Management of per- and polyfluoroalkyl substances (PFAS)-laden wastewater sludge in Maine: Perspectives on a wicked problem

This article discusses the challenges and potential solutions for managing wastewater sludge that contains per- and polyfluoroalkyl substances (PFAS), using the experience in Maine as a guide toward addressing the issue nationally. Traditional wastewater treatment, designed to remove excess organic waste and nutrients, does not eliminate persistent toxic pollutants like PFAS, instead partitioning the chemicals between discharged effluent and the remaining solids in sludge. PFAS chemistry, the molecular size, the alkyl chain length, fluorine saturation, the charge of the head group, and the composition of the surrounding matrix influence PFAS partitioning between soil and water. Land application of sludge, incineration, and storage in a landfill are the traditional management options. Land application of Class B sludge on agricultural fields in Maine peaked in the 1990s, totaling over 2 × 106 cu yd over a 40-year period and has contaminated certain food crops and animal forage, posing a threat to the food supply and the environment. Additional Class A EQ (Exceptional Quality) composted sludge was also applied to Maine farmland. The State of Maine banned the land application of wastewater sludge in August 2022. Most sludge was sent to the state-owned Juniper Ridge Landfill, which accepted 94 270 tons of dewatered sludge in 2022, a 14% increase over 2019. Between 2019 and 2022, the sum of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) concentrations in sludge sent to the landfill ranged from 1.2 to 104.9 ng/g dw. In 2022, the landfill generated 71.6 × 106 l of leachate. The concentration of sum of six PFAS in the leachate increased sixfold between 2021 and 2022, reaching 2 441 ng/l. The retention of PFAS within solid-waste landfills and the potential for long-term release of PFAS through liners into groundwater require ongoing monitoring. Thermal treatment, incineration, or pyrolysis can theoretically mineralize PFAS at high temperatures, yet the strong C-F bond and reactivity of fluorine require extreme temperatures for complete mineralization. Future alternatives may include interim options such as preconditioning PFAS with nonpolar solvents prior to immobilization in landfills, removing PFAS from leachate, and interrupting the cycle of PFAS moving from landfill, via leachate, to wastewater treatment, and then back to the landfill via sludge. Long-term solutions may involve destructive technologies such as electron beam irradiation, electrochemical advanced oxidation, or hydrothermal liquefaction. The article highlights the need for innovative and sustainable solutions for managing PFAS-contaminated wastewater sludge.

Spatial and temporal trends of mercury in the aquatic food web of the lower Penobscot River, Maine, USA, affected by a chlor-alkali plant

Mercury (Hg) concentrations in aquatic biota, including fish and shellfish, were measured over the period 2006-2012 in the lower Penobscot River and upper estuary (Maine, USA). The Penobscot is a system contaminated with Hg by a chlor-alkali plant that operated from 1967 to 2000, discharging 6-12 tons of mercury into the river. Mercury levels in aquatic biota were highest at sites downstream of the chlor-alkali plant and spatial trends were similar to those of sediments. Mean total Hg concentrations in fish muscle (adjusted for size or age) in the most affected areas were 521 (480, 566; 95% CI) ng/g ww in American eels, 321 (261,395) in mummichog, 121 (104, 140) in rainbow smelt, 155 (142,169) in tomcod, 55.2 (42.7,71.4) in winter flounder, and 328 (259,413) in American lobster tail and 522 (488,557) ng/g dw in blue mussel. Levels exceeded the 50 ng/g ww considered protective for piscivorous predators and were of concern for human health, with American eels and American lobster exceeding Maine's mercury action level of 200 ng/g ww. Calculations of trophic position (using nitrogen isotopes) suggested that the spatial patterns observed in total Hg concentrations were not due to changes in feeding habits of the species. Fish feeding in benthic food webs, as defined by stomach content and stable carbon isotope analyses, showed no change in Hg concentrations over time. In contrast, declining trends in Hg were found in two species dependent on pelagic food webs. The absence of declines in Hg concentrations in the benthically-based food webs, despite the fact that most Hg was discharged into the system >40 years ago, is consistent with the long recovery predicted from dated sediment cores and from similar studies elsewhere.

Distribution and biogeochemical controls on net methylmercury production in Penobscot River marshes and sediment

The distribution of mercury and methylmercury (MeHg) in sediment, mudflats, and marsh soils of the Hg-contaminated tidal Penobscot River was investigated, along with biogeochemical controls on production. Average total Hg in surface samples (0-3 cm) ranged from 100 to 1200 ng/g; average MeHg ranged from 5 to 50 ng/g. MeHg was usually highest at or near the surface except in highly mobile mudflats. Although total Hg concentrations in the Penobscot are elevated, it is the accumulation of MeHg that stands out in comparison to other ecosystems. Surface soils in the large Mendall Marsh, about 17 km downstream from the contamination source, contained particularly high %MeHg (averaging 8%). In Mendall marsh soil porewaters, MeHg often accounted for more than half of total Hg. Salt marshes are areas of particular concern in the Penobscot River, for they are depositional environments for a Hg-contaminated mobile pool of river sediment, hot spots for net MeHg production, and sources of risk to marsh animals. We hypothesized that exceptionally low mercury partitioning between the solid and aqueous phases (with log K_d averaging ~4.5) drives high MeHg in Penobscot marshes. The co-occurrence of iron and sulfide in filtered soil porewaters, sometimes both above 100 μM, suggests the presence of nanoparticulate and/or colloidal metal sulfides. These colloids may be stabilized by high concentrations of aromatic and potentially sulfurized dissolved organic matter (DOM) in marsh soils. Thus, Hg in Penobscot marsh soils appears to be in a highly available for microbial methylation through the formation of DOM-associated HgS complexes. Additionally, low partitioning of MeHg to marsh soils suggests high MeHg bioavailability to animals. Overall, drivers of high MeHg in Penobscot marshes include elevated Hg in soils, low partitioning of Hg to solids, high Hg bioavailability for methylation, rapidly shifting redox conditions in surface marsh soils, and high rates of microbial activity.

Tidal fluxes of mercury and methylmercury for Mendall Marsh, Penobscot River estuary, Maine

Tidal marshes are both important sites of in situ methylmercury production and can be landscape sources of methylmercury to adjacent estuarine systems. As part of a regional investigation of the Hg-contaminated Penobscot River and Bay system, the tidal fluxes of total suspended solids, total mercury and methylmercury into and out of a regionally important mesohaline fluvial marsh complex, Mendall Marsh, were intensively measured over several tidal cycles and at two spatial scales to assess the source-sink function of the marsh with respect to the Penobscot River. Over four tidal cycles on the South Marsh River, the main channel through which water enters and exits Mendall Marsh, the marsh was a consistent sink over typical 12-h tidal cycles for total suspended solids (8.2 to 41 g m^-2), total Hg (9.2 to 47 μg m^-2), total filter-passing Hg (0.4 to 1.1 μg m^-2), and total methylmercury (0.2 to 1.4 μg m^-2). The marsh's source-sink function was variable for filter-passing methylmercury, acting as a net source during a large spring tide that inundated much of the marsh area and that is likely to occur during approximately 17% of tidal cycles. Additional measurements on a small tidal channel draining approximately 1% of the larger marsh area supported findings at the larger scale, but differences in the flux magnitude of filter-passing fractions suggest a highly non-conservative transport of these fractions through the tidal channels. Overall the results of this investigation demonstrate that Mendall Marsh is not a significant source of mercury or methylmercury to the receiving aquatic systems (Penobscot River and Bay). While there is evidence of a small net export of filter-passing (<0.4 μm pore size) methylmercury under some tidal conditions, the mass involved represents <3% of the mass of filter-passing methylmercury carried by the Penobscot River.

Current and historical rates of input of mercury to the Penobscot River, Maine, from a chlor-alkali plant

Mercury inputs by surface and ground water sources to Penobscot River from a defunct Hg-cell chlor-alkali plant were measured in 2009-10 and estimated for the entire period of operation of this facility. Over the measured interval (422 days) approximately 2.3 kg (5.4 g day^-1) of mercury was discharged to the Penobscot River by the two surface streams that drain the site, with most of the combined loading (1.8 kg Hg, 78%) associated with a single storm with rainfall in excess of 100 mm. Groundwater seepage rates from the site, as estimated from both a radon tracer and seepage meter methods were in the range of 3 to 4 cm day^-1 and, when combined with a best estimate of the area of groundwater discharge (11,000 m²) and average seepage/porewater mercury concentration (242 ng L^-1, UCL95), yielded a loading of 0.11 g day^-1 for site groundwater. None of the municipal or other industrial point sources of mercury to the river between Veazie and Bucksport, Maine exceeded 1 g day^-1 individually, nor was the aggregate loading of all such sources >3 g day^-1 (based on State of Maine data). Mercury loadings for the three largest tributaries downstream of Veazie Dam were estimated to contribute 4.2, 3.7 and 2.5 g day^-1, respectively, to the Penobscot River. Based on sampling (total Hg ~ 2 to 4 ng L^-1) and historical mean discharge data (340-460 m³ s^-1), the Penobscot River upstream of the plant site contributes as much as 160 g day^-1 to the downstream reach depending on river discharge. Estimates of historical (1967-2012) mercury loading using both generic emission factors and measured releases ranged from 2.6 to 27 MT while the mass of mercury found in downstream sediments amounted to 9 MT.

Elevated mercury in blood and feathers of breeding marsh birds along the contaminated lower Penobscot River, Maine, USA

Mercury (Hg) concentrations in the blood and feathers of five species of migratory marsh birds, Nelson's sparrow (Ammodramus nelson subvirgatus), song sparrow (Melospiiza melodia), swamp sparrow (Melospiza geogiana), red-winged blackbird (Agelaius phoeniceus), and Virginia rail (Rallus limicola), breeding in marshes along the lower Penobscot River, Maine, far exceeded reference concentrations, exceeded concentrations associated with reproductive health, and are the highest Hg concentrations reported to date for several species. Blood Hg concentrations in adult Nelson's sparrows were greatest in 2007, at 6.6μg/gww (geometric mean) and in 2012, at 6.5μg/gww and greatest in red-winged blackbirds in 2012, 8.0μg/gww. Mercury in blood increased with residence time on the contaminated marshes at an estimated rate of 0.04 to 0.07μg/gww per day. Feather mercury concentrations in specific primary, secondary and tail feathers (P1, S2, R6) were strongly associated with exposure location at the time of feather formation. Geometric mean Hg concentrations in primary feathers (P1) reached 39.6μg/gfw in 2010 in Nelson's sparrows. The paper documents the dynamic nature of Hg concentrations in avian blood and feathers, an important consideration in contaminant study design, and the increased risk to marsh birds posed by Hg deposition from upstream sources.

Mercury in wintering American black ducks (Anas rubripes) downstream from a point-source on the lower Penobscot River, Maine, USA

Waterfowl wintering along the lower Penobscot River, Maine continue to be exposed to elevated Hg concentrations from the HoltraChem chlor-alkali plant that operated along the river between 1967 and 2000. In American black ducks (Anas rubripes) total Hg in duck breast muscle increased with residence time on contaminated marshes, reaching means of 0.82±0.21μg/g ww (wet weight) by the end of the fall hunting season, and prompting Maine to issue a human consumption advisory on duck breast muscle. Methyl Hg comprised over 99% of the total Hg in breast muscle. The ratio of Hg concentrations in blood and muscle were strongly correlated and approached 1:1 after extended residence times. Primary feather (P1) total Hg concentrations averaged 2.2±1.3μg/g fw (fresh weight), verifying low Hg exposure during feather growth on distant breeding grounds the preceding summer. Mercury concentrations in black ducks, following winter residence along the lower Penobscot exceeded levels associated with reproductive toxicity. Carry-over of Hg to summer breeding grounds may limit the subsequent reproductive success of black ducks.