[en] Groundwater in the 100-NR-2 Operable Unit (OU) has been contaminated with various radionuclides, metals, and ionic constituents because of waste water disposal practices and spills associated with I 00-N Reactor operations. Of primary concern are the presence of high levels of strontium-90 in the groundwater and the discharge of ⁹⁰Sr contaminated groundwater to the nearby Columbia River through historic river bank seeps known as 'N-Springs'. In accordance with the requirements of the Hanford Federal Facility Agreement and Consent Order (Tri-Party Agreement), remediation alternatives are currently being evaluated to address the hazardous substances and constituents in the soil and groundwater at the 100-NR-1/100-NR-2 OUs. Because of concern regarding the release of ⁹⁰Sr to the river, the Washington State Department of Ecology (Ecology) and the US Environmental Protection Agency (EPA) issued an action memorandum on September 23, 1994, to the U.S. Department of Energy (DOE), Richland Operations Office (RL), to take immediate actions consisting of a combination of a groundwater pump-and-treat system and a sheet pile barrier wall at N-Springs. By letter, dated March 23, 1995, Ecology and EPA agreed that a sheet pile construction test in December 1994 showed that the installation of the jointed hinge sheet pile wall could not be achieved in the manner specified. Ecology and EPA directed RL to proceed with the installation of the Expedited Response Action (ERA) pump-and-treat system and to (1) continue assessing accurately the flux of strontium to the river, (2) further characterize geologic and hydrologic conditions, and (3) assess design and installation alternatives related to modified barriers and expected performance