OPINION OF THE COURT

VANASKIE, Circuit Judge.

"A request for attorney's fees should not result in a second major litigation." Hensley v. Eckerhart, 461 U.S. 424, 437, 103 S.Ct. 1933, 76 L.Ed.2d 40 (1983). Regrettably, requests for attorneys' fees in this protracted environmental clean-up case have resulted not only in a second major litigation, but a third as well. An earlier multi-million dollar fee award previously brought before us was vacated and remanded for additional review by the District Court. Interfaith Cmty. Org. v. Honeywell Int'l, Inc. (ICO II), 426 F.3d 694 (3d Cir.2005). We are now confronted with a challenge to another multi-million dollar award. This latest appeal calls upon us to decide whether offers of judgment pursuant to Fed.R.Civ.P. 68 may be made in the context of attorney's fee disputes under the fee-shifting provisions of the Resource Conservation and Recovery Act ("RCRA"), 42 U.S.C. §§ 6901, et seq. We are also called upon once again to determine whether the fee award is excessive. Because we conclude that Rule 68 offers of judgment may be made in this context, we will reverse the District Court's declaration that the offers of judgment in this case are null and void as well as its decision to bar any further offers of judgment. And, while we uphold as not clearly erroneous the District Court's decisions with respect to the appropriate hourly rates in this case, we are unable to sustain its conclusions with respect to the number of hours claimed by counsel because the District Court's findings lack sufficient explanation. Accordingly, we will vacate the fee award and remand for further proceedings.

I. Facts and Procedural History

Mutual Chemical Company of America ("Mutual") operated a chrome manufacturing plant in Jersey City, New Jersey from 1895 to 1954. During that time, the company deposited approximately 1.5 million tons of industrial waste residue containing hexavalent chromium into wetlands along the Hackensack River. (Joint Appendix ["J.A."] 1082-83.) In 1954, Allied Corporation purchased the plant and ended the dumping. Allied Corporation was succeeded by AlliedSignal, Inc., and later by Honeywell International, Inc. ("Honeywell"). Although the dumping stopped, the contaminated area was not cleaned up.

In 1995, the Interfaith Community Organization and five residents of the nearby community (collectively, "ICO"), represented by the Washington, D.C. law firm of Terris, Pravlik & Millian, LLP ("Terris"), filed the original suit against AlliedSignal, then the owner of the site, seeking the cleanup of a contaminated area designated "Study Area 7." ICO sued AlliedSignal under the citizen suit provision of RCRA, which allows individuals to bring a civil action against any person "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 imminent and substantial endangerment to health or the environment...." 42 U.S.C. § 6972(a)(1)(B).

The District Court entered judgment for ICO in 2003, ordering Honeywell (which had succeeded AlliedSignal) to clean up Study Area 7. Interfaith Cmty. Org. v. Honeywell Int'l Inc., 263 F.Supp.2d 796, 802 (D.N.J.2003). This Court affirmed. See 399 F.3d 248, 252 (3d Cir.2005).

In 2004, the District Court awarded ICO more than $4.5 million in fees and expenses for litigating the 1995 action, and also required Honeywell to pay the future fees and costs incurred by ICO in monitoring Honeywell's cleanup. Interfaith Cmty. Org. v. Honeywell Int'l, Inc. (ICO I), 336 F.Supp.2d 370, 403-04 (D.N.J.2004). We affirmed in part and vacated in part. ICO II, 426 F.3d 694, (3d Cir.2005). Specifically, we sustained as not clearly erroneous the District Court's decision with respect to the hourly rates sought by ICO's counsel, id. at 707-10, but rejected as inadequate the District Court's review of the hours for which compensation was claimed. Id. at 711-14. Accordingly, we vacated the fee award and remanded for further proceedings.

In 2005, Hackensack Riverkeeper ("Riverkeeper"), also represented by Terris, filed companion cases against Honeywell stemming from the same contamination but relating to areas adjacent to Study Area 7, designated as "Study Area 5" and "Study Area 6." (J.A. 1140.) The parties entered into a number of consent decrees in which Honeywell conceded responsibility, and agreed to remediate the additional contaminated sites. As part of the consent decrees, Honeywell also agreed to pay $5 million in fees and costs for the expenses incurred prior to the decrees, and to pay "reasonable" future fees and expenses incurred in connection with monitoring Honeywell's remediation efforts. (J.A. 334-35.)

Initially, the parties were able to reach agreement on fees and expenses. Beginning in the fall of 2009, however, Honeywell, on the one hand, and ICO and Riverkeeper (collectively, "Appellees") on the other, failed to reach agreement with respect to the fees sought for monitoring Honeywell's work.

Terris subsequently filed two separate fee applications, totaling more than $700,000, for its monitoring work performed in 2009 and the first half of 2010 in connection with the 1995 case. Terris filed a separate application, seeking almost $2.5 million, for work performed on the 2005 litigation. Honeywell filed objections to the fee applications. Specifically, Honeywell renewed its previously-rejected arguments that the forum rate rule should be applied so that the hourly rates sought by Terris should be based upon the rates charged by New Jersey lawyers as opposed to Washington, D.C. lawyers, and that, even if D.C. rates were used, Appellees applied the wrong method for calculating prevailing D.C. market rates. In addition, Honeywell once again contested the reasonableness of the hours and expenses claimed by counsel. Honeywell also served offers of judgment pursuant to Rule 68 for the disputed fees. In response, Appellees asked the District Court to issue a declaratory judgment that Honeywell's Rule 68 offers are null and void in the context of RCRA citizen suits.

On September 8, 2011, the District Court issued an opinion that substantially upheld the Appellees' fee request. First, the District Court once again ruled that the forum-rate rule need not be applied in this case so that Terris could be paid Washington, D.C. rates for work relating to a dispute in New Jersey. Interfaith Cmty. Org. v. Honeywell Int'l, Inc. (ICO III), 808 F.Supp.2d 744, 749-50 (D.N.J. 2011). Second, the District Court evaluated two different methods for calculating prevailing D.C. market rates, and approved the method requested by Appellees. Id. at 750-51. Third, the District Court rejected most of the challenges to the reasonableness of the hours expended by Appellees' counsel. Id. at 751-55. And, finally, the District Court held that Rule 68 offers of judgment cannot be made in citizen suits filed under RCRA, concluding that application of Rule 68 to RCRA citizen suits would violate the Rules Enabling Act, 28 U.S.C. § 2072, by discouraging the very citizen suits that Congress intended to promote. Id. This appeal followed.

II. Discussion

The District Court had jurisdiction under 42 U.S.C. § 6972(a), the citizen suit provision of RCRA, and we have appellate jurisdiction under 28 U.S.C. § 1291. We "review the legal interpretation of procedural rules de novo." United Auto. Workers Local 259 Soc. Sec. Dep't v. Metro Auto Ctr., 501 F.3d 283, 286 (3d Cir.2007). The standard the district court should "apply in calculating a fee award is a legal question subject to plenary review," Evans v. Port Auth. of N.Y. & N.J., 273 F.3d 346, 358 (3d Cir.2001), but "[t]he determination of the appropriate billing rate is a factual finding which [this Court] review[s] for clear error." ICO II, 426 F.3d at 709. Finally, the amount of a fee award is within the district court's discretion, and we will not disturb the district court's "determination of ... the number of hours reasonably expended absent clear error." Evans, 273 F.3d at 358.

A. Rule 68 Offers of Judgment

We must first decide whether offers of judgment made pursuant to Federal Rule of Civil Procedure 68 apply to attorney's fee disputes brought under the citizen suit provision of RCRA. As the Supreme Court has instructed, "[w]e give the Federal Rules of Civil Procedure their plain meaning, and generally with them as with a statute, [w]hen we find the terms unambiguous, judicial inquiry is complete." Pavelic & LeFlore v. Marvel Entm't Group, 493 U.S. 120, 123, 110 S.Ct. 456, 107 L.Ed.2d 438 (1989) (quotation marks omitted). Thus, we begin, as we must, with the text of the rule. See United States v. Gonzales, 520 U.S. 1, 4, 117 S.Ct. 1032, 137 L.Ed.2d 132 (1997).

Federal Rule of Civil Procedure 68 provides, in pertinent part:

Fed.R.Civ.P. 68(a), (c)-(d)

Rule 68 does not exempt from its purview any type of civil action. See 12 C. Wright & A. Miller, Federal Practice & Procedure § 3001.1 (2d ed.1987). Moreover, Rule 1 of the Federal Rules of Civil Procedure states that the rules apply to "all suits of a civil nature," unless exempted by Rule 81. Fed.R.Civ.P. 1. Rule 81, in turn, does not set forth any restrictions on Rule 68's applicability to citizen suits under RCRA, or to suits seeking equitable relief generally. Thus, by its plain terms, Rule 68 is applicable to RCRA citizen suits.

The District Court, however, held that Rule 68 is so incompatible with Congress' purpose in enacting RCRA that its application to cases brought under § 6972 would violate the Rules Enabling Act, 28 U.S.C. § 2072. The Rules Enabling Act gives the Supreme Court the power to "prescribe general rules of practice and procedure and rules of evidence for cases in the United States district courts ... and the courts of appeals," provided that "such rules [do] not abridge, enlarge or modify any substantive right." § 2072(a)-(b). Thus, if applying Rule 68 to § 6972 citizen suits abridges or modifies a substantive right, then Rule 68 offers are void in this context notwithstanding the plain meaning of the rule.

A rule of procedure does not run afoul of this statutory limitation merely because it "affects a litigant's substantive rights; most procedural rules do." Shady Grove Orthopedic Assocs., P.A. v. Allstate Ins. Co., 559 U.S. 393, 130 S.Ct. 1431, 1442, 176 L.Ed.2d 311 (2010) (Scalia, J., plurality opinion). As Justice Scalia elaborated: "What matters is what the rule itself regulates: If it governs only `the manner and the means' by which the litigants' rights are `enforced,' it is valid; if it alters `the rules of decision by which [the] court will adjudicate [those] rights,' it is not." Id. (quoting Mississippi Pub'g Corp. v. Murphree, 326 U.S. 438, 446, 66 S.Ct. 242, 90 L.Ed. 185 (1946)).

Applying the criterion that a rule of procedure impermissibly "abridge[s], enlarge[s], or modif[ies] [a] substantive right," 28 U.S.C. § 2072(b), only if it alters the rules for adjudicating a litigant's rights, we readily conclude that application of Rule 68 in the specific context of this case does not violate the Rules Enabling Act. No rule of decision governing the adjudication of the attorney's fee dispute that is the subject of Honeywell's offers of judgment is affected by application of Rule 68. The amount of the fee to be awarded remains governed by the same rules of decision regardless of the interposition of an offer of judgment. At best, the only impact that Rule 68 has on the ultimate outcome of the attorney's fee dispute is to require Appellees to bear their post-offer costs, including counsel fees, if the fee award is less favorable than the offer of judgment.1 See Fed.R.Civ.P. 68(d); Marek v. Chesny, 473 U.S. 1, 11-12, 105 S.Ct. 3012, 87 L.Ed.2d 1 (1985).2

In light of Rule 68's laudatory purpose of facilitating settlement, Delta Airlines, Inc. v. August, 450 U.S. 346, 352, 101 S.Ct. 1146, 67 L.Ed.2d 287 (1981), the requirement that a plaintiff bear the fees incurred after it rejects an offer of judgment simply cannot be said to abridge some substantive right. In this regard, fees incurred after a party rejects an offer of judgment and recovers less than the offer are properly viewed as being unreasonable.

The District Court, relying upon Public Interest Research Group of New Jersey v. Struthers-Dunn, Inc., Civ. A. No. 87-1773, 1988 WL 147639 (D.N.J. Aug. 16, 1988), reasoned that Rule 68 offers would discourage citizens from bringing suit and firms from accepting the cases, because there is no possibility for monetary relief in citizen suits and, therefore, the only source of compensation for law firms representing plaintiffs in these cases comes in the form of an award of attorney's fees. ICO III, 808 F.Supp.2d at 756-57. The impact on a decision to pursue litigation, however, has nothing to do with whether the offer of judgment rule abridges or modifies some substantive right. It may very well be that a Rule 68 offer of judgment in the context of a RCRA attorney's fee dispute will require a plaintiff to make a hard choice between accepting what has been offered versus adjudicating the issues that are in dispute in such a case — the appropriate hourly rate and the reasonableness of the hours expended. That Appellees may feel compelled to take less than the amount to which they believe they are entitled, however, has nothing at all to do with the determination of the appropriate hourly rate and the reasonableness of the hours expended before the offer of judgment was made. Settlement offers often present difficult choices for a plaintiff, but that fact neither abridges nor modifies the substantive rights at issue. Speculation as to the potential "chilling" effect of allowing Rule 68 offers of judgment in citizen suits under RCRA, advanced in Struthers-Dunn and embraced by the District Court in this case, is simply irrelevant to the pertinent inquiry: whether the rules of decision are altered by the offer of judgment.

The Supreme Court has not considered Rule 68's impact on § 6972 citizen suits, but it has addressed the interaction between Rule 68 and the fee-shifting statute applicable to civil rights litigation, 42 U.S.C. § 1988. See Marek, 473 U.S. 1, 105 S.Ct. 3012. While the majority opinion in Marek did not address whether application of Rule 68 in the context of civil rights litigation violated the Rules Enabling Act, its rationale is indeed instructive here. In Marek, the Court considered whether post-offer of judgment "costs" to be borne by the plaintiff included plaintiff's attorney's fees when the ultimate recovery was less than the offer. Id. at 3, 105 S.Ct. 3012. Stated otherwise, the issue in Marek was whether a plaintiff who received a verdict that was less than the offer of judgment could recover the fees incurred after the offer was made. The plaintiffs in Marek argued that a recovery of less than the offer of judgment should not preclude an award of all counsel fees, including post-offer fees. Id. at 4, 105 S.Ct. 3012. Much like the argument advanced by Appellees in this case, the Marek plaintiffs asserted that a different reading of Rule 68 would unfairly burden civil rights plaintiffs by discouraging attorneys from bringing meritorious claims at the risk of losing attorney fees. Id. Notwithstanding the strong policy arguments favoring the encouragement of suits to vindicate important constitutional rights, the Court applied the plain meaning of Rule 68 to foreclose recovery of post-offer fees. The Court concluded that the purpose of § 1988 (to encourage meritorious civil rights claims) was distinct from and compatible with the purpose of Rule 68 (to encourage settlement). Id. at 11, 105 S.Ct. 3012. Thus, the Court held that "nothing... in the policies underlying § 1988 constitutes `the necessary clear expression of congressional intent' required `to exempt... [the] statute from the operation of Rule 68.'" Id. at 11-12, 105 S.Ct. 3012 (citing Califano v. Yamasaki, 442 U.S. 682, 700, 99 S.Ct. 2545, 61 L.Ed.2d 176 (1979)).

Appellees argue that Marek is distinguishable because civil rights plaintiffs are often motivated by the potential for personal gain, in contrast to RCRA plaintiffs, who seek injunctive relief in furtherance of a purely public gain.3 Although Appellees are correct that citizen plaintiffs suing under § 6972 cannot recover monetary damages, while plaintiffs in civil rights cases often can, they miss the point of the Court's analysis in Marek. The Court sustained application of Rule 68 to civil rights cases even though it could chill the pursuit of litigation intended to vindicate important rights. The Court concluded that, notwithstanding such potential, Rule 68 applied because the policies underlying Rule 68 and the fee shifting statute at issue there were compatible.4

The fact that only equitable relief is available under section 6972 does not alter this conclusion. Courts have applied Rule 68 to suits seeking equitable relief despite arguments that doing so would discourage such claims. See, e.g., NAACP v. Town of East Haven, 259 F.3d 113, 121 n. 9 (2d Cir.2001) ("[N]othing in the Rule suggests that it applies only to cases seeking damages or other relief amenable to simple comparisons."); Chathas v. Local 134 Int'l Bhd. of Elec. Workers, 233 F.3d 508, 512 (7th Cir.2000) ("Rule 68 offers are much more common in money cases than in equity cases, but nothing in the rule forbids its use in the latter type of case.")

The fee shifting provision of section 6972 encourages plaintiffs to bring meritorious suits to enforce environmental laws, while Rule 68 encourages settlement of civil suits. See Delta Air Lines, 450 U.S. at 352, 101 S.Ct. 1146. "There is nothing incompatible with these two objectives." Marek, 473 U.S. at 4, 105 S.Ct. 3012 (comparing purposes of § 1988 with purposes of Rule 68). Accordingly, we conclude that nothing in the text of Rule 68 suggests that such an exemption is warranted, and application of Rule 68 in the context presented here does not violate the Rules Enabling Act.

Our Rule 68 inquiry is not yet complete, however. Appellees raise another challenge to Rule 68's applicability in this case, contending that the rule does not apply to proceedings after judgment has been rendered on liability. Appellees point to the text of the rule, which states that an offer of judgment must be made "at least 14 days before the date set for trial," or, if "one party's liability to another has been determined but the extent of liability remains to be determined by further proceedings... it must be served within a reasonable time ... before the date set for a hearing to determine the extent of liability." Fed.R.Civ.P. 68(a), (c). Appellees interpret this language to mean that the rule only applies in two situations: first, before a trial; and, second, in a bifurcated proceeding after judgment has been rendered but before the extent of liability is determined.

The first situation plainly does not apply here, and Appellees claim that this case does not fall within the second situation because attorney's fees cannot be regarded as part of Honeywell's liability. Specifically, Appellees assert that the word "liability" — even within the phrase "extent of liability" — does not encompass a dispute over attorney fees. To support this interpretation of the text, Appellees point to Federal Rule of Civil Procedure 54, which provides definitions of "judgment" and "costs." See Fed.R.Civ.P. 54. Because Rule 54 includes attorney fees within the definition of "costs," Appellees argue that fees cannot also be included within the definition of "liability." (Appellee's Br. 55). Further, Appellees observe that "RCRA provides that `costs of litigation' include `reasonable attorney and expert witness fees,'" and note that, under Marek, Rule 68 "costs" must also include attorney fees. (Appellee's Br. 55-56). Because "costs" include attorney fees for purposes of Rule 68, Appellees argue, attorney fees cannot also be part of "liability." (Id.)

Given the ordinary meaning of "liability," see Black's Law Dictionary 997 (9th ed.2009), the phrase "extent of liability" encompasses all legal responsibilities. This appeal is evidence that the extent of Honeywell's liability has yet to be determined.

This conclusion is consistent with our approach in Public Interest Research Group of New Jersey v. Windall (PIRG), 51 F.3d 1179 (3d Cir.1995), where we implicitly treated a Rule 68 offer made at the attorney's fee stage of litigation as valid. In PIRG, we vacated a fee award and directed the district court to consider on remand whether the plaintiff reasonably continued to litigate the attorney's fee issue after refusing the defendant's Rule 68 offer. 51 F.3d at 1190. See also Sanchez v. Prudential Pizza, 709 F.3d 689, 691 (7th Cir.2013) (indicating that Rule 68 offers may include attorney's fees so long as the offer clearly states that it includes fees and costs).

Moreover, the policies underlying Rule 68 support this interpretation. Rule 68 was created to "encourage the settlement of litigation." Delta Air Lines, 450 U.S. at 352, 101 S.Ct. 1146. The benefits of settlement are highest in the context of attorney fee disputes, which the Supreme Court has warned "should not result in a second major litigation." Hensley, 461 U.S. at 437, 103 S.Ct. 1933. Incentive to settle is beneficial where, as here, the "litigation to resolve fee disputes has ... taken on a life of its own." ICO III, 808 F.Supp.2d at 749. As the District Court noted, "the amount of litigation engendered by the present fee dispute has probably cost as much as the contested amount." Id. at 748. We are confident that encouragement to settle is warranted in this context. Thus, because we believe that a Rule 68 offer in this context both comports with the ordinary meaning of the phrase "extent of liability" and is consistent with the fee-shifting provision of RCRA, we conclude that Rule 68 offers of judgment apply to disputes over attorney fees after liability has been determined.

B. Forum-Rate Rule

We now turn our attention to the District Court's departure from the forum-rate rule. The forum-rate rule holds that "in most cases, the relevant rate [for calculating attorney fees] is the prevailing rate in the forum of the litigation." ICO II, 426 F.3d at 705. We have recognized two exceptions to the rule: "first, when the need for the special expertise of counsel from a distant district is shown; and, second, when local counsel are unwilling to handle the case." Id. (internal quotations omitted) (citing Report, of the Third Circuit Task Force on Court Awarded Attorney Fees, 108 F.R.D. 237, 261 (1985)). We sustained as not clearly erroneous the District Court's decision in ICO I that the forum-rate rule should not be applied in this case. See ICO II, 426 F.3d at 707.

Our prior ruling presents us now with the question of whether further litigation of the forum-rate rule is foreclosed by the doctrine of collateral estoppel. We have "identified four standard requirements for the application of collateral estoppel ... (1) the identical issue was previously adjudicated; (2) the issue was actually litigated; (3) the previous determination was necessary to the decision; and (4) the party being precluded from relitigating the issue was fully represented in the prior action." Jean Alexander Cosmetics, Inc. v. L'Oreal USA, Inc., 458 F.3d 244, 249 (3d Cir.2006) (quotation marks omitted). We are satisfied that each element is met in this case.

In ICO II, we adjudicated the identical issue presented here; namely, whether the "lack of northern New Jersey counsel willing to handle RCRA citizen suits without an immediate payment of fees and advanced out-of-pocket costs foreseeably amounting to over $1,000,000" justifies a departure from the forum-rate rule. (Appellee's Br. 12.) We held that departure was justified in this case. Thus, the first element is met. Second, the issue was actually litigated in this District Court and, on appeal, in this Court. See ICO II, 426 F.3d at 703-07. Third, a determination about the forum-rate rule was necessary to this Court's decision about the proper rate for calculating attorney fees. Finally, Honeywell was represented in the prior case. Honeywell fully litigated this issue in that case, and our decision was a valid, final judgment.

Honeywell argues that collateral estoppel cannot apply because the issue in the first case was whether a departure from the forum-rate rule was justified "in the 1995 litigation," asserting that Appellees must prove that New Jersey counsel is not available to take the case now. (Reply Br. 8.) Honeywell mischaracterizes this litigation. This Court determined in 2005 that a departure from the forum-rate rule was warranted. ICO II, 426 F.3d at 707. The instant litigation is part of the same case that began in 1995, including the now-consolidated case brought by Riverkeeper in 2005. As the District Court noted, "it would have been unreasonable to expect Plaintiffs to change horses in midstream, and suddenly find, or need to document the inability to find, competent local counsel to continue the litigation in Study Areas 5 and 6." ICO III, 808 F.Supp.2d at 749. Thus, we conclude that further litigation concerning the applicability of the forum-rate rule is barred by collateral estoppel.

C. The Laffey Matrix

Because we have sustained the District Court's decision to allow Terris to be compensated on the basis of Washington, D.C. rates, we must now determine whether the District Court erred in deciding what these rates are. In this case, the District Court applied what is known as the "Laffey Matrix" for purposes of determining the appropriate hourly rates.5 The Laffey Matrix "provides billing rates for attorneys in the Washington, D.C. market with various degrees of experience." ICO II, 426 F.3d at 708. The original Laffey Matrix set forth the prevailing market rates from 1981-1982. In 1989, the Laffey Matrix was updated to account for inflation. See Trout v. Ball, 705 F.Supp. 705, 709 n. 10 (D.D.C.1989) (approving updated Laffey Matrix). Since 1989, courts have approved various methods for updating the Laffey Matrix. Compare Salazar v. District of Columbia, 123 F.Supp.2d 8, 15 (D.D.C. 2000) (utilizing legal services component of the Consumer Price Index) with M.R.S. Enters., Inc. v. Sheet Metal Workers' Int'l Ass'n, No. 05-1823, 2007 WL 950071, at *5 (D.D.C.2007) (applying U.S. Attorney Office Matrix).

The parties agree that the initial Laffey Matrix was a valid index of Washington, D.C. rates in 1982. The parties further agree that an updated version of the Laffey Matrix, which accounts for the rise in prevailing rates based on inflation, would be a valid vehicle for determining the applicable hourly rates in the D.C. legal market today. The parties disagree, however, about the proper method of updating the matrix. Appellees favor the Legal Services Index ("LSI") method, which accounts for "shifts in the consumer price index for legal services nationwide." ICO III, 808 F.Supp.2d at 750. Honeywell prefers the U.S. Attorney Office ("USAO") matrix, which "regularly updates the Laffey Matrix using the Consumer Price Index for the D.C. area." (Appellant's Br. 31.) These divergent methods result in very different prevailing rates.6 This Circuit has not specifically approved of either method, and both parties cite cases from the District of Columbia in support of their respective preferred methods.

Appellees point to Salazar in support of their preference for the LSI index. See Salazar, 123 F.Supp.2d at 15. In that case, the District Court for the District of Columbia weighed the advantages and disadvantages of both the LSI index and the USAO matrix and determined that the LSI method "more accurately reflects the prevailing rates for legal services in the D.C. community."7 Id. In contrast, Honeywell cites multiple decisions applying the USAO matrix rather than the LSI-updated matrix. See, e.g., Heller v. District of Columbia, No. 03-213, 2011 WL 6826278, at *8-10 (D.D.C.2011); Hayes v. D.C. Pub. Sch., 815 F.Supp.2d 134, 143 (D.D.C.2011); M.R.S. Enters., Inc. v. Sheet Metal Workers' Int'l Ass'n, No. 05-1823, 2007 WL 950071, at *5 (D.D.C.2007).

The District Court, recognizing that "our Circuit has yet to specifically approve either version of updating the Laffey Matrix," was persuaded by the methodology in Salazar. ICO III, 808 F.Supp.2d at 750. Furthermore, the District Court relied on this Court's prior opinion affirming use of the LSI methodology. See ICO II, 426 F.3d at 709-10; ICO III, 808 F.Supp.2d at 750 ("[T]he Court will rely on the holding in the previous [ICO II]."). In ICO II, we stated:

ICO II, 426 F.3d at 709-10.

We review the District Court's determination of the appropriate billing rate for clear error. ICO II, 426 F.3d at 709. Clear error exists only where factual findings "are unsupported by substantial evidence, lack adequate evidentiary support in the record, are against the clear weight of the evidence or where the district court has misapprehended the weight of the evidence." United States v. 645 Acres of Land, 409 F.3d 139, 145 n. 10 (3d Cir.2005). In light of our prior decision affirming the LSI methodology, as well as the District Court's assessment of the Salazar court's reasoning as persuasive, we will not now hold that it was clear error to once again rely on the LSI method. We thus affirm the District Court's use of the LSI-updated Laffey Matrix to determine the prevailing rates in the Washington, D.C. market.

D. The Reasonableness of the Hours Expended

Although we have a sufficient record for sustaining the District Court's determinations as to the appropriate hourly rates, we cannot say the same with respect to the other component of the fee calculation: the reasonableness of the hours expended by Terris. As we remarked in ICO II,

426 F.3d at 711 (quoting PIRG, 51 F.3d at 1188) (internal quotation marks omitted).

The District Court "has `a positive and affirmative function in the fee fixing analysis, not merely a passive role.'" Id. at 713 (quoting Loughner v. Univ. of Pittsburgh, 260 F.3d 173, 178 (3d Cir.2001)). And, where, as here, an objecting party has challenged specific types of work and states why it is contended that the hours claimed are excessive, the reviewing court must support its findings with a sufficient articulation of its rationale to allow for meaningful appellate review. Id.

Here, Honeywell did identify specific categories of work for which the hours claimed were purportedly unreasonable. Specifically, Honeywell objected to the following:

Although decrying the litigation tactics employed by Terris as "distasteful," "aggressive," and "unsavory," id. at 751, 753, the District Court nonetheless chose to "credit[] [Appellees'] arguments ... as to the reasonableness of the legal and expert fees, expenses and hours charged," explaining that it "will not second guess the staffing decisions of either the Terris firms or its experts...." Id. at 754-55. This perfunctory statement does not allow for meaningful appellate court review. As we said in ICO II, "where the opinion of the District Court `is so terse, vague, or conclusory that we have no basis to review it, we must vacate the fee-award order and remand for further proceedings.'" 426 F.3d at 713 (quoting Gunter v. Ridgewood Energy Corp., 223 F.3d 190, 196 (3d Cir. 2000)). Accordingly, we must once again vacate the District Court's latest fee awards and remand for further proceedings.10

III.

For the foregoing reasons, we will reverse the District Court's ruling that Rule 68 offers of judgment are inapplicable in the context of environmental citizen suits brought under RCRA, direct that the previously made offers of judgment be reinstated, affirm the District Court's departure from the forum-rate rule because review of this issue is barred by collateral estoppel, affirm the District Court's application of the LSI-updated Laffey Matrix, vacate the District Court's fee award, and remand the case for further proceedings consistent with this opinion.