Sediment Transport Mitigation in Watersheds on Los Alamos National Laboratory - 18669
Chemicals released into the environment from historical activities at Los Alamos National Laboratory are still present in surface water and groundwater. In streams, which are mostly ephemeral on Laboratory land, chemicals tend to bind to sediment and are transported downstream via entrainment in storm water runoff during precipitation events. In several watersheds across the Laboratory, sediment transport mitigations have been installed to stymie the movement of contaminated sediments. These mitigations include: grade-control structures, sediment detention basins, riparian and in-channel vegetation plantings, wetland restoration and promotion, and vegetative buffers. These mitigations have been implemented with the overall goals of minimizing the potentially erosive nature of storm water runoff, enhancing deposition of sediment, and reducing access of contaminated sediments to storm water. These mitigations are monitored using stream gaging stations, precipitation gaging stations, and storm water sampling, including monitoring locations that bound the mitigation sites. Stage height/discharge is monitored at 5 minute intervals at a series of gaging stations. Precipitation data are collected across the Laboratory by means of 5 meteorological towers and an extended network of 14 precipitation gages. Sampling for analytical suites specific to each reach of the watershed is conducted using portable automated samplers. Sampling equipment and the extended rain gage network are deactivated during the winter/spring months (December to May) and reactivated in the summer/fall months (June to November) to capture monsoonal precipitation events and associated streamflow and to avoid damage to the equipment. Attenuation of flow and associated sediment transport are primary goals of the sediment transport mitigation activities. Decreasing flow velocity allows for increased infiltration, thus reducing peak discharge, reducing the distance the flood bore travels downstream, and reducing the distance suspended sediment and associated contaminants entrained in the storm water travel downstream. Data analyses of flow and suspended sediment concentrations performed over the past seven years indicate that these mitigations are performing as designed. (authors)