No abstract

For some sites, it may be an attractive and viable option.

The potential application of monitored natural attenuation (MNA) as a remedy for ground water contaminated with arsenic (As) is examined for a subset of contaminated sites, specifically those where naturally occurring As has been mobilized due to localized anthropogenic organic carbon (OC) releases. This includes sites subject to petroleum releases, exposure to landfill leachates, and OC additions for biostimulation of reductive dechlorination of chlorinated solvents. The key characteristic of these sites is that, under conditions prevailing before the anthropogenic OC introduction, the naturally occurring As in the subsurface was not mobile and did not adversely affect ground water quality. This suggests that, in the far‐field (where background conditions are (re) established), As may be sequestered upon contact of the contaminated ground water with either or both the (uncontaminated) ambient ground water and the background aquifer minerals. The observed extents of elevated concentrations (or “footprints”) of As and other chemical species, such as dissolved OC and iron (Fe), and related parameters, such as redox potential (Eh) and dissolved oxygen, and their evolution over time can be used to assess the mobilization and sequestration of As and the potential feasibility of MNA as a remedial option. Ultimately, the capacity for As sequestration must be assessed in the context of the OC loading to the site, which may require “active” measures for source control. Monitoring is needed to confirm the continuing effectiveness of the MNA remedy or to indicate if contingency measures must be implemented.

An advanced chemical fingerprinting approach, called nonaqueous phase liquid (NAPL)-groundwater (GW) finger printing, is presented for relating NAPL, soil, and groundwater data. This approach is accomplished by utilizing the "effective solubility" relationship and a hypothetical n-component NAPL to convert NAPL, soil (containing residual NAPL), and groundwater data to common frames of reference with respect to both phase and dilution. The resulting NAPL-GW fingerprints make it possible to perform detailed forensic analysis incorporating all three data types (assuming minimal confounding effects of attenuation processes and non-ideal solution behavior). Three case histories using the NAPL-GW fingerprinting approach are presented, illustrating its utility in forensic analysis when appropriately applied.

The chemical composition and acute toxicity of whole digested municipal sewage sludges from twelve water pollution control plants were collected, fractionated, and analyzed. The chemical composition and acute toxicity of whole sewage sludges were compared with those of liquid sewage sludge phases. The chemical constituents analyzed included nutrients, conventional parameters, and U.S. Environmental Protection Agency consent decree priority pollutants. Results indicated that most toxic constituents were associated with suspended solids and were therefore dramatically reduced in the liquid phase sludge fractions. Despite the dramatic reduction of metals, pesticides, and other organic compounds in the liquid sludge phases, there was a distinct trend towards equivalent phase toxicity to Atlantic silversides (Menidia menidia) and grass shrimp (Palaemonetes pugio). However, whole sludges were generally more toxic to mysids (Mysidopsis bahia) than were the liquid phase sludges. Evidence to support the hypothesis that ammonia may be the major contributor to overall toxicity of the sewage sludges tested is presented and discussed.