This page presents sediment chemistry thresholds for seven different contaminant classes, used by the Dredged Material Management Office (DMMO) for determining when bioaccumulation testing will typically be required for dredged material proposed to be discharged at unconfined open water disposal sites in San Francisco Bay. These same thresholds are also used by DMMO to determine when additional analysis of the post-dredge sediment surface (“residual” or “z-layer” sediments) may be warranted. The June 9, 2011, Essential Fish Habitat Agreement between USACE, USEPA, and NMFS established the approach used to determine the testing thresholds for San Francisco Bay sediments.

The individual chemical thresholds presented in the table below are of two types:

  1. Thresholds for mercury, total PCBs, and total PAHs are based on San Francisco Bay ambient sediment concentrations determined via the Regional Monitoring Program (RMP), and are recalculated and updated when new data are available. Similar calculations are used to update TMDL in-Bay dredged material disposal limits for mercury and total PCBs when new data are available. (Details on how these ambient-based thresholds are calculated are provided in the 2016 ambient sediment concentrations memo.)
  2. Thresholds for total DDTs, total chlordane, Dieldrin, and dioxins/furans are based on similar values in use in other parts of the country and generally remain the same year-to-year.

Dredged Material Testing Thresholds

  Mercurya(mg/kg dw) Total PCBs (μg/kg dw) Total PAHs (μg/kg dw) Total DDTs (μg/kg dw) Total Chlordane (μg/kg dw) Dieldrin (μg/kg dw) Dioxins/ Furans (pg/g dw)
Bioaccumulation Trigger 0.35 18 4,700 50 37 1.9 10
TMDL Limit 0.43 26.5 -- -- -- -- --
Basis b b b c c d e

a. DMMO no longer requires bioaccumulation testing for mercury above the BT. See Amendment to EFH consultation.

b. Threshold based on San Francisco Bay ambient sediment concentrations, as described further below.

c. Published bioaccumulation trigger for Puget Sound marine sediments.

d. Published marine SL value from the Pacific Northwest Sediment Evaluation Framework.

e. Toxicity Equivalency Quotient (TEQ) based on WHO 1998 Toxicity Equivalency Factors (TEFs). Value is consistent with the published Puget Sound limit for unconfined aquatic disposal, and is ½ the established limit for placement at the Hamilton Wetlands Restoration Project site.


What data are used in calculating the threshold concentrations for Mercury, PCBs, and PAHs?

RMP sediment data collected since 2002 form the basis for calculating both the Dredged Material Management Office (DMMO) bioaccumulation testing trigger concentrations (BTs) for mercury, PCBs and PAHs, and the TMDL-based in-Bay disposal limits for mercury and PCBs. The BTs are calculated using the 90% Upper Tolerance Limit (UTL) of the 90th percentile concentrations of mercury, PCBs, and PAHs (rounded to two significant figures), while the TMDL-based limits for mercury and PCBs are calculated using the 90% Upper Tolerance Limit of the 99th percentile concentrations.

These values are intended to be based on the most recent 13 years of RMP sediment data. In previous years, these values were calculated on the most recent 10 years of data. However, because the frequency of RMP sediment sampling has been changed from every two years to every four years, the current 13-year window is needed to ensure that each threshold update is based on at least four sediment sampling events. In 2002, the RMP adopted a probabilistic (random, spatially balanced) sampling design to obtain a more representative characterization of conditions throughout the Bay, and only the newest 13-year data are used in calculating the BTs and TMDL-based limits. The individual threshold values are expected to change slightly each year and may be higher or lower than the previous year.


Which calendar year threshold applies for making suitability determinations?

The DMMO will make suitability determinations for in-Bay disposal using the BT and TMDL thresholds in effect at the time of the determination. In general, BT and TMDL thresholds in effect at the time a suitability determination is made will remain valid as long as the suitability determination holds: for example, even if the material is not dredged until a subsequent year or if the determination includes a Tier I testing exclusion for a future dredging episode. However, if conditions have changed substantially enough for the DMMO to require a new round of testing and a new suitability determination, the thresholds in effect at the time of the new suitability determination will be used.

The summary table below presents the BTs for mercury, PCBs, and PAHs and TMDL-based in-Bay disposal limits for mercury and PCBs, after removal of any statistical outliers. Further details on the calculation of these thresholds are provided in the links below. The actual BTs used by DMMO agencies are based on the 90th UTL of the 90th percentile concentrations shown, but rounded to the nearest two significant figures. Thresholds for prior years are shown in the link below the table.

In order to make direct comparisons of dredging project sediment chemistry results to BTs and TMDL limits, it is important that dredgers report total PCBs (40 congeners) and total PAHs (25 compounds) in the same manner as the RMP. The target analytes are shown in this table. A discussion of DMMO’s accepted method for analyzing PCB congeners in San Francisco Bay area dredged material is presented here.


For questions, contact Jazzy Graham Davis([email protected].)

Ambient-Based Thresholds (Based on 2006-2018 RMP Data)

Contaminant 90% UTL of 
90th percentile
DMMO BTs 90% UTL of 
99th percentile 
(TMDL Disposal Limits)
Hg (mg/kg dry wt.)3 0.348 0.35 0.43 Total Mercury
PCB (μg/kg dry wt.)1 18.2 18 26.5 Sum of 40 congeners2
PAH (μg/kg dry wt.) 4,737 4,700 -- Sum of 25 PAHs2

The ambient-based thresholds for PCBs are based on 2007-2018 RMP data. The 2004-2006 PCB data are not included in the analysis because of a recently identified issue with sample processing at the laboratory. PCB results for 2004-2006 passed routine laboratory and RMP Quality Assurance; however, were censored in 2014 due to a low bias identified in the extraction method.  

The RMP reports total PCBs and PAHs using slightly different analyte lists and/or analytical methods than those shown in the DMMO's “Guidelines for Implementing the Inland Testing Manual in the San Francisco Bay Region”. For PCBs, instead of the Aroclor mixtures, the RMP reports a sum of 40 PCB congeners. For dredged material testing, the DMMO recommends USEPA Method 8082 which employs gas chromatography/electron-capture detection (GC/ECD) capable of achieving per congener detection limits in the 0.1 – 0.6 μg/kg range and reporting limits in the 0.5 – 1.0 μg/kg range. Alternative analytical methods may be acceptable as long as the detection and reporting limits are comparable to those of Method 8082. For PAHs, the RMP reports a slightly expanded list of PAH compounds (25 compounds vs. the previous DMMO list of 16). Since all the PAHs in this expanded list are measured by the same analytical method most commercial laboratories already use when analyzing sediment samples (EPA 8270 SIM), there should be little or no cost increase for reporting the additional PAHs.

The ambient-based thresholds for Hg were previously based on a 10 year window (2003-2012). Beginning with the current threshold calculation, a 13-year time window will be used, in order to account for the recent reduction in sediment sampling frequency from every two years to every four years. However, to maintain consistency with the previous threshold calculation, the current ambient-based thresholds for Hg were calculated based on the 2006-2018 time window.

In 2015, SFEI also updated the Bay-wide ambient sediment concentrations for the full range of chemicals regularly surveyed by the RMP, based on 10 years of monitoring results.  The last previous comprehensive update was in 1998.  While these updated ambient sediment concentrations are not intended for direct regulatory application, they provide valuable technical support for a variety of agency considerations in the future.  The 2015 ambient sediment concentrations report may be downloaded here.

Ambient-Based Thresholds from Prior Years

Associated Staff: 
Programs and Focus Areas: 
Clean Water Program
Bay Regional Monitoring Program
Data Services
Location Information
General Project Location(s): 
San Francisco Bay Area