A variety of geochemical tools are available to assist in the performance of environmental monitoring at Subtitle D landfills. These include alternative data analysis methods, environmental isotopes, selection of indicator parameters, sampling methodologies, and investigation of alternative sources of apparent groundwater impacts such as landfill gas. The use of available tools is further advanced by a better understanding of the dynamics of landfills and the performance of their liner systems. Recent studies have shown an absence of leachate impacts downgradient of Subtitle D lined facilities.

States differ in their approach to implementing Subtitle D requirements, thus the use of certain approaches may be precluded by a State's rules. A challenge faced by industry and regulatory agencies is utilizing these tools effectively within the often regimented framework of a state-regulated Subtitle D monitoring program. In many instances, these tools may be effective alternatives to the monitoring prescribed by Subtitle D.

In the time since Subtitle D was passed, much has been learned about the geochemistry of landfills, potential sources of environmental impacts, chemical signatures, and the effectiveness of Subtitle D liner systems. In some instances, deviation from the strict Subtitle D model is warranted to attain monitoring goals. Variation in state approaches has resulted in inconsistent interpretation of monitoring results. For example, facility data for a site in one state may places a site in “detection monitoring” whereas that same data could result in “assessment monitoring” in another state.

Two areas of Subtitle D monitoring which have generated much discussion over the years are (1) approaches to data analysis such as statistics; and (2) the so-called alternate source demonstration (ASD). Statistical procedures for detection monitoring have evolved significantly since 1993, and has even spawned landfill-specific statistical analysis software. Experience has shown that statistical analysis is not an effective tool in many circumstances and alternative approaches not discussed in Subtitle D need to be considered. Performing an ASD will often provide better information regarding the cause of anomalous monitoring data than the “assessment” required by State rules.