Following the 1989 Exxon Valdez oil spill (EVOS), the Prince William Sound Regional Citizens' Advisory Council began the Long-Term Environmental Monitoring Program (LTEMP) in 1993 to track oil hydrocarbon chemistry of recovering sediments and mussel tissues along the path of the spill in Prince William Sound (PWS) and across the Northern Gulf of Alaska (NGOA) region. The program also samples sites near the Alyeska Marine Terminal (AMT) within Port Valdez, primarily to monitor tanker operations and the resulting treatment and discharge of oil-contaminated tanker ballast water.
Over the last 28 years, the program has documented EVOS oil's disappearance at the spill-impacted sites (albeit buried oil still exists at a few unique sheltered locations in PWS). Within the Port, a few tanker- and diesel-spill incidents have been documented over the years, but all were minor and with recovery times of < 1 yr. Of highest concern has been the permitted chronic release of weathered oil from tankers' ballast-water that is treated and discharged at the Alyeska Marine Terminal (AMT). In earlier years (1980s–90s), with discharge volumes reaching 17–18 MGD, up to a barrel of finely dispersed weathered oil would be released into the fjord daily. Over the last two decades, total petrogenic inputs (TPAH43) into the Port have declined as measured in the monitored mussels and sediments. This trend reflects a combination of decreased Alaska North Slope (ANS) oil production and thus, less tanker traffic, plus less ballast from the transition to double-hulled tankers with segregated ballast tanks, and improved treatment-facility efficiency in removing PAH. From the 2018 collections, mussel-tissue hydrocarbon concentrations from all eleven LTEMP stations (within Port Valdez as well as PWS and NGOA regions) were below method detection limits and similar to laboratory blanks (TPAH43 < 44 ng/g dry wt.). At these low background levels, elevated TPAH values from a minor 2020 spill incident at the Terminal were easily detected at all three Port Valdez stations.
In the aftermath of the 1989 Exxon Valdez oil spill, the Oil Pollution Act of 1990 (OPA 90) mandated the Prince William Sound Regional Citizen's Advisory Council (PWSRCAC) to establish the Long-Term Environmental Monitoring Program (LTEMP) with the goal of monitoring petroleum hydrocarbons for environmental impacts from the EVOS and oil transportation activities. Begun in 1993, the program samples intertidal mussel tissues at selected sites in Prince William Sound (PWS) and across the northwest Gulf of Alaska (GOA) region (Figure 1) as well as mussels and sediments in Port Valdez adjacent to the Alyeska Marine Terminal (operated by Alyeska Pipeline Service Company) and from a nearby reference station across the Port (Figure 2). Initially, samples were collected biannually, but with consistently dropping hydrocarbon signals in the outer PWS and GOA regions, efforts were curtailed in 2010 to a single, annual sampling inside the Port and a five-year cycle for the outer stations. Within Port Valdez, three stations, Saw Island (AMT) and Jackson Point (JAP) adjacent to the terminal, and the Gold Creek (GOC) reference site (6 km across the Port) are now sampled midyear. In the five-year cycle, the Port Valdez stations are augmented by mussel sampling at two non-spill-impacted sites, Knowles Head (KNH) and Sheep Bay (SHB); along the path of the oil spill within PWS at Disk Island (DII), Zaikof Bay (ZAB), and Sleepy Bay (SLB); and along the extended EVOS path at three Gulf of Alaska stations, Aialik Bay (AIB), Windy Bay (WIB), and Shuyak Harbor (SHH) (Figure 1).
A previous summary of early LTEMP results (through 2006) has been published (Payne et al., 2008), and annual LTEMP Reports are available on the PWSRCAC Website (PWSRCAC, 2019). This paper summarizes more recent events, results, trends, and insights.
Modelled after the NOAA Mussel Watch Program and the EVOS Natural Resources Damage Assessment, LTEMP uses slightly modified sampling and analytical chemistry protocols (Short et al., 1996 and Payne and Driskell, 2017). Briefly, three replicates of 30 mussels are collected by hand at each site while triplicate subtidal sediment samples are collected from the two Port locations (AMT and GOC) using a modified Van Veen grab. During the 28 years of LTEMP, the program was initially run by Kinnetic Laboratories, Inc. (KLI) until Payne Environmental Consultants, Inc. (PECI) assumed the program in 2002. Chemical analyses were provided sequentially by the Geochemical and Environmental Research Group (GERG) in College Station, TX, the NOAA Auke Bay Laboratory in Juneau, AK, and currently Alpha Analytical/NewFields Environmental Forensics in Mansfield, MA. Interlaboratory calibration and Method Detection Limit (MDL) studies have been conducted to ensure data comparability, accuracy, precision, and MDLs (Payne et al., 2003; and Payne and Driskell, 2017).
Currently measured variables in whole mussel (Mytilus trossulus) tissues and sediments include polycyclic aromatic hydrocarbons (PAH), saturated hydrocarbons (SHC), and more recently, oil biomarkers (S/T). Semi-volatile compounds, the PAH, alkylated PAH, and petroleum biomarkers, are analyzed using selected-ion-monitoring gas chromatography/mass spectrometry (SIM GC/MS) via a modified Environmental Protection Agency (EPA) Method 8270 (aka 8270M). This analysis provides the concentrations of parent PAH, alkylated PAH homologues, individual PAH isomers, and sulfur-containing aromatics (now reporting 50 analytes vs. 43 previously; summed as TPAH50 or TPAH43) plus ~70 tricyclic and pentacyclic triterpanes, regular and rearranged steranes, and triaromatic and monoaromatic steroids. SHC in sediments and tissues are quantified as 33 n-alkanes (C9–C40) and selected (C15–C20) acyclic isoprenoids (e.g., pristane and phytane) using modified EPA Method 8015B. A sample's high-resolution gas chromatography-flame ionization detector (GC/FID) fingerprint is examined to track microbial degradation as well as marine biogenic and terrestrial plant-wax input. Sediment particle grain size (PGS) distributions and total organic carbon (TOC) are also reported.
Beginning in 2017, low-density polyethylene, passive sampling devices (PSD) were deployed at Port Valdez mussel sites to sample dissolved PAH and other oxygenated hydrocarbons. Following protocols used at Oregon State University's (OSU) Food Safety and Environmental Stewardship Program (Huckins et al., 2006 and O'Connell et al., 2013), LTEMP PSD were constantly submerged for ~30 days while anchored adjacent to the LTEMP mussel sites (Figure 2).
Generally, the PSD is intended to only sample the most bioavailable hydrocarbons, the dissolved compounds and labile complexes that diffuse into the membrane. OSU's lab reports 61 PAH isomers as their normal analyte list, but the list was expanded in 2018 to include 40 parent and alkylated PAH homologs routinely used for forensic interpretations. A critical part of the PSD design involves pre-infusing various deuterated surrogate compounds into the membrane. Following deployment, their known rate of diffusion out of the membrane while local dissolved hydrocarbons diffuse in enables the back-calculation of hydrocarbon concentration exposure in the water column. Laboratory handling, sample extraction and analyses of the PSDs follow respective OSU's standard operating procedures.
Overall, LTEMP chemistry data are viewed from two perspectives: 1) characterizing the hydrocarbon profiles as to the likely source, dissolved- vs. oil-droplet phase, and degradation state; and 2) trends in TPAH concentration. The results, combined with other process and environmental knowledge, are interpreted as likely fate and transport scenarios. Analyte concentrations are presented graphically as bar plot profiles for each sample; however, the bars for alkylated PAH actually represent the sum of alkylated homologue components. Also, in this paper, for each sample's profile, an appropriate reference is overlaid as either the sample's MDL or as a source reference, e.g., ANS oil or treated ballast effluent, scaled to a conservative, non-degrading component (typically, hopane).
Alyeska Marine Terminal BWTF effluent sampling and analyses
Unrelated to the 1989 EVOS, the primary oil contamination source in Port Valdez has historically been partially degraded, ANS crude oil discharged from the Alyeska Marine Terminal's ballast-water treatment facility (BWTF) (Payne et al., 2005). Over the last two decades, petrogenic inputs into the Port, as measured by TPAH43 concentrations, have been declining in nearby mussels and sediments (Payne and Driskell, 2017, 2021). These trends correlate with production and process changes as the Trans-Alaska Pipeline oil throughput has dropped from 85.3 million gallons per day (MGD) at its peak in 1988 to current levels of ~20 MGD ( Table 1) Likewise, treated, oil-contaminated, ballast-water discharge to the Port has dropped from a maximum of around 15 MGD in 1990 to currently only ~1 MGD ( Table 1). This is partially due to 1990 regulations to phase out single-hull tankers for safer double-hulled tankers with segregated ballast. Cargo-tank ballast water is oily and requires treatment while segregated ballast water comprises uncontaminated seawater and is simply dumped dockside. Aboard segregated-ballast vessels, empty cargo tanks are only filled with supplemental seawater ballast when operationally necessary (e.g., during winter storms). As a result, during summer months when minimal extra ballasting is required, more than half of the BWTF effluent discharge is from the terminal's stormwater runoff (Rich Loftin, pers comm, 2016).
BWTF discharge samples were analyzed as both whole (raw unfiltered) effluent and after partitioning into particulate/oil-phase droplets distinct from the dissolved-phase constituents using a portable filtration system (Payne et al., 1999, Payne and Driskell 2017, 2021). In July 2016, the effluent turned out to be nearly 80% freshwater with a low salinity that reflects the collected runoff from the terminal and reduced treated tanker-ballast volumes during the summer. Effluent samples were again collected in March 2017 to reflect winter operations, a period with less runoff and higher ballast-water content.
As expected, the winter effluent had higher TPAH values (7,605 ng/L vs. 2,885ng/L), and was less weathered and biodegraded relative to fresh ANS crude oil (Figure 3). The dominant profiles of non-water-soluble S/T biomarkers in the whole (unfiltered) samples from both seasons (right-hand profiles in Figure 3) also suggests the presence of free oil droplets in the effluent, as is confirmed by the filtered samples (Figure 4 B) where particulate/oil-phase droplets were present in the effluent during both seasons at similar concentrations (1,659 and 2,084 ng/L) and with similar degrees of weathering. On the other hand, the less-degraded, bioavailable, dissolved-phase PAH in the filtered winter Biological Treatment Tank (BTT) effluent sample had order-of-magnitude higher concentrations (11,296 vs. 1,957 ng/L) compared to the summer (Figure 4 C). During winter, additional ballast in the tanker cargo holds produce higher volumes and throughput for the BWTF. Also, there is reduced freshwater runoff at the terminal during the colder winter months.
Lower-molecular-weight SHC are subject to both dissolution/evaporation losses and microbial degradation (NAS 1985; Payne et al., 2005). In a sample's SHC profile, microbial degradation processes initially appear as decreases in the more easily assimilated n-alkanes, n-C17 and n-C18, relative to the branched-chain isoprenoids, pristane and phytane. As expected, ratios n-C17/pristane and n-C18/phytane demonstrate that during both seasons the microbial processes in the BTT are very effective at removing SHC (Payne et al. 2005 and Payne and Driskell 2017 and 2021).
Port Valdez sediment trends 1993–2020
We consider TPAH concentrations to be a very rough proxy of oil contamination (like discussing weather systems but only talking about the temperature highs and lows); a truer picture is in the profile details and interpretations. But for historical and trend perspectives, TPAH43 concentrations are presented and briefly discussed in the following sections.
With sediment TPAH43 levels dropping from historic highs in the hundreds if not thousands of ng/g dry weight (DW) between 1993 and 2004 (Figure 5) (including a spike from the 1995 Eastern Lion tanker spill), average concentrations at the 68–72m deep, terminal Berth 4 site (Figure 2) continued to decrease from values in the low hundreds of ng/g in the 2002–2004 period until they dropped to around 50–60 ng/g in March 2005. The decline continued in a range between 20–50 ng/g until 2013 when the concentrations dropped to all-time lows, ~4 ng/g, and then rebounded up to a range of 30 – 114 ng/g between 2016 and 2020 (Figure 5). During some years, the PAH patterns reflect a weathered ANS oil profile, while at other times there is a dominant pyrogenic (combustion derived) signal (Figure 6). The SHC show marine biogenic constituents plus higher-molecular-weight petrogenic waxes. Oil biomarkers (begun in 2011) confirmed BWTF accumulation in sediments near the terminal regardless of the shifting petrogenic and pyrogenic PAH content (Payne and Driskell 2021).
At the shallower (28–30 m) GOC reference site, sediment TPAH43 trends have generally tracked with those observed at the terminal, but since 2016, the concentrations are usually 2–4 times lower, 20–40 ng/g (Figure 5). In recent years, GOC profiles (not shown) reflect only PAH from combustion products and marine/terrestrial biogenic SHC (Payne and Driskell, 2021).
Mussel tissue trends
Although historically, TPAH43 concentrations in mussel tissues sampled from both near the terminal and at the background-reference site were commonly reported in hundreds of ng/g DW (excepting the 1995 tanker spill), by 2002 the concentrations dropped to ~80 ng/g levels (Figure 7). Exceptions to this post-2002 range occurred with a diesel spill at GOC in the fall of 2004 (TPAH ~1,000 ng/g), likely from commercial fishing vessels. By 2005, the residuals were long purged, and concentrations were back in the pre-spill range at both locations until 2009 through 2013 when only exceptionally low, near-MDL, traces of petrogenic components were present. Due to a program hiatus, no samples were collected in 2014 but, in 2015, the PAH tissue burdens at both locations remained very low while the profiles further transitioned into primarily dissolved-phase, background patterns (Figure 8). TPAH43 concentrations at GOC increased again in July 2016 due to another localized diesel spill (Payne and Driskell 2017).
In April 2020, an estimated 635 gallons (16 bbl) of ANS crude oil from an overflow sump at the terminal reached the intertidal zone (see the red arrow in Figure 2 , ADEC 2020). This resulted in elevated TPAH (438, 256, and 194 ng/g) at all three Port Valdez sites, JAP, AMT and GOC, respectively (Figure 7). The PAH profiles at the traditional LTEMP stations on either side of the spill site (e.g., AMT in Figure 8 D), showed a distinct petrogenic signal, but by June 2020 the levels had dropped back to the 33–71 ng/g range (Figure 7). Mussels immediately adjacent to the spill were heavily oiled (approaching 230,000 ng/g) with ANS crude oil profiles. Another paper is in preparation addressing mussel depuration rates and transcriptomic responses.
Outside of Port Valdez, TPAH43 concentrations since 2002, have generally remained below 40 ng/g with some in single digits. In addition, in 2018, the PAH profiles from all 11 mussel stations in Port Valdez, PWS and the NGOA regions were identical (e.g., see the 2018 AMT profile in Figure 8 C). At these exceptionally low levels, the individual PAH components in all the 2018 samples were below MDLs and essentially indistinguishable from laboratory method blanks. SHC profiles from all 2018 stations contained primarily biogenic hydrocarbons of marine origin and occasionally, odd-carbon-numbered terrestrial plant waxes (NAS 1985).
Taken together, the most recent PAH and SHC data demonstrate that except for occasional diesel spills and the most recent crude oil spill at the terminal, there is very little detectable petroleum contamination in the sentinel mussel tissues from any of the sampled areas in PWS or the NGOA. Compared to the most recent National Oceanographic and Atmospheric Administration (NOAA) West Coast Mussel Watch data (2004–05) and the more recent 2008–10 Alaska Mussel Watch sites, the 2020 LTEMP mussel-tissue results continue to demonstrate that the sampled region is exceptionally clean. Data from these other studies show 10–1,000 times higher TPAH concentrations than those observed with LTEMP.
Passive Sampling Devices
Beginning in 2016, PSDs were deployed at the three Port Valdez sites to sample concurrently with the LTEMP mussel collections (Figure 2). The goal was to compliment the LTEMP tissue data with integrative, longer-term sampling (30 days) using these high-sensitivity devices. In 2018, two more sites were added, KNH, a clean site originally located near a tanker anchorage, and DII, a site known to contain residual EVOS oil (Figure 1). Results from all deployments showed a low-level, dissolved PAH profile dominated by heavily-weathered, water-washed naphthalene components plus traces of higher-molecular weight PAH. Because KNH is essentially an undisturbed control site and DII is a known residual EVOS oil site, the profiles were expected to be different from the Port samples; however, both profiles and levels were similar to those within Port Valdez. Thus, the PSD appear to be acquiring a ubiquitous background of dissolved PAH (i.e., proximity to the BWTF effluent diffuser had little effect on the dissolved PAH patterns). After the April 2020 intertidal spill at the terminal, the PSD patterns were again identical, but there was a 10-fold higher signal at JAP. For comparison, the June 2018 and June 2020 mussel data, which include both the particulates as well as dissolved hydrocarbon portions, were also primarily just background level PAH. These data suggest the dissolved hydrocarbons detected by PSD devices are well below any known toxicity thresholds for sensitive marine organisms and life stages and unlikely to have significant impact to local biota (Dr. Sarah Allan, pers comm).
“Oxygenated” hydrocarbons (OPAH) are primarily aromatic hydrocarbons that have been altered by adding oxygen, nitrogen or sulfur molecules through photo-oxidation, microbial, or chemical processes. Here, we focus on the microbial process as most relevant to ballast-water treatment. OPAH have been mostly ignored during the last three decades as most are not amenable to analysis with common-usage GC instruments. They are, however, currently considered fundamental to understanding oil-degradation products and thus, fate and transport (Aeppli et al., 2012).
Conceptually, because hydrocarbon oxygenation is the preliminary step in most aerobic microbial biochemical pathways, the effluent from a ballast-water treatment facility designed to promote oil biodegradation would be ideal to explore aspects of OPAH production and fate. In July 2016 and March 2017, three effluent samples from the BWTF discharge pipe, raw, filtered (particulate oil droplets), and dissolved phase, were traditionally analyzed as described herein. They were also screened for oxygenated products by Dr. Christoph Aeppli of Bigelow Laboratory (Boothbay, Maine) using an Iatroscan (TLC-FID) method to separate components into saturated, aromatic, mono-oxygenates, and di-oxygenates. Results from fresh ANS crude oil and the three 2016 BWTF effluent samples (Figure 9) showed the expected depletion (conversion) of the saturates and aromatics found in fresh ANS oil and their subsequent appearance as mono- and di-oxygenated products.
Because of the increased water solubility of oxygenated products, the highest relative concentrations of mono- and di-oxygenated constituents were observed in the filtered, dissolved-phase fraction sample. Over 93% of the measured components in the dissolved phase sample were oxygenates, compared to only 36% in the starting oil. A series of alcohols, carboxylic acids, diols, and dioic acids were detected, although explicit compound identifications have not been completed. These results confirm our expectations and again document the biological treatment tank's efficacy in converting hydrocarbons into water-soluble, biodegradation products.
In summary, from LTEMP mussel sampling across the region, EVOS oil is no longer detected (albeit not entirely removed from certain locations; Lindeberg, et al. 2018). Except for the most recent spill, samples related to terminal and tanker operations within Port Valdez continue to show substantial reductions in hydrocarbon concentrations and profile complexity due to less oil coming through the pipeline, less tanker traffic, cleaner ballast, and a reconfigured ballast-water-treatment process at the Alyeska terminal. The latest LTEMP mussel-tissue results continue to demonstrate that the sampled region (particularly outside the Port) is exceptionally clean. When there is a spill, the mussels quickly assimilate both dissolved- and particulate oil-phase hydrocarbons. Under non-spill conditions, ambient mussel and PSD profiles reflect primarily background-level PAH while Port Valdez sediments still show low hydrocarbon inputs from treated ballast-water effluent. Oxygenated and other heterocyclic hydrocarbons are abundant in the treated ballast discharge and are of continuing interest.