James T. Waples

Measuring low concentrations of 234Th in water and sediment

Th/ 238 U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle- reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of 234 Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of 234 Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a 229 Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low 234 Th/ 228 Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure 234 Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity. D 2002 Elsevier Science B.V. All rights reserved.

Particle delivery to the benthos of coastal Lake Michigan

A 2-D non-steady state model was applied to measured profiles of 234Th/238U and 90Y/90Sr disequilibria in a shallow (22 m) water column of coastal Lake Michigan. Downward fluxes of 234Th and 90Y were primarily driven by onshore horizontal advection. Concordance between 234Th and 90Y-derived mass flux estimates from the water column could only be realistically achieved under a nuclide scavenging scenario dominated by direct sorption on bottom or near-bottom sediment and vertical convection in the water column—not sinking particles. An estimated vertical 234Th/90Y flux ratio of ∼0.31 in the water column agreed with measured 234Th/90Y activity ratios on collected ejecta from bottom dwelling dreissenid mussels (0.26 ± 0.05) and not with water column particles (3.3 ± 1.3). A similar 238U/90Sr parent nuclide activity ratio of 0.30 ± 0.02 suggests that both 234Th and 90Y are scavenged in toto below the maximum sampling depth (17 m) and near the sediment/water interface. Determining the mechanism by which particles are transported to the bottom is important for understanding not only how benthos are supplied with water column material, but also how particle fluxes should be measured and calculated.

Real time observation of the thermal bar and spring stratification of Lake Michigan with the GLUCOS coastal observatory

During the spring of 2008 a subset of the WATER Institutes Great Lakes Urban Coastal Observing System (GLUCOS) was deployed to demonstrate the ability of the observatory to remotely monitor, in real time, the seasonal transition from a mixed state to a stratified state in coastal Lake Michigan. A prominent feature of this transition is the spring thermal front, also known as the thermal bar, a boundary of 4degC water that extends from top to bottom and separates near-shore stratified water from colder, mixed water further off-shore. Three Pioneer II buoys were deployed in a west to east line at 20 m, 40 m and 60 m depths and at distances of 2 km, 6 km and 12 km from shore respectively. Each buoy was equipped with a string of temperature sensors spaced one meter apart from the bottom to about 3 meters below the surface. The buoys communicated with a shore station using 900 MHz radio modems. An Internet connection was established between the WATER Institute and the buoys via the shore station permitting two-way communication for real-time data retrieval and control of the buoys. The spring thermal transition was remotely observed at the 20 m buoy along with its characteristic 4degC thermal bar. This paper will present the data on the spring thermal evolution of the lake as measured by the GLUCOS observatory and will describe the design of the new Pioneer II buoy that made-up the buoy array for this experiment.

Using Naturally Occurring Radionuclides To Determine Drinking Water Age in a Community Water System

Drinking water quality in a community water system is closely linked to the age of water from initial treatment to time of delivery. However, water age is difficult to measure with conventional chemical tracers; particularly in stagnant water, where the relationship between disinfectant decay, microbial growth, and water age is poorly understood. Using radionuclides that were naturally present in source water, we found that measured activity ratios of (90)Y/(90)Sr and (234)Th/(238)U in discrete drinking water samples of known age accurately estimated water age up to 9 days old (σest: ± 3.8 h, P < 0.0001, r(2) = 0.998, n = 11) and 25 days old (σest: ± 13.3 h, P < 0.0001, r(2) = 0.996, n = 12), respectively. Moreover, (90)Y-derived water ages in a community water system (6.8 × 10(4) m(3) d(-1) capacity) were generally consistent with water ages derived from an extended period simulation model. Radionuclides differ from conventional chemical tracers in that they are ubiquitous in distribution mains and connected premise plumbing. The ability to measure both water age and an analyte (e.g., chemical or microbe) in any water sample at any time allows for new insight into factors that control drinking water quality.