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    Completed Exposure to Dioxin in the Blue Lake Water Supply

Dioxins have been detected at low concentrations in the sediment of Blue Lake, but have not been detected in water samples. Because dixons are hydrophobic (only slightly soluble in water), strongly adhere to soil /sediment, and the concentrations detected in sediment in Blue Lake are low, it is unlikely that dioxin would be detected in the water of Blue Lake. Concentrations of these compounds in water tend to be absorbed onto particulate matter in water. Dioxins in bodies of water are primarily associated with other substances: 70-80% associated with the particulate phase and the remainder associated with dissolved organic substances. Therefore, the principal route of exposure to dioxin via the Blue Lake water supply is consumption of particulate material that might be suspended in the water.

The Sitka municipal water supply is not filtered. The potential does exist for particulate material to be suspended in the water that is delivered to customers. In fact, because community members in Sitka had observed particulate material in the water delivered to their homes, the community requested ATSDR to evaluate this potential exposure pathway to determine if the water supply is safe.

    Environmental Data

ATSDR reviewed available environmental data for water and sediment samples collected from Blue Lake. In addition, ATSDR reviewed data for turbidity and total suspended solids (TSS). Turbidity is a measure of the amount of suspended material such as clay, silt, or organic and inorganic materials in the water. Turbidity may be caused by a wide variety of suspended materials which range from colloidal to coarse dispersions. The term total suspended solids relates to the solids that are visible and physically held in suspension in the water column by agitation or flow.

    Water Samples from Blue Lake

Water samples were collected from Blue Lake on two occasions. The EPA collected two water samples from Blue Lake in 1990. The samples were analyzed for total dioxins/furans these results were reported to be at nondetectable levels. The detection limit was 0.0005 parts per trillion (ppt) [4] . The 1995 Expanded Site Inspection Report includes results for the analysis of one water sample from the Blue Lake supply tunnel prior to entry into the APC Filter Plant. No detectable levels of three dioxin congeners as well as cadmium, chromium, copper and lead were reported [3].

    Sediment Samples from Blue Lake

Sediments in Blue Lake have been sampled on three occasions. The first sampling was conducted by EPA in 1990 [4]. A single sediment sample was collected from the southern end of the lake. The sample was analyzed for dioxin/furans, metals, and organic compounds. Dioxin/furans were the only contaminants of concern detected. The toxic equivalence (TEQs) concentration (see Appendix A, Methods Summary, for an explanation of TEQs) for dioxin was reported as 9 nanograms per kilogram (ng/kg) or 9 parts per trillion (ppt). In 1993, Environmental Toxicology International collected two composite sediment samples from Blue Lake that were analyzed for dioxin/furans [5]. The TEQs for dioxin/furans averaged 1.4 ng/kg or 1.4 ppt.

As part of the remedial investigation at the APC, four sediment samples were collected in December, 1996, from Blue Lake [6]. Sample locations are shown in Figure 5 and results in Figure 6 in Appendix B. The samples were analyzed for dioxin/furans. The TEQs ranged from 0.45 - 1.17 ng/kg (0.45 - 1.17 ppt).

    Measures of Suspended Particulate Material: Turbidity and Total Suspended Solids

The City of Sitka has monitored turbidity and total suspended solids to determine the amount of suspended material in the water. Turbidity in water is caused by suspended matter, such as clay, silt, finely divided organic and inorganic matter, soluble colored inorganic compounds, plankton, and other microscopic organisms. Turbidity is a key indicator of water quality. Turbidity comes from the word "turbid" meaning cloudy or hazy. It is the optical property of the interaction between light and suspended particles in water. The units of measurement of turbidity are Nephelometric Turbidity Units (NTU). Nephelometers are used to measure low turbidities and are considered the standard instrument for measurement of low turbidities. [7].

The City of Sitka's Public Water System conducts several activities to ensure that turbidity in the drinking water supply remains within the SDWA acceptable range of 0-5 NTUs. Raw water turbidity is continuously monitored with a HACH (Hach Company) model 1720C instrument and chart recorder at the water plant. A turbidity chart recorder located at the Fire Hall also records high/low turbidity levels and has failure alarms. A turbidity reading that is above the acceptable range results in notification of the system operator and collection of additional samples to ensure that the system is meeting the SDWA standards. A grab sample of finished water (water following chlorination; water delivered to public) is also collected daily. Turbidity readings of the finished water have been similar to the raw water samples. [8]

Turbidity readings from January 1991 to April 1992 ranged from 0.2 NTUs to 5.81 NTUs with a median of 0.8 NTUs [1]. Monthly turbidity data from 1992 through 1996 are shown in Table 1 in Appendix B [9]. Data collected during these time periods indicate the typical average TSS is less than 1.0 mg/L. Maximum turbidity readings occur during heavier rainfall, typically in the months of September and October. In 1997, turbidity averaged 0.7 (NTUs); no events above five NTUs were reported [8].

The turbidity in Blue Lake is thought to be associated with glacial sources and typically has a low level of total suspended solids (TSS). ATSDR reviewed TSS samples collected from October 9, 1996, through January 7, 1998. Samples were collected approximately weekly. During this time period, the level of Blue Lake was lowered to approximately 263 feet mean sea level, leaving exposed sediments along the shoreline. The level of the lake was approximately 326 feet mean sea level in October 1996, reached a low of approximately 263 feet mean sea level in April 1997, and again was at overflow (approximately 342 feet mean sea level) in November 1997. ATSDR considers TSS samples collected during this period as representative of maximum levels expected in the Blue Lake water. The average TSS was 7.6 milligrams per liter (mg/L) with a range of 1.0 to 54.0 mg/L. The elevated TSS levels occurred during heavy rainfall or storm events.

It was necessary to lower the level of Blue Lake in late 1996 to accomplish maintenance of the intake structure. During the time frame when the lake level was below normal, turbidity and total suspended solids were elevated above average following heavy rainfall. Heavy rainfall along with exposed sediments at the lake were correlated with a higher total suspended solids for a given level of turbidity. In May 1997, the City of Sitka implemented actions to reduce turbidity and total suspended solids in the lake following heavy rainfall. An area was identified where rainfall (sheet flow) induced erosion resulted in increased turbidity in the lake above the intake. The removal of mud and sediment down to bedrock on shore at this point (above the water intake and rainfall diversion ditch) helped to reduce the turbidity increases [8].

    Public Health Implications

If the concentration of dioxins in the total suspended solids (TSS) is assumed to be similar to the concentration of dioxins in sediments, an estimate of the amount of dioxin that a person might be exposed to from consumption of a specific amount of drinking water can be calculated. In 1990, EPA reported maximum concentrations of dioxin (TEQs) in sediments at Blue Lake were approximately 10 ppt (0.00001 nanogram/milligram (ng/mg)). In comparison, samples collected in 1993 and 1996 reported TEQ concentrations about 5-10 times lower than the concentrations reported in 1990. Because only one sample was collected in 1990, two in 1993, and four in 1996, ATSDR cannot determine which results are most representative of actual concentrations of dioxins in sediment. Therefore, to be most protective of public health ATSDR selected the maximum value of 10 ppt to determine the public health impact from exposure to dioxins in the water supply. In addition, ATSDR used TSS data reported during the time the lake level was lowered (time when TSS was elevated). This approach provides a worse case exposure estimate.

The average amount of dioxin in a liter of water would be 0.000076 ng (0.00001 ng dioxin /mg of sediment x 7.6 mg/L average TSS). That is, for each liter of water ingested a person would be exposed to approximately 0.00008 ng of dioxin. Therefore, the average estimated exposure to dioxin from consumption of two liters of water per day would be 0.00016 ng/day or 0.000002 ng/kg/day for a 70 kilogram adult or 0.00001 ng/kg/day for a 15 kilogram child. Because both the dioxin TEQ concentrations reported in 1990 and the average TSS used in the calculations may be over- estimated, actual exposures may be as much as 10 times less than this estimate.

The minimum risk level (MRL) for dioxins is 0.001 ng/kg/day. The MRL is the estimated daily exposure that is considered unlikely to be associated with any appreciable risk of adverse noncancerous health effects. The average estimated exposures to dioxin from using water from the Blue Lake supply is 100-500 times less than the MRL. Based on data from 1993 and 1996, the exposures may be 500-2500 times less than the MRL. Therefore, the potential exposure to dioxin in the water is not of public health concern.

    ATSDR Child Health Initiative

ATSDR considers children in the evaluation of all exposures. When ATSDR evaluated levels of contaminants from the data reported in this document, ATSDR used health guidelines that are protective for children. ATSDR did not identify any chemical contaminants at levels of health concern for children.

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