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Historical Document

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EXPOSURE INVESTIGATION

GREAT POND
WEYMOUTH, NORFOLK COUNTY, MASSACHUSETTS


BACKGROUND AND SITE DESCRIPTION

In response to community concerns and at the request of local officials and residents of the Town of Weymouth, the Massachusetts Department of Public Health's (MDPH) Bureau of Environmental Health Assessment (BEHA), Environmental Toxicology Program (ETP) conducted sampling of raw drinking water, surface water, and sediment in Great Pond in Weymouth to evaluate possible opportunities of exposure to arsenic. The environmental sampling of Great Pond was the most recent activity conducted by MDPH in a series of activities undertaken in response to concerns over reports of elevated urinary arsenic levels in several residents, first raised in the Spring of 2001. A pediatrician with training in environmental medicine and working with one of the families who reported elevated urinary arsenic levels hypothesized that drinking water could be the source of the arsenic. While initial efforts did not suggest the presence of arsenic in the municipal water supply, a local environmental consultant stressed the potential for arsenic to go undetected until the periods of pond turnover. He hypothesized that if arsenic was present in the pond sediment, pond turnover could provide a means of intermittent exposure to arsenic. For that reason, further sampling should be conducted to address seasonal variation. Residents were particularly concerned that possible sediment contamination in the reservoir could be impacting the municipal drinking water supply. To address this concern, MDPH asked the U.S. Agency for Toxic Substances and Disease Registry (ATSDR) for funding support to conduct an exposure investigation to evaluate possible arsenic contamination in Great Pond. ATSDR agreed to provide partial support for this effort.

Great Pond in Weymouth is the primary source of drinking water for South Weymouth. The depth of the pond ranges from approximately 12 to 24 feet (Weston Geophysical Corporation, 1998). To supplement the water in Great Pond during periods of low precipitation (i.e., usually late summer/fall), the Town pumps water from the South Cove of Whitman's Pond, located across town, into Great Pond in proximity to the Great Pond pumping station (see Figure 1). Although Weymouth's water supply had been monitored for arsenic in accordance with applicable federal and state regulations, concern had been expressed by the community that this monitoring was not frequent enough (e.g., at times 18 month gaps occurred for drinking water testing as required by state and/or federal regulations) to detect possible sporadic elevations in the levels of arsenic in the municipal supply that may occur at certain times of the year (e.g., during the periods of spring and fall turnover). As previously mentioned, an environmental engineer working for one of the families suggested that if sediment in Great Pond contained arsenic, it was possible that this sediment could be stirred up during spring and fall turnover of the pond and that arsenic-contaminated water could then enter the water supply's distribution system.

METHODS

Protocol Development
In order to address the possibility of sporadic elevations of arsenic that may result from various environmental conditions (e.g., turnover events in Great Pond), BEHA/ETP developed a protocol for testing samples of surface water (water taken directly from the reservoir), sediment, and pre-treated or raw drinking water (water taken from the intake, that is the raw water tap inside the water treatment plant). A draft of the protocol was shared for comment with the Massachusetts Department of Environmental Protection (MDEP), the U.S. Geological Survey (USGS), and the environmental engineer consultant previously mentioned (Michael Pierdinock of Lightship Engineering). The protocol was designed to answer the following six questions:
  1. What arsenic concentrations are seen in raw (pre-treatment) drinking water samples originating from the Great Pond intake?
  2. What arsenic concentrations are seen in sediment and surface water samples collected in proximity to the Great Pond Weymouth Treatment Plant (WTP) intake in Great Pond?
  3. What arsenic concentrations are seen in sediment and surface water samples collected from the area where water from South Cove enters Great Pond?
  4. Do arsenic concentrations in surface water or sediment vary with the seasons, e.g., fall (the turnover period for the lake), versus summer (when the pond is likely to be more stratified).
  5. Are there variations in arsenic concentrations in sediment or surface water during times when water from South Cove is being pumped into Great Pond (e.g., fall and winter) and when water from South Cove is not being pumped (e.g., spring and summer)?
  6. Could the levels of arsenic in water or sediment present intermittent exposure opportunities or health concerns via drinking water to area residents?
Definition of Pond Turnover
Turnover of a water body (lake or pond) occurs when temperature changes (such as those that occur when winter transitions to spring or summer transitions to fall) cause changes in the density of water molecules. For example, in the fall, the surface layer cools to a point that its density exceeds the density of the lower layers and it sinks. In the spring, the frozen surface breaks up and warms and reaches a density equivalent to the lower layers making it easier for wind action at the surface to cause mixing of the water. The USGS commented that a likely mechanism for seasonal variations in water-column arsenic concentrations (where arsenic originates from bottom sediment) would be arsenic release from sediments in oxygen-depleted bottom water during the summer followed by mixing of this water higher into the water column during fall turnover. Oxygenation of the entire water column following turnover of a pond would reduce water column arsenic concentrations (arsenic is released under oxygen-poor conditions).

Selection of Sample Locations
Collection of sediment and surface water samples was focused on the northeast area of the pond where both the water treatment plant intake is located and where surface water from South Cove enters Great Pond (see Figure 1). Samples of pond surface water were taken at the same time sediment samples were collected in order to relate any findings of arsenic in sediment samples to arsenic concentrations in the water itself in the vicinity of the sediment samples. After several discussions with USGS and the environmental consultant, five locations were chosen for sampling (see Figure 1). Raw Drinking Water
Raw drinking water samples were collected from the intake to the water treatment plant prior to the water undergoing treatment for approximately one year every other week, beginning in July 2002. These samples should reflect the quality of the water entering the treatment system directly from the Great Pond water treatment plant intake under all conditions throughout the year. At the same time that each raw water sample was collected, a sample of finished water (post-treatment) was also collected but was not analyzed unless an unusual result was found in the pre-treatment sample.

Sample Collection

Raw Drinking Water
Raw drinking water samples were collected every other week for one year in 250 milliliter (ml.) plastic bottles containing nitric acid as a preservative. The samples were placed in a cooler and transported to the laboratory by BEHA/ETP staff. Water samples were analyzed for arsenic using EPA Method 200.9, with a detection limit of 5 parts per billion (ppb).

Sediment and Surface Water
In July 2002, a field visit was conducted in order to map proposed sampling locations with a GPS. Paul Milone (Weymouth Harbormaster) and Michael Smart (Weymouth Selectman) accompanied MDPH staff. The GPS enabled MDPH to sample, approximately, the same locations during each sampling round. The Town of Weymouth's Mayor's office helped to arrange a boat for these events.

Further discussions were held with USGS and the environmental consultant regarding the timing of the sampling events. The specific time periods for sampling were chosen because they were likely to capture both periods of pond stratification and turnover.

On August 30, 2002, surface water and sediment samples were collected from four sample locations (Locations 1 to 4, see Figure 1 and 2), and on September 6, 2002, samples from Location 5 were taken. At each location, one surface water sample was collected at one foot below the water's surface and at one foot above the sediment, and one sediment sample was collected from the top six inches of the sediment. Surface water samples were collected prior to sediment samples to avoid disturbing sediment that could potentially interfere with the water sample. The water samples were collected in 250 ml. plastic bottles containing nitric acid as a preservative. The sediment samples were collected using a metal borer and placed in sterile 250 ml. glass jars. During each sampling event, duplicate water and sediment samples were also collected (two water samples and one sediment sample). All of the samples were split with Lightship Engineering. Five surface water parameters (i.e., conductivity, dissolved oxygen, pH, temperature, and turbidity) were measured for each surface water sample collected. The following equipment was used to measure surface water parameters: 3000 TLC meter (conductivity), YSI O2 meter (dissolved oxygen), Hanna Laboratory pH Meter (pH), 3000 TLC Meter (temperature), and a Cole Parmer Turbidimeter (turbidity). In addition, the depth of the water was measured at each location using a tape measure attached to a weight.

On April 18, 2003, surface water and sediment samples were collected from Locations 1-5 (see Figures 1 and 3). At each location, one surface water sample was collected at one foot below the surface and at one foot above the sediment, and one sediment sample was collected from the top six inches of the sediment. Duplicate water and sediment samples were also collected (two water samples and one sediment sample). Five surface water parameters (i.e., conductivity, dissolved oxygen, pH, temperature, and turbidity) were measured for each surface water sample as well as the depth of the water.

Water samples were analyzed for arsenic using EPA Method 200.9, with a detection limit of 5 ppb. Sediment samples were analyzed for arsenic using EPA Method SW-846 6010B. An equipment rinsate blank was also collected during each of the three field sampling events. The duplicates and blanks were also analyzed for arsenic.

RESULTS

Raw Drinking Water
From July 2002 through June 2003, 26 pre-treatment raw drinking water samples were analyzed for arsenic. No arsenic was detected in any of these samples (see Table 1).

Sediment and Surface Water Samples
Results showed that arsenic was not detected in any of the 26 surface water samples taken at Great Pond (see Table 2). Two surface water samples were taken at each of five locations in the pond during each of the two sampling events. An additional six duplicate samples were also analyzed for arsenic for quality control purposes.

One sediment sample was taken from each of the same five locations in the pond that surface water samples were taken from during each of the two sampling events. In addition, three duplicate sediment samples were analyzed for arsenic. One sediment sample taken from Location 3 (see Figure 1) during the first sampling event had an arsenic concentration of 6.4 parts per million (ppm) (see Table 3). This concentration of arsenic in sediment is well within typical background concentrations of arsenic in soil or sediment for eastern U.S. soils (Shacklette and Boerngen, 1984). It is important to note that arsenic was not detected in the two water samples taken above this sediment sample location, which means that the arsenic is not getting into the water column and thus is unlikely to enter the municipal water supply. No other sediment sample including the sample taken from this location during the second sampling round, contained any detectable levels of arsenic (reporting limits for these samples varied from 1.2 and 6.0 ppm).

Results for the split samples taken by Lightship Engineering during the first round (August 30 and September 6) reportedly were consistent with MDPH results (Talbot, pers. comm., April, 2003). The data were not provided to MDPH for review.

Pond Stratification
Water and sediment samples were collected at times designed to maximize the ability to detect arsenic in the water column. Summer is typically when lower oxygen conditions exist that would allow for release of any arsenic from sediment into water at the bottom. The Summer 2002 sampling event showed that for 4 of the 5 locations, dissolved oxygen was indeed lower in the summer than in the spring (in the spring, lake water typically becomes more oxygenated because mixing of water can occur). Hence, this indicates that the summer conditions of lower dissolved oxygen should have facilitated greater release of arsenic into the water if arsenic was indeed present.

Spring typically sees changing water temperatures from the seasonal shift from winter, which would allow for mixing of water and hence, mixing of any 'stirred up' arsenic-containing sediment throughout the water column. The Spring 2003 sampling event showed little temperature gradient with depth (see Table 2). Similar results were also seen in the summer sampling. Hence, these data indicate that the pond in the sampled locations does not appear to stratify. Lack of stratification can occur if a water body such as Great Pond is shallow because the surface and deeper layers are continuously mixed by the wind (the greatest depth measured in all sampling events combined was 18 feet at Location 1, see Table 2).

DISCUSSION

The key findings from this investigation were as follows: Surface water samples taken at the location of the single detection of arsenic in sediment and collected at the same time as the sediment sample showed no detection of arsenic. Therefore, despite its presence in this one sediment sample, the arsenic was not released into surface water. In addition, arsenic was not detected in sediment from this location during the second sampling event. The level detected in the sediment sample is not unusual based upon known background levels for soil. For example, ATSDR reports that background concentrations in soils range from about 1 to 40 parts per million (ppm), with a mean value of 5 ppm (ATSDR 2000). USGS reports that the average background arsenic concentration in eastern U.S. soils is 7.4 ppm, and ranges from less than 0.1 ppm to 73 ppm (Shacklette and Boerngen 1984). Nevertheless, even if the level in sediment was higher, there must be a route of human exposure for it to pose a risk and the water sampling, as already mentioned, indicates that water in the vicinity of where this sediment sample was taken from, was not affected. Furthermore, arsenic was never detected in any of the raw drinking water samples collected biweekly from the water treatment plant.

In addition to this exposure investigation, the Town of Weymouth conducted supplemental drinking water testing between April 2001 and December 2002 at various residential and commercial locations, the water treatment plant, public water supply wells, and water pumping stations. MDPH reviewed data provided by MDEP that represented 397 drinking water samples (see Appendix A). Arsenic was not detected in any of these samples. Results from this supplemental testing conducted in 2001 - 2002 were consistent with the results of testing conducted from 1994 - 2001, per regulatory requirements for municipal drinking water supplies (see Table 4). Of 26 samples taken from 1994 - 2001, all but two samples were non-detect for arsenic. Two samples with detectable arsenic showed levels of 2 and 6 ppb, both in 1997. These levels are below both the current (50 ppb) and future federal and state drinking water standards for arsenic in drinking water (10 ppb, effective in 2006).

CHILD HEALTH CONSIDERATIONS

ATSDR and MDPH, through ATSDR's Child Health Initiative, recognize that the unique vulnerabilities of infants and children demand special emphasis in communities faced with contamination of their environment. Children are at a greater risk than adults from certain kinds of exposure to hazardous substances emitted from waste sites. They are more likely to be exposed because they play outdoors and because they often bring food into contaminated areas. Because of their smaller stature, they might breathe dust, soil, and heavy vapors close to the ground. Children are also smaller, resulting in higher doses of chemical exposure per body weight. The developing body systems of children can sustain permanent damage if certain toxic exposures occur during critical growth stages. Most importantly, children depend completely on adults for risk identification and management decisions, housing decisions, and access to medical care.

CONCLUSIONS

The data for the water and sediment samples that were analyzed for this investigation indicate that Great Pond sediment is not contributing detectable levels of arsenic to the Town of Weymouth's water supply. Arsenic was analyzed in water samples using a laboratory method approved by EPA for use by public water suppliers to determine whether water meets federal and state arsenic in drinking water standards. Separate testing for arsenic in the municipal water supply was also conducted by the Town of Weymouth. Samples were collected from the water treatment plant intake and at various points along the distribution system and no arsenic was detected during the April 2001 to December 2002 supplementary testing.

RECOMMENDATIONS

At this time, MDPH does not recommend any further evaluation of potential arsenic sources in Great Pond or the Weymouth drinking water.

PUBLIC HEALTH ACTION PLAN

MDPH will make this report available to the public on the agency's website. In addition, hard copies of the report will be distributed to the Town of Weymouth's Mayor's office, health department, and public library.

This document was prepared by the Bureau of Environmental Public Health Assessment of the Massachusetts Department of Public Health. If you have any questions about this document, please contact Suzanne K. Condon, Assistant Commissioner, 7th Floor, 250 Washington Street, Boston, Massachusetts 02108.

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