Regulation and Risk Assessment of 1,4-Dioxane - Environmental Investigation and Remediation - page 282

Chapter 7, the treatment technology selected for removing solvents often is not suitable for removing
1,4-dioxane. For example, the 1,4-dioxane concentration in air-stripper efl uent is often only slightly
less than in air-stripper inl uent. Discharge of concentrations of 1,4-dioxane well below aquatic
toxicity thresholds may nevertheless be restricted to uphold antidegradation policies. State and fed-
eral regulators have generally arrived at a pragmatic framework for resolving the question of appro-
priate efl uent limits for 1,4-dioxane in a manner that considers local conditions.
In California, the San Francisco Bay Regional Water Quality Control Board (SFB Water Board)
uses a concentration-based trigger of 5.0 μg/L for discharges of 1,4-dioxane from groundwater
treatment systems; concentration triggers are also set for 146 other compounds, including another
solvent stabilizer, nitromethane. The SFB Water Board requires a one-time sample for 1,4-dioxane
in treated groundwater efl uent upon system start-up and twice per year thereafter where chlori-
nated solvents are present in groundwater. If 1,4-dioxane is not detected above the SFB Water
Board's 5.0 μg/L trigger for 1,4-dioxane, dischargers may reduce the monitoring schedule to once
every 3 years (Cal EPA, 2004a). If the trigger is exceeded, three additional samples are required
within the next quarter; in case of additional detections, the discharger must report the median con-
centration and mass load discharged or provide a technical rationale for not monitoring 1,4-dioxane
in the efl uent. If the calculated median and mass discharge of 1,4-dioxane in the efl uent exceed the
trigger, the discharger must evaluate the technical feasibility and cost of treatment alternatives for
1,4-dioxane. If treatment is deemed not feasible, then the SFB Water Board requires an evaluation
of impacts to benei cial uses of the receiving water. If that evaluation indicates that benei cial uses
are threatened, the discharger then has the option of proposing alternatives, such as discharging to
the sewer instead of surface waters, using in situ treatment instead of groundwater extraction, or
other means of eliminating or reducing the impact.
In practice, the SFB Water Board has restricted 1,4-dioxane discharge to streams where there is
a potential to impact drinking water sources. The groundwater and surface-water cleanup standards
at a large solvent cleanup site were set to 3 μg/L, equivalent to California's NL (formerly, action
level), because groundwater at the site discharges to streams, and the streams drain to a large
reservoir for drinking water (Cal EPA, 2004b). The Los Angeles Water Board applies 3 μg/L as a
screening level for 1,4-dioxane in efl uents regulated by general NPDES permits (Cal EPA, 2002a).
Table 6.12 summarizes discharge limits set for 1,4-dioxane at several groundwater cleanup sites and
industrial facilities in the United States and Canada.
The volume of groundwater efl uent discharged to surface-water bodies is usually small com-
pared to the volume of receiving water l ow, resulting in large dilution ratios. 1,4-Dioxane in ground-
water treatment system efl uent may quickly drop below detection thresholds because of dilution in
receiving waters; consequently, detection of 1,4-dioxane in surface-water bodies is usually limited
to large-volume industrial discharge scenarios or to watersheds in which treated municipal
wastewater efl uent makes up a substantial proportion of the l ow. Table 6.13 lists a few examples of
1,4-dioxane detections in surface water.
6.2.5 R EGULATING 1,4-D IOXANE IN I NDUSTRIAL W ASTEWATER D ISCHARGES AND R ECYCLED W ATER
1,4-Dioxane is not easily removed through aerobic wastewater treatment, the primary means used
to treat domestic sewage (Abe, 1999). Discharge of 1,4-dioxane to sewers should be restricted where
the discharge would cause a detectable concentration in the plant inl uent or efl uent. In addition to
water quality considerations, discharge of 1,4-dioxane to sanitary sewers is potentially hazardous
because 1,4-dioxane is l ammable.
1,4-Dioxane is not routinely analyzed in industrial pretreatment programs. * Title 33 of the Code
of Federal Regulations governs navigable waters and includes Chapter 26, which addresses water
* Mohr et al. (2007) reviewed two decades of organic compound analyses at i ve POTWs in the San Francisco Bay area as
part of a study on historical dry cleaner discharges.