DFN-M field characterization of sandstone for a process-based site conceptual model and numerical simulations of TCE transport with degradation.

Plumes of trichloroethene (TCE) with degradation products occur at a large industrial site in California where TCE as a dense non-aqueous phase liquid (DNAPL) entered the fractured sandstone bedrock at many locations beginning in the late 1940s. Groundwater flows rapidly in closely spaced fractures but plume fronts are strongly retarded relative to groundwater flow velocities owing largely to matrix diffusion in early decades and degradation processes in later decades and going forward. Multiple data types show field evidence for both biotic and abiotic dechlorination of TCE and its degradation products, resulting in non-chlorinated compounds. Analyses were conducted on groundwater samples from hundreds of monitoring wells and on thousands of rock samples from continuous core over depths ranging from 6 to 426 metres below ground surface. Nearly all of the present-day mass of TCE and degradation products resides in the water-saturated, low-permeability rock matrix blocks. Although groundwater and DNAPL flow primarily occur in the fractures, DNAPL dissolution followed by diffusion and sorption readily transfers contaminant mass into the rock matrix. The presence of non-chlorinated degradation products (ethene, ethane, acetylene) and compound specific isotope analysis (CSIA) of TCE and cis-1,2-dichloroethene (cDCE) indicate at least some complete dechlorination by both biotic and abiotic pathways, consistent with the observed mineralogy and hydrogeochemistry and with published results from crushed rock microcosms. The rock matrix contains abundant iron-bearing minerals and solid-phase organic carbon with large surface areas and long contact times, suggesting degradation processes are occurring in the rock matrix. Multiple, high-resolution datasets provide strong evidence for spatially heterogeneous distributions of TCE and degradation products with varying degrees of degradation observed only when using new methods that achieve better detection of dissolved gases (i.e., Snap Sampler™) and contaminant mass stored in the low permeability rock matrix (i.e., CORE-DFN™). Simulations using a discrete fracture-matrix (DFN-M) numerical model capable of rigorously simulating flow and transport in both the fractures and matrix, including interactions, show that even slow, first-order degradation rates (i.e., 5- to 20-year half-lives) informed by site-derived parameters can contribute strongly to natural attenuation, resulting in TCE plumes that become stationary in space and might even retreat after 50 to 100 years, if the DNAPL sources become depleted due to the combination of diffusion and degradation processes.

Children's understanding of their research rights before and after debriefing: informed assent, confidentiality, and stopping participation.

This study explored children's understanding of their research rights in the context of an investigation of how children cope with peer provocation. Participants were 178 children (97 girls and 81 boys) who had finished the second, fourth, and sixth grades (average ages: 8, 10, and 12, respectively). After children agreed to take part in research on "how kids get along together," as well as after debriefing, their perceptions of free assent, their understanding of what they would be doing and why, their belief in voluntary participation and freedom to withdraw, and their comprehension of confidentiality were assessed. The vast majority of participants gave assent freely and reported no undue pressure from their families or the experimenters to participate. Older children knew before participating in the research what their participation would involve, comprehended confidentiality, and understood the study's purpose after their participation, but second graders were less likely to understand these issues than fourth and sixth graders were. Following debriefing, children's understanding of their research rights was largely unchanged; most children still had difficulty describing the research goals.