Prediction and interpretation of rates of hypolimnetic oxygen depletion

The areal hypolimnetic oxygen deficit model (AHOD) developed by Strom (1931) and Hutchinson (1938) was tested by examining the predictions which the model makes and by testing the assumptions which were made during the formulation of that model. The model was found to be incorrect. Rates of hypolimnetic oxygen depletion are strongly influenced by the morphometry and temperature of the hypolimnion. Lake morphometry influences at least two processes which affect oxygen concentrations. Significantly more oxygen is turbulently transferred into the hypolimnia of shallow lakes than deep lakes. However the maximum rates of vertical transport are always less than 15% of the observed rate of oxygen depletion. The morphometry of the hypolimnion exerts a much stronger influence upon the amount of respiration which occurs within the hypolimnetic water column. The amount of respiration measured in the water column increases as the thickness of the hypolimnetic water column increases. In oligotrophic lakes with shallow hypolimnia, less than 20% of the total amount of oxygen consumed in the hypolimnion is respired in the water column. In deep lakes more than 60% of the total amount of oxygen consumed is respired within the water column. Measured rates of water column respiration are strongly correlated with the temperature and amount of particulate organic matter present in the water column.The rate of oxygen consumption within the hypolimnion is constant throughout the period of thermal stratification. Respiration does not depend upon the ambient concentration of oxygen present within the hypolimnion. Changes in oxygen concentration within any stratum of the hypolimnion of a lake can be predicted from a knowledge of the retention of phosphorus by the lake's sediments (Rp), the average temperature of the stratum during the period of thermal stratification (T), and the ratio of the volume of the stratum to the area of lake sediments horizontally contiguous to the stratum (V:SA). A simple statistical model developed from published estimates of rates of oxygen consumption (VOD mg/m('3)/day) predicts thatVOD = -6.6 + .0081T*Rp + 11.07T - 2.32T*Ln(V:SA).The predictions of this equation agree very well with rates of oxygen depletion measured in 12 lakes which possess a diversity of physical and chemical characteristics. During the period of stratification, hypolimnetic oxygen concentrations can be estimated from the predicted rate of oxgyen depletion and an estimate of the initial oxygen concentration within the hypolimnion at the onset of stratification

A Rapid and Efficient Autoclave Pre-Treatment to Extract Iodine-129 from Urine Samples for AMS Analysis

A new method was developed to extract 129I from urine samples and then measure it by AMS. The samples were pre-treated in an autoclave with hydrogen peroxide to remove unwanted compounds from the urine samples and were acidified with nitric acid, followed by precipitation of iodine as silver iodide (AgI) for measurement by AMS. This new procedure is substantially faster than previous methods for the extraction of iodine from urine and results in less chemical waste. The efficiency and reproducibility of this method were evaluated by using 125I as a yield tracer, eventually giving a recovery above 99%. To achieve this, several iterations of the method were required. The method was then successfully applied to measure 129I/127I isotopic ratios and 129I concentrations in 25 human urine samples. The AMS results for 129I in urine ranged 3.3 x 106 atoms/L to 884 x 106 atoms/L and the isotope ratio (129I/127I) in human urine ranged from 7.38 x 10-12 to 3.97 x 10-10 with a median of 1.29 x 10-10. This new method will be useful for investigations into the sources of iodine in the human diet and their relative importance for iodine sufficiency. </p

Potential increased risk of ischemic heart disease mortality with significant dose fractionation in the Canadian Fluoroscopy Cohort Study.

Risks of noncancer causes of death, particularly cardiovascular disease, associated with exposures to high-dose ionizing radiation, are well known. Recent studies have reported excess risk in workers who are occupationally exposed to low doses at a low dose rate, but the risks of moderately fractionated exposures, such as occur during diagnostic radiation procedures, remain unclear. The Canadian Fluoroscopy Cohort Study includes 63,707 tuberculosis patients exposed to multiple fluoroscopic procedures in 1930-1952 and followed-up for death from noncancer causes in 1950-1987. We used a Poisson regression to estimate excess relative risk (ERR) per Gy of cumulative radiation dose to the lung (mean dose = 0.79 Gy; range, 0-11.60). The risk of death from noncancer causes was significantly lower in these subjects compared with the Canadian general population (P &lt; 0.001). We estimated small, nonsignificant increases in the risk of death from noncancer causes with dose. We estimated an ERR/Gy of 0.176 (95% confidence interval: 0.011, 0.393) (n = 5,818 deaths) for ischemic heart disease (IHD) after adjustment for dose fractionation. A significant (P = 0.022) inverse dose fractionation effect in dose trends of IHD was observed, with the highest estimate of ERR/Gy for those with the fewest fluoroscopic procedures per year. Radiation-related risks of IHD decreased significantly with increasing time since first exposure and age at first exposure (both P &lt; 0.05). This is the largest study of patients exposed to moderately fractionated low-to-moderate doses of radiation, and it provides additional evidence of increased radiation-associated risks of death from IHD, in particular, significantly increased radiation risks from doses similar to those from diagnostic radiation procedures. The novel finding of a significant inverse dose-fractionation association in IHD mortality requires further investigation

Potential increased risk of ischemic heart disease mortality with significant dose fractionation in the Canadian Fluoroscopy Cohort Study.

Risks of noncancer causes of death, particularly cardiovascular disease, associated with exposures to high-dose ionizing radiation, are well known. Recent studies have reported excess risk in workers who are occupationally exposed to low doses at a low dose rate, but the risks of moderately fractionated exposures, such as occur during diagnostic radiation procedures, remain unclear. The Canadian Fluoroscopy Cohort Study includes 63,707 tuberculosis patients exposed to multiple fluoroscopic procedures in 1930-1952 and followed-up for death from noncancer causes in 1950-1987. We used a Poisson regression to estimate excess relative risk (ERR) per Gy of cumulative radiation dose to the lung (mean dose = 0.79 Gy; range, 0-11.60). The risk of death from noncancer causes was significantly lower in these subjects compared with the Canadian general population (P &lt; 0.001). We estimated small, nonsignificant increases in the risk of death from noncancer causes with dose. We estimated an ERR/Gy of 0.176 (95% confidence interval: 0.011, 0.393) (n = 5,818 deaths) for ischemic heart disease (IHD) after adjustment for dose fractionation. A significant (P = 0.022) inverse dose fractionation effect in dose trends of IHD was observed, with the highest estimate of ERR/Gy for those with the fewest fluoroscopic procedures per year. Radiation-related risks of IHD decreased significantly with increasing time since first exposure and age at first exposure (both P &lt; 0.05). This is the largest study of patients exposed to moderately fractionated low-to-moderate doses of radiation, and it provides additional evidence of increased radiation-associated risks of death from IHD, in particular, significantly increased radiation risks from doses similar to those from diagnostic radiation procedures. The novel finding of a significant inverse dose-fractionation association in IHD mortality requires further investigation

Anthropogenic activity in the Halifax region, Nova Scotia, Canada, as recorded by bulk geochemistry of lake sediments

<p>Dunnington DW, Spooner IS, Krkošek WH, Gagnon GA, Cornett RJ, Kurek J, White CE, Misiuk B, Tymstra D. 2018. Anthropogenic activity in the Halifax region, Nova Scotia, Canada, as recorded by bulk geochemistry of lake sediments. Lake Reserv Manage. 34:334–348.</p> <p>Separating the timing and effects of multiple watershed disturbances is critical to a comprehensive understanding of lakes, which is required to effectively manage lacustrine systems that may be experiencing adverse water quality changes. Advances in X-ray fluorescence (XRF) technology has led to the availability of high-resolution, high-quality bulk geochemical data for aquatic sediments, which in combination with carbon, nitrogen, δ¹³C, and δ¹⁵N have the potential to identify watershed-scale disturbance in lake sediment cores. We integrated documented anthropogenic disturbances and changes in bulk geochemical parameters at 8 lakes within the Halifax Regional Municipality (HRM), Nova Scotia, Canada, 6 of which serve as drinking water sources. These data reflect more than 2 centuries of anthropogenic disturbance in the HRM that included deforestation, urbanization and related development, and water-level change. Deforestation activity was documented at Lake Major and Pockwock Lake by large increases in Ti, Zr, K, and Rb (50–300%), and moderate increases in C/N (>10%). Urbanization was resolved at Lake Fletcher, Lake Lemont, and First Lake by increases in Ti, Zr, K, and Rb (10–300%), decreases in C/N (>10%), and increases in δ¹⁵N (>2.0‰). These data broadly agree with previous paleolimnological bioproxy data, in some cases identifying disturbances that were not previously identified. Collectively these data suggest that bulk geochemical parameters and lake sediment archives are a useful method for lake managers to identify causal mechanisms for possible water quality changes resulting from watershed-scale disturbance.</p

RFQ Reaction Cells for AMS: Developments and Applications

The use of anion-gas interactions in Radiofrequency Quadrupole (RFQ) ion guide reaction cells has been shown to be very effective in the elimination of a number of atomic and molecular isobars which have caused difficulties for Accelerator Mass Spectrometry (AMS) measurements [1,2]. This presentation begins with a review of the early work leading to the use of ion-gas reactions and continues with a discussion the recent measurements of the efficacy of this technique, some of which involve fluoride molecular anions. However, the transformation of the equipment used for these proof-of-principle measurements into a system suitable for routine analysis has required attention to aspects of the ion beam transport and gas handling subsystems. For example, the cross sections of the ion-gas reactions, involving both the analyte ion as well as the isobar, are critically dependent on the ion energy which has to be reduced from the ion source energy, usually between 20 and 80 keV, to energies typically in the range of several eV, a task complicated by the energy spread and divergence of beams from AMS sputter sources. With simulations using SIMION 8.1 [3] and tests of promising configurations in a laboratory system, principles for the design of the retarder optics have been developed. These are discussed, along with their planned implementation in a next generation analytical system