F. Ward Whicker

Contaminant Transport through Agroecosystems: Assessing Relative Importance of Environmental, Physiological, and Management Factors

Agroecosystems can become contaminated by atmospherically released radionuclides. The subsequent concentrations of radionuclides in foods are dependent on numerous environmental, physiological, and management factors. We compared four approaches for estimating the relative importance of several of these factors in determining concentrations of 131 I and 137 Cs in milk. A series of sensitivity analyses with Monte Carlo and full-factorial sampling designs was conducted on the PATHWAY model, which simulates radionuclide transport through an agroecosystem. Sensitivity of time-integrated concentrations in milk was estimated as a function of the time of year that fallout was deposited and as a function of time following a spring deposition. The dominant parameters affecting time-integrated concentrations of 131 I in milk were the initial fraction of radionuclides deposited on vegetation, timing and amount of pasture consumption, and the production rate of milk. For time-integrated concentrations of the longer-lived 137 Cs in milk, resuspension was a dominant parameter and pasture use was less important. The sampling designs were compared by ranking the parameters to which the model output is sensitive. The three sampling designs based on parameter variances produced sets of ranks that were similar to each other but differed from the ranking produced by the sampling design based on parameter magnitude. The results indicate which data are most crucial for real-time calculations following an accident and how subsequent dose from ingestion can be most effectively reduced, provide insight into model behavior, and help prioritize future research. This paper demonstrates the importance of variance-based sensitivity analysis.

Effects of ionizing radiation on terrestrial plant communities

Publisher Summary This chapter describes the effects of ionizing radiation on terrestrial plants communities. Factors frequently characteristic of radioresistant plants include polycentric or diffuse centromeres, multinucleate cells, high chromosome number, polyploidy, and asexual reproduction. Under conditions of chronic irradiation, plant radiosensitivity can be increased if cells are dormant or slowly dividing, if meiotic stages are present, and if meiosis and premeiosis are slow. Under conditions of acute radiation, however, plant radiosensitivity is normally increased with mitotic activity. The general effects of ionizing radiation on plants can be classified as—(1) genetic or, (2) somatic or physiological. Both classes of effects are considered to be caused largely by chromosome damage and gene mutations.

Sorption of Cs and Sr to profundal sediments of a Savannah River Site reservoir

Laboratory measurements of the sorption and desorption of 134Cs and 85Sr to sediments were conducted. These sediments were sampled from the top 1 cm of the profundal zone of Par Pond at the Savannah River Site, Aiken, South Carolina. The isotopes 134Cs and 85Sr were used to trace the sorption properties of the main contaminants found in the reservoir, which are 137Cs and 90Sr, respectively. The sorption behavior of these two elements was studied using spiked sediment/water slurries of a variable, but known, mass to volume ratio. The results reveal that Sr undergoes significant reversible sorption while a fraction of Cs irreversibly sorbs to the sediment. The Kd for 137Cs was a function of the slurry ratio, pH, conductivity, and contact time. These factors were inter-related since the sediments released ions to the slurry mixture which simultaneously decreased the pH and increased the conductivity. The calculated Kd at equilibrium was 2.8 × 104 for 134Cs after 60 days and 1 × 103 for 85Sr after 7 days at a pH ~ 6 and a slurry ratio of 1:1000 g/ml.

Food chains and biogeochemical pathways: contributions of fallout and other radiotracers.

This paper reviews examples of how measurements of global fallout in the environment and related tracer radionuclides have been used to enhance our basic knowledge of biogeochemical processes and food-chain pathways. Because it is these fundamental, natural processes that control the transport and accumulation of such trace substances in the environment, direct measurements of trace substances over time and space reveal strong insights into these processes. The necessity to monitor global fallout transport, although largely motivated by human health concerns, gave rise to a plethora of new information about plants, animals, and natural and agricultural ecosystems and how they function. This review provides a small selection of examples in the areas of plant and animal physiology, productivity and energy transfer in food chains, biogeochemical cycles of certain elements and their analogues, feeding relationships and movements of organisms, and the agriculture-based human food chain. It is concluded that if society is to cope successfully with continued growth of the human population and resource consumption, more knowledge is still required about these fundamental processes. The use of radiotracers can contribute greatly to this need, but current funding priorities, societal attitudes, and onerous regulations on the use of radioactivity may continue to limit such applications.

Variations of 137Cs depositions and soil concentrations between alpine and montane soils in northern Colorado.

Abstract 137 Cs was measured in soil samples taken from two high-elevation sites in the northern Front Range of Colorado. The first site was in an alpine meadow adjacent to a glacial lake, while the second site was in a nearby montane forest at a slightly lower elevation. The samples were all of equal volume and represented an equal ground surface area. The coefficients of variation of 137 Cs soil activity concentration (Bq g −1 ) were similar for the two sites, as were the CVs for activity deposition (Bq m −2 ). The mean 137 Cs depositions we found were generally in agreement with values reported in the literature for similar elevations and higher than values found at lower elevation sites in the same region. Activity deposition was less variable than activity concentration at both sites. The alpine site had a significantly higher mean 137 Cs concentration than the montane site, but a significantly lower mean deposition value.

137Cs in sediments of Utah Lakes and reservoirs: Effects of elevation, sedimentation rate and fallout history

Abstract Cesium-137 in lentic sediments was measured in 15 impoundments in Utah, two in Oregon, and one in Colorado, Cesium-137 sediment profiles were used to investigate areas that may have received additional fallout from aboveground nuclear weapons testing at the Nevada Test Site (NTS). Factors that affected sedimentation rates and patterns in the impoundments were explored. Estimated cumulative deposition and mean sedimentation rates were correlated, and both were inversely related to impoundment elevation. There was evidence for enhanced fallout 137 Cs deposition during the period of most intense aboveground testing from 1951 to 1957 at the Nevada Test Site (NTS) at two locations in southwestern Utah. However, our technique for estimating relative NTS fallout generally yielded greater within-lake variability than between-lake variability. Furthermore, most of the observed 137 Cs deposition in sediments deposited in the 1951–1957 time period could be attributed to global rather than NTS fallout. Thus, in general, differences between impoundments in the relative deposition from NTS fallout could not be statistically demonstrated.

Validation of pathway, a simulation model of the transport of radionuclides through agroecosystems

ABSTRACT Kirchner, T.B. and Whicker, F.W., 1984. Validation of PATHWAY, a simulation model of the transport of radionuclides through agroecosystems. Ecol. Modelling, 22: 21–44. PATHWAY is a dynamic simulation model of the transport of radionuclides from fallout through the agricultural foodchain. The model was developed to fulfill the need for a realistic, testable, completely time dependent model to estimate radionuclide ingestion by humans exposed to fallout patterns originating at the Nevada Test Site during the 1950s. The model simulates the fate of radionuclides in various management units (pastures, gardens, croplands, etc.) of an agro-ecosystem. The agricultural products currently considered in the model are pasture grass, alfalfa, grains, garden vegetables, milk, eggs, beef, and poultry. Harvested and stored products, such as hay and canned vegetables, are also represented in the model. Animal diets are specified on a daily basis. Thus, the model can be used to simulate the fate of radionuclides from both acute and chronic fallout deposition. The utility and credibility of radionuclide transport simulation models are severely limited without rigorous testing of their predictive capabilities against real observations. This paper reviews exercises testing the overall accuracy and temporal dynamics simulated by the PATHWAY model against 37 sets of observed data. These data include concentrations of 137 Cs, 90 Sr, 140 Ba, and 131 I in milk, beef, alfalfa, and pasture grass from several western states following deposition of the radionuclides in fallout. In addition, the model is tested against observations of concentrations of 131 I in milk following the accident at the Windscale reactor in England. The statistical tests used to compare the predictions of PATHWAY to the observations include a correlation analysis, a paired t-test, and a binomial test. We use the correlation coefficient between observations and predictions through time to compare the dynamics of the simulated and real world system. Plots of the residuals from regression are then examined for bias between the predictions and observations. The significance of any trends in the residuals is evaluated using a runs test. The paired t-test and the binomial test are used to evaluate the accuracy of PATHWAYs predictions. The hypothesis for the paired t-test is that the ratio of predictions to observations is 1. The paired t-test can be used to test hypotheses about ratios because the distributions of observations and predictions appear to be lognormal. However, the paired t-test does not consider uncertainty in the predictions of the model. We use a binomial test to compare the observed data to an interval estimate from PATHWAY. The interval corresponds to a 95% confidence interval on the prediction, and is derived from uncertainty analyses that have been conducted on PATHWAY. PATHWAYs predictions are significantly correlated with observed levels of 137 Cs and 90 Sr in pasture and alfalfa. PATHWAY also simulates the dynamics of 131 I, 140 Ba, and 137 Cs in milk well, but fails to predict what appears to be a long term accumulation of 90 Sr in the agro-ecosystem. PATHWAY predicts the absolute concentrations of 131 I in milk quite well, but tends to predict levels of 140 Ba, 90 Sr, 137 Cs in milk that are different from those observed by factors of 2 to 7. PATHWAY predicts levels of 137 Cs and 90 Sr in pasture and beef within a factor of 2 of those observed.

Temporal trends of 137Cs in an abandoned reactor cooling reservoir

Abstract A comprehensive resampling study was initiated for the purpose of gaining insight into 137 Cs mobility and retention in Pond B, an abandoned reactor cooling reservoir at the United States Department of Energys Savannah River Site in Aiken, South Carolina. Measurements made during this study were compared to those made ten years earlier. Cesium-137 inventories were estimated in water, sediment, and biotic components. The total measured 137 Cs inventory in Pond B decreased from 4.6 × 10 11 Bq in 1984 to 2.3 × 10 11 Bq in 1994. This decline was largely driven by the decrease measured in the sediment inventory, which approximates the total inventory. The results suggest a 10-year effective half-time for 137 Cs in Pond B, which is significantly more rapid than the 28-year estimate made by Whicker et al. (1990). However, it is likely that the water turnover rate between 1984 and 1994 may have been higher than the rate of 0.3 year −1 used for this estimate. Concentration ratios for the sediment and biota were very similar for both studies. A general trend of 137 Cs penetration into the sediment profile and of sediment transport to deeper water was observed.

Estimation of the Metabolic Rate of the Desert Iguana (Dipsosaurus dorsalis) by a Radionuclide Technique

We describe a method for measuring metabolic rates (MRs) of ectotherms using successive measurements of radionuclide body burdens, which may provide an alternative means of measuring field metabolic rate (FMR). Although MRs can be measured in the laboratory, variability in food supply and quality, temperature, activity, and other factors preclude the direct application of such data to field conditions. Recently, the doubly labeled water (DLW) technique has been successfully applied to estimate FMR in a variety of animals, but this method is expensive, requires special equipment, necessitates the sampling of blood or other body fluids, and may be unsuitable for certain species. We compared the rates of elimination of seven radionuclides with estimates of MRs (CO₂ production) from DLW measurements in the desert iguana, Dipsosaurus dorsalis. We observed temperature-dependent elimination for five of the radionuclides, and found a high correlation (r² = 0.93) between the slopes of individual 86Rb elimination curves and the MRs estimated by the DL W technique. This correlation appeared to be relatively insensitive to differences in water turnover and ingestion of the stable nutrient analog (K). The results suggest that a practical and inexpensive method for measuring FMRs of ectotherms may be derived from this relationship.

137Cs contamination of plants used for traditional medicine and implications for human exposure

Abstract This paper discusses the use of medicinal plants in traditional medicine practices in the Marshall Islands and measurement data of 137 Cs in plants used in medicinal remedies. This previously unexplored contribution to radiation exposure was recognized as one of several potentially important considerations in determining present-day risks to the Marshallese population from residual weapons’ test radioactivity in the environment. The leaves or fruit of five species of plants were sampled at 14 of the 29 atolls of the Marshall Islands. 137 Cs concentrations were measured in 295 samples. The concentration of 137 Cs in each species generally decreased with increasing distance and decreasing latitude from the atomic weapons testing area in the northern area of the country. Of the species sampled, Polypodium scolopendria exhibited the highest mean 137 Cs concentration by several fold. Other species could not be differentiated in terms of 137 Cs uptake. The results of calculations to assess possible radiation doses under specific intake scenarios indicate that at most locations in the Marshall Islands, ingestion of these plants for medicinal purposes will not contribute significantly to the total dose received (above background), although the frequency and amount of consumption of these plants is not well known. This study provides the first information of its kind for the assessment of radiation dose to Marshall Islands’ residents from locally grown plants used in traditional medicine.