Ingvar L. Larsen

Direct analysis of 210Pb in sediment samples: Self-absorption corrections

Abstract A procedure for the direct γ-ray instrumental analysis of 210Pb in sediment samples is presented. The problem of dependence of self-absorption on sample composition is solved by making a direct transmission measurement on each sample. The procedure has been verified by intercalibrations and other tests.

Pollutant—particle associations and dynamics in coastal marine environments: a review☆

Abstract A wide variety of substances, including known or potential pollutants, become associated with particles in coastal marine environments. This association may result from: (1) ion exchange, precipitation, or hydrophobic interactions with the particle surface, (2) co-precipitation with iron and manganese hydrous oxide coatings, (3) complexation with organic substances bound or aggregated with particles, (4) incorporation into mineral lattices, organisms or fecal material, or (5) flocculation of colloidal organic and inorganic matter during river and sea water mixing. The transport, accumulation and fate of such chemically-reactive pollutants in coastal marine environments are governed to a great extent by particle dynamics. Understanding of the mechanisms and processes affecting pollutant-particle associations and dynamics allows the prediction of the fate of a wide variety of pollutants in estuarine, coastal and marine environments.

The effect of sediment mixing on Pb-210 accumulation rates for the Washington continental shelf

Abstract Nine cores from the Washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from Pb-210 profiles. Th-234 profiles indicate mixing coefficients of 140 cm 2 yr −1 for the seabed offshore from the Columbia River and 47 cm 2 yr −1 for the seabed of the Mid-Shelf Silt Deposit (75 km north of the Columbia River). These large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer (∼ 10 cm) within one year after emplacement at the seabed surface. Observed depths of Cs-137 penetration within the seabed are compared with depths predicted from the surface mixed-layer thickness and the Pb-210 accumulation rate. The observed and predicted depths agree well at all but one station. At this station a combination of Th-234, Co-60, Pb-210 and Cs-137 profiles suggests that active mixing occurs below the intensely mixed surface layer. The agreement of Pb-210 and Cs-137 data at the other stations, however, indicates a general absence of deep mixing on the Washington shelf. Thus, the apparent accumulation rates calculated from Pb-210 profiles (below the intensely mixed surface layer) on the Washington shelf generally reflect the true rates of sediment accumulation.

Stable Isotopes of Oxygen and Natural and Fallout Radionuclides Used for Tracing Runoff During Snowmelt in an Arctic Watershed

Stable isotopes of oxygen, and natural and fallout radionuclides, have been used, respectively, to identify water sources and to quantify watershed fluxes of precipitation-borne solutes and particles that come in contact with tundra vegetation at Imnavait Creek, Alaska (68° 37′N, 149° 17′W). Oxygen 18/oxygen 16 ratios of water in the snowpack, stream, and unfrozen soil during the peak of snowmelt showed that less than ∼14% of streamwater was derived from sources other than snow. Within a month of snowmelt, 18O/16O ratios in the stream indicated that only negligible amounts of water derived from snow remained in the watershed, in contrast to the low degree of mixing between snow and underlying soil water, greater than 90% of the atmospherically derived 7Be (53-day half-life) that was deposited in the snowpack was adsorbed onto the surface 2–3 cm of frozen organic soils and vegetation. A slightly lower degree of absorption (∼2/3) on tundra was observed for atmospherically derived 35S (87-day half-life) released during snowmelt. These data indicate that atmospherically derived substances in the snowpack are often retained strongly by sorption or chemical interaction with vegetation and soil during snowmelt. This conclusion is also supported by the vegetation and soil inventories of 137Cs (30.2-year half-life), which are similar to values expected as a result of atmospheric deposition over the past 40 years.

Air-to-vegetation transfer rates of natural submicron aerosols

Abstract Submicron aerosol deposition to outdoor vegetation was evaluated by measuring vegetation and air concentrations of 212Pb, 214Pb, and 7Be attached to atmospheric aerosols. A biomass-normalized deposition velocity (VD), with units of m3 kg−1 s−1, was used to compare species and isotopes with respect to air-to-vegetation transfer rates. For 212Pb ( t 1 2 = 10·64 h ), higher night-time air concebtrations dominate deposition, while for 214Pb ( t 1 2 = 26·8 min ), deposition measurements over shorter time periods relative to changing atmospheric conditions were possible. Calculated VD values were usually higher for 214Pb, possibly reflecting wind-enhanced deposition during the afternoon period of sampling. Evergreen species, including pines, were not appreciably different from deciduous species. Most striking was the narrow range of results (factor of 6 for 34 212Pb measurements of 20 species). Beryllium-7 ( t 1 2 = 53·3 d ) was used to evaluate time-integrated deposition by sampling vegetation in California near the end of dry summers. Similar deposition rates were found for this isotope. The results of the study indicated that naturally radioactive atmospheric aerosols can be used to understand the dynamics of submicron aerosol deposition to ecosystems.

The Concept of an Equilibrium Surface Applied to Particle Sources and Contaminant Distributions in Estuarine Sediments

Studies have shown that many chemically-reactive contaminants become associated with fine particles in coastal waters and that the rate, patterns, and extent of contaminant accumulation within estuarine systems are extremely variable. In this paper, we briefly review our findings concerning the accumulation patterns of contaminants in several estuarine systems along the eastern coastline of the United States, and have applied a well-established concept in geology, that is “an equilibrium profile,” to explain the observed large variations in these patterns. We show that fine-particle deposition is the most important factor affecting contaminant accumulation in estuarine areas, and that accumulation patterns are governed by physical processes acting to establish or maintain a sediment surface in dynamic equilibrium with respect to sea level, river discharge, tidal currents, and wave activity. Net long-term particle and particle-associated contaminant accumulations are negliglible in areas where the sediment surface has attained “dynamic equilibrium” with the hydraulic regime. Contaminant, accumulation in these areas primarily occurs by the exchange of contaminant-poor sedimentary particles with contaminant-rich suspended particles during physical or biological mixing of the surface sediment. Virtually the entire estuarine particulate and contaminant load bypasses these “equilibrium” areas to accumulate at extremely rapid in relatively small areas that are temporally out of equilibriums as a result of natural processes or human activities. These relatively small areas serve as major sinks for particles from riverine and marine sources, and for biogenic carbon formed in situ within estuaries or on the inner shelf.

Geochemical and environmental processes affecting radionuclide migration from a formerly used seepage trench

The disposal of liquid waste, containing about 0.3 million curies (107 GBq) of fission nuclides, activation products, actinides and transuranics, in a shallow land seepage trench from 1962 to 1966, provided a field opportunity for investigating the chemical, geological, and hydrological processes which affect contaminant migration in soils and weathered bedrock. Gamma-log profiles of wells near the trench indicate that the waste liquids seeped along discrete layers parallel to bedding and along the strikes of faults and folds. Most of the radioactivity measured in the groundwaters consisted of 3H, 99Tc, 60Co, and 233U. The mobility of 99Tc, 60Co, and 233U has been attributed to low molecular weight anionic complexing. Concentrations of 90Sr and 137Cs in the groundwaters were extremely low because of the chemical treatments and precautions taken to establish and maintain an alkaline environment near the trench, which allows for 90Sr sorption and precipitation, and because of the strong tendency for 137Cs to be selectively sorbed by illite. Plutonium isotopic ratios indicate that much of the plutonium contamination near the trench results from the migration of 242Cm and 244Cm and their subsequent decay to 238Pu and 240Pu. Radionuclide concentrations in the groundwaters near the north end of the trench undergo seasonal variations, with the highest activities occurring in the spring and after prolonged rainfall. This suggests that contamination may be leached from the trench or from the relict waste migration layers when the groundwater level rises to saturate these zones or when precipitation infiltrates into the trench or along these relict migration layers during drainage. Suspected transport pathways from the trench to a nearby seep area appear to be associated with fault zones and limbs of a plunging limestone fold.

Evidence for re-distribution of137Cs in Alaskan tundra, lake, and marine sediments

Tundra sampling conducted during 1989–1990 at Imnavait Creek, Alaska (68°37′N, 149°17′W) indicated that inventories of137Cs were close to expectations, based upon measured atmospheric deposition for this latitude band. Typically observed at a depth of 4–10 cm, peak accumulations of137Cs were associated with organic materials, above any mineral soil. Accumulated inventories of137Cs in tundra decreased by up to 50% along a transect to Prudhoe Bay (70°13′N, 148°30′W). Atmospheric deposition of137Cs decreased with latitude in the Arctic, but declines in deposition would have been relatively small over this distance (200 km). This finding suggests a recent loss of137Cs from tundra over the northern parts of the transect between Imnavait Creek and Prudhoe Bay. This hypothesis is supported by observations of maximum137Cs accumulations occurring in surface layers of the more northern tundra rather than at depth (as at Imnavait Creek), as well as by indications of higher137Cs accumulations in a lake inlet. On the other hand, marine sediments collected in the Bering Sea and Chukchi Seas, including the Yukon River delta region, show apparently lower inventories compared to tundra or lake sediments, although complete inventories are unavailable due to radiocesium buried to depths (> 20 cm) beyond the sampling capabilities used.

Inventories and Distribution of Radiocaesium in Arctic Marine Sediments: Influence of Biological and Physical Processes

Abstract Extensive surveys of sediment burdens of radiocaesium, specifically 137Cs, and other radioactive contaminants in the Arctic during the 1990′s, indicate that almost all anthropogenic radionuclides buried on continental shelves adjacent to Alaska are derived from global bomb fallout. the 137Cs (half-life: 30.2y) activities observed in surface (0–4 cm) marine sediments however, vary widely, albeit much less than the expected current inventory resulting from bomb fallout at this latitude (∼100mBq cm−2). This observed geographical variation provided the opportunity to evaluate physical and biological mechanisms that may affect caesium biogeochemistry on Arctic continental shelves. We investigated whether high biological productivity in portions of the Bering and Chukchi Seas is effective in removing dissolved radiocaesium from the water column, and whether biological production in overlying water affects total radiocaesium inventories in sediments. Based upon C/N ratios in the organic fraction of shal...

Inventories and sorption-desorption trends of radiocesium and radiocobalt in James River estuary sediments

Anthropogenic radionuclides (137Cs,134Cs,60Co) have been introduced to the James River estuary as a result of low-level releases from the Surry Reactor site since 1973 and worldwide atmospheric fallout from nuclear weapons tests since the early 1950s The total radionuclide burden in the estuary sediments has been estimated by integrating radionuclide activities in 29 box cores and extrapolating these integrated values over surface areas subdivided on the basis of sediment type, rates of accumulation, and proximity to the reactor release site. Our results indicate that 30% of the60Co, but only 15% of the134Cs released from the reactor site, has been retained in the estuary sediments, and about 40% of the134Cs and60Co sediment inventory is in areas that represent less than 5% of the total estuarine surface area. Depletion of the134Cs in downstream sediments forms a noticeable trend in the James River estuary, and we postulate that seawater cation competition and exchange is primarily responsible.