Impact of Sea-Level Rise on Sea Water Intrusion in Coastal Aquifers

Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.     

Simulation of the Vapor Intrusion Process for Nonhomogeneous Soils Using a Three-Dimensional Numerical Model

This paper presents model simulation results of vapor intrusion into structures built atop sites contaminated with volatile or semivolatile chemicals of concern. A three-dimensional finite element model was used to investigate the importance of factors that could influence vapor intrusion when the site is characterized by nonhomogeneous soils. Model simulations were performed to examine how soil layers of differing properties alter soil-gas concentration profiles and vapor intrusion rates into structures. The results illustrate difference in soil-gas concentration profiles and vapor intrusion rates between homogeneous and layered soils. The findings support the need for site conceptual models to adequately represent a site's geology when conducting site characterizations, interpreting field data, and assessing the risk of vapor intrusion at a given site. For instance, in layered geologies, a lower permeability and diffusivity soil layer between the source and building often limits vapor intrusion rates, even if a higher permeability layer near the foundation permits increased soil-gas flow rates into the building. In addition, the presence of water-saturated clay layers can considerably influence soil-gas concentration profiles. Therefore, interpreting field data without accounting for clay layers in the site conceptual model could result in inaccurate risk calculations. Important considerations for developing more accurate conceptual site models are discussed in light of the findings.     

美国联邦紧急事务管理局的新方向

我们所承担的义务自我担任联邦紧急事务管理局局长以来 ,这个组织就已经面临着重新确定其职责 ,建立最强大的、装备精良的集应急管理、消防和防灾为一体的社区的许多挑战。面对地震、火灾、水灾和飓风的挑战 ,我们已经做了非常实际的准备和响应工作。不幸的是 ,如今我们又必须增加打击恐怖分子的任务。当我们迈进新的一年进入 2 1世纪之际 ,我们的主要职责仍然是减轻所有人为灾害和自然灾害造成的生命和财产损失。灾害事件一旦发生 ,联邦紧急事务管理局会始终与各州和地方部门一道开展应急和灾后恢复工作。 3项重点工作“9.1 1”事件后 ,我…     

强震记录中速度大脉冲模拟及其对反应谱的影响

在近断层地震中,具有速度大脉冲的强震动对建筑物的影响较大。为了描述速度大脉冲,Makris和Roussos提出了由简单连续函数构成的模型,该模型参数较少、形式简单,用来描述实际地震记录中的速度脉冲大多取得较好的效果,但还是有一些记录模拟的结果会出现“丢失”现象,存在一些缺陷。针对此问题,本文提出了2个改进的方法——组合法和镜像法,用来扩展Makris和Roussos模型的适用范围,使其具有更好的适应性。