Summary of MWRA mercury monitoring data A. Rex November 27, 2006

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This summary focuses on measurements made in MWRA wastewater flows at Deer 

Island, and MWRA environmental monitoring. In Massachusetts, most mercury (about 

90%) in the rivers, lakes and oceans comes from atmospheric deposition and about 10% 

from “point sources” like treatment plants. 

I. DITP Wastewater:

There have been statistically significant decreases in both concentration and total 

loadings of mercury to DITP since the beginning of the decade. Effluent concentrations 

and loadings have also decreased. (Some but not all of the decrease in effluent 

concentration can be explained by the high flows in 2005 and 2006.)

Mercury Concentrations: Influent

0

.1

.2

.3

.4

.5

Mercury concentration (ug/L)

2000 2001 2002 2003 2004 2005 2006

South System Influent

North System Influent

Error Bars: 95% Confidence Interval 

Figure 1 Changes in influent mercury concentration since 2000. Influent 

concentration measurements are highly variable, but are substantially lower in 

2006 than in previous years. 
Summary of MWRA mercury monitoring data A. Rex November 27, 2006

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Mercury Concentrations: Effluent

Average Effluent Mercury Concentration

0.000

0.005

0.010

0.015

0.020

0.025

0.030

1999 2000 2001 2002 2003 2004 2005 2006 2007

Mercury (ug/g)

Mercury Loadings:

Mercury Loadings (pounds/day) from DITP Effluent and Biosolids pellets: 

1999-2006 (2006 data through August)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1999 2000 2001 2002 2003 2004 2005 2006

Effluent Pellets

Figure 2 Mean annual effluent concentration of mercury at DITP since September 

2000 has dropped from 0.026 ug/L in 2002 to 0.01 ug/L in 2006 (through October).

(The means were estimated using Helsel’s Regression on Order Statistics which 

assumes a lognormal distribution and handles the non-detects appropriately.)

Figure 3 Total mercury loadings in DITP effluent and biosolids pellets. Because flows affect 

concentration and are highly variable from year to year, mercury loadings measurements give a better 

idea of the total amount of mercury entering and leaving the plant. Effluent +biosolids can be a 

reasonable surrogate for influent. 
Summary of MWRA mercury monitoring data A. Rex November 27, 2006

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[Note: Our NPDES reporting data (based on fiscal year) do not show exactly the same 

trend in influent loadings suggested by the effluent + pellet data (Figure 4), this is likely 

due to the variation in flows. Sampling error for influent is also relatively high.

DITP Influent:

In FY 02, influent loadings to DITP were 0.55 lbs/day

In FY 06, influent loadings to DITP were 0.43 lbs/day

DITP Effluent:

Effluent loads also decreased, from about 0.1 lb/day to <0.05 lb/day]

0

.1

.2

.3

.4

.5

.6

.7

.8

Average Mercury Load (lbs/d)

2001 2002 2003 2004 2005 2006

Effluent

Influent

Mercury Loadings at Deer Island Treatment Plant

Fiscal Year

Figure 4 Influent and effluent mercury loadings to DITP FY01-

FY06
Summary of MWRA mercury monitoring data A. Rex November 27, 2006

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II. Biosolids Pellets:

Wastewater treatment removes the mercury from the water to the sludge. Therefore it is 

important to look at mercury in the solids removed.

[Technical note: for mercury measurements in the pellets, there are discrepancies in 

results between the contract lab now doing the work and MWRA’s DLS results: the 

contract lab is giving results about 25% lower than are obtained in-house. It is not clear 

what the cause is, split samples are still being analyzed. Therefore, although the pellet 

concentrations are likely lower than in previous years, we are not yet certain of the 

amount of decrease.] 

Concentration:

Figure 4 shows that the concentration in biosolids pellets from Fore River has 

significantly decreased over the past 5.5 years: from 4.6 mg/kg to 1.9 mg/kg.

Figure 5 Change in concentration of mercury in MWRA 

biosolids pellets since 2000, (data are through June, 2006).

0

1

2

3

4

5

6

Mervury (mg/Kg)

2000 2001 2002 2003 2004 2005 2006

Mercury concentrations in sludge pellets

Error Bars: 95% Confidence Interval
Summary of MWRA mercury monitoring data A. Rex November 27, 2006

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Mercury Loadings in MWRA Biosolids Pellets 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Pounds/Day

Figure 6 Loadings of mercury in biosolids have dropped by more 

than 50% since 1999.

III. Environmental Mercury: Sediments, Mussels, Lobster, Flounder

Sediments:

Boston Harbor Sediments: 

NOAA sediment quality guidelines for mercury suggest that at less than 0.15ug/g, 

adverse effects are rare, while above 0.71 ug/g adverse effects are often observed. In 

Boston Harbor in 1994, 7 out of 12 stations had average mercury levels exceeding 

0.71ug/g. In 2002, 4 out of 12 stations exceeded 0.71.

In the Outer Harbor, average mercury levels have remained relatively stable since 1994, 

at 0.5-0.8 ug/g (ppm) dry weight. In the Inner Harbor which is relatively more affected by

runoff, there has been a decrease in sediment mercury from about 0.75 ug/g in 2001 to 

about 0.6 ug/g in 2004. 

Massachusetts Bay 

There has been little change in mercury levels either near the MWRA outfall or far from 

the outfall over the monitoring period (since 1992). In the nearfield (sites nearest the 

outfall) average mercury levels have remained at 0.2-0.3 ug/g from 1992-2004; in the 

farfield mercury levels have stayed at about 0.15-0.25 ug/g. 

Fish and Shellfish Comparison to Regulatory Standards:

EPA recommends that states develop site-specific standards for water quality based on 

estimated human fish consumption from the site. (Massachusetts has not done this yet.) 

EPA’s “default” water quality standard for methyl-mercury is 0.3 mg/kg (parts per 
Summary of MWRA mercury monitoring data A. Rex November 27, 2006

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thousand) wet weight. Flounder and lobster tissue1 from Boston Harbor and the outfall 

site have total mercury (likely the same as methyl mercury) ranging from 0.07 to 0.17 

parts per million in lobster and flounder, well below the default standard. Mussels

deployed at the outfall site and in Boston Harbor are much lower, 0.0122 to 0.0173 parts 

per million. These levels are also well below the FDA action level of 1.0 ppm.

Fish and Shellfish Patterns in Mercury Concentration in Over Time:

There have been no significant changes in mercury concentration in lobster, flounder and 

mussels measured in Boston Harbor and at MWRA’s outfall site in Massachusetts Bay 

when data before and after the outfall came on line are compared. This reflects that most 

mercury entering the harbor and bay comes from atmospheric sources which have not 

changed substantially. 

Summary:

Our best estimates are that that mercury entering DITP has decreased about 50% since 

2000, from 0.76 lbs/day to 0.35 lbs/day. Concentrations of mercury in DITP effluent and 

biosolids pellets have both decreased. 

In 1988, EPA’s Supplemental Environmental Impact Study for DITP and the outfall 

predicted that the plant would discharge 216 kg mercury annually after full build-out. 

However, over the past 5 years, DITP has averaged a discharge of 12 kg mercury 

annually—95% less than the assumptions upon which the plant was planned and built. 

The relatively low loading of mercury from DITP is consistent with the negligible

changes measured in sediments fish and shellfish in the area near the outfall discharge. 

Meanwhile, Boston Harbor sediments in the inner harbor may be beginning to show 

small decreases in mercury, as it is removed by natural processes.

 1 Mercury bioaccumulates, which means that higher level predatory fish like tuna accumulate more than 

animals like lobster and flounder, that consume prey lower in the “food chain.” These types of fish which 

are found in Massachusetts waters are also highly migratory. 
Summary of MWRA mercury monitoring data A. Rex November 27, 2006

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More detail on fish and shellfish2

Mussels:

Mussels deployed in Boston Harbor:

There have been no significant changes in mercury levels in mussels deployed either near 

the previous outfall site in Boston Harbor or in the Inner Harbor. Levels have remained at 

0.13 to 0.18 ug/g dry weight. (A slight increase since 2000 is due to increased 

“background” concentrations in the mussels before they were deployed.)

Flounder caught in Boston Harbor:

Overall, there has been no significant trend in mercury levels in flounder in Boston 

Harbor since 1998; the average has been about 0.3-0.4 ug/g dry weight. 

Lobster caught in Boston Harbor:

Overall, there has been no significant trend in mercury levels in lobster in Boston Harbor 

since 1998; the average has been about 0.8-0.9 ug/g dry weight. 

Massachusetts Bay:

Mussels deployed in near the outfall:

At the outfall site, average mercury concentrations in mussels have increased from 0.09 

parts per million to 0.15 parts per million. However, this increase is due to increased 

“background” concentrations in the mussels before they were deployed.

Flounder caught near the outfall:

Overall, there has been no significant trend in mercury levels in flounder caught near the 

outfall since 1998. The mean as been 0.4-0.5 ug/g dry weight. However, in 2003-2004 

mercury in flounder at the outfall site was relatively elevated at about 0.6-0.7 ug/g. 

Lobster caught near the outfall:

Overall, there has been no significant trend in mercury levels in lobster caught near the 

outfall since 1998. The mean as been about 0.9-1.0 ug/g dry weight.

 2 MWRA analyzes both wet weight and dry weight mercury. Dry weight represents the concentration of 

mercury without interference by the variable amount of water in a sample, so MWRA looks at spatial and 

temporal trends based on dry weight. However FDA and EPA standards for human consumption are based 

on wet weight.