Incidental Occurrence of MTBE and Other Oxygenates in U.S. Gasolines after State-Level MTBE Bans, 2004-2020

Abstract The rapid rise in the use of MTBE in automotive gasoline in the U.S. in the early 1990s was matched only by its rapid decline in the early 2000s, the latter stemming from its unsettling and detrimental impact on groundwater and drinking water supplies following gasoline leaks and spills. Its decline was promulgated through implementation of numerous legislated, state-level bans, which along with the Energy Policy Act of 2005’s elimination of a minimum oxygen requirement for reformulated gasoline, effectively ended the “MTBE era” by the Spring of 2006. Despite the bans, most of which only prohibited the intentional use of MTBE, opportunities remained for its incidental occurrence in gasoline. In this study, we report on the existence, concentration, and frequency of incidental MTBE and other oxygenates (TAME, ETBE, DIPE, and TBA) in an ad hoc suite of 76 “post-ban” gasolines collected from retail service station dispensers in multiple states between 2004 (in states with early bans) and 2020. These results are compared to those for 75 “pre-ban” (oxygenated, reformulated, and conventional) gasolines. Using GC/MS full scan data with a method detection limit of 10 mg/kg, the results confirm the incidental occurrence of MTBE in 34% of the “post-ban” gasolines at concentrations between 980 and 17 mg/kg (avg. ± σ; 209 ± 307 mg/kg), which generally decreased between 2004 and 2020. MTBE’s origin(s) in “post ban” gasolines is reasonably attributed to cross-contamination with neat MTBE or MTBE-laden gasoline destined for U.S. export, and/or its otherwise lingering presence within the gasoline distribution system. Although less frequently detected, low concentrations (mostly < 1,000 mg/kg) of TAME, ETBE, DIPE and TBA were also found in 17, 14, 13 and 4% of the “post-ban” gasolines studied, respectively. The forensic implications of these results on constraining the “age” of gasoline-derived NAPL containing MTBE or of MTBE-impacted groundwater, which are equally applicable to the other oxygenates measured, are discussed. Of overriding relevance is that any presumption that MTBE completely disappeared from the U.S. gasoline pool after the implementation of the state-level bans and Energy Policy Act is false.


The Rise and Fall of MTBE
Except for the troubled public health and environmental legacy of organic lead, methyl-tert-butyl ether (MTBE) was perhaps the most notorious blending agent ever used in gasoline.Despite its positive impact toward reducing air pollutant emissions from gasoline engines due to more efficient combustion (Shearman 1991), MTBE's notoriety stemmed from its unsettling and detrimental impact on groundwater and drinking water supplies (Lapham et al. 1997;Stikkers 2001) owing to its propensity to partition out of leaked gasoline into water, biodegrade slowly, and migrate significant distances (U.S. Environmental Protection Agency 1998).
The use of MTBE as a blending agent in U.S. gasoline originated in 1979 when, owing to its high octane value [(R þ M)/2 110], the continuing phaseout of organic lead that began in the early 1970s, an increasing demand for higher octane premium grade gasoline, and the U.S. Environmental Protection Agency's (EPA) approval of an industry consortium's waiver request for MTBE's use in gasoline in March 1979(Federal Register 1979), its production and use was both profitable and legal.The EPA's approval of the consortium's waiver, commonly referred to as the ARCO waiver, allowed up to 7.0 volume percent (vol%) MTBE to be blended in gasoline and refiners quickly commenced the production and use of MTBE as a gasoline blending agent.After the EPA issued the "substantially similar" ruling in 1981 the maximum limit for MTBE was increased to 11.0 vol% (Gibbs 1990).MTBE's use as an octane booster grew slowly through the 1980s but drastically increased in the early 1990s following passage of the Clean Air Act of 1990 (CAA), wherein the clean fuels program for gasoline was adopted by the U.S. EPA.The CAA required that, beginning in November 1992, wintertime gasoline sold in 39 US markets with high ambient carbon monoxide concentrations (CO nonattainment areas) must contain a minimum of 2.7 weight percent (wt%) oxygen.MTBE was widely used to meet this requirement by blending 15 wt% MTBE in this wintertime oxygenated gasoline, referred to as Oxy-Fuel, and led to dramatic increases in both production and import of MTBE in the U.S. in the early 1990s (Figure 1).
In 1995, the federal Reformulated Gasoline (RFG) program was implemented to reduce ozone-forming or toxic emissions from gasoline engines in the nine largest U.S. markets year-round, with approximately 20 additional metropolitan markets "opting-in" to participate in the program.Among other parameters, RFGs were required to contain a minimum of 2.0 wt% oxygen per gallon (or 2.1 wt% on average over 180 days with a lower cap of 1.5 wt%).Again, MTBE was widely used to meet the 2.0 wt% oxygen RFG requirement through the blending of 11 wt% MTBE in RFG.The use of ethanol and alternative ethers, such as tert-amyl-methyl ether (TAME), in RFG was remotely secondary behind MTBE.Through a combination of both U.S. domestic production and imports the volume of MTBE used in blending both Oxy-Fuel and RFG remained high throughout the 1990s and early 2000s (Figure 1).In the early 2000s, however, the detrimental impact of MTBE on water resourcesalong with politically-favoured use of ethanol in some statesprompted 18 state-level bans on MTBE's use in Oxy-Fuel and RFGs that were implemented between 2000 and 2005 (Table 1).
The Energy Policy Act of 2005, which became effective nationwide May 5, 2006(April 24, 2006 in California; U.S. Environmental Protection Agency 2008), additionally eliminated the minimum oxygen requirements for RFG and introduced a new renewable fuel standard program, the latter of which provided tax incentives for gasoline-ethanol blends containing up to 5.7 wt% (10 vol%) ethanol.Notably, the Energy Policy Act of 2005 did not impose a federal ban on the use of MTBE, but only eliminated the minimum oxygen requirements for RFG.Around this same time most CO non-attainment areas discontinued the wintertime Oxy-Fuel programs with those few remaining prohibiting the use of MTBE in favour of ethanol.
Thus, through the combined effects of the Energy Policy Act's repeal of the minimum oxygen requirement for RFG, the concurrent reduction in wintertime Oxy-Fuel programs, and in light of the widespread realization of MTBE's negative impact on water resources, MTBE's use in all U.S. gasolines (conventional, Oxy-Fuel, and RFG) was voluntarily and dramatically reducedeven as additional state-level bans were being legislated or implemented after 2005 (Table 1).Not surprisingly U.S. refineries' production and imports of MTBE rapidly declined through the early 2000s (Figure 1) and by the Spring of 2006 the "MTBE-era" in U.S. gasoline was effectively endedand consequently ushered in the "ethanol-era".

MTBE after the Spring of 2006
Incomplete understanding or inaccurate assumptions surrounding the regulations and market forces driving the use/presence of MTBE in U.S. gasoline described above promulgate the presumption that gasoline sold in the U.S. after the Spring of 2006 did not, or even (by law) could not, contain any MTBE.However, a few facts render this presumption incorrect.First, 22 of the 25 state-level bans imposed in the 2000s were not complete bans, but instead were only partial bans that allowed between 0.3 and 1.0 vol% MTBE to be present in the gasoline sold within those states (Table 1).Only three states' regulations prohibited the presence of MTBE at any concentration, viz., Colorado, Michigan, and Minnesota (Table 1).The intent of the 22 state-level partial bans was to allow some de minimus concentration of MTBE in gasoline to account for the unintentional (incidental) crosscontamination of MTBE-free gasoline with increasingly rarer MTBE-laden gasoline still in production or lingering within the shared distribution system.Second, because of the de facto elimination of MTBE from most gasoline in the Spring of 2006 the remaining 25 states that hadn't already passed regulations banning MTBE actually never did, meaning its intentional use in these states was never strictly prohibited.Third, although production declined sharply (Figure 1), the U.S. production of MTBE did not universally end in the Spring of 2006.In fact, between May 2006 and December 2018 U.S. production of MTBE still averaged over 1 million barrels per month (Figure 1) with the majority destined for export (mostly to Mexico and South America; Energy Information Administration 2018).Finally, unlike ethanol, neat MTBE or MTBE-laden gasoline (for export) can be shipped by tankers, barges, and pipelines that might also be used to carry MTBE-free gasoline (for ethanol blending) allowing opportunity of incidental mixing and cross-contamination of MTBE-free gasoline with MTBE-laden gasoline.Thus, in summary, despite MTBE's marked decrease in production and imports in the early-to-mid 2000s (Figure 1), opportunities remained for MTBE to still exist within the U.S. gasoline pool after the Spring of 2006.As expanded upon below, this study investigates the existence, concentration, and frequency of MTBE in U.S. gasoline resulting from these opportunities.

Measuring MTBE Concentrations in Gasoline
Numerous analytical methods are available to measure the concentration of MTBE in gasoline (Table 2).Multiple ASTM methods were developed during the 1990s when MTBE was intentionally being added to gasoline in high concentrations suitable for Oxy-Fuels and RFGs (150,000 and 110,000 mg/kg, respectively).Understandably, these earlier ASTM methods (ASTM D4815, D5599, D5845, D6729, and D6730) are not suitable for measuring incidental MTBE in gasoline at trace-to-low concentrations (<1000 mg/kg).However, two ASTM methods developed in the early 2000s (ASTM D7423 and D7754) are capable of measuring trace concentrations of MTBE (Table 2) through a combination of oxygen selective chromatographic  1) or after May 2006 in those states without legislated bans.We measured concentrations of MTBE using U.S. EPA Method 5030 in combination with modified U.S. EPA Method 8260 capable of detecting MTBE in gasoline at a method detection limit of 10 mg/kg using gas chromatography-mass spectrometry (GC-MS) operated in the full scan acquisition mode (Table 2; and described below).The results provide the first published dataset of the concentrations of MTBE in individual, dispensed gasoline formulations sold in the U.S. after MTBE was either legislatively banned (Table 1) or eliminated for practical (economic) reasons after the Spring of 2006.Although this study is limited to a relatively small population of dispensed gasoline, and thereby does not represent a statistical or market-based survey, the results inform on the existence, concentration, and frequency of lingering incidental MTBE in gasoline sold in the U.S. for more than a decade after being banned.The forensic implications these results have on constraining the "age" of gasoline-derived NAPL containing MTBE or of MTBE-impacted groundwater are discussed.

Retail Dispensed Gasoline Samples
Retail dispensed gasoline was targeted for study because it reflects the actual properties of the gasoline in use, which may differ from the gasoline batches leaving any given refinery, gasoline importer, or oxygenate blender due, at least in part, to the aggregation that occurs within the fungible gasoline distribution system serving most U.S. markets.All of the samples studied were collected directly from active service station dispensers into glass sample vials (25 to 200 mL) and shipped directly (usually overnight) for laboratory analysis.Samples were stored refrigerated (<4 C) prior to analysis.The samples were collected over nearly 30 years and analysed (as described in Section 2.2) shortly after their collection, typically within 14 days.
Table 3 provides a summarized inventory of the 151 retail dispensed gasoline samples analysed within the study, which are classified in three broad categories described in the following paragraphs.Additional details for the individual samples [viz., the state where each was collected and the dispenser octane grade; (R þ M)/2] are provided in the Supporting Information (Table S1).
The 151 samples were classified within two categories of "pre-ban" gasoline and one category of "postban" gasoline (Table 3)."Pre-ban" refers to the time prior to the beginning of any state-level ban specific to the state where the sample was collected (Table 1) or prior to May 2006 in those states where no ban was ever officially adopted.Oppositely, "Post-ban" refers to the time after the beginning of the state-level ban specific to the state where the sample was collected or after May 2006 in those state where no ban was officially adopted.
The first category of pre-ban gasoline included 18 samples collected between 1993 and 2002 from regions or states where wintertime oxygenated gasoline (Oxy-Fuel) or year-round reformulated gasoline (RFG) was in use either as mandated by the CAA or through opt-in programs.There was no distinction made herein between Oxy-Fuel and RFG in this group of 18 pre-ban gasoline samples.The second group of pre-ban gasolines included 57 gasolines collected between 1993 and 2004 from regions or states where Oxy-Fuel or RFG were not required by the CAA and thereby conventional gasoline was assumed to have been in use (Table 3).Finally, 76 "post-ban" gasoline samples were collected between 2004 and 2020 after each state's legislated state-level bans had taken effect (Table 1), or after May 2006 in those states where no ban was ever officially adopted.

Analysis of Gasoline Samples
All 151 samples were analysed using a combination of U.S. EPA Method 5035, Closed-System Purge-and-Trap and Extraction for Volatile Organics in Soil and Waste Samples, and U.S. EPA Method 8260, Volatile Organic Compounds by Gas Chromatography-Mass Spectrometry (GC/MS) (Table 2).The details of these methods in regard to the analysis of gasoline (or other liquid petroleum samples) have been described in detail elsewhere (Uhler et al. 2003;Douglas et al. 2015).In brief, aliquots of gasoline (20 mg) are stabilized in HPLC grade methanol (10 mL) and the solution is spiked (50 mL) into a sealed 40 mL VOA vial containing 20 mL of reagent water.The vial is then fortified with a methanol solution containing appropriate surrogate and recovery internal standards prior to purge-and-trap and instrument analysis via EPA Method 8260.EPA Method 8260 employed a gas chromatograph-mass spectrometer (GC/MS) operated in a full scan acquisition mode.The GC's capillary split/splitless injection port was directly interfaced with a robotic autosampler (Varian Archon, Tekmar Precept or Solatek, or EST Centurion) and concentrator (Tekmar 3100 or Velocity or EST Encon) system and was equipped (over time) with various narrow bore, fused silica capillary columns (e.g., Restek RTX-1 PONA 50 m, 0.32 mm i.d., 0.5 mm film thickness, or equivalent) connected directly to the MS.
Although MTBE was the focus of our study other ether oxygenates used in gasoline also were targeted, which included tert-amyl-methyl ether (TAME), diisopropyl ether (DIPE), and ethyl-tert-butyl ether (ETBE).In addition, tert-butyl alcohol (TBA) was also targeted in 135 of the samples studied; the 16 gasoline samples collected prior to 1998 were not analysed for TBA.[Approximately 80 additional volatile hydrocarbons common to gasoline were also targeted but these results are not discussed herein.] The concentrations of MTBE and the other target compounds were determined using a 5-point calibration of a solution that contained all of the target analytes, allowing for the development of internal standard-based average response factor for each target analyte.All concentration were reported in milligrams per kilogram (mg/kg), or parts per million (gasoline).Sample specific reporting limits (SSRL) for MTBE and the other ether oxygenates were approximately 90 to 100 mg/kg with a method detection limit (MDL) of 10 mg/kg.TBA's SSRL and MDL were higher (1000 and 200 mg/kg, respectively).Concentrations between the SSRL and MDL were considered estimated (Jqualified) and concentrations below the MDL were non-detected (nd).If a sample contained a target analyte at a concentration that exceeded the working range of the initial calibration, the sample was re-analysed at a more dilute concentration.All gasoline sample analyses were accompanied by analysis of method blank (B), lab control sample (LCS), and lab control sample duplicate (LCSD) for quality control.

Results and Discussion
The measured concentrations of MTBE, TAME, DIPE, ETBE and TBA in each of the 151 gasoline samples studied are provided in Supporting Information (Table S1).A summary of these results for each of the three categories of gasoline studied is presented in Table 4.
3.1.Oxy-Fuels and Reformulated Gasolines (Pre-Bans) Only a small number of Oxy-Fuels or RFGs from the pre-ban category (18) were analysed herein, which precludes any detailed analysis of these results.However, these limited results provide some context for comparison to the post-ban results discussed below (Section 3.3).MTBE concentrations in the 18 samples studied ranged from 60,000 to 133,000 mg/kg with an average concentration of 98,600 mg/kg (± 23,100 mg/kg; Table 4).These 18 dispensed retail gasoline results are slightly higher but generally consistent with monthly average, batch-level (pre-retail) RFGs reported by the U.S. EPA for all U.S. refineries and import facilities between 1999 and 2004, which averaged 90,500 mg/kg (Figure S1).[Details of this EPA dataset are provided in the Supporting Information.]Notably, few of the "pre-ban" Oxy-Fuels and RFGs studied herein contained MTBE at concentrations that approached or exceeded the concentrations of MTBE necessary to meet the minimum oxygen requirements of the Oxy-Fuel and RFG programs (150,000 mg/kg and 110,000 mg/kg, respectively).This also was evident in the EPA's batch-level dataset wherein the EPA attributed the apparent deficit largely to the trading of oxygen credits, whereby ethanol blenders sold credits to MTBE blenders to help the latter meet the 2.1 wt% average minimum oxygen requirements for Oxy-Fuels and RFGs (U.S. Environmental Protection Agency 2008).
In addition, gasoline producers generally chose to comply with oxygen content requirements on an average basis, not on an individual (per gallon) gasoline basis, e.g., blending higher than required oxygen (MTBE) in premium gasoline and less than required in regular gasoline.Finally, MTBE may not represent the entire source of oxygen in some gasoline samples, e.g., other ether oxygenates or alcohols may also be present.Thus, variation in the concentrations of MTBE around, and particularly below, the minimum oxygen requirements for individual, retail-dispensed Oxy-Fuels and RFGs, such as is evident in the 18 samples studied herein is not unexpected.The three highest MTBE concentrations in our dataset (124,000 to 133,000 mg/kg), were measured in three 100-octane gasoline samples from California (Table S1), wherein excess MTBE, with its high octane value [(R þ M)/2 110], was evidently used to boost octaneand not just to meet any minimum oxygen requirements of Oxy-Fuel or RFG.Even upon including these three 100-octane gasolines, however, there was only a poor correlation between MTBE concentration and octane rating (r 2 ¼ 0.25).This poor correlation testifies to the complexity of blending different gasoline components (e.g., alkylate, reformate, etc.) to achieve the final octane of Oxy-Fuel or RFG, wherein MTBE's role in blending to meet the oxygen minimum requirements was only one factor in a gasoline's final octane.Interestingly, of the "pre-ban" Oxy-Fuel or RFG samples studied containing among the lowest concentrations of MTBE (60,000 to 85,600 mg/kg) also contained TAME in lower concentrations (3,040 to 10,100 mg/kg; Table S1).The concentrations of MTBE and TAME are weakly correlated (r 2 ¼ 0.79) with TAME consistently comprising about 9% of their total (8.9 ± 2.4%).Although TAME reportedly can comprise a minor impurity (<1%) formed during the production of MTBE (Petrisor 2006) its presence around 9% suggests it is authentically present as a discrete oxygenate.Thus, the presence of both MTBE and TAME in these five gasoline samples, all of which were obtained from service stations in New York and Massachusetts, demonstrates that (at least) some Oxy-Fuel/RFG gasolines dispensed from retail stations contained mixtures of multiple ether oxygenates.None of the 18 Oxy-Fuels or RFGs studied contained any detectable DIPE or ETBE (Table 4).
Finally, TBA was detected in only three of the eight Oxy-Fuels or RFGs analysed for this compound at an average concentration of 9,610 mg/kg (±2,230 mg/kg; Table 4).The three gasoline samples were (again) the three 100-octane gasoline formulations from California that contained the highest MTBE among all samples studied (Table S1).Additional assessment of TBA's presence in these three 100-octane gasoline formulations is discussed in Section 3.6.

Conventional Gasoline (Pre-Bans)
Thirty-five of the 57 conventional gasoline samples (61%) collected from retail stations in markets not requiring the sale of Oxy-Fuel or RFG, and prior to any state-level bans, contained MTBE (Table 4).The concentration of MTBE in conventional gasoline containing MTBE averaged 11,800 mg/kg (± 16,500 mg/kg) and ranged from 95 to 59,600 mg/kg (Table 4).These 35 dispensed retail gasoline results are slightly higher but generally consistent with monthly average, batch-level (pre-retail) RFGs reported by the U.S. EPA for all U.S. refineries and import facilities between 1999 and 2004, which averaged 8,900 mg/kg (Figure S2).These concentrations were markedly lower than in Oxy-Fuel and RFG markets, but nonetheless notable as they serve to emphasize that MTBE-laden gasoline was not restricted to Oxy-Fuel or RFG markets (and the associated opt-in markets) but in fact were common in conventional gasoline markets.This is not surprising since MTBE was a known octane booster and no regulations precluded its use in conventional gasoline (so long as emission standards remained met).In addition, the relatively common occurrence of MTBE in conventional gasoline may also be in part due to the fungibility of the U.S. gasoline distribution system wherein it was sometimes economically impractical to separate conventional gasoline from Oxy-Fuel or RFG.
Only eight of the 57 conventional gasoline samples (14%) studied contained TAME, and none contained DIPE or ETBE (Table 4).The concentrations of TAME in these eight samples ranged from 228 to 57,600 mg/kg with an average concentration of 10,600 mg/kg (±20,500 mg/kg; Table 4).TAME was never detected in samples that did not also contain MTBE (Table S1).However, unlike the five RFG/Oxy-Fuels containing both MTBE and TAME (Section 3.1) there was no correlation between the concentrations of TAME and MTBE in the eight gasoline samples containing both (r 2 ¼ 0.33) and the percentage of TAME of their total varied widely (range: 2-52%; 27 ± 21%; Table S1).The latter still exceeded the 1% reasonably attributable to TAME impurity in MTBE further suggesting these eight conventional gasolines also contained authentic mixtures of MTBE and TAME.
Finally, TBA was detected in 15 of the 51 conventional gasoline samples analysed for this compound at an average concentration of 3,540 mg/kg (± 2,450 mg/kg; Table 4).With one exception all of the conventional gasoline samples containing TBA also contained MTBE.The single exception was a conventional 91-octane gasoline from Missouri collected in 2002 that contained 2,627 mg/kg of TBA with no detection of MTBE (or other ether oxygenates).The association of TBA with MTBE is discussed in Section 3.6.

Post-Ban(s) Gasoline or Post-May 2006 Gasoline
Twenty-six of the 76 gasoline samples (34%) collected from retail stations after the beginning of the statelevel ban specific to the state where the sample was collected or after May 2006 in those states where no ban was officially adopted contained MTBE (Table 4).
The concentration of MTBE in these 26 "post-ban" gasoline samples containing MTBE averaged only 290 mg/kg (±307 mg/kg) and ranged from 17 to 980 mg/kg (Table 4).These MTBE concentrations were markedly lower than in Oxy-Fuel/RFG and conventional gasoline markets discussed above ( In contrast, we detected MTBE in 34% of the "post-ban" dispensed gasoline samples studied, which is, at least in part, due to the 10-times lower method detection limit afforded by the US EPA Method 8260 used herein (10 mg/kg; Section 2.2).Notably, MTBE was detected (38 mg/kg) in a dispensed gasoline from New York as recently as 2020, the most recent year from which dispensed gasoline samples were included in our study (Table 3; Table S1).The detection of MTBE in dispense gasoline as recently as 2020 testifies to the lingering and apparently incidental (non-intentional) presence of MTBE, albeit at trace-to-low concentrations, within the U.S. gasoline pool despite its disuse for more nearly 15 years, which is discussed further in Section 3.4.Most state-level MTBE bans did not apply to the other ether oxygenates (Table 1) and, therefore, it is not surprising that TAME, DIPE, and ETBE were also detected at trace-to-low concentrations in 13 to 17% of the "post-ban" gasoline samples studied (Table 4).Specifically, in the 76 "post-ban" samples studied (1) TAME was detected in 13 (17%) samples at concentrations ranging from 28 to 727 mg/kg (154 ± 179 mg/kg), (2) DIPE was detected in 11 (14%) samples at concentrations ranging from 43 to 3,480 mg/kg (1,300 ± 1,430 mg/kg), and (3) ETBE was detected in 10 (13%) samples at concentrations ranging from 16 to 320 mg/kg (136 ± 103 mg/kg; Table 4).Comparison among the four ethers' mean and maximum concentrations measured in post-ban gasoline samples might suggest that DIPE was present in higher concentrations than MTBE, TAME, and ETBE (Table 4).However, DIPE's relatively higher concentration in our data is likely biased high due to its relative prevalence in five dispensed gasoline samples from service stations in Maryland (2008) and Virginia ( 2009), perhaps representing a regional influence on DIPE's use (Table S1).By the same note, ETBE was only detected in 10 dispensed gasoline samples from service stations in New York and three more in Massachusetts (Table S1), again perhaps representing a regional influence on ETBE's use (e.g., import from western Europe where ETBE was widely manufactured; CONCAWE 2012).
Trace concentrations of only one of the four ether oxygenates (MTBE, TAME, DIPE, or ETBE) were detected in only 13 samples (6, 1, 5, and 1, respectively) whereas mixtures of two or more ether oxygenates were present in 21 samples (Table S1).The most frequently detected mixtures were MTBE and ETBE, which occurred in nine gasoline samples dispensed in New York and Massachusetts between 2005 and 2016 and mixtures of MTBE, TAME and DIPE, which occurred in seven gasoline samples dispensed in Maryland, Virginia, and Georgia between 2008 and 2014 (Table S1).Despite these interesting observations we acknowledge that the limited number of samples analysed in our study does not allow for any marketbased conclusions.Rather, and as noted above, the mere frequency of detection of one or more ether oxygenates, particularly MTBE, in multiple dispensed gasoline samples from multiple locations approximately 15 years after implementation of the various state-level bans (Table 1) or Energy Policy Act in May 2006 is significant since it proves MTBE (and other ether oxygenates) lingered in the gasoline pool long after its intentional use was legislatively banned or effectively eliminated.
Finally, TBA was detected in only three of the 76 "post-ban" gasoline samples analysed for this compound at an average concentration of 739 mg/kg (± 128 mg/kg; Table 4).All three samples were three different grades of gasoline (87, 89 and 91 octane) collected from a single service station in Washington state in July 2004 (Table S1).These three gasoline samples also contained trace concentrations of MTBE (and TAME; Table S1), which all seem attributable to their incidental occurrence only months after MTBE was banned in Washington on January 1, 2004 (Table 1).

MTBE
The results surrounding MTBE discussed in Sections 3.1 to 3.4 and summarized in Table 4 are graphically depicted in Figure 2. Inspection shows the 18 "preban" RFGs and Oxy-Fuels studied exhibit the highest concentrations of MTBE near (above and below) the 10 wt% (100,000 mg/kg) required of RFG.The "preban" conventional gasoline samples studied exhibit much more variability in the concentration of MTBE present (than RFG and Oxy-Fuel), the range and concentration of which arguably broadened and decreased in the early 2000s (Figure 2) Of greatest importance to our study is the occurrence of trace-to-low concentrations of MTBE in 34% the "post-ban" gasoline samples studied (Table 4), which tend to show an overall decrease in concentration between the years shortly after the bans and 2020 (the last year for which data were collected).Although clearly limited due to the small number of samples studied this decrease would seem predictable given that most state-level bans were only partial bans, wherein MTBE was still allowed in gasoline at concentrations less than or 10,000 mg/kg so long as it was not being intentionally added by refiners or oxygenate blenders (Section 1.2; Table 1).Clearly, the concentrations of MTBE in the "post-ban" gasoline samples studied did not approach (let alone exceed) these partial ban thresholds and thereby would seem entirely explained by the incidental (unintentional) addition of MTBE (or MTBE-laden gasoline) to these "post-ban" gasoline at some point during their production or distribution.Incidental addition of MTBE would seem reasonably attributable to the fact that MTBE and MTBE-laden gasoline continued to be, and still is, produced for export (Figure 1) or that it remains lingering within the U.S. gasoline distribution system.In the first few years following the state-level bans the concentration of incidental MTBE was mostly in the 100 to 1000 mg/kg range but over time the concentration of incidental MTBE was reduced to mostly below 100 mg/kg.With this decreasing trend in mind it might be argued that MTBE lingering within the gasoline distribution system is predominantly responsible (rather than crosscontamination with MTBE for export), and thereby may eventually become non-detectable.By analogy, it is noteworthy that unleaded gasoline produced in the 1970s, 1980s, and early 1990s also demonstrated a decreasing amount of incidental lead due to crosscontamination with the decreasing volume and lead content of the leaded gasoline pool over these decades (Gibbs 1993).A comparable trend appears evident for MTBE (Figure 2).
None of our 76 "post-ban" samples were collected from the three states with total bans on the presence of MTBE (CO, MI, and MN; Table 1), so we cannot determine if incidental MTBE may have been present in "post-ban" gasolines from these states.

TAME, DIPE and ETBE
Although our data revealed fewer detections of TAME, DIPE and ETBE (than MTBE) in the dispensed gasoline samples studied, they nonetheless confirmed the presence of these ether oxygenates in some of "post-ban" gasoline (Table 4).The results for TAME are shown in Figure 3, which demonstrates a trend similar to that of MTBE (Figure 2).Owing (at least in part) to TAME's fewer overall detections and its frequent co-occurrence with MTBE in "pre-ban" gasoline samples (Table S1), the concentration of  3).All data from Supplemental Information, Table S1.See text for description.
TAME in "pre-ban" gasoline samples varied widely.The concentration of TAME in the 13 "post-ban" gasoline samples containing TAME generally remained around 100 mg/kg and arguably may have decreased slightly over time (Figure 3), akin to what was evident for MTBE (Figure 2).
Both DIPE and ETBE were only detected in 11 and 10 "post-ban" gasoline samples, respectively, and were not detected in any "pre-ban" gasoline samples (Table 4).Thus, discerning any temporal trend(s) in these ethers' use is not possible (see Figure S3).However, their mere detection in some "post-ban" gasolines is noteworthy since it shows their lingering presence within the gasoline pool after the bans on MTBE.

Tert-Butyl Alcohol
TBA was frequently analysed for in the conventional gasolines and "post-ban" gasoline samples only (Table 4).Notably, it was detected in 29% of the 51 conventional gasolines but only 4% of the 76 "post-ban" gasolines.The latter were represented by three gasoline samples collected from a service station in Washington state in 2004 (Table S1), only months after that state's MTBE ban commenced (Table 1).This suggests TBA's frequency of use decreased quickly after state-level MTBE bans were imposed (Table 1) or after May 2006 in those states without legislated bans (see Figure S3).The absence of detectable TBA in the remaining 73 "post-ban" gasoline samples collected between 2004 and 2020 is perhaps the most notable result surrounding TBA.

Forensic Implications of Incidental MTBE in
Post-Ban Gasolines

Age-Dating Implications for NAPLs
Owing to its propensity to partition into groundwater, the absence of MTBE in non-aqueous phase liquids (NAPLs) recovered from the subsurface is equivocal in regard to the NAPL's "age".Oppositely, however, the presence of MTBE in NAPLs, as well as soil or groundwater, is often used as evidence in constraining the "age" of the leaked parent gasoline (e.g., Petrisor 2006).This approach would seem to be straightforward whereupon if MTBE were present in the subsurface environment (at concentrations in excess of attributable to non-point sources) it could reasonably only have been derived from a release of MTBE-laden gasoline (except, of course, rare situations where neat MTBE may be released).In practice, determining the oldest possible date that MTBE-laden gasoline may have been handled at a given site (e.g., service station), and thereby possibly released, often is not well constrained.In the absence of detailed, site-specific records that might yield precise dates (e.g., bills of lading on gasoline deliveries, pipeline shipment records, or other documentation sometimes available through litigation) MTBE's first use at a given site might only be constrained by the onset of regulations, e.g., implementation of Oxy-Fuels and RFG programs in designated metropolitan (non-attainment) areas in 1992 and 1995, respectively.Oppositely, MTBE's last possible use in gasoline at a given site thereby might be assumed to coincide with the date that the corresponding state-level ban  3).All data from Supplemental Information, Table S1.See text for description.
was implemented (Table 1) or, in states with no bans, shortly after May 6, 2006 when the Policy Act of 2005 eliminated the minimum oxygen requirements for RFG.However, this assumption is unwarranted as it would be true only if MTBE had been completely eliminated from the "post-ban" U.S. gasoline pool on these dates.As explained in Section 1.2, MTBE's presence in gasoline after (all but three of) the state-level bans was not completely prohibited.Rather, only its intentional addition to gasoline was prohibited with the maximum permissible concentrations remaining at 5,040 mg/kg (0.5 vol%) in most states (Table 1).The results reported herein for the 76 "post-ban" gasoline samples showed MTBE was relatively common, being detected in 34% of the samples all at concentrations less than 1,000 mg/kg (980 to 17 mg/kg; Figure 2), i.e., well below the 5,040 mg/kg maximum permissible concentrations after being banned.Nonetheless, the presence of these trace-to-low concentrations of incidental MTBE clearly has persisted in some fraction of dispensed gasoline for the nearly 15 or so years since most state-level bans commenced.Although limited to our sample set, the concentration of this incidental MTBE when present generally appears to have decreased over this time to less than 100 mg/kg between 2014 and 2020 (Figure 2).
The forensic "age-dating" implication of the results obtained is that the mere detection of MTBE in a NAPL, soil, or groundwater at a given site does not necessarily indicate the parent gasoline's "age" must pre-date the corresponding state-level MTBE ban or, more generally in those states without legislative bans, the Spring of 2006.Our results also indicate that NAPL containing less than 1,000 mg/kg could be entirely attributable to a release of "post-ban" gasoline containing incidental concentrations of MTBE.Alternatively, and owing to MTBE's propensity to partition out of NAPL and into groundwater, NAPLs containing less than 1,000 mg/kg could also arise from "pre-ban" gasoline from which MTBE has been reduced via water-washing.Additionally, mixing of an older MTBE-laden gasoline with a gasoline devoid of MTBE could also produce NAPL containing less than 1,000 mg/kg.Thus, the minimum approximate "age" of NAPL containing MTBE at concentrations below 1,000 mg/kg cannot be confidently determined based upon this one feature.It remains reasonable, however, that NAPL containing MTBE at a concentration(s) greater than 1,000 mg/kg, or certainly greater than the maximum permissible concentration allowed in a given state (5,000 mg/kg; Table 1), has a minimum approximate "age" that must pre-date that state's MTBE ban, or more generally in states without bans, the Spring of 2006.

Theoretical Impact of Incidental MTBE in Post-
Ban Gasolines on Groundwater As noted above, it is well established that MTBE will preferentially partition out of leaked gasoline into water, which was the principal feature that led to its eventual disuse in the U.S. gasoline pool (Section 1.1).MTBE's propensity to partition from gasoline into water raises the question of the potential impact on groundwater from leaked "post-ban" gasoline containing incidental concentrations of MTBE.Our results indicate that incidental concentrations of MTBE in "post-ban" gasoline containing MTBE ranged less than 1,000 mg/kg (17 and 980 mg/kg; Table 4), being generally below 100 mg/kg after 2014 (Figure 2).With these results in mind, equilibrium partitioning theory can be used to calculate the maximum concentration of MBTE that may occur in groundwater impacted by gasoline only containing incidental MTBE at concentrations of 1,000 and 100 mg/kg.
The maximum concentration of MTBE in groundwater at a given temperature (C; mg/L) is a function of its aqueous solubility (S; 50,000 mg/L at 25 C; Mackay, Shui, and Ma 1993) and its mole fraction in the parent gasoline (X; unitless).Because MTBE's molecular weight approximates that of gasoline, X for MTBE will closely approximate its concentration in the gasoline (Squillance et al., 1997).Additionally, any added effect of MTBE's chemical properties relative to gasoline as a whole is negligible (Squillance et al., 1997).Thus, the maximum concentration of MTBE in groundwater equilibrated with an MTBE-laden gasoline can be approximated as; Table 5 contains the results for a series of calculations predicting the maximum concentration of MTBE in gasoline containing a wide range of MTBE concentrations.Historic gasolines containing on the order of 100,000 mg/kg (10 wt%) MTBE, e.g., "preban" gasoline (Oxy-Fuel and RFG; Figure 2), could result in concentrations theoretically as high as 5,000 mg/L (at 25 C) in groundwater.In practice, however, maximum concentrations of MTBE historically measured in groundwater near real-world Oxy-Fuel or RFG release sites are usually much lower than the theoretical maximum.For example, in surveys of leaking underground storage tank (LUST) sites Davidson (1995) reported a maximum concentration of only 200 mg/L and Beckenbach et al. (1995) reported a 90 th percentile maximum of 100 mg/L, i.e., 50-fold lower than the calculated theoretical maximum.This marked difference between the theoretical and real-world maximums is attributed to the dilution of MTBE-impacted groundwater with uncontaminated groundwater and gradual reduction of MTBE from the source gasoline over time (Squillance et al., 1997).
Of relevance herein, "post ban" gasoline containing only 1,000 mg/kg (0.1 wt%) or 100 mg/kg (0.01 wt%) of incidental MTBE (Figure 2) are predicted to yield theoretical maximum concentrations of MTBE in groundwater of only 50 mg/L and 5 mg/L, respectively, per Equation 1 (Table 5).Assuming an anticipated 50-fold decrease attributable to dilution with uncontaminated groundwater, in practice the real-world maximum concentrations of MTBE in groundwater proximal to a recent spill of "post-ban" gasoline containing incidental MTBE are expected to be less than 1.0 mg/L and 0.1 mg/L, respectively (Table 5).Downgradient concentrations would, of course, be even less due to attenuation processes.If the apparent decrease in the concentration of incidental MTBE within the 2016 to 2020 time interval evident in our limited dataset is assumed correct (Figure 2)and concentrations of any incidental MTBE are expected to be <100 mg/kg in these gasolinesthen the maximum concentration of MTBE that might be reasonably expected in groundwater proximal to and shortly following any recent release of such gasoline should not exceed 0.1 mg/L (Table 5).Higher concentrations would implicate the residual impact of an older release of gasoline containing higher concentrations of MTBE.Though clearly estimated, even this low concentration (0.1 mg/L) is not insignificant considering that regulatory thresholds protective of human health are on the order of 0.013 to 0.070 mg/L (e.g., California and Massachusetts;California State Water Resources Control Board, 2017 andMassachusetts Department of Environmental Protection, 2021).In summary, considerable caution is warranted in constraining the "age" of a parent gasoline that may have given rise to MTBE in groundwater.It is perhaps most clear, however, that concentrations of MTBE dissolved in groundwater exceeding those predicted to be attributable to a recent release of "post-ban" gasoline containing only incidental MTBE (Table 5) can only reasonably be attributed to historic release of a "pre-ban" gasoline.
TBA's detection in many of the "pre-ban" gasoline samples and its rarity in "post-ban" gasoline samples (Table 4) is interesting and warrants some added discussion.TBA has been used as an octane-boosting blending agent in gasoline by some refiners since about 1969 (Gibbs. 1990).TBA is commercially produced via the acid-catalysed hydration of iso-butylene (Gehlawat and Sharma 1968) or as a co-product of propylene oxide production (Kornfeld 1977).Initially, refiners were reportedly blending TBA into unleaded gasoline at 1 to 5 vol% throughout the 1970s.TBA's use in unleaded gasoline became (legitimized and) more widespread after the EPA officially approved its use at concentrations up to 7 vol% in 1979 (Gibbs 1990; U.S. Environmental Protection Agency 2000) even though, as just noted, many refiners already had been using it for years.In 1979 and 1981, EPA issued waivers allowing up to 2.75 and 4.75 vol% TBA in unleaded gasoline when it was blended 50:50 with methanol (Guetens et al. 1982;U.S. Environmental Protection Agency 2000).Thus, TBA was intentionally blended in some gasolines throughout the 1970s and 1980s although its use was generally supplanted by the rise in MTBE use (Section 1.1).Most, but not all, refiners that produced TBA for gasoline simply shifted to use iso-butylene as a feedstock needed for MTBE production (instead of TBA).Specifically, MTBE is produced through a reaction of methanol with iso-butylene (Adams et al. 1981) as follows;

MTBE: CH
ETBE is produced similarly except that ethanol is substituted for the methanol feed and the reactor temperatures are slightly higher than during MTBE production (Adams et al. 1981).The chemical reaction used in the production of ETBE is: During the production of both MTBE and ETBE hydration of the iso-butylene feedstock will allow production of some TBA by the following reaction (P.Beall, personal communication, 2002): As such, gasoline-grade MTBE and ETBE will include impurities, among which TBA can predominate at concentrations up to 2.0 wt% of each (U.S. Environmental Protection Agency 2000; American Petroleum Institute 2012).(Notably, during the production of TAME or DIPE there is no opportunity for the production of TBA, since iso-butylene is not used as a feedstock in either's production; Adams et al. 1981.)Thus, gasoline containing 100,000 mg/kg of MTBE or ETBE may contain up to 2,000 mg/kg of TBA as an impurity.Turning attention to the three 100-octane gasoline samples collected in 2002 from California that contained high concentrations of both MTBE and TBA discussed in Section 3.1 (Table S1), the TBA in these gasoline was present in concentrations between 5.6 and 8.9 wt% of the MTBE, i.e., significantly exceeding 2 wt% of the MTBE that could be attributed to an impurity in the MTBE.The excess proportions of TBA (5.6 to 8.9 wt% of the MTBE) indicates that TBA was intentionally and directly blended into these 100-octane gasolines in 2002along with MTBEas a means to boost octane.
Similarly, all but one of the 15 "pre-ban" conventional gasoline samples studied herein that contained TBA had TBA concentrations that equalled or significantly exceeded the concentration of any co-occurring MTBE (Table S1).Thus, these gasolines' TBA also cannot be attributed to an impurity in the MTBE but rather to the direct blending of TBA as an octane booster.Like the three 100-octane gasolines described above, the one exception among the 15 "pre-ban" conventional gasolines contained TBA at a concentration of 6.4 wt% of the MTBE, i.e., also too high to attribute to an impurity thereby indicating it too contained directly blended TBA.
The relative lack of low concentrations of TBA in the MTBE-laden gasoline samples studied herein that could be attributable to impurity of the MTBE results, at least to some degree, from the relatively high sample specific reporting limits and method detection limit for TBA using EPA Methods 5035/8260 (1000 and 200 mg/kg, respectively; Section 2.2).As such, dispensed gasolines would need to contain at least 10,000 mg/kg of MTBE to even detect TBA that could be present as an impurity only (2 wt% of 10,000 mg/kg is 200 mg/kg).However, even among the 28 dispensed gasoline samples studied that contained MTBE at concentrations above 10,000 mg/kg (Table S1), which excludes the four samples that contained elevated TBA due to direct blended (see preceding paragraphs), none contained TBA in a concentrations near or below 400 mg/kg (2 wt%) of the MTBE (Table S1).Albeit limited to our small sample set, the lack of low concentrations of TBA in the MTBE-laden gasoline samples studied suggests that TBA's presence in dispensed gasoline as a measurable impurity in the MTBE present was uncommon.
In summary, based on the history of TBA's use and MTBE and ETBE production chemistry described above, there are two routes by which TBA could possibly occur in dispensed gasolines, namely, (1) as a direct blending agent (sometimes as a methanol blend, 1:1) and (2) as an impurity in MTBE and ETBE.However, for our limited dataset, the former appears to be more commonly observed.Our dataset also demonstrates the production and intentional addition of TBA persisted in some unleaded gasolines, at least, into the early 2000s but was not evident in any of the 67 gasoline samples studied after 2004 (Table S1).

Conclusions
MTBE's rise and fall within the U.S. gasoline pool is well established.Its fall stemmed from its unsettling and detrimental impact on groundwater and drinking water supplies following gasoline leaks and spills, which promulgated the implementation of numerous legislated, state-level bans beginning in the early 2000s.These bans, along with the Energy Policy Act of 2005s elimination of a minimum oxygen requirement for reformulated gasoline, effectively ended the "MTBE era" by the Spring of 2006.However, most state-level bans still allowed for the incidental (nonintentional) occurrence of MTBE with permissible concentrations ranging from 0.3 to 1.0 vol%, but mostly below 0.5 vol% (5,040 mg/kg).Such an accommodation was necessary because opportunities remained for the incidental occurrence of MTBE in U.S. gasoline due to cross-contamination with neat MTBE or MTBE-laden gasoline that was still being produced in the U.S. for export, and/or its otherwise lingering presence within the U.S. gasoline distribution system.
Our ad hoc survey of 76 "post-ban" dispensed gasoline samples collected directly from retail service station dispensers in multiple states between 2004 (in states with early bans) and 2020 showed, to our knowledge for the first time, that "post-ban" gasolines still frequently contained MTBE (above the full-scan GC/MS method detection limit of 10 mg/kg).Specifically, MTBE was present in one-third (34%) of the gasoline samples studied at concentrations below 1,000 mg/kg, specifically, between 980 and 17 mg/kg (avg.± r; 209 ± 307 mg/kg), i.e., well below the statelevel bans' permissible thresholds.Although admittedly limited by the small number of samples studied, the MTBE concentrations generally declined between 2004 and 2020.In the first few years following the state-level bans the concentration of MTBE mostly range between 100 to 1,000 mg/kg but over time the concentration of incidental MTBE was reduced to mostly below 100 mg/kg.With this decreasing trend in mind, the incidental presence of MTBE in "post-ban" gasoline would seem to be predominantly derived from (assumedly) declining amounts of MTBE lingering within the gasoline distribution system, as opposed to direct cross-contamination with MTBE or MTBE-laden gasoline still produced for export, the volumes of which have not substantially been reduced over this time.If true, incidental MTBE in U.S. gasoline may eventually become non-detectable.
Although less frequently detected, TAME, ETBE, DIPE and TBA were also found in 17, 14, 13 and 4% of the "post-ban" gasoline samples studied, respectively, at concentrations mostly below 1,000 mg/kg.Like MTBE, TAME arguably exhibited an overall decrease in concentration in "post-ban" gasolines over time.Notably, TBA was detected in 29% of the 51 "pre-ban" conventional gasoline samples but only 4% of the 76 "post-ban" gasoline samples, with the latter all collected in 2004 (and none thereafter).This marked decline of TBA's detection in "post-ban" gasoline samples suggests TBA's frequency of use decreased quickly after state-level MTBE bans were imposed.Additionally in regard to TBA, its concentration relative to any MTBE (or ETBE) also present in both "pre-ban" and "post-ban" gasolines suggests that TBA was present as a blending agent directly added to these gasolines, rather than a minor (up to 2 wt%) contaminant within the MTBE (or ETBE).
The forensic implications these results have on constraining the "age" of gasoline-derived NAPL containing MTBE or of MTBE-impacted groundwater include: 1. Any presumption that MTBE completely disappeared from the U.S. The "age" of gasoline-derived NAPLs containing less than 1,000 mg/kg MTBE remains equivocal (based on MTBE alone) because they may arise from "pre-ban" gasoline, now depleted in MTBE, and/or "post-ban" gasoline.4. Based on equilibrium partitioning theory and typical dilution, groundwater impacted by a recent and proximal release of "post-ban" gasoline containing only 1,000 or 100 mg/kg of MTBE would not give rise to dissolved MTBE exceeding 1.0 or 0.1 mg/L, respectively.MTBE in groundwater exceeding these concentrations can only reasonably be attributed to some historic release(s) of "pre-ban" gasoline.The "age" of gasoline giving rise to lower concentrations of MTBE in groundwater remains equivocal since these may arise from attenuation of MTBE derived from either a historic release of "pre-ban" gasoline or from a recent release of "post-ban" gasoline.

Figure 1 .
Figure 1.Monthly production, imports, and exports of MTBE in the United States (Jan.1993-Dec.2018; the last date figures are available).Data Source: U.S. Energy Information Admin.; eia.gov.

Figure 2 .
Figure 2. Concentration of MTBE in dispensed gasolines containing MTBE by category (per Table3).All data from Supplemental Information, TableS1.See text for description.

Figure 3 .
Figure 3. Concentration of TAME in dispensed gasolines containing TAME by category (per Table3).All data from Supplemental Information, TableS1.See text for description.

Table 1 .
Inventory of legislated state-level bans of MTBE in U.S. automotive gasoline.
a Ban(s) also apply to ETBE and TAME.b Ban(s) also apply to ETBE, DIPE, TAME, and TBA.c Ban(s) also apply to ETBE, DIPE, TAME, TBA and other alcohols (except ethanol).d There is no universal weight-to-volume and volume-to-weight conversion since different gasoline blends' densities vary albeit narrowly; herein weight percent and mg/kg calculated assume densities of MTBE and gasoline to be 0.746 and 0.740 g/ml, respectively.e

Table 2 .
Analytical methods available for measuring the concentration of MTBE in gasoline.

Table 3 .
Classification of the 151 dispensed automotive gasolines analysed in this study.Additional details provided in Supporting Information TableS1.
a refers to individual state-level bans (Table1) or May 2006.

Table 4 .
Summary of results for multiple oxygenates in 151 dispensed gasolines studied by category.Additional details provided in Supporting Information TableS1.
Table4).The frequency of detecting tens-to-hundreds of parts per million MTBE in 34% of the 76 samples studied was initially surprising.However, after further research, and as explained in Section 1.2, we came to realize a significant opportunity for incidental concentrations of MTBE to occur within the U.S. gasoline pool despite the proliferation of state-level bans (Table1) or implementation of the Energy Policy Act in May 2006 that eliminated the minimum oxygen requirement from RFG.Our results substantiate this fact and, to our knowledge, represent the first report on the existence, concentration, and frequency of incidental MTBE in gasoline dispensed after state-level bans or after May 2006 in states without bans.The EPA's batch-level, monthly average data for all U.S. gasolines show rare detections of MTBE after 2006 (see insets to FiguresS1 and S2).Specifically, the EPA dataset shows MTBE was detected (>100 mg/kg; method(s) not specified) in only seven of 290 batches of RFG (2.4%) and 58 of 335 batches of conventional gasoline (17%), or combined, 65 of 625 (10%) of all gasoline batches refined or imported after May 2006.

Table 5 .
Examples showing MTBE concentrations in gasoline and the theoretical and predicted concentrations that may be found in groundwater proximal to the gasoline when spilled.See text for description.
gasoline pool after the implementation of the state-level bans and Energy Policy Act is false.2. The mere detection of MTBE in a NAPL (soil or groundwater) at a given site does not necessarily indicate the parent gasoline's "age" must pre-date the corresponding state-level MTBE ban or, more generally in those states without legislative bans, the Spring of 2006.3. It remains reasonable, however, that the "age" of gasoline-derived NAPL containing MTBE at a concentration(s) above 1,000 mg/kg, or certainly above the maximum permissible concentration allowed in a given state, must pre-date that state's MTBE ban, or more generally in states without bans, the Spring of 2006.