.2 The case was consolidated for discovery
and pretrial purposes with twelve similar cases, but each case is to be tried
individually. This is the first, and thus far only, case to proceed to trial.


        The complaint alleged three theories of recovery: nuisance, negligence, and
strict liability. In each claim, Marmo asserted that she had been damaged by hydrogen
sulfide gas emitted from the wastewater treatment lagoons at IBP’s beef processing
plant. Marmo later attempted to amend the complaint to add an unjust enrichment
claim based on a pollution easement theory. Specifically, Marmo claimed that IBP
inequitably saved $70 million by not installing appropriate pollution control
equipment. The district court rejected the proposed amendment as futile, concluding
that the unjust enrichment claim based on the facts presented was a novel cause of
action that the Nebraska courts had not recognized.


      1
      The Honorable Paul A. Magnuson, United States District Judge for the District
of Minnesota, sitting by designation.
      2
         The Honorable Laurie Smith Camp, United States District Judge for the
District of Nebraska.
       The initial progression order established a staggered schedule for expert witness
disclosures. The order required Marmo to submit her expert disclosures first,
followed by IBP’s expert disclosures. Thereafter, either party could disclose
additional expert witnesses if “necessary to refute the disclosed opinions of any expert
witness of an opponent.” (Marmo App. in Appeal No. 05-1906 at 77.) The order was
amended numerous times to provide both parties extensions to the disclosure
deadlines.


      In March 2002, Marmo submitted disclosures for seven expert witnesses,
including Dr. Kaye Kilburn, a physician who examined Marmo, and Dr. Harriet
Ammann, a toxicologist. IBP disclosed its expert witnesses on various dates between
August and November 2002. Marmo disclosed several rebuttal experts, including Dr.
William Meggs, in April 2004.


        In May 2004, IBP moved to strike several of Marmo’s rebuttal witnesses,
including Dr. Meggs, on the ground that their opinions offered replacement, and not
rebuttal, testimony. In opposition to the motion, Marmo asserted that “the rebuttal
reports . . . disclose quintessential rebuttal testimony” and that Dr. Meggs’s reports
were “classic rebuttal testimony.” (IBP App. in Appeal No. 05-1906 at 94, 96.) The
magistrate judge denied the motion to strike without prejudice, deferring the issue
until trial.


       In November 2004, IBP filed motions in limine to exclude the testimony of Dr.
Kilburn and to preclude Dr. Ammann from offering an opinion on medical causation.
In response, Marmo withdrew Dr. Kilburn as a witness for her case-in-chief and
moved to re-designate Dr. Meggs as a primary witness. The district court denied the
motion to re-designate, finding that modification of the progression order was not
supported by good cause and would have substantially prejudiced IBP. The district
court also ruled that Dr. Ammann was not qualified to testify on medical causation.


                                           3
       IBP moved for partial summary judgment on the strict liability and negligence
claims, as well as on the issue of permanent physical injury. The district court granted
summary judgment on the strict liability claim, finding that IBP did not engage in an
ultrahazardous activity in the operation of its wastewater treatment facility. The
district court also granted summary judgment on the physical injury issue, reasoning
that Marmo could not present necessary medical causation evidence, in light of the
denial of re-designation and the in limine ruling relating to Dr. Ammann. However,
the district court denied summary judgment on the negligence claim, concluding that
issues of fact remained concerning when the applicable four-year limitations period
commenced.


       Trial occurred in February 2005. During opening statements, IBP’s counsel
remarked that no physician would testify about Marmo and her health. The next day,
Marmo objected to the statement, arguing that it created a false inference that no
medical evidence existed to support her claims. To rebut the inference, Marmo
requested that Dr. Meggs testify in her case-in-chief or that she elicit testimony from
IBP’s experts. The district court denied the request. Thereafter, Marmo attempted to
call Dr. Meggs as a rebuttal expert. However, the district court refused to allow Dr.
Meggs to testify, reasoning that IBP had presented no evidence for Dr. Meggs to
rebut.


       During trial, Marmo admitted that she was aware in the early 1990s of her
alleged difficulties with hydrogen sulfide and linked her symptoms to emissions from
IBP. Accordingly, the district court granted a directed verdict for IBP on Marmo’s
negligence claim, finding that the statute of limitations had run well before Marmo
filed suit in September 2000.


       Marmo’s nuisance claim for the four-year period preceeding September 2000
was submitted to the jury, which returned a verdict of $17,500.00 for Marmo. The
district court then awarded Marmo $23,006.56 in costs.
                                           4
DISCUSSION


A.    Denial of Leave to Amend to Add an Unjust Enrichment Claim


       Marmo sought leave to amend her complaint to add an unjust enrichment claim
based on the theory that IBP unjustly profited from its failure to install adequate
pollution control equipment. The district court denied leave, reasoning that the
attempt was futile because Nebraska law did not recognize an unjust enrichment claim
based on the theory advanced by Marmo.


      A district court should grant leave to amend freely “when justice so requires.”
Fed. R. Civ. P. 15(a). However, denial of leave to amend may be justified when the
amendment is futile. United States ex rel. Joshi v. St. Luke’s Hosp., Inc., 

, 557-58 (8th Cir. 2006) (citing United States ex rel. Gaudineer & Comito, L.L.P.
v. Iowa, 

, 936 (8th Cir. 2001)). Although we review a denial of leave to
amend for an abuse of discretion, we review de novo the underlying legal conclusion
of whether the proposed amendment to the complaint would have been futile. 

(citations omitted).


       An unjust enrichment claim embodies the equitable doctrine that one will not
be allowed to profit or enrich oneself unjustly at the expense of another.3 Hoffman
v. Reinke Mfg. Co., 

, 219 (Neb. 1987) (citing Haggard Drilling, Inc.
v. Greene, 

, 845 (1975)). When the inequitable and unconscionable
retention of a benefit occurs, Nebraska law requires the recipient to pay for the

      3
        Courts recognize unjust enrichment claims “when there is no contractual
relationship, but when, on the grounds of fairness and justice, the law compels the
performance of a legal and moral duty to pay.” 66 Am. Jur. 2d Restitution & Implied
Contracts § 9 (2001). Unjust enrichment implies a contract so that a party may
recover damages from the benefitted party. 

express contract, intent is
irrelevant. 

2-5.
                                         5
reasonable value of the benefit received. Bush v. Kramer, 

, 369
(1969). “Unjust enrichment requires restitution, which measures the remedy by the
gain obtained by the defendant, and seeks disgorgement of that gain.” Trieweiler v.
Sears, 

, 834 (Neb. 2004) (citations omitted). Thus, a defendant will
be liable for the unjust benefit it receives — and not the harm sustained by the
plaintiff. 

(citation omitted).


      Marmo admits that neither the Nebraska Supreme Court nor the Nebraska
Legislature has recognized an unjust enrichment claim based on a pollution easement
theory, but nonetheless argues that Nebraska case law supports her claim. Nebraska
courts have recognized an unjust enrichment claim to allow a purchaser who made
valuable improvements to a property to recover the reasonable value of the
improvement. See McIntosh v. Borchers, 

, 203 (Neb. 1978). They
have also recognized an unjust enrichment claim to require payment for land use when
an individual disavowed an obligation to pay for the use. 

.



      However, no Nebraska state court has recognized a negative unjust enrichment
claim based on the pollution easement theory, which seeks disgorgement of profits
unjustly saved by a polluter.4 Rather, Nebraska courts focus on how the pollution


      4
        The theory is not entirely novel, as other jurisdictions have recognized it. See
Branch v. Mobil Oil Corp., 

, 35-36 (W.D. Okla. 1991); N.C. Corff
P’ship, Ltd. v. Oxy USA, Inc., 

, 295 (Okla. Ct. App. 1996); Evans v. City
of Johnstown, 

, 205-07 (N.Y. Sup. Ct. 1978); see also generally
Allan Kanner, Unjust Enrichment in Environmental Litigation, 20 J. Envtl. L. & Litig.
111 (2005). Indeed, since we heard oral argument in this case, another district court
from the District of Nebraska ruled that Nebraska law supports such a claim. Schwan
v. CNH Am. LLC, No. 4:04CV3384, 

, at *34 (D. Neb. May 4,
2006). Notably, however, the district court in Schwan relied heavily on Oklahoma
law — not Nebraska law. 

have not interpreted unjust enrichment
claims so broadly as to include the pollution easement theory.
                                           6
injures the plaintiff, and that claim is properly brought under the law of nuisance. See
Bargmann v. Soll Oil Co., 

, 486 (Neb. 1998); Karpisek v. Cather &
Sons Constr., Inc., 

, 326-27 (Neb. 1962). There is no indication that
the Nebraska Supreme Court would recognize an unjust enrichment claim on the facts
of this case. Thus, the proposed amendment would have been futile, and the district
court did not err in denying Marmo leave to amend the complaint.


B.     Dismissal of the Negligence Claim


       The district court found that Marmo learned of the irritant effects of hydrogen
sulfide and linked her symptoms with the emissions from IBP by November 1994.
Because Marmo did not commence her action until September 2000, the district court
directed a verdict on the negligence claim, finding it barred by the statute of
limitations.


       We review the grant of a motion for directed verdict de novo. Randall v.
Federated Retail Holdings, Inc., 

, 787 (8th Cir. 2005) (citations omitted).
A directed verdict is appropriate “if during a trial by jury a party has been heard on
an issue and there is no legally sufficient evidentiary basis for a reasonable jury to find
for that party on that issue.” 

R. Civ. P. 50(a)(1)).


       Under Nebraska law, a four-year statute of limitations applies to negligence
claims. See Neb. Rev. Stat. § 25-207(3). The statute of limitations in a negligence
action generally commences when the injurious act or omission occurs. Carruth v.
State, 

, 580 (Neb. 2006). Thus, the limitations period runs from the
date that the injured individual has the right to commence an action, even if the
individual is ignorant about whether a cause of action exists or is unaware of the
nature or extent of the damage. Cavanaugh v. City of Omaha, 

, 544-
45 (Neb. 1998).

                                            7
        Marmo maintains that IBP repeatedly breached its duty to exercise due care in
operating its facility and that the district court erred by failing to apply the continuous
tort doctrine to her negligence claim. Nebraska recognizes the continuing treatment
doctrine, which provides that the limitations period in medical malpractice claims runs
from the date on which a continuing course of treatment ends, even if the injury was
inflicted earlier in the course of treatment. 

. However,
Nebraska courts refuse to extend the continuous tort doctrine beyond the medical
malpractice context. See Anonymous v. St. John Lutheran Church of Seward, 

, 925 (Neb. Ct. App. 2005) (refusing to apply continuing tort doctrine to
negligence cause of action); see also Grand Island Sch. Dist. No. 2 v. Celotex Corp.,

, 608 (Neb. 1979) (no continuous injury despite repeated failures to
repair negligently installed roof). Moreover, the policy considerations underlying the
continuing treatment doctrine do not apply in this case. See Casey v. Levine, 

, 487-88 (Neb. 2001) (primary considerations underlying the continuing
treatment doctrine are to allow a physician an opportunity to correct any malpractice
and not disrupt the physician-patient relationship) (citations omitted). Accordingly,
the district court properly refused to apply the continuous tort doctrine and
appropriately directed a verdict on the negligence claim.


C.     Expert Witnesses


       The district court granted IBP partial summary judgment because Marmo failed
to proffer admissible expert testimony to demonstrate causation with regard to
permanent physical injury. Initially, Marmo identified Dr. Kilburn as an expert to
render an opinion on medical causation. However, after IBP filed a motion in limine
challenging the reliability of Dr. Kilburn’s opinion and a summary judgment motion
on permanent physical injury, Marmo withdrew Dr. Kilburn as an expert. Thereafter,
Marmo proffered the testimony of both Dr. Ammann and Dr. Meggs, but the district
court ruled that neither expert could testify on causation.


                                            8
      We review evidentiary rulings for an abuse of discretion, according such
decisions substantial deference. Morgan v. United Parcel Serv. of Am., Inc., 

, 467 (8th Cir. 2004) (citation omitted); Life Plus Int’l v. Brown, 

, 803 (8th Cir. 2003) (citation omitted).


1.    Dr. Ammann


      The district court allowed Dr. Ammann to testify that Marmo’s alleged injuries
were “consistent with” hydrogen sulfide exposure, but precluded Dr. Ammann from
opining on causation. Marmo submits that the district court improperly limited Dr.
Ammann’s testimony.


       District courts must ensure that all scientific testimony is both reliable and
relevant. Daubert v. Merrell Dow Pharms., Inc., 

, 580 (1993); Fed. R.
Evid. 702. The inquiry as to the reliability and relevance of the testimony is a flexible
one designed to “make certain that an expert, whether basing testimony upon
professional studies or personal experience, employs in the courtroom the same level
of intellectual rigor that characterizes the practice of an expert in the relevant field.”
Kumho Tire Co. v. Carmichael, 

, 152 (1999). Expert testimony is
inadmissible if it is speculative, unsupported by sufficient facts, or contrary to the
facts of the case. Concord Boat Corp. v. Brunswick Corp., 

, 1056-57
(8th Cir. 2000).


       To satisfy the reliability requirement, the proponent of the expert testimony
must show by a preponderance of the evidence both that the expert is qualified to
render the opinion and that the methodology underlying his conclusions is
scientifically valid. 

. To show that the expert testimony
is relevant, the proponent must show that the reasoning or methodology in question
is applied properly to the facts in issue. 

Courts should resolve doubts
regarding the usefulness of an expert’s testimony in favor of admissibility. Clark v.
                                            9
Heidrick, 

, 915 (8th Cir. 1998); see also Arcoren v. United States, 

, 1239 (8th Cir. 1991) (noting that Rule 702 “is one of admissibility rather
than exclusion”). However, a court should not admit opinion evidence that “is
connected to existing data only by the ipse dixit of the expert.” Gen. Elec. Co. v.
Joiner, 

, 146 (1997). When the analytical gap between the data and
proffered opinion is too great, the opinion must be excluded. 

previously held that a toxicologist may testify that exposure to a
chemical caused a person’s symptoms and injuries. Bonner v. ISP Techs., Inc., 

, 928-31 (8th Cir. 2001); Loudermill v. Dow Chem. Co., 

, 569-
70 (8th Cir. 1988); see also Reference Manual on Scientific Evidence 401-31 (2d ed.
2000) (recognizing that toxicologists may offer expert opinions on whether exposure
to a chemical caused an individual’s injury). However, neither Bonner nor Loudermill
provides a blanket rule that toxicologists are qualified to render an opinion on
causation. Rather, under the circumstances of each case, we found that the opinion
of each toxicologist was reliable. In Bonner, we emphasized that the causation
opinion on the plaintiff’s immediate, acute symptoms was reliable because of the
temporal connection between the exposure and the 

.
In Loudermill, we noted that the expert had considerable academic and practical
knowledge, including clinical experience examining patients and conducting autopsies
to determine toxicological related events in 

.


       In this case, Dr. Ammann interviewed Marmo but did not examine her and did
not inquire about other toxic exposures. See Reference Manual on Scientific

(“One of the basic and most useful tools in diagnosis . . . of
disease is the patient’s medical history. . . . [I]t is widely recognized that a thorough
medical history involves questioning and examination of the patient as well as
appropriate medical testing.”). Moreover, Dr. Ammann did not exclude confounding
factors, which “leaves open the possibility of competing causes of the disease” and
raises questions about the competency of expert testimony. 

Finally, Dr.

                                           10
Ammann admitted that the causation standard she employed was not subject to
expression in terms of a potential rate of error and was a much lower standard than
medical causation. Because of these analytical flaws, the district court acted within
the bounds of discretion when precluding Dr. Ammann from testifying on medical
causation.


      2.     Dr. Meggs


      The district court refused to allow Marmo to call Dr. Meggs as a witness in her
case-in-chief and later ruled that Dr. Meggs could not testify as a rebuttal witness.
Marmo challenges these rulings on several grounds.


       First, Marmo argues that the progression order did not dictate the presentation
of evidence at trial. To ensure that trial does not proceed higgledy-piggledy, the
district court has wide discretion to determine the order in which parties adduce proof.
Geders v. United States, 

, 86 (1976). Under the terms of the progression
order, Marmo disclosed Dr. Meggs as an expert witness “necessary to refute the
disclosed opinions” of IBP’s expert witness. The district court properly construed Dr.
Meggs as a rebuttal witness. See Black’s Law Dictionary 1295 (8th ed.) (defining
“rebut” as “to refute, oppose, or counteract (something) by evidence, argument, or
contrary proof”). Indeed, Marmo described Dr. Meggs as providing “classic” rebuttal
testimony.


       “The function of rebuttal testimony is to explain, repel, counteract or disprove
evidence of the adverse party.” United States v. Lamoreaux, 

, 755 (8th
Cir. 2005) (citation omitted); Faigin v. Kelly, 

, 85 (1st Cir. 1999) (“The
principal objective of rebuttal is to permit a litigant to counter new, unforeseen facts
brought out in the other side’s case.”) (citations omitted). As such, rebuttal evidence
may be used to challenge the evidence or theory of an opponent — and not to
establish a case-in-chief. Cates v. Sears, Roebuck & Co., 

, 685 (5th Cir.
                                          11
1991) (“Rebuttal must be kept in perspective; it is not to be used as a continuation of
the case-in-chief.”); see also John Henry Wigmore, Evidence in Trials at Common
Law § 1873 (1976) (a district court should allow rebuttal evidence only if it is
necessary to refute the opponent’s case).


       The district court reasonably construed the progression order as dictating both
the timing and substantive nature of expert witness testimony, and it was well within
its discretion to rule that witnesses disclosed as rebuttal witnesses under the
progression order would testify as rebuttal witnesses at trial. To construe the order
otherwise would eviscerate the distinction between primary and rebuttal witnesses.


       Second, Marmo argues that the district court abused its discretion in refusing
to re-designate Dr. Meggs as a witness in her case-in-chief. Marmo identified Dr.
Meggs as an expert witness in April 2004, two years after the deadline to disclose
primary witnesses and after IBP had filed its motion in limine challenging the
admissibility of Dr. Kilburn and its summary judgment motion based on lack of
admissible evidence on causation. The district court refused to re-designate Dr.
Meggs because Marmo failed to show good cause to modify the progression order.


       Adherence to progression order deadlines is critical to achieving the primary
goal of the judiciary: “to serve the just, speedy, and inexpensive determination of
every action.” Fed. R. Civ. P. 1; see also Bradford v. DANA Corp., 

, 809
(8th Cir. 2001) (“As a vehicle designed to streamline the flow of litigation through our
crowded dockets, we do not take case management orders lightly, and will enforce
them.”) (citation omitted). Accordingly, the district court has broad discretion in
establishing and enforcing the deadlines. Fed. R. Civ. P. 16, 37. To modify a
progression order, a party must show good cause for the modification. 

. To establish good cause, a party must show its diligence in attempting
to meet the progression order. 

A district court may also


                                          12
consider the existence or degree of prejudice to the party opposing the modification.

ample opportunity to develop her expert testimony on the causation
issue. She chose to keep Dr. Kilburn as the primary witness on that issue for over two
years while litigation proceeded, attempting to re-designate Dr. Meggs only after
withdrawing Dr. Kilburn. The tactical decision to withdraw Dr. Kilburn in response
to a motion challenging his opinion did not provide good cause to substitute Dr.
Meggs. See Nelson v. Tenn. Gas Pipeline Co., 

, 250 (6th Cir. 2001)
(“fairness does not require that a plaintiff, whose expert witness testimony has been
found inadmissible under Daubert, be afforded a second chance to marshal other
expert opinions and shore up his case”). Similarly, that new scientific evidence
relating to hydrogen sulfide exposure was published after the deadline for primary
expert disclosure does not provide good cause to re-designate Dr. Meggs as a primary
witness. Instead, Marmo should have sought leave to present supplemental opinions
by her existing primary experts. See Fed. R. Civ. P. 26(e).


       Furthermore, IBP would have suffered significant prejudice if the district court
allowed Dr. Meggs to testify as a primary expert witness. IBP based its litigation
strategy on the initial disclosure of primary experts, including the designation of Dr.
Kilburn to opine on causation. It selected its own expert witnesses and developed
motion and trial strategies based on that expert witness evidence. See Trost v. Trek
Bicycle Corp., 

, 1008-09 (8th Cir. 1998) (because defendant had
already prepared its summary judgment motion at least partially based on the lack of
expert opinion to support plaintiff’s claim, the district court did not abuse its
discretion in excluding plaintiff’s untimely disclosed expert). To cure the prejudice
in allowing Dr. Meggs to testify as a primary witness, the district court would have
had to re-progress the case and practically start anew. Within two months of trial, the
district court did not abuse its discretion in refusing to allow the re-designation.


                                          13
       Third, Marmo argues that the district court erroneously refused to allow Dr.
Meggs to testify as a rebuttal witness. During opening statements, IBP’s counsel
remarked that no doctor would testify about Marmo and her health. Marmo claims
that the statement created a false inference that no medical evidence supported her
claims. She contends that the district court should have allowed Dr. Meggs to testify
to rebut the allegedly false inference.


       We disagree. “Allowance of a party to present additional evidence on rebuttal
depends upon the circumstances of the case and rests within the discretion of the
individual most able to weigh the competing circumstances, the trial judge.” Gossett
v. Weyerhaeuser Co., 

, 1156 (8th Cir. 1988) (citations omitted). The
district court instructed the jury that the arguments of counsel are not evidence. This
instruction cured any prejudice that might have been caused by the remark. 

Billingsley v. City of Omaha, 

, 997 (8th Cir. 2002) (an
admonishment to the jury that statements made by an attorney are not evidence
remedies any prejudice). Moreover, IBP presented no evidence on medical causation,
and therefore, there was no evidence for Dr. Meggs to rebut. Accordingly, the district
court did not abuse its discretion in refusing to allow Dr. Meggs to testify in rebuttal.5
See Life Plus 

(a party may not “get admitted through the back
door of rebuttal evidence that which the district court correctly barred . . . at the front
door”).



      5
        IBP contends that we must review this issue for plain error because Marmo
contemporaneously failed to object during the opening statement. Federal Rule of
Evidence 103(a)(1) requires objections to evidence to be “timely.” For an objection
to be timely, “it must be made at the earliest possible opportunity after the ground of
objection becomes apparent, or it will be considered waived.” Terrell v. Pollard, 

, 638-39 (8th Cir. 1984). We need not address whether Marmo waived the
objection by waiting until the day following opening statements to lodge her objection
because we affirm the district court even under the less deferential abuse-of-discretion
standard.
                                            14
D.    Jury Instructions


       Marmo advances several arguments relating to jury instructions. In this
diversity case, Nebraska law governs the substance of jury instructions. Smith v.
Tenet Healthsystem SL, Inc., 

, 885-86 (8th Cir. 2006). “District courts
have ‘wide discretion’ in drafting jury instructions.” 

(quoting Omega
Healthcare Investors, Inc. v. Lantis Enters., Inc., 

, 776 (8th Cir. 2001)).
“Our review is limited to whether the instructions, viewed on the whole, fairly and
adequately represent the evidence and applicable law in light of the issues presented
to the jury.” 

on Damages


       The district court instructed the jury: “If you return a verdict for Carol Marmo,
then you must decide how much money will compensate Carol Marmo for the
discomfort, annoyance, and inconvenience that she experienced in the use and
enjoyment of her residential property.” Marmo argues that this instruction was
insufficient for two reasons. First, she contends that the district court should have
instructed the jury that she was not required to prove a specified dollar amount of
damages. Second, she argues that the instruction was too narrow because it did not
expressly explain that the jury could award damages for physical and mental injury.
In particular, Marmo contends that the district court should have instructed the jury
that she was entitled to damages for “the nature and extent of the injury, including
whether the injury is temporary or permanent and whether any resulting disability is
partial or total.”


       The district court’s instruction was based on well-settled Nebraska law. See
Thomsen v. Greve, 

, 57 (Neb. Ct. App. 1996). Moreover, the
instruction proposed by Marmo includes consideration of factors that were no longer
part of the case, such as permanent injury. Even if there were an instructional error,
                                          15
Marmo has not made any showing that it affected her substantial rights. To the
contrary, the instruction left Marmo’s counsel free to argue that Marmo experienced
a multitude of damages within the general categories — and he did so. The
instruction adequately represented the evidence and applicable law in light of the
issues presented to the jury.


      2.     Instruction on Pre-Existing Conditions


       Marmo argues that the district court erred in failing to include a pre-existing
condition instruction. We disagree. A pre-existing condition instruction should be
given rarely and only when supported by the evidence. See NJI2d Civ. § 4.09 cmt.
II. The instruction proposed by Marmo related to pre-existing respiratory and
neurological symptoms, yet there was no evidence at trial that Marmo suffered from
either respiratory or neurological problems. Thus, there is no factual basis for the
proposed instruction.


      3.     Instruction on Concurrent Cause


       Marmo submits that the district court erred in refusing to give a concurring
cause instruction in light of IBP’s arguments and evidence of other sources of
hydrogen sulfide in her neighborhood.6 She contends that the small verdict suggests
that the jury reduced the award to reflect the existence of twelve other potential
sources of hydrogen sulfide. However, the district court instructed the jury to award
Marmo for all the discomfort, annoyance, and inconvenience she experienced; the
court did not instruct the jury to apportion damages. Moreover, Marmo’s counsel


      6
        Evidence showed that there were thirteen potential sources of hydrogen sulfide
gas in the Dakota City area. IBP argued that those sources may have caused the
hydrogen sulfide in Marmo’s home and that it was undeterminable how much the
other potential sources contributed to the problem.
                                         16
informed the jury that other sources were inconsequential. The exclusion of a
concurrent cause instruction was not erroneous.


      4.     Instruction on Loss of the Sense of Smell


      In the partial summary judgment order, the district court ruled that the lack of
expert testimony precluded Marmo from recovering for permanent medical injury.
IBP asked for a specific instruction informing the jury that it could not award damages
for Marmo’s permanent loss of the ability to smell. The district court refused the
request, reasoning that a reasonable jury would conclude that the loss is not
“discomfort, annoyance, or inconvenience.”


      During closing arguments, Marmo’s counsel used a demonstrative slide that
asked the jury to award damages for “the loss of one of the five senses.” The district
court sustained IBP’s objection to the slide, instructed the jury to not award any
damages based on an alleged loss of the sense of smell, and directed that the slide be
removed.


      Marmo purports that these actions left the jury with the false inference that
Marmo had contravened the court’s orders and deprived the jury of the ability to
award damages even for temporary loss of the sense of smell. However, Marmo had
not presented causation evidence to permit recovery for a loss of her sense of smell.
Thus, the district court properly instructed the jury.


E.    Taxable Costs


      At the completion of trial, Marmo sought $165,159.00 in expenses and costs.
The district court awarded Marmo $23,003.56, based on her limited success at trial


                                          17
and her failure to provide sufficient information to support the bill of costs. Marmo
argues that the district court improperly reduced the award.


       Costs other than attorneys’ fees “shall be allowed as of course to the prevailing
party unless the court otherwise directs.” Fed. R. Civ. P. 54(d); see also 28 U.S.C. §
1920 (enumerating costs that are recoverable). Rule 54(d) presumes an award of costs
to the prevailing party; however, the district court has substantial discretion in
awarding costs. Computrol, Inc. v. Newtrend, L.P., 

, 1072 (8th Cir.
2000) (citations omitted). Indeed, Rule 54(d) is phrased in permissive terms and
generally grants a federal court the discretion to refuse to tax costs in favor of the
prevailing party. 

Accordingly, we review the district court’s
decision concerning the award of costs for abuse of discretion. Martin v.
DaimlerChrysler Corp., 

, 695 (8th Cir. 2001).


      1.     Transcription Costs


       A district court may tax transcription costs if a deposition was “necessarily
obtained for use in a case” and was not “purely investigative.” 

; see also 28 U.S.C. § 1920. The Bill of Costs Handbook used by the District of
Nebraska details what deposition transcripts are necessary. It provides that costs
incurred in taking depositions generally will be taxed in favor of the prevailing party
“if the taking of the depositions was reasonably necessary at the time it was taken,
even though they may not have been used at trial.” (IBP Add. in Appeal No. 05-3649
at 3.) However, it also requires that a prevailing party show that the deposition was
introduced into evidence, relied upon for cross-examination or impeachment purposes,
or otherwise useful in assisting a resolution of contested issues. (Id.) The Handbook
further provides that depositions taken solely for discovery are not taxable as costs.
(Id.)



                                          18
        The district court refused to tax costs associated with witnesses whom Marmo
voluntarily withdrew or whom the district court ruled inadmissible.7 When a party
withdraws a witness in response to a motion in limine, or when the district court
concludes that a witness does not have the expertise under Federal Rules of Evidence
104 or 702 to assist the jury in determining an issue, the district court is well within
its discretion to deny costs associated with that witness. Moreover, the district court
is in a better position than the Court of Appeals to assess whether the depositions of
these witnesses were necessary. Accordingly, we find no abuse of discretion.


      2.     Witness Fees


        A district court may award witness fees if it determines that the witness’s
testimony “was crucial to the issues decided and the expenditures were necessary to
the litigation.” Neb. Pub. Power Dist. v. Austin Power, Inc., 

, 975 (8th
Cir. 1985). The Handbook provides that witness fees will not be taxed if the witness
is subpoenaed for trial but does not testify, or if the witness is deposed but the
transcript is not used at trial or in support of a motion. (IBP Add. in Appeal No. 05-
3649 at 5.) The district court did not abuse its discretion in refusing to award costs
for witnesses who did not testify at trial. Likewise, the district court did not abuse its
discretion in declining to award transportation costs due to insufficient explanation.




      7
         After IBP objected to the bill of costs, Marmo sought to supplement the bill
of costs to demonstrate how each deposition was necessary to the litigation. The
district court struck the supplement, reasoning that “it is imperative that counsel
provide materials to the Court at the appropriate juncture in the proceedings and not
in reaction to opposing counsel’s observation of the deficiency.” (Marmo App. in
Appeal No. 05-3649 at 400.) The district court was well within its discretion to strike
the untimely supplementation.
                                           19
      3.     Exemplification and Photocopy Costs


       Marmo sought $124,914.47 in exemplification and photocopy costs. Because
this case was one of thirteen consolidated for pretrial purposes, the district court
apportioned these costs among the thirteen cases. It noted that many of the materials
prepared for trial in this action — such as graphic and visual aids, as well as materials
prepared for electronic display — will be useful in the other twelve cases. In addition,
the district court refused to award costs that were described insufficiently.


       Copy and exemplification fees may be awarded if the fees were incurred for
items “necessarily obtained” for use in the case. 28 U.S.C. § 1920(4). The Handbook
explains that expenses for copying papers are “necessarily obtained” if “the copies
were received as evidence, prepared for use in presenting evidence, or obtained for
service on the other parties in the litigation and the court.” (IBP Add. in Appeal No.
05-3649 at 8.) The Handbook further provides that routine copy expenses made for
service, filing, or convenience of counsel are not taxable absent prior court approval.
(Id.) Marmo challenges the Handbook provisions as contrary to § 1920. However,
that section provides the district court with discretion to assess costs, and the
Handbook is a product of exercising that discretion. The district court appropriately
followed the Handbook provisions and limited photocopying expenses to those
necessarily incurred in this litigation.


       In addition, a division of copy and exemplification costs among the thirteen
cases was equitable. The plaintiffs in each of these cases have incurred costs, and the
district court may not award Marmo costs for expenses that other plaintiffs incurred.
Moreover, the apportionment reduced the risk of duplicative cost recovery.
Accordingly, Marmo’s reliance on In re Derailment Case, 

(8th Cir.
2005), is misplaced. In that case, we held that the district court should have held the
taxation award in abeyance until all related actions were resolved. 

However, that case was based on joint and several liability of the losing parties. In
                                           20
this case, IBP, the losing party, is the only party that may be held liable for costs. The
reasoning of that case, therefore, does not apply here.


CONCLUSION


       Nebraska law does not recognize an unjust enrichment claim based on the
pollution easement theory advanced by Marmo. Accordingly, the district court
properly denied leave to amend the complaint to add an unjust enrichment claim.
Likewise, the district court appropriately refused to apply the continuing tort doctrine
to the negligence claim and properly found that the statute of limitations barred the
claim. The district court did not abuse its discretion in finding the opinion of Dr.
Ammann unreliable and in refusing to allow Dr. Meggs to testify as either a primary
or rebuttal witness. We further find no error in the jury instructions. Finally, the
district properly exercised its discretion to reduce and apportion Marmo’s request for
costs because she did not comply with the Bill of Costs Handbook, requested
reimbursement of nontaxable expenses, and failed to include sufficient detail to
substantiate certain items. In addition, the district court did not abuse its discretion
in finding that the requested cost amount was disproportionate to the verdict and
inequitable under the circumstances of this case.


For the foregoing reasons, we affirm the judgment of the district court.


ARNOLD, Circuit Judge, dissenting.


       I respectfully disagree with the court's conclusion that the courts of the State of
Nebraska would not recognize a claim for unjust enrichment in the circumstances of
this case. The law of restitution, as the court seems to recognize, is well established
in Nebraska. Generally speaking, restitution is available whenever a person acquires
a benefit that in justice he or she ought not to retain. Ahrens v. Dye, 

,

                                           21
133, 

, 684-85 (1981). The generality of this principle may account
for the court's reluctance here to entertain this suit, but in truth the principle is not
unbounded and there are well-defined and well-developed categories into which
claims for restitution fit. One of those is when a person wrongfully uses the property
of another for profit. See Restatement (Third) of Restitution and Unjust Enrichment
§ 40 (Tentative Draft No. 4, 2005). Nebraska law specifically recognizes, moreover,
that in a proper case a person who is unjustly enriched must disgorge the profits
produced by the wrongful act that he or she committed. See Trieweiler v. Sears, 

, 979, 

, 834 (2004).


       These are the principles, well established in Nebraska law, that the plaintiff
seeks to have applied to her case. She claims that the defendant has wrongfully used
her property and that allowing them to keep the gains that they realized from their tort
would unjustly enrich them. There is nothing particularly exotic or radical about this
claim. It is true that no Nebraska court has recognized a claim that is the exact
duplicate of the one raised here, but the present claim is constructed from the basic
building blocks of the law of restitution, which is very much a part of Nebraska law.
Our job is simply to apply the law that we believe the Supreme Court of Nebraska
would apply in the circumstances. See Rucci v. City of Pacific, 

, 652-53
(8th Cir. 2003); Williams v. Crews, 

, 265 (8th Cir. 1977). For the
reasons given, I have no doubt that that court would recognize this claim.


       The court notes, correctly, that the plaintiff cites no Nebraska case directly on
point. But it is also true that the defendant does not point to any Nebraska case that
rejects a claim like the plaintiff's. In fact, though the court acknowledges numerous
cases from other jurisdictions that provide restitutionary relief in the present
circumstances, it is highly suggestive that the court does not advert to a single instance
in which a court turned away a similar claim. The reason for the lack of such
authority, I believe, is that the plaintiff's case presents essentially a mine-run, routine
restitutionary claim that calls for a relatively straightforward application of familiar

                                            22
legal principles. Another federal district court in Nebraska has correctly recognized
this. See Schwan v. CNH Am. LLC, No. 4:04CV3384, 

, at *34 (D.
Neb. May 4, 2006).


        The court also points out that Nebraska courts provide an action of nuisance for
damages in the present circumstances; but it is unclear exactly why the court thinks
that the nuisance remedy would be exclusive. It is certainly not true, as the defendant
repeatedly suggests in its brief, that restitution is an equitable remedy and therefore
is available only when an action at law is not. Restitution is not an equitable remedy:
It is part of the substantive law of obligations, like the law of tort and contract, and the
Nebraska courts have recognized that it is in ordinary courts of law that restitutionary
actions lie. See Collection Bureau of Grand Island, Inc. v. Fry, 

, 282-
84, 

, 446-48 (2000). Nor is there any other principled reason why the
mere existence of a nuisance remedy for damages should oust the plaintiff from her
restitutionary remedy. Restitution is simply there at the plaintiff's option: She may
waive the tort and sue for unjust enrichment. As Judge Posner has said, "Restitution
is available in any intentional-tort case in which the tortfeasor has made a profit that
exceeds the victim's damages (if the damages exceed the profit, the plaintiff will prefer
to seek damages instead)." Williams Electronics Games, Inc. v. Garrity, 

, 576 (7th Cir. 2004).


       Though the availability of the restitutionary claim is plain enough, it is not
altogether clear what remedy the Nebraska courts would provide in the present case.
If an injunction would not have been issued under Nebraska law to enjoin the
defendant's emissions, it is likely that disgorgement of the profits realized from them
would not be available as a remedy in this case. Instead, a Nebraska court might well
enter a judgment in an amount equal to the reasonable cost of a license to trespass or
commit a nuisance--the market price, in other words, of a so-called pollution
easement. See Restatement (Third) of Restitution and Unjust Enrichment § 40, Illus.
14 (Tentative Draft No. 4, 2005); cf. Daniel Friedmann, "Restitution of Benefits

                                            23
Obtained Through the Appropriation of Property or the Commission of a Wrong," 80
Colum. L. Rev. 504, 531-32 (1980). This is a matter that the district court ought to
explore on a more fully developed record.


      For the reasons given, I would reverse the district court's summary judgment
on the plaintiff's unjust enrichment claim and remand for further proceedings.


                       ______________________________




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