A risk assessment for this National Priority List (NPL) site evaluated the magnitude of the public health impact, assuming that no remediation had or would occur at a landfill. The purpose of the groundwater modeling effort was to predict future groundwater concentrations after liner failure at a residential well currently using the ground water. A two‐dimensional, transient, instantaneous‐release, multipoint‐source model was used to estimate the groundwater concentration at the residential well for 120 years after the release. A worst‐case concentration in ground water was calculated by assuming that no retardation or degradation (chemical or biological) would occur. A more realistic concentration was determined by developing a retardation factor. The modeled groundwater concentrations that were simulated did not exceed standards. If no remediation were to occur and worst‐case liner failure did occur, then the population would still not be at an unacceptable risk from consuming impacted ground water as a potable water supply. In reality, the predicted worst‐case exposure concentrations are unlikely to be reached, and the population (i.e., few individuals) might not be exposed.

During several investigations of ground water contamination, relatively large fluctuations in ground water levels—which occurred in a time interval shorter than that necessary to collect a round of water levels—were observed. However, because of their short-term nature, it was not possible to collect the data necessary to characterize fully the effects of these fluctuations when using standard water level measurement techniques. Without such a characterization, these short-term fluctuations in ground water levels can seriously impair interpretation of the ground water flow system. In a case history presented, tidal and other environmental influences were large enough to make a normal round of water level data useless in interpreting ground water flow. Monitoring of water levels at the site for a period of eight days, using electronic water level recording instruments, allowed evaluation of the fluctuations and determination of an acceptable method of collecting site water levels. To evaluate ground water levels properly, multiple rounds of water levels had to be collected and averaged to filter the noise, or short-term fluctuations, from the water level data. Monitoring for these short-term fluctuations in ground water levels should be considered good engineering practice in any ground water investigation.

A risk assessment for this National Priority List (NPL) site evaluated the magnitude of the public health impact, assuming that no remediation had or would occur at a landfill. The purpose of the groundwater modeling effort was to predict future groundwater concentrations after liner failure at a residential well currently using the ground water. A two-dimensional, transient, instantaneous-release, multipoint-source model was used to estimate the groundwater concentration at the residential well for 120 years after the release. A worst-case concentration in ground water was calculated by assuming that no retardation or degradation (chemical or biological) would occur. A more realistic concentration was determined by developing a retardation factor. The modeled groundwater concentrations that were simulated did not exceed standards. If no remediation were to occur and worst-case liner failure did occur, then the population would still not be at an unacceptable risk from consuming impacted ground water as a potable water supply. In reality, the predicted worst-case exposure concentrations are unlikely to be reached, and the population (i.e., few individuals) might not be exposed.