This court mandated collaborative project between USM, TAMUG and the Flett Laboratory has evaluated the potential of natural attenuation of the mercury contaminated Penobscot River and Estuarine System in Maine through determination of sedimentation and sediment mixing rates.
This court mandated collaborative project between USM, TAMUG and the Flett Laboratory evaluated the potential of natural attenuation of the mercury contaminated Penobscot River and Estuarine System in Maine through determination of sedimentation and sediment mixing rates. The gradual reduction of surface sediment mercury concentrations by deposition of cleaner particulates will lead to a gradual reduction in the rate of methyl mercury (MeHg) production in the ecosystem, which would be followed by lower MeHg concentrations in biota. This natural attenuation option would obviously also be very cost effective because it would require no active remediation. The age of sediments was determined by using several radioisotopes that have been deposited from past atmospheric nuclear testing events (cesium-137, plutonium-239, 240) as well as ongoing deposition of naturally occurring radioisotopes (beryllium-7, lead-210). The concentration profiles of these isotopes versus depth was used to determine the rate of long term sediment accumulation.
Sediment core dating using the bomb test fallout radionuclides 137Cs and 239,240Pu as 1963 event markers and the steady-state atmospherically delivered tracer radionuclide 210Pbxs, (210Pbxs = Excess-210Pb = Total 210Pb – 226Ra-supported 210Pb) revealed time-averaged sedimentation rates for each site. Mercury (Hg) profiles in the same sediment cores showed maxima at depths that can be attributed to a 1967 release date, and decreased to lower surface sediment Hg concentrations. Hg(o) values at different sites were quite similar, even though individual Hg profiles are, at times, quite heterogeneous. For example, while highest Hg(o) values were found in Penobscot River (PBR) cores, mean (average) Hg(o) values decrease from OR>PBR~MM>ES. However, PBR, OR and MM cores are statistically not different (1 SD of mean value is given below), averaging about 600-700 ng/g, while ES cores are lower. PBR: 742±88 ng/g; MM: 639±75 ng/g; OR: 892±156 ng/g; ES: 513±53 ng/g. Only six out of 24 PBR cores have Hg(o) values ≥1000 ng/g, and only 1 out of 11 MM cores have Hg(o) values ≥1000 ng/g. Furthermore, only 1 out of 5 OR cores have Hg(o) values ≥1000 ng/g, and none of the 17 ES cores have Hg(o) values ≥1000 ng/g.
One could ask the question in what form Hg was deposited to the sediments. In agreement with the recent literature, one can assume that the carrier phases for Hg are sulfur (S) containing compounds (e.g., iron sulfides and thiols). However, these compounds are present at much higher concentrations than Hg, and thus, relationships between Hg and sulfur (S) cannot necessarily be expected. Indeed, no relationships have been found between Hg concentrations and concentrations of total organic carbon (TOC) or S in sediments, except for Hg values at the peak depth and immediately above and below. This strongly suggests that initial dispersal of Hg and deposition was aided by S in TOC, most likely from S and TOC compounds in co-occurring paper mill effluents. However, the dispersal and deposition of Hg in recent times is controlled by lateral processes, which redistributes Hg from sites with higher to sites with lower Hg concentrations in surface sediments, which also explains the slowing down of the decrease in Hg concentration in most surface sediments.
Sedimentation rates determined from radionuclide profiles agreed, within the errors of the measurements, with the assumption of a major Hg input into the Penobscot River in 1967. Thus, Hg peaks were also useful for sediment dating and sedimentation rate calculations. Apparent sedimentation rates calculated from event tracers Hg, 137Cs and 239,240Pu, and steady state tracer 210Pbxs, agree well. Since vertical sediment mixing is restricted to the upper 3 cm or so, historic input rates of radionuclides and Hg to a particular site are only minimally distorted by vertical mixing. The Hg profiles are more influenced by lateral processes, as evidenced by uneven tracer profiles that are likely reflecting lateral inputs of sediments and associated tracers.
Therefore, we are confident that radioactive dating of sediments established that one can take Hg profiles and interpret them as a temporal input record into sediments from a particular site. Average sedimentation rates from all 57 sediment cores, using either 137Cs (or 239,240Pu) or Hg, are 0.6±0.06 cm/yr (1 SD of mean). Calculated average sedimentation rate (SR, cm/y) for each core assuming constant SR over the length of the sediment core, are then used to reconstruct the input history of Hg to that site.
Hg tracer profiles reveal sharp peaks around this maximum Hg deposition event assumed to have occurred in 1967, strongly suggesting that initially, the apparent recovery after this Hg pulse input was quite fast. However, afterwards, in most cores, Hg decreased relatively more slowly with time (in cores where Hg concentrations were elevated), stayed constant, or even increased towards the present time (when Hg concentrations were generally lower), which strongly suggests input from redistribution of the Hg from sites with higher surface concentrations, Hg(o), to sites with lower Hg(o) concentrations. Therefore, the Hg profiles were divided into two sections: the first 21 years (1967-1988) with relatively fast recovery, and the recent 21 years (1988-2009), with a relatively slower recovery rate. The recent 21 years of input history was then given close attention to reveal ‘apparent’ Hg recovery rates. ‘Apparent’ is used here to indicate that evaluated recovery rates depend on assumptions and degree of extrapolation. Calculated apparent recovery half times (T1/2 = ln2/α) were calculated from an exponential (Hg(t) = Hg(t=21)*exp(-α*t)) fit to the Hg concentration profiles over the past 21 years. Hg concentrations from the past 21 years (1988-2009) were then evaluated to reveal apparent recovery rates and apparent half times (T1/2). These calculations were first carried out assuming a recovery to near 0 ng/g Hg in surface sediments. When T1/2 are calculated assuming a non-zero asymptotic level (Hg(∞)) of 400 ng/g Hg, apparent recovery half times would appear to be faster, but the time scale to reach a certain target concentration of Hg(∞) would be more similar to the Hg(∞) approach, depending on what is assumed for Hg(∞).
Suitable Hg profiles would need to indicate that these coring sites have been in close communication with the system, and thus, can be taken as an indicator what the system as a whole has experienced in the last 21 years. For the recent 21 year time period, there is a large spread of T1/2 values at PBR, but not as much at MM sites, in the cores that were deemed to be representative of the recovery of the Penobscot River ‘system’. Mean values of T1/2 at PBR were 31±6 years for 16 of our 24 cores, and 22±3 years in 9 out of 11 cores at MM (where semi-exponential decreases were observed). Eight out of 24 PBR cores, and 2 out of 11 MM cores (with generally lower Hg concentrations) do not show any Hg decrease in the past 21 years (or even show Hg increases towards the surface), thus indicating that they were in much slower ‘communication’ with the rest of the system. In 12 out of 18 ES cores, T1/2 values range from 20 to 120 years (mean of 78±13 years). In 5 out of ES 18 cores (with lower Hg concentrations), Hg values actually increase towards the surface or stay constant. In 4 out of 5 OR cores, T1/2 values average 77±21 years. In the remainder (1) of the OR cores, Hg values either stay constant, or increase towards the surface. One can conclude that such sites with lower and/or Hg concentrations that increase towards the surface are sites, which are not in close communication with the rest of the system, and thus, are also not representative of that system as a whole. The Penobscot system, as a whole, however, clearly has recovered a great deal since the late 1960’s. It appears then that sedimentary Hg(o) values in cores that are deemed to be representative of the system as a whole are currently converging to values close to 600-700 ng/g.
Of course, calculated apparent half times of several decades do not mean that after that time, sediments have fully recovered. Estimated recovery times depend on what acceptable Hg(o) concentration one assumes. Regardless, one would have to assume several of these half times to estimate system recovery. For example, one can estimate that to get from Hg(o) of about 700 ng/g to a value of Hg(∞) of about 50 ng/g at Mendall Marsh (MM), it would take about 4 half lifes of 21 y, or 84 years.