Full opinion text
MEMORANDUM DECISION AND ORDER RE: BAC DEFENDANTS’ MOTION FOR PARTIAL SUMMARY JUDGMENT; DAUBERT MOTIONS OLIVER W. WANGER, District Judge. I. INTRODUCTION. This lawsuit arises out of a now-closed cooling tower manufacturing facility (the “BAC site”) that pressure treated wood and was operated by entities formerly owned by the BAC Defendants (“BAC”). Plaintiffs, current or former residents of residential neighborhoods (“Beachwood”) near the BAC Site, allege that two contaminants from the BAC Site migrated from the treating area via groundwater, surface water, surface soil, private well, and air pathways to locations where plaintiffs were exposed to them. Also named as defendants are various municipalities, water districts, and developers, including the Franklin County Water District, Merced Irrigation District, the City and County of Merced, and the Meadowbrook Water District. Before the court for decision is BAC Defendants’ motion to summarily adjudicate Plaintiffs’ tenth, twelfth, thirteenth, and fourteenth causes of action. According to Defendants, Plaintiffs have failed to present any admissible evidence of actual exposure to contaminants from the BAC site, via any pathway, which was required under “Phase 1” of the Court’s “Order Modifying Scheduling Conference Order.” (Doc. 540.) The first phase of discovery was to focus on “whether contaminants from the former [] BAC Site, Franklin County Water District or the April 2006 Flood have ever reached any location where plaintiffs could have been exposed to them, and if so, when such contaminants arrived, how such contaminants arrived at the location, how long they were present, and at what levels they were present.” (Id. at 1:14-1:28.) By this motion, the BAC Defendants assert that Plaintiffs have not met their “Phase 1” or “general exposure” burden, entitling them to partial summary judgment. Plaintiffs opposed the motion on July 1, 2010, submitting over 3,000 pages of documentation and 168 exhibits. Plaintiffs’ opposition is founded on their claim that they have presented “substantial evidence” in the form of expert opinion and analysis to show that contaminants migrated from the BAC facility to Plaintiffs’ homes and/or properties. Plaintiffs further argue that the BAC Defendants’ motions to exclude certain expert testimony fail because their criticisms go to its weight, not its admissibility. Oral argument on the BAC Defendants’ motions was held on October 6, 7, 13, 14, and 15, 2010, during which the parties presented argument and evidence relevant to the Phase 1 pathway exposure issues. The parties were permitted to introduce expert testimony on key scientific issues, namely the methodologies and assumptions used to model contamination via the groundwater and air pathways. The testifying experts were examined by counsel for Plaintiffs and Defendants, as well as the Court. On October 22 and November 1, 2010, the parties were advised the Court intended to appoint independent experts under Fed.R.Evid. 706 to assist in the understanding and resolving the complex scientific disputes over groundwater and air modeling. The analysis included determining concentration levels of hexavalent chromium and/or arsenic in Meadowbrook Well No. 2 and surrounding areas (i.e., monitor networks and private wells), as well as the assumptions/caleulations underlying the air model. The Fed.R.Evid. 706 Experts were appointed on October, 26. 2010 (Kenneth D. Schmidt, Ph.D.—groundwater) and November 1, 2010 (Chatten Cowherd Jr., Ph.D. and Richard Countess, Ph. D. — air). The parties and the Court jointly prepared a list of questions for the experts, which were transmitted to the experts in early November. The independent expert reports were submitted to the Court on November 22, 2010. The Fed.R.Evid. 706 experts were examined by the Court and parties on December 2, 3, and 15, 2010. Rebuttal testimony was permitted on a limited basis. The Rule 56 motions were submitted for decision following summation arguments on December 28 and 29, 2010. II. FACTUAL BACKGROUND? The facts underlying this case are summarized in the Court’s previous Memorandum Decisions in this case, filed on November 13, 2008, May 18, 2009, and July 15, 2009, in brief: approximately 2,100 Plaintiffs seek damages relating to two occurrences: (1) an April 2006 flood; and (2) alleged long-term contamination released from the former site of a cooling tower manufacturing facility operated by entities formerly owned by the BAC Defendants. The now-closed facility, which is the alleged source of contamination, is located in Merced, California, approximately I, 600 feet southwest from Meadowbrook Well No. 2 (“MWC-2”), which was the primary well supplying domestic water to the Beachwood community, until it was taken out of service in March 2008. Plaintiffs allege that MWC-2, which was operated by the Meadowbrook Water District, was contaminated by hexavalent chromium and total chromium in the well and from the aquifer, which drew on the contaminated plume during times of high water production, and exposed Plaintiffs to contaminants. It is further alleged that the remaining defendants contributed to Plaintiffs’ exposure to carcinogens and/or toxins based on their operation of a nearby drainage system (City and County of Merced), a collection system and ponds (Franklin County Water District), and an irrigation canal (Merced Irrigation District). According to Plaintiffs, these municipalities and water districts were “direct participants” in the contamination based on their location and significance to the local community’s water supply. A. History and Remediation of BAC Site From 1969 until 1984, the BAC Site housed a cooling tower manufacturing facility and BAC-Pritchard, Inc., the operator/owner, used pressure-treated wood to make cooling tower frames. The wood was treated in an on-site cylindrical vessel (retort), using two different solutions. Specifically, from 1969 until 1980, the wood was treated with a solution of chromium, copper, and arsenic (“CCA”). In 1980, BAC-Pritchard stopped using arsenic and instead used a solution of acid, copper, and chromium (“ACC”). BAC-Pritchard ceased treating wood at the site in May 1991 and the facility was closed in early 1994. The BAC Defendants acknowledge that elevated levels of hexavalent chromium and arsenic were discovered at the BAC Site as early as 1986. That same year, BAC Defendants hired an environmental consultant to investigate and implement a number of remedial measures associated with contamination at the Site. The retained consultant found elevated levels of hexavalent chromium and arsenic in the soil at the BAC Site. Subsequent consultants found hexavalent chromium and arsenic in groundwater beneath the Site and in a stormwater pond located on the southeastern portion of the Site. In 1991, IT Corporation, a remediation consultant hired by Merck and Amsted, completed a “clean closure” of the storm water pond. The closure involved excavating contaminated soil from the pond and disposing of it at a landfill under the supervision of the California Department of Toxic Substances Control (“DTSC”). In 1993 and 1994, IT Corp., under the supervision of the Regional Water Quality Control Board (“RWQCB”), installed a pump- and-treat groundwater remediation system to control migration of contaminants beneath and off of the Site and to remove hexavalent chromium and arsenic from groundwater. IT Corp. continued to expand and modify the groundwater treatment system through 2005. In 1994, as part of the cleanup efforts, IT Corp. prepared a health risk assessment for the BAC Site. According to BAC Defendants, the health risk assessment employed “conservative exposure assumptions” to protect public health and was premised on the fact that no further remediation would take place. Based on these assumptions, IT Corp. determined that contamination at the Site created a cancer risk of two in one million (ratio of 2:1,000,000). The BAC Defendants assert that the DTSC employs a ratio of one in one million (1:1,000,000) to determine whether additional remediation should be performed. IT Corp.’s risk assessment did not evaluate the risk posed by the Site in the past (pre-1994), but looked prospectively at the future risk the Site might create if no further remediation was performed. IT Corp.’s risk assessment determined that if soils in the area of the former pressure-treating system were excavated or capped, that would “effectively eliminate the exposure pathways (and subsequent risk).” In 1996, IT Corp. completed excavation of contaminated soil from the area of the former pressure-treating system and installed a four-inch thick asphalt cap over the entire area. The BAC Defendants maintain that the excavation and cap “effectively eliminated the risks identified in the 1994 risk assessment.” Additional remediation at the BAC Site was performed in the mid to late-2000’s by Arcadis, Inc., an environmental consulting firm to the BAC Defendants, which was paid $17 million to complete the cleanup of the BAC Site. This included excavation of the contaminated soil beneath the asphalt cap installed in 1996. Once the soil was removed, Arcadis re-paved the exposed soil. Arcadis also treated the groundwater with methanol to convert the hexavalent chromium to the more benign trivalent chromium. The groundwater treatment is expected to be completed within a few years, at which point the BAC Site will be monitored by the RWQCB until its final closure. BAC Defendants assert that they have spent approximately $39 million remediating the BAC Site. B. RWQCB Involvement — 2007 In 2007, the RWQCB announced the availability for public review of a revised cleanup plan for the BAC Site. According to the BAC Defendants, the RWQCB solicited comments from the community and held an informational meeting to discuss the status of the BAC Site remediation. Following the public comment period, in May 2007, the RWQCB published a “fact sheet” regarding remediation of the Site, which purportedly expressed the RWQCB’s opinion that contaminants “posed no health risk to residents in the vicinity of the Site.” The BAC Defendants assert that the RWQCB’s “fact sheet” expressed several additional opinions re: the BAC Site: 1. Groundwater supply wells in the vicinity of the Site have been tested and have not been impacted by the pollution associated with the former BAC facility; 2. Impacted groundwater is not being used for drinking water supply; and 3. The Site is currently not a threat to public health. (Doc. 677-2 at 7:4-7:7.) C. 2009 RWQCB Briefing to Senator Dianne Feinstein On January 15, 2009, the RWQCB provided a “briefing paper” on the BAC Site in response to an inquiry by Senator Diane Feinstein. (Doc. 725-4.) The brief “provided a description of the environmental conditions at the site, as well as a summary of the regulatory actions and cleanup responses taken at the site to address hexavalent chromium contamination.” (Id.) In the brief, the RWQCB summarized its regulatory involvement, which commenced in 1987, and concluded that “[biased upon a thorough review of all of the data currently in the Central Valley Water Board’s files, Board staff believe that the public is not being exposed to harmful levels of hazardous substances originating from the BAC site.” Key details of the RWQCB’s briefing are summarized as follows: * In 1989, the Board determined that sediments in a storm water pond at the BAC site were contaminated with hexavalent chromium. The Board required the BAC Defendants to excavate the pond and further remediate the pond area. Storm water monitoring indicates that the BAC Defendants’ remedial actions were effective in reducing total chromium levels in the storm water that was discharged from the site to below California’s MCL for total chromium in drinking water of 50 ppb. * In 1992, the U.S. EPA conducted an investigation to determine if the site warranted actions pursuant to the federal Superfund law (“CERCLA”). Based on its evaluation, the EPA recommended that no further remedial action under CERCLA was required because it found no hazardous substances were detected in drinking water wells or private wells and no designated water intakes or fisheries within 15 miles. RWQCB notified residents of the BAC Defendants’ cleanup activities through public notices and a fact sheet. * In 1993, under the regulatory oversight of the RWQCB, a groundwater monitoring extraction system was installed. The system commenced operations in 1994 and was regulated pursuant to a permit issued by the Central Valley Water Board. The system was active in January 2009 and has extracted and treated over 2,200 gallons of water. It has removed over 5,400 pounds of hexavalent chromium from that water. * Merck installed a network of 65 monitoring wells that are used to measure the extent of hexavalent chromium pollution in the groundwater. A thorough review, conducted by Board staff, of the sampling and analyses results from the monitoring well network has determined that pollution from the BAC site has not impacted drinking water supply wells in the vicinity of the BAC site. Testing of the water supply wells indicate that hexavalent chromium levels are consistent with background levels for the area. * In 2000, RWQCB staff reviewed and concurred with the BAC Defendants’ reports, which indicated that no domestic supply wells were impacted by pollution originating from the BAC site. * RWQCB staff believes storm water discharges from the site since the cleanup of the pond occurred in 1991 do not present a significant risk for exposure to the public of hazardous contaminants because: (1) wood treating operations ceased in 1991; (2) all significant sources of hexavalent chromium have been eliminated [and] are not exposed to storm water drainage from the site; (3) soils and sediment in the storm water pond that were contaminated with hexavalent chromium have been excavated and disposed off site; (4) total chromium concentrations have consistently been below the drinking water standard for total chromium. (Doc. 725-4 at pgs. 2-3.) The RWQCB’s briefing contained a detailed review of the groundwater pollution and cleanup at the BAC Site: Groundwater at the site is located about 40 feet below the ground surface. Hexavalent chromium has seeped into the groundwater, which has migrated away from the area of release with the flow of the groundwater, which is generally to the north. Seasonal migration of contamination to south, beneath a small portion of the Beachwood neighborhood has also occurred [...] The contamination has remained confined to the upper regions of the groundwater due to the occurrence of a clay layer at about 90 to 100 feet below the ground surface. The clay layer has helped to prevent the migration of contaminants to deeper levels that serve as the drinking water supply for the local community. The two nearby drinking water supply wells owned by the Meadowbrook Water Company, which provided water to the Beachwood neighborhood, take water from deeper groundwater zones. The closest well to the BAC site, MWC-4, draws water from a depth 210 feet below the ground surface and is below the clay layer. To ensure shallow groundwater is not pumped into the drinking water supply, the well is sealed with cement grout to 200 feet below ground surface. A second supply well, MWC-2, takes it water from more than 140 feet below the ground, but the well is older and the depth of the seal against shallow groundwater entering the well is unknown. MWC-2 is no longer in service. Monitoring wells emplaced between the hexavalent chromium plume and the two supply wells indicate that the plume has not reached the location of the supply wells in groundwater situated above and below the clay layer. The community’s drinking wells, MWC-2 and MWC^f, have been tested a number of time and hexavalent chromium levels in the wells are within the expected background concentration range for hexavalent chromium. (Id. at pgs. 7-8.) The RWQCB attached a table summary of water supply well data for hexavalent chromium from public water supply systems near the Beachwood neighborhood, specifically, Meadowbrook Water Company, the City of Atwater and Winton Water District. All of the Meadowbrook wells tested within background levels, including MWC-2 and MWC-4. The RWQCB noted: “For comparison purposes, a number of supply wells for the City of Los Banos, located approximately 25 miles southwest of the site, contain higher levels of hexavalent chromium [than the three closest public water supply systems].” In its “closing observations,” the RWQCB stated that “the public is not currently being exposed to harmful substances originating from the BAC Site,” and that the cleanup actions “appear reasonable under the circumstances and the cleanup is proceeding in a satisfactory manner.” (Id. at pg. 12.) D. Plaintiffs’ Response to BAC Site Histoi"y/Remediation Plaintiffs do not specifically dispute the BAC Defendants’ recitation of the BAC Site history, including the remediation and RWQCB involvement, rather they take issue with the testing data used to support the BAC Defendants’ motions. In particular, Plaintiffs argue that the data from the monitoring wells and MWC-2 Well is unreliable and/or inaccurate for the following reasons, among others: (1) the monitoring wells are too shallow; (2) the use of “selective” and/or “interval sampling”; and (3) the “honor system” employed by the Meadowbrook Water District lacks the necessary formalities to assure accurate records. Based on these factors, Plaintiffs’ groundwater modeler, Mr. Douglas Bartlett, allegedly excluded portions of the sampling data from his modeling and analyses. The exclusion, if any, of the MWC-2 and monitoring well testing data is heavily disputed among the parties and forms the basis for BAC Defendants’ motion to exclude Bartlett’s expert testimony and model. Plaintiffs also allege that several public entities, including the RWQCB and Defendants Meadowbrook Water District, Merced Irrigation District, and Merced County conspired with the BAC Defendants to: avoid collecting data from the MWC-2 Well, hide documents from third parties, and conceal the true danger/risk of the contaminants at or near the BAC Site. In particular, Plaintiffs contend that the BAC Defendants “conceal[ed] from the Regional Board [RWQCB] and plaintiffs [ ] the Arcadis groundwater modeling findings concerning the impact of MWC-2 on the contaminated plume.” Plaintiffs further allege: “The draft report addressing [Arcadis’] model which was to be submitted to the Regional Board in December of 2007, was altered by Merck by deleting references to the findings of the model and by deleting a graphic illustration of contaminants flowing from the BAC directly into MWC-2.” III. PROCEDURAL BACKGROUND. On March 8, 2007, Plaintiffs commenced this civil action against the current public entity defendants, alleging property damage caused by the April 2006 flood. (Doc. 1.) On September 13, 2007, in the second amended complaint, Plaintiffs named Merck & Co., Inc., Amsted Industries, Inc., Baltimore Aircoil Company, and Track Four, Inc. as Defendants in this action. (Doc. 35.) The operative complaint, the eighth amended complaint, was filed by Plaintiffs on March 26, 2010. (Doc. 633.) The eighth amended complaint alleges ten claims against the BAC Defendants: (1) violation of 42 U.S.C. § 6972(a)(1) [RCRA]; (2) violation of 42 U.S.C. § 6972(a)(1)(b) [RCRA]; violation of 33 U.S.C. § 1311(a) [CWA]; (4) violation of 33 U.S.C. § 1342(a) and (b) [CWA]; (5) negligence; (6) trespass; (7) nuisance; (8) wrongful death; (9) fraud and deceit; and (10) civil conspiracy. On March 23, 2009, BAC Defendants filed a “Motion for Case Management Order Re: Exposure” to “compel plaintiffs to make a prima facie showing of exposure.” (Doc. 355.) The motion was denied on July 6, 2009, however, on August 12, 2009, the Court established a multi-phase trial plan in which case-wide exposure issues were to be tried first (“Phase 1”), before general medical causation (“Phase 2”) and plaintiff-specific exposure and causation (“Phase 3”). The August 12, 2009 “Order Modifying Scheduling Conference Order” provides, in relevant part: Discovery and expert disclosures shall be conducted in phases. Phase 1 shall focus on the issue of general exposure; that is, whether contaminants from the former [] BAC Site, Franklin County Water District or the April 2006 Flood have ever reached any location where plaintiffs could have been exposed to them, and if so, when such contaminants arrived, how such contaminants arrived at the location, how long they were present, and at what levels they were present. (Doc. 540 at 1:14-1:28.) On June 1, 2010, BAC Defendants moved for partial summary judgment on Plaintiffs’ state law tort claims for personal injury and property damages, i.e., Plaintiffs’ claims for negligence (Claim X), trespass (Claim XII), nuisance (Claim XIII), and wrongful death (Claim XIV). (Doc. 677.) According to Defendants, Plaintiffs have failed to produce evidence sufficient to show that any plaintiff was exposed to contaminants allegedly released from the former BAC Site located in Merced, California. BAC Defendants argue that since there is no contamination evidence, there is no genuine issue of material fact on the threshold issue of causation. In support of their motion, BAC Defendants submit: (1) a Memorandum of Points and Authorities (“Memorandum”); (2) a Statement of Undisputed Facts in Support of its Motion; (3) the declarations of attorney R. Morgan Gilhuly and experts Scott Fendorf, Daniel B. Stephens, John L. Wilson, and Paolo Zanetti; and (4) separate motions to exclude the testimony of Plaintiffs’ groundwater expert, Douglas Bartlett, and air expert, Camille Sears. Plaintiffs opposed the motion on July 1, 2010, submitting over 3,000 pages of documentation and 168 exhibits. Plaintiffs oppose summary judgment on grounds that they have “substantial evidence” supporting their contention that “contaminants from the BAC facility have historically migrated from the [BAC] facility via groundwater, surface water and air pathways to locations were exposed to them and at levels which could cause harm.” In particular, Plaintiffs argue that Sears and Bartlett’s expert testimony is admissible and creates “competing expert opinions as to the migration of the contaminants from the BAC facility.” According to Plaintiffs, Sears and Bartlett’s expert opinions, by themselves, are sufficient to withstand a Rule 56 challenge. BAC Defendants filed their reply briefs on August 13, 2010. (Docs. 834 & 835.) IV. LEGAL STANDARD. Summary judgment/adjudication is appropriate when “the pleadings, the discovery and disclosure materials on file, and any affidavits show that there is no genuine issue as to any material fact and that the movant is entitled to judgment as a matter of law.” Fed.R.Civ.P. 56(c). The movant “always bears the initial responsibility of informing the district court of the basis for its motion, and identifying those portions of the pleadings, depositions, answers to interrogatories, and admissions on file, together with the affidavits, if any, which it believes demonstrate the absence of a genuine issue of material fact.” Celotex Corp. v. Catrett, 477 U.S. 317, 323, 106 S.Ct. 2548, 91 L.Ed.2d 265 (1986) (internal quotation marks omitted). Where the movant will have the burden of proof on an issue at trial, it must “affirmatively demonstrate that no reasonable trier of fact could find other than for the moving party.” Soremekun v. Thrifty Payless, Inc., 509 F.3d 978, 984 (9th Cir.2007). With respect to an issue as to which the non-moving party will have the burden of proof, the movant “can prevail merely by pointing out that there is an absence of evidence to support the non-moving party’s case.” Soremekun, 509 F.3d at 984. When a motion for summary judgment is properly made and supported, the nonmovant cannot defeat the motion by resting upon the allegations or denials of its own pleading, rather the “non-moving party must set forth, by affidavit or as otherwise provided in Rule 56, ‘specific facts showing that there is a genuine issue for trial.’ ” Soremekun, 509 F.3d at 984. (quoting Anderson v. Liberty Lobby, Inc., 477 U.S. 242, 250, 106 S.Ct. 2505, 91 L.Ed.2d 202 (1986)). “A non-movant’s bald assertions or a mere scintilla of evidence in his favor are both insufficient to withstand summary judgment.” FTC v. Stefanchik, 559 F.3d 924, 929 (9th Cir.2009). “[A] non-movant must show a genuine issue of material fact by presenting affirmative evidence from which a jury could find in his favor.” Id. (emphasis in original). “[SJummary judgment will not lie if [a] dispute about a material fact is ‘genuine,’ that is, if the evidence is such that a reasonable jury could return a verdict for the nonmoving party.” Anderson, 477 U.S. at 248, 106 S.Ct. 2505. In determining whether a genuine dispute exists, a district court does not make credibility determinations; rather, the “evidence of the non-movant is to be believed, and all justifiable inferences are to be drawn in his favor.” Id. at 255, 106 S.Ct. 2505. V. DISCUSSION. A. Introduction Defendants move for summary judgment on Plaintiffs’ negligence, trespass, nuisance, and wrongful death causes of action. Defendants’ claim there is no evidence of exposure via any pathway, i.e., groundwater, private domestic wells, surface water, air, or soil. Plaintiffs oppose each facet of Defendants’ motion, arguing that their expert evidence clearly shows that “contaminants from the BAC have historically migrated from the facility via groundwater, surface water, and air pathways to locations were plaintiffs were exposed to them and at levels which could cause harm.” (Doc. 792 at 1:6—1:9.) The substance of the parties’ briefing, more than 5,000 pages of argument and expert/scientific reports, focuses on two pathways, groundwater and air. As to these pathways, Plaintiffs support their Phase 1 exposure burden with the testimony of Douglas Bartlett, groundwater hydrologist (groundwater modeler), and Camille Sears, meteorologist (air modeler). Defendants move to exclude these expert opinions and testimony pursuant to Federal Rule of Evidence 702 and two United States Supreme Court cases, Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 118 S.Ct. 2786, 125 L.Ed.2d 469 and Kumho Tire Co. v. Carmichael, 526 U.S. 137, 119 S.Ct. 1167, 143 L.Ed.2d 238 (1999). In particular, Defendants challenge Bartlett and Sears’ expert testimony on grounds that it cannot pass Daubert’s “gatekeeping” requirement. B. Daubert Legal Standard Rule 702 governs the admissibility of expert testimony. Pursuant to Rule 702, a witness qualified as an expert in “scientific ... knowledge” may testify thereto if: “(1) the testimony is based upon sufficient facts or data; (2) the testimony is the product of reliable principles and methods; and (3) the witness has applied the principles and methods to the facts of the case.” Fed.R.Evid. 702. The trial court acts as a gatekeeper to the admission of expert scientific testimony under Rule 702. Daubert, 509 U.S. at 579-580, 113 S.Ct. 2786. The court must conduct a preliminary assessment to “ensure that any and all scientific testimony or evidence admitted is not only relevant but reliable.” Id. at 589, 113 S.Ct. 2786. This two-step assessment requires consideration of whether (1) the reasoning or methodology underlying the testimony is scientifically valid (the reliability prong); and (2) whether the reasoning or methodology properly can be applied to the facts in issue (the relevancy prong). Id. at 592-93, 113 S.Ct. 2786; Kennedy v. Collagen Corp., 161 F.3d 1226, 1228 (9th Cir.1998). Reliable testimony must be grounded in the methods and procedures of science and signify something beyond “subjective belief or unsupported speculation.” Daubert, 509 U.S. at 590, 113 S.Ct. 2786. The inferences or assertions drawn by the expert must be derived by the scientific method. Id. In essence, the court must determine whether the expert’s work product amounts to “ ‘good science.’ ” Daubert v. Merrell Dow Pharms., Inc., 43 F.3d 1311, 1315 (9th Cir.1995) (“Daubert II”) (quoting Daubert, 509 U.S. at 593, 113 S.Ct. 2786). In Daubert, the Supreme Court outlined factors relevant to the reliability prong, including: (1) whether the theory can be and has been tested; (2) whether it has been subjected to peer review; (3) the known or potential rate of error; and (4) whether the theory or methodology employed is generally accepted in the relevant scientific community. Daubert, 509 U.S. at 593-94, 113 S.Ct. 2786. The Supreme Court emphasized the “flexible” nature of this inquiry. Id. at 594, 113 S.Ct. 2786. As later confirmed in Kumho Tire Co. v. Carmichael, 526 U.S. 137, 119 S.Ct. 1167, 143 L.Ed.2d 238 (1999): “Daubert’s list of specific factors neither necessarily nor exclusively applies to all experts or in every case. Rather the law grants a district court the same broad latitude when it decides how to determine reliability as [the court] enjoys in respect to its ultimate reliability determination.” Id. at 141-42,119 S.Ct. 1167. The relevancy, or “fit,” prong requires that the testimony be “relevant to the task at hand, ... i.e., that it logically advances a material aspect of the proposing party’s case.” Daubert II, 43 F.3d at 1315 (quoting Daubert, 509 U.S. at 597, 113 S.Ct. 2786). Relevancy requires opinions that would assist the trier of fact in reaching a conclusion necessary to the case. See Kennedy, 161 F.3d at 1230. The Daubert analysis focuses on the principles and methodology underlying an expert’s testimony, not on the expert’s conclusions. Daubert, 509 U.S. at 595, 113 S.Ct. 2786. However, the Supreme Court has cautioned that “conclusions and methodology are not entirely distinct from one another.” General Elec. v. Joiner, 522 U.S. 136, 146, 118 S.Ct. 512, 139 L.Ed.2d 508 (1997). As such, “[a] court may conclude that there is simply too great an analytical gap between the data and the opinion proffered.” Id. Nothing in either Daubert or the Federal Rules of Evidence requires the admission of opinion evidence connected to existing data “only by the ipse dixit of the expert.” Id. C. Pathway # 1—Air Plaintiffs allege that soil contaminated with hexavalent chromium and/or arsenic was transported to their homes or properties via wind and other airborne pathways. To satisfy their Phase 1 burden on the air pathway, Plaintiffs submit: (1) the expert testimony of air modeler Camille Sears (air exposure for the years 1969-1993); and (2) a 1994 Risk Assessment completed by IT Corp (air exposure posH993 years). Defendants contend that summary judgment is appropriate with respect to any air pathway because Plaintiffs’ expert’s testimony is inadmissible under Daubert. Specifically, Defendants move to exclude Ms. Sears’ testimony on grounds that: (1) she is not qualified to perform soil calculations; (2) her entire emission scenario is flawed and lacks scientific reliability; and (3) her input parameters are scientifically unsound. As to the Risk Assessment, Defendants assert that it has no evidentiary value because “it was based on a hypothetical scenario that never occurred, and it does not purport to accurately calculate actual exposure or risks.” (Doc. 677-2 at 23:18-23:19.) To support their Daubert motion, Defendants rely on the declaration and testimony of Dr. Paulo Zanetti, an air modeler, and Dr. Scott Fendorf, a chemist. Both these experts opine that Ms. Sears’ model is scientifically unreliable. Plaintiffs rejoin that Ms. Sears has performed similar soil calculations in modeling air dispersion, her emission scenario is well-accepted in the air modeling community and the dispute over her input parameters bear on the weight of the opinion rather than on its admissibility. 1. Fed.R.Evid. 706 Based on the extent of the dispute between the parties’ experts and the complexity of the scientific issues presented by air modeling, the Court retained two independent experts, Chatten Cowherd Jr., Ph.D. and Richard Countess, Ph.D., under Fed.R.Evid. 706, to assist in the understanding and analyzing the air pathway dispute. Drs. Countess and Cowherd prepared a joint expert report, submitted on November 22, 2010. (Doc. 942.) The joint independent expert report and the opinions contained therein are discussed in the context of Defendants’ criticisms of the air model. 2. Supplemental Briefing & Fed.R.Evid. 706 Experts The parties filed supplement briefing concerning the air expert reports on November 29, 2010. The supplemental arguments mirror those advanced in the parties’ earlier briefing, however, Plaintiffs reiterate that “the FRE 706 reports serve only to confirm the grave concerns plaintiffs have repeatedly expressed to this Court related to these Daubert proceedings.” Plaintiffs contend that the Court “may be tempted to act as a trier of fact by weighing the relative strengths and weaknesses of factual and scientific assumptions made by the experts.” They further assert that the air model disagreements “go to the weight of their opinions, and not their admissibility.” Defendants rejoin that the 706 expert reports “confirm[ ] that Plaintiffs’ groundwater and air models contain serious methodological flaws, including faulty assumptions unsupported by science or facts, and are not reliable.” Responding to Plaintiffs’ “weight” arguments, Defendants assert that if Ms. Sears’ expert report is either unreliable or irrelevant, it is inadmissible. An inadmissible report cannot create a disputed issue of fact. BAC Defendants also include a string citation to a number of federal cases holding that, under Daubert, an expert opinion cannot be “based on assumptions of fact without evidentiary support, or on speculative or conjectural factors.” 3. Testimony/Model of Camille Sears (1969-1993) Plaintiffs designated Sears as an air pathways expert who will opine on whether Plaintiffs’ homes were exposed to significantly elevated air concentrations of hexavalent chromium and arsenic. Sears’ declaration specifies she has worked in the air quality field since January 1982, following her graduation from the University of California at Davis (M.S. and B.S. degrees in atmospheric science). Prior to forming her own consulting firm in 1992, Sears worked as a private air consultant/scientist (Dames & Moore, URS Consultants) and Air Toxics Program Coordinator (Santa Barbara Air Pollution Control District). Sears has been a testifying expert for twenty years, since 1990, and provided air modeling testimony in California Dept. of Toxic Substances Control v. Interstate Non-Ferrous Corp., No. 97-CV-5016-OWW-LJO, an environmental contamination case involving, among other things, the alleged air dispersion of dioxin-eontaining ash released from incendiary operations at a smelting site in Mojave, California. According to her expert reports, declarations, and testimony, Sears’ followed a three-step process. First, she calculated air concentrations and surface deposition of hexavalent chromium and arsenic resulting from the BAC site based on “widely-accepted air dispersion modeling techniques.” Second, she input these calculations into an air flow model endorsed by the U.S. Environmental Protection Agency called AERMOD to determine if Beach-wood residents were exposed to particulates from fugitive dust emissions sources at the former BAC site. Third, she provided opinions re: whether, from 1969 to 1993, areas surrounding the BAC facility were exposed to elevated air concentrations of arsenic and hexavalent chromium. Sears built her model by extrapolating data from chromium at the BAC site, in a treating solution that was released to the ground at the retort area where it entered the soil, and that this exposed solution in and on the soil was later disturbed by forklifts driving on the site. She offered the following opinions of exposures caused by the BAC facility wood storage area emissions: • During the period 1969 through 1993, areas surrounding the BAC facility were exposed to significantly elevated air concentrations of hexavalent chromium; • During 1969, and the period 1991 through 1993, areas surrounding the BAC facility were exposed to elevated air concentrations of hexavalent chromium, but not as high as 1970 through 1990; • During the period 1969 through 1983, areas surrounding the BAC facility were exposed to significantly elevated air concentrations of arsenic; • During 1969, and the period 1981 through 1983, areas surrounding the BAC facility were exposed to elevated air concentrations of arsenic, but not as high as 1970 through 1980; • During the period 1969 through 1983, areas surrounding the BAC facility were exposed to increased arsenic surface deposition levels; • During the period 1969 through 1993, areas surrounding the BAC facility were exposed to increased hexavalent chromium surface deposition levels. (Doc. 781 at 5:21-6:9.) Defendants move to exclude these opinions because her testimony rests on a number of unsupported assumptions about the chemical and physical properties of soils at the Site during the relevant time period (1969-1993). According to Defendants, Sears is unqualified to make these assumptions because she “is an atmospheric scientist and meteorologist, not an expert in chemistry, geology, geochemistry, or soil science.” In addition, as part of her calculation of fugitive emissions, Sears allegedly employed an improper EPA emission technique, namely the paved road emission scenario (AP-42, § 13.2.1). The flawed assumptions, enumerated below, relate solely to Sears’ “emission rate” calculations: 1. Valence — Sears assumes that all of the chromium that dropped from treated wood at the BAC site was hexavalent chromium; 2. Particle Size — She assumes the contaminated dust was comprised of particles that were less than 10 microns in diameter, making them small enough to blow off the site; and 3. Accumulation — She assumes that one year’s worth of drippage accumulated and remained on the surface of the site, in the form of dust that is 90 percent pure (900,000 parts per million) chromium and arsenic. Defendants contend that these assumptions are demonstrably false and Sears does not have the expertise to make them. To support their Daubert motion, Defendants rely on the declaration and testimony of Dr. Paulo Zanetti, an air modeler, and Dr. Scott Fendorf, a chemist. Dr. Zanetti disputes Sears’ use of the EPA’s “paved road” emission scenario and, critically, the assumption that drippage would accumulate and remain on the surface of the wood storage area in the form of dust at a concentration of 900,000 parts per million. Dr. Fendorf opines that chromium does not drip off only in hexavalent state and, assuming it did, the chromium particles would be at least 100, not 10, microns in size. Sears responds first to Defendants’ claims that she is unqualified to calculate fugitive dust emissions from a wood processing facility: While this is the first time I have calculated fugitive dust emissions from a wood treating facility, the principles involved are no different than any other paved surface fugitive dust source. Furthermore, I have calculated air emissions of hexavalent chromium dozens of times — from cooling towers, annealing operations, chrome plating and anodizing, surface coatings (including primers and pigmented — paints), welding operations and cold fire combustion. I also have experience calculating air emissions of arsenic — from combustion sources, fugitive dust and mining activities. (Doc. 785 at ¶ 15.) On October 7, 2010, the second day of Daubert hearings, Sears testified that while she lacks formal “soil science” training, she has calculated air emissions in a number of environmental contamination cases and teaches a class in chemistry: First of all, I heard [defense counsel] say that I had no education in chemistry or geology or soil science. That’s— that’s never anything that I’ve said. I have taken a number of chemistry courses and I’ve been involved in a number of soil analyses over the years. I’ve taught chemistry to air toxic students through the UCSB extension as part of my air toxics class. I never said I had no education in chemistry. And I don’t believe it’s correct for [defense counsel] to say so. Over the last 30 years, which is the time that I’ve been doing these types of analyses, I’ve been involved in the use of chemistry on a weekly basis. It’s part of the job experience. You don’t need to be a chemist or a geologist, soil scientist or geochemist to calculate air emissions from basically soil releases or surface releases of dust. On the contrary, Dr. Fendorf, who they offer as their expert on this issue, I don’t believe has ever calculated air emissions, or at least I did not see any of his qualifications stating that he has ever calculated air emissions. (RT, Oct. 7, 2010, 257:7-257:25.) Ms. Sears detailed her experience calculating fugitive air emissions during counsel’s direct examination on October 13, 2010. Ms. Sears’ air emission experience, which is quite extensive and includes review of industrial sites and hexavalent chromium, rules out any characterization of her as “unqualified” in the field of calculating air emissions: Q. Ms. Sears, are you qualified to calculate air emissions? A. Yes, I believe so. Q. And why do you feel that you’re qualified to do that? A. Well, first, I want to correct — or give my viewpoint on the issue of whether a meteorologist is qualified to calculate air emissions. We heard testimony last week that Ms. Sears is a meteorologist and atmospheric scientist and therefore she’s not qualified to calculate air emissions. I believe that’s untrue. First of all, meteorologists, atmospheric scientists, we need to take all the chemistry, all the physics that basically any engineer would take. And basically the rest of it is real world experience. I started calculating air emissions in 1983. I was hired by the Santa Barbara county air pollution control district as an air pollution engineer. And my job was to calculate emissions, not only for permitting purpose, but also for fee calculations. Our district became a fee based entity, which the fees were generated by the amount of emissions that companies emitted. And during that time, when I was at the air pollution control district, not only did I calculate emissions for a number of different sources, I also had to develop what we called emission estimating techniques, or EETs for many sources for which we never calculated emissions before. Q. Let me ask you just on this subject. What types of emissions did you calculate when you were with the Santa Barbara County Air Pollution Control District? A. I was just about to answer that. Thanks. The types of emissions I would calculate were generally by the — I’ll do it by type of pollutant. Originally in the early '80s, the only thing we dealt with would have been called criteria air pollutants. And those are pollutants for which there were ambient air quality standards. For example, nitrogen dioxide, sulphur dioxide, carbon monoxide, lead and particulate matter were the ones where we would calculate emissions. And for ozone, we would calculate emissions of volatile organic compounds as well. Later in the '80s, when air toxics, we also called non-criteria pollutants became the focus of regulation, we had to calculate air emissions of essentially all the toxic chemicals that you can think of from every source in the district. And those would include oil and gas processing facilities, excuse me, medical device manufacturers, diatomaceous earth mining facilities, we had Casmalia class one landfill. We had mining operations. There were no end to the different types of sources that we had to calculate emissions from. There’s several hundred different facilities I was responsible for calculating air emissions. Q. And give us an example as to how you did calculate the emissions at that time. A. Well, I — I could think of a lot of them. However, the type of emissions that — the type of sources that we would generally deal with for air toxics, you might think of as an oil and gas processing facility. And we had offshore and on shore components, stationary and mobile sources. So we had to identify all the emission sources for all the different pollutants and then quantify them using emission calculation methods. Sometimes we did source testing. And other times, like I said, we have to use what we call emission factors. And again, for many of these sources, for air toxics, none of these emission factors existed. We had to develop them. And I was on the committee for the State of California, the criterion guidelines regulation committee, which established a number of emission estimating techniques or EETs for air toxics. Q. And how many cases have you testified in either a courtroom like this or in state court or an administrative hearing, wherein you have rendered opinions and testimony on the calculation of air emissions, Ms. Sears? A. About 15 cases. And I outlined them in my opposition declaration. So about 15 cases where I testified and where I’ve actually calculated emissions that went into my model. Q. And what experience do you have in calculating air emissions of hexavalent chromium? A. Over the years, probably starting in the late '80s, I calculated air emissions of hexavalent chromium dozens of times from cooling towers, where zinc chromate was added as an — vis-á-vis an anti-corrosion compound. Calculated hexavalent chromium emissions from chrome anodizing and chrome plating operations, both hard chrome and soft chrome. And also from sparging, which is air injection into chrome plating operations. Q. Now, in terms of the central valley of California here, what experience do you have in evaluating the impact of hexavalent chromium in the air here in the central valley of California? Which is where the BAC plant is located. A. Okay. Q. Former BAC plant. A. I’ll answer that. But I also want to say I’ve also calculated hex chrome emissions from a number of surface coating operations, such as spray painting and aircraft refurbishing and that type of thing. Certain coatings, zinc chromate, strontium chromate, lead chromate have hexavalent chromium in the formulation. Now, as far as the Central Valley goes, I have calculated emissions from cooling towers. This was from a case where the emissions were in the past. And we calculated — or I calculated hexavalent chrome emissions. And then I modeled these emissions and calculated the air concentrations. And then we were also involved in looking at the background levels of hexavalent chromium in the central valley, basically in the '90s. Because that’s when we had data. And so I’ve had quite a bit of experience looking at the data from the hexavalent chrome in the air from Stockton, Modesto, Fresno, Bakersfield, the site where the Air Resources Board was collecting this data in the 1990s. Q. What experience — there was some mention last week by [defense counsel] about your background or lack of background in regard to the chemistry of air pollutants. Do you remember that testimony? A. Ido. Q. What is your experience in regard to the chemistry of air pollutants, Ms. Sears? A. Well, again, I want to say that, again, I’m a meteorologist, and I would never say I’m a chemist, but I would never say I don’t have any education in chemistry. I do. And — but I — and again, a lot of it, though, is on the job training in the last 30 years since I graduated with a masters from UC Davis. And essentially what you have to think of is that when you’re looking at air toxics, toxic air pollutants, you need to know something about the chemistry of these chemicals. You know, chlorinated alkanes, chlorinated alkines, valence of metals, all these different things come into play as well as the reactivity of volatile organic compounds in ozone formation. All these are issues that we have to deal with consistently in the air pollution field. And so it’s a matter of needing to know certain details of the chemistry, but not having to be a chemist to do so. Q. So let me ask this more specifically. What is your education, training and background in regard to the subject of understanding the chemistry of air pollutants? A. Well, I’ve taken the basic, you know, again, I’m not a chemist, but I’ve taken the basic freshman, sophomore chemistry classes. And essentially I was an atmospheric physicist in — at Davis, that was my specialty. I wasn’t an air modeling student at UC Davis. And so we dealt a lot with the issues of electricity, magnetism, electromagnetic issues, that would deal with individual compounds as well. And again, after I graduated and became working in the air toxics field, I had to refresh my memory and, you know, go back into dealing withal canes, al keens, al kinds, air mat I can hydrocarbons and so forth and how all these compounds relate to each other. So again, I’m not a chemist, but I’m not a completely ignorant chemistry either. Q. In the opinions that you have rendered in other cases and in other courts, have your opinions included the subject of chemistry in regard to air pollutants? A. Yes. I can give you some examples. Q. Why don’t you do that. A. One case that I was working on, it was a federal court case. It was Akee versus Dow, it was in Hawaii. It dealt with soil fumigants. And the soil fumigants were applied to pineapple fields on the island of Oahu from 1946 to — 1946 to 2001. I calculated air emissions from volatilization of Dibromochloropropane, ethylene dibromide, 1, 3-trichloro-propene, 1, 2-dichloropropane, and epichlorohydrin from 319 pineapple fields for 56 years. And, of course, the defendants’ experts challenged everything I did. They didn’t file any motions to exclude any testimony, they just disagreed with me [....] (RT, Oct. 13, 2010, 560:12-567:4.) Camille Sears is qualified to offer the testimony about how to calculate fugitive air emissions from a former wood-treatment facility. She has substantial experience in the air emission field, having calculated air emission rates for over twenty years in and around Northern and Central California. Sears has calculated fugitive air emissions for public and private employers and has experience with industrial sites and a variety of harmful pollutants, including hexavalent chromium. By education and experience, Ms. Sears is qualified to opine on air emissions, analysis and modeling in this case and her calculations can be challenged through cross-examination and presentation of contrary evidence. See Robinson v. GEICO General Ins. Co., 447 F.3d 1096, 1100 (8th Cir.2006) (“Gaps in an expert witness’s qualifications or knowledge generally go to the weight of the witness’s testimony, not its admissibility.”). Defendants take an overly-narrow view of the degree of expertise necessary to calculate fugitive air emissions in the context of air modeling. An air modeler, who is also an experienced atmospheric scientist and meteorologist, does not need to have a “sub-specialty” or advanced degree in chemistry or soil science before that person has sufficient qualifications to perform air emissions. Defendants cite no authority for such a proposition. Rather, Rule 702 only requires that an expert possess “knowledge, skill, experience, training, or education” sufficient to “assist” the trier of fact, which is “satisfied where expert testimony advances the trier of fact’s understanding to any degree.” See Lauria v. Nat'l R.R. Passenger Corp., 145 F.3d 593, 598 (3rd Cir.1998) (holding trial court abused its discretion by excluding testimony simply because the trial court did not deem proposed expert to be the best qualified or because proposed expert did not have the specialization that the trial court considered most appropriate); see also United States v. Newmont USA Ltd., No. CV-05-020-JLQ, 2007 WL 4856859, at *2 (E.D.Wash. Nov. 16, 2007) (in a mining enterprise case, holding that although the expert “is a historian, as opposed to an expert in the specific areas of ‘corporate organization’ is not disqualifying in this case, as his background appears to provide sufficient expertise upon which his opinions, as a general matter, are based.”). Ms. Sears has engaged in the field of air modeling and analysis as a regulator and expert for over twenty years. She has worked on sites that have toxic dust emissions. She satisfies Rule 702’s foundational standards and her testimony will assist the trier of fact. Defendants next argue that Ms. Sears’ entire emission scenario is flawed and lacks scientific reliability. According to Dr. Zanetti, Ms. Sears’ flawed methodology begins and ends with her use of the “paved road” or “parking lot” emission scenario, AP-42 § 13.2.1. Dr. Zanetti opines that there is no scientific basis to model the paved portions at the BAC Site because they do not qualify as “roads.” Rather, according to Dr. Zanetti, the proper emission scenario in this case is the EPA equation for unpaved surfaces. The selection of AP-42 as an emission scenario was fully developed during defense counsel’s direct examination of Dr. Zanetti on October 14, 2010: A: In Ms. Sears’ scenario, she did all the work by herself, my understanding, she developed this new theory, she developed emission rates, she applied the AERMOD model. She applied the AP-42 equations. Certainly she has the capability to select an AP-42 equation and to use it. The problem is is this the correct methodology? It’s not a question of her lacking the mathematical skills to use those equations, the question is is the equation or the methodology appropriate [...] I tried to summarize in a way that is easy to understand, my reading of Ms. Sears’ work in this case. And in particular, the emission scenario. So there are seven points. And I can start giving a brief description. Q. Okay. Before you do that, if you would, please, explain to us all what you mean by the phrase “emission scenario.” A. An air pollution model is a computer program that requires basically two main inputs. The emissions and the meteorology. Q. The emission? A. The meteorology, like the wind speed, the wind direction, the temperature. So these are the two main inputs. Emission and meteorology. And the results will be the concentration data in the community. So the meteorological data are generally easy to collect. There are airports all over the United States providing meteorological data, like wind speed, wind direction that we can collect easily. So generally this is not a big task. But the emission calculation is often a major task and that has enormous consequences in air pollution modeling application because if the emissions are incorrect, the concentration will be incorrect in the same proportional amount. Q. Okay. So emission scenario refers in the BAC case to what? A. In Ms. Sears’ theory, she is considering emissions from paved surface in the wood storage area. So task number one is my description of her work, that for the period '69 to 1993, she assumes that contaminated liquid, liquid contaminated with heavy metals, arsenic and chromium 6, is dripping from the treated wood in the wood storage area and this liquid will drip over — or this paved surface of the wood storage area. That’s step number one. (RT, Oct. 14, 2010, 703:8-707:2.) Dr. Zanetti argued that there was insufficient “road traffic” at the wood treatment area to support Ms. Sears’ selection of the emission scenario for paved roads. He also argued that AP-42 explicitly states that the AP^42 formula should not be applied to “stop and go” traffic: A: And she is using the AP formulas for road traffic. And this formula has been used many, many times [...] so this formula has been used by hundreds of times all over the world. It’s a simple formula. Q. But it’s used for, you said, road traffic? A. Road traffic. And my comment is that, first of all, I look at the picture of the wood storage area. There is no road. Q. Okay. A. First of all. Second, there is a picture of, in the AP-42 document, which I have in my files, and it shows a road with traffic. And no forklifts in the traffic, of course. This formula is designed for calculating emissions from traffic, not for forklifts. I don’t think I ever seen, and I don’t think Ms. Sears has cited one literature study where this formula has been applied to forklifts. And in order to apply to forklifts, you need really to stretch enormously the assumption that are inherent in the development of this formula. The most important one is the speed. The formulas was developed for vehicles that travel with an average speed, average, between 10 miles per hour up to 60 miles per hour, if I remember correctly. But 10 is the main one. There is no way you can claim a forklifts have an average speed of 10 miles per hour. It’s stop and go. It’s— Q. Does AP-42 apply to stop and go traffic? A. No. It explicitly say, the formula that Ms. Sears applied, the formula should not be applied for stop and go traffic. So it is very questionable, the use of AP^2 for forklifts. And again, the most important thing, there is no literature support for doing that. I haven’t found one single study. (RT, Oct. 14, 2010, 715:15-717:1.) During counsel’s re-direct examination on October 14, 2010, the fourth day of Daubert hearings, Ms. Sears testified that AP-42 is appropriate in these circumstances given the lack of an emission scenario for forklift use at industrial sites: A: Now, I think it’s important to describe that — the process that I used to calculate the air emissions from the fugitive dust, the treated wood storage area. I used an EPA factor. It’s from a document called AP 42, which is number 42 in an air pollution document series prepared by U.S. EPA. And it’s a compilation of emission factors. Which are techniques used to calculate air emissions from various types of activities. And in this case, Section 13.2.1 deals with fugitive dust from paved roads or parking lots. It’s a technique that can be used to any paved road or parking lot. There’s nothing unique about fugitive dust coming off a paved surface area at a wood treatment plant or a treated wood storage area paved surface. It’s the same technique. And I used that same technique scores and scores and scores of times. I’ve used it for refineries, I’ve used it for oil and gas processing facilities, for steel mills, for quarries, or battery recycling plants. The list goes on. I’ve done it scores and scores of times. And there is a very simple, I think, a very simple determination based on my experience that it would also apply to this facility. Q. You mean the type of surface from which fugitive dust would be sitting upon and then emitted through some type of activity? A. Right. What happens is that you have an amount of surface dust on the surface of the soil. That then can be released into the air due to mechanical disruption by vehicular tires, mainly. And the vehicle doesn’t matter, it’s the weight of the vehicle that matters. The fact that Ms. Sears used a different algorithm to calculate fugitive air emissions than Dr. Zanetti does not render her unqualified nor does it suggest her emissions scenario is invalid. Rather, based on the current record, her emissions scenario is relevant, admissible, and can be challenged through cross-examination and presentation of contrary evidence. In this Circuit, an expert’s decision to use one form of scientific methodology over another goes to the expert’s credibility rather than the admissibility of the testimony. See, e.g., United States v. Garcia, 7 F.3d 885, 889-90 (9th Cir.1993). This is such a case. In addition, the Fed.R.Evid. 706 air experts, Drs. Countess and Cowherd, agree