In the process of screening a soil against a certain contaminant, we define the health-risk based preliminary remediation goal (PRG) as the contaminant concentration above which some remedial action may be required. PRG is thus the first standard (or guidance) for judging a site. An over-estimated PRG (a too-large value) may cause us to miss some contaminated sites that can threaten human health and the environment. An under-estimated PRG (a too-small value), on the other hand, may lead to unnecessary cleanup and waste tremendous resources. The PRGs for soils are often calculated on the assumption that the contaminant concentration in soil does not change with time. However, that concentration usually decreases with time as a result of different chemical and transport mechanisms. The static assumption thus exaggerates the long-term exposure dose and results in a too-small PRG. We present a box model that considers all important transport processes and obeys the law of mass conservation. We can use the model as a tool to estimate the transient contaminant concentrations in air, soil and groundwater. Using these concentrations in conjunction with appropriate health risk parameters, we may estimate the PRGs for different contaminants. As an example, we calculated the tritium PRG for residential soils. The result is quite different from, but within the range of, the two versions of the corresponding PRG previously recommended by the U.S. EPA.

The critical height for DNAPL entry into a partially water saturated, dead-end fracture is derived and compared to laboratory observations. Experiments conducted in an analog, parallel-plate fracture demonstrate that DNAPL accumulates above the water until the height of the DNAPL overcomes the sum of the capillary forces at the DNAPL-air interface and at the DNAPL-water interface. These experiments also show that DNAPL preferentially enters the water at locations where DNAPL has previously entered, and the entry heights for these subsequent entries are lower than the heights measured for the initial invasion. The wetting contact angle at the DNAPL-water interface becomes larger at the locations where the DNAPL has already entered the water because of residual DNAPL on the fracture walls, which results in lowering the critical entry height at those locations. The experiments also demonstrate that a DNAPL lens can remain nearly immobile above the water for a period of time before eventually redistributing itself and entering the water.

This study of the hydrogeology of Chicken Creek Canyon was conducted by the Environmental Restoration Program (ERP) at Lawrence Berkeley National Laboratory (LBNL). This canyon extends downhill from Building 31 at LBNL to Centennial Road below. The leading edge of a groundwater tritium plume at LBNL is located at the top of the canyon. Tritium activities measured in this portion of the plume during this study were approximately 3,000 picocuries/liter (pCi/L), which is significantly less than the maximum contaminant level (MCL) for drinking water of 20,000 pCi/L established by the Environmental Protection Agency.There are three main pathways for tritium migration beyond the Laboratory s boundary: air, surface water and groundwater flow. The purpose of this report is to evaluate the groundwater pathway. Hydrogeologic investigation commenced with review of historical geotechnical reports including 35 bore logs and 27 test pit/trench logs as well as existing ERP information from 9 bore logs. This was followed by field mapping of bedrock outcrops along Chicken Creek as well as bedrock exposures in road cuts on the north and east walls of the canyon. Water levels and tritium activities from 6 wells were also considered. Electrical-resistivity profiles and cone penetration test (CPT) data were collected to investigate the extent of an interpreted alluvial sand encountered in one of the wells drilled in this area. Subsequent logging of 7 additional borings indicated that this sand was actually an unusually well-sorted and typically deeply weathered sandstone of the Orinda Formation. Wells were installed in 6 of the new borings to allow water level measurement and analysis of groundwater tritium activity. A slug test and pumping tests were also performed in the well field.

In the late 1980s, groundwater contamination was detected at the site of the Lawrence Berkeley National Laboratory (LBNL). A detailed investigation was conducted to locate the source and the extent of the contamination. Interim corrective measures were initiated where appropriate and required, typically directed towards removing the source of contamination, excavating contaminated soil, and limiting further spreading of contaminants. As the first step for predicting the fate of remaining contaminants, a three-dimensional transient groundwater flow model was developed for the complex hydrogeological situation. This flow model captured strong variations in thickness, slope, and hydrogeological properties of geologic units, representative of a mountainous groundwater system with accentuated morphology. The flow model accounts for strong seasonal fluctuations in the groundwater table. Other significant factors are local recharge from leaking underground stormdrains and significant water recharge from steep hills located upstream. The strong heterogeneous rock properties were calibrated using the inverse simulator ITOUGH2. For validation purposes, the model was calibrated for a time period from 1994 to 1996, and then applied to a period from 1996 to 1998. Comparison of simulated and measured water levels demonstrated that the model accurately represents the complex flow situation, including the significant seasonal fluctuations in water table and flow rate. Paths of particles originating from contaminant plumes in the simulated transient flow fields were obtained to represent advective transport.