Perchlorate Contamination in the Mather Air Force Base Water Service Area
RANCHO CORDOVA, SACRAMENTO COUNTY, CALIFORNIA
In late January and early February 1997, Aerojet, as a part of their ongoing monitoring of certain off-site public drinking water wells, detected perchlorate in five off-site public drinking water wells west of Aerojet and north of Mather (none of these wells were Mather wells) (11). To analyze these water samples, Aerojet used a refined or improved analytical method such that instead of a reporting level of 400 ppb, they were able to obtain a detection limit of 35 ppb. (For a more complete history of the perchlorate discovery see Aerojet General Health Consultation- Perchlorate Groundwater Contamination.) These detectable levels of perchlorate exceeded the concentration (4 to 18 ppb) suggested by the USEPA provisional reference dose (1 to 5E-4 mg/kg/day) based on a 70 kg individual consuming 2 liters of water a day (12).
Based on subsequent testing by Aerojet that showed that Main Base well 2 had a level of 120 ppb perchlorate, this well was taken off-line on March 14, 1997. Mather Main Base well 1 was reported to have a level of 67 ppb and it was taken off-line on March 21, 1997.
In March 1997, the Sacramento District field staff of the CDHS Division of Drinking Water (DDW) sampled 41 public water supply wells in the area of the known perchlorate contaminated wells, including seven wells located at Mather Air Force Base and the hosebib at one of the Main Base buildings (13). The well samples were processed by the CDHS's Radiation and Sanitation Laboratory with a quantitation limit of 4 ppb (Table 1). In March 1997, Mather Main Base well #3 had a level of 14 ppb, while well #4 had no detectable levels of perchlorate. Because well #4 was not contaminated, well #4 was put as the lead well to supply water to the ground level reservoir serving the Main Base area and well #3 was placed in a backup mode (14). Mather Family Housing wells #1, #4, and #5 had no detectable levels of perchlorate.
The DDW field staff have continued to play the lead role in monitoring the perchlorate contamination. In April, DDW staff sampled 22 wells, including the four Main Base wells, two irrigation wells serving the Mather Air Force Base golf courses, and three monitoring wells that Mather has installed near a location where they were planning to put another drinking water well (14). In May, DDW staff sampled 43 locations, including five Mather wells (14). In June, DDW staff sampled 47 locations, including four Mather wells (14). In July, DDW staff analyzed water from 40 locations, including five Mather wells (15). In August, DDW staff analyzed water from 42 locations, including five Mather wells (16). No perchlorate has been detected in Family Housing wells #1, 4, and 5, the golf course wells, Main Base well #4, and in the three monitoring wells (Table 1). The Main Base well #3 is in-line as a backup and it has levels of perchlorate around 18 ppb; in July, perchlorate was detected at 19 ppb in Main Base well #3 (Table 1).
The Mather Air Force Base Conversion Agency of the U.S. Air Force send periodic updates to interested persons and nearby residences and commercial businesses about the Mather site investigations and cleanup activities. In the spring 1997 fact sheet, the Base Conversion Agency summarized the perchlorate contamination that had been found at Mather Field and the actions that had and would be taken to protect the water supply (17). They also offered the perchlorate fact sheet created by CDHS cooperative agreement staff to anyone who requested a copy (18). Air Force Base Conversion Agency staff and people associated with the county redevelopment effort have not reported much interest in the perchlorate problem (9).
It is not clear when the perchlorate contamination reached the Mather wells, because Aerojet had previously been using an analytical method to monitor for perchlorate that was not sensitive enough to adequately assess the migration of perchlorate. In fact, until recently, Aerojet had a perchlorate reporting level to RWQCB of 400 ppb, based on the fact that the older method had a practical quantitation limit for perchlorate of 400 ppb (19). It was not until Aerojet improved upon the analytical method they had been using and were able to obtain lower detection limits, that the perchlorate contamination could be adequately addressed.
Though we do not have good monitoring information, we do know that Aerojet began reinjecting water from their treatment plants on the west boundary of the site in 1984 and 1985, which continues to this day (20). Thus, assuming that it took a couple of years for the perchlorate to move from the reinjection wells to the Main Base wells, perchlorate has probably been a contaminant in the Main Base wells since 1987/1988.
The exposure to the perchlorate contamination in Mather Main Base wells #1 ceased on March 14, 1997 when the Mather Air Force Base Conversion Agency learned of the perchlorate contamination and took it off-line (14). The exposure to the perchlorate contamination in Mather Main Base wells #2 ceased on March 21, 1997 when the Mather Air Force Base Conversion Agency took it off-line (14).
Main Base well #3, which is contaminated with perchlorate in the range of 14-19 ppb, has been the back-up well to Main Base well #4 since April 1, 1997 (14). Main Base well #3 has been used periodically, but it is anticipated that 35-50% blending would occur in the ground level reservoir, thus reducing perchlorate level below 18 ppb prior to entering the distribution system (9).
In August, the upgraded intertie between Family Housing and the Main Base system went on-line. Since that time, the Family Housing intertie has been the lead source for Main Base water with Main Base well #4 as the first backup source, and Main Base well #3 as the second backup source.
From the time when perchlorate may have first affected the Main Base wells, approximately 1987/1988, until the base was closed on September 3, 1993, the Main Base supplied water to 6,200 personnel (8). Since September 30, 1993, there has been a steady increase in the reuse and development of the former base. For instance in 1994, it was estimated that 1,000 people were being served by the Main Base wells (8), while, in 1997, it is estimated that Main Base serves water to 1,900 employees and approximately 70 people using the transitional housing for up to a two year period (21). (Customers of the Citizens Utilities were also served water from the Main Base water system in parts of 1995 and 1996. That exposure is reviewed in a separate health consultation.)
For a target population to be exposed to environmental contamination, there must be a mechanism by which that contamination comes into direct contact with the target population. An exposure pathway is the description of this mechanism (22). A completed exposure pathway consists of five parts: a source of contamination, an environmental medium and transport mechanism, a point of exposure, a route of exposure, and a receptor population. For a population to be exposed to an environmental contamination, a completed exposure pathway (all five elements) must be present.
In the next few paragraphs, CDHS will describe how we evaluated the completed exposure pathway related to the perchlorate contamination of the Main Base well water for four different receptor populations: worker in the Main Base Area, frequent adult customer/visitor to a business served by the Main Base well system, adult using the transitional housing, and adult patient at the McClellan Hospital in the Main Base Area (Table 2). As of early April when Main Base well #4 became the permanent lead well, there has been no water contaminated with perchlorate being added to the Main Base water system. Thus, this exposure does not currently exist, it is a completed exposure pathway in the past.
When evaluating the potential health impact from exposure to contaminated potable water, CDHS considered all routes of exposure to perchlorate in the water. The most important route of exposure is through ingestion of the water. We did not evaluate exposure from eating homegrown fruits and vegetables that were irrigated with perchlorate-contaminated water, because we do not believe that there are any residential gardens or agricultural uses of the Main Base water. We did not evaluate inhalation exposure to perchlorate in the potable water because perchlorate is not volatile (does not become a gas).
For certain chemicals, skin contact with contaminated water can be an important route of exposure. Generally speaking, skin absorption of a chemical is based on how much that chemical likes to be in fat-like surroundings. Inorganic ions like perchlorate do not like being in fat-like surroundings and thus their uptake by the skin, a fat-like environment, are typically less than 10% and frequently less than 1%.Since the permeability characteristic for perchlorate is not known, we used the permeability characteristic of another anion, chloride (1 x 10-10 cm/sec) to evaluate skin exposure to perchlorate (23). We found that skin contact would result in an exposure dose estimate that is less than 0.0005% of the dose estimate that would be received by ingesting the water. Therefore, CDHS focused on ingestion in calculating dose estimates.
The amount of Main Base water system perchlorate-contaminated water that is ingested will be determined for each exposure pathway; however, when the route of exposure is ingestion, it will be assumed that there is 100% absorption of perchlorate into the body from the gut from the amount water that is ingested.
This health consultation focuses on perchlorate exposure and thus the toxicological evaluation will focus on perchlorate. CDHS acknowledges that there very, low levels, well below the drinking water standard, of nitrates and nitrite, naturally-occurring, agriculturally-related and perhaps site-related, in the well water; however, the affect of nitrates/nitrites in combination with perchlorate will not be evaluated due to lack of toxicological information that would allow such an evaluation.
Most of the information about the toxicity of perchlorate comes from studies of potassium perchlorate as a treatment for hyperthyroidism, resulting from Graves' Disease. Perchlorate inhibits the secretion of thyroid hormones (and can thus relieve the symptoms of Graves' Disease) by competitively inhibiting the accumulation of iodide in the thyroid (24). Discontinued administration of the ammonium perchlorate to Graves' Disease patients does result in a return to their hyperthyroid condition (25). People who have been treated with perchlorate have reported gastrointestinal irritation, skin rash, and hematological effects including agranulocytosis, aplastic anemia, and lymphadenopathy (24). The severe hematological effects seem to be more likely to occur when large doses of more than 1,000 mg/day (approximately 14 mg/kg/day for a 154 pound man) are used (26).
Potassium perchlorate was extensively used for treatment of Graves' Disease patients in the late 1950s and 1960s. After the reports of the severe hematological effects, potassium perchlorate was not used for many years (27). In the early 1980s, physicians in Europe began using it again for the treatment of Graves Disease, and reporting no serious side effects occurring as long as the dose was kept below 1,000 mg/day (approximately 14 mg/kg/day for a 154 pound man) (26). In addition, potassium perchlorate has also been found helpful in treating thyrotoxicosis resulting as a side effect from other drug therapies (28-32).
There are only a few studies of the short-term exposure in persons without Graves Disease (33). The animal studies that have been conducted have also involved short-term exposures and the doses were too high to see a level where there was no effect on the thyroid. Both human and animal studies have primarily examined the effects of perchlorate on the thyroid, interference with the production of thyroid hormones resulting in a below normal level of thyroid hormone in circulation (hypothyroidism). The effect of perchlorate on systems other than the thyroid needs to be explored, especially, effects on the blood system (described above) and developmental effects (described below).
Children are not little adults, their bodies are not fully developed, and may not respond to a perchlorate in the same manner as an adult. For instance, thyroid hormone is critical to normal brain and physical development, and the critical period for this dependency on thyroid hormone begins in the uterus and extends up until three years of age. After the age of 3, thyroid hormone continues to play a primary role in physical development until puberty.
Thus, a low level or absence of thyroid hormone in utero or in childhood may lead to irreversible mental retardation and retarded physical growth.
Perchlorate can cross the placenta and thus could affect the developing fetus, though these effects have not been studied in humans. It is known, however, that drugs currently being used to treat Graves' Disease such as propylthiouracil do cross the placenta and can produce neonatal hypothyroidism (34, 35) and fetal in utero goiter (enlargement of the thyroid) (36-38). In fact, because the developing fetus's thyroid is immature, propylthiouracil is a more potent suppressor of thyroid function in the fetus than in the mother (39).
In a study of the effects of potassium perchlorate (740 mg/kg/day for the mother) fed to pregnant guinea pigs during pregnancy, a 15-fold enlargement of thyroid of the newborns was noted, even though no increase in size of the mother's thyroids occurred (40). Thyroid hormone levels of the newborn guinea pig were not measured in this study. Another animal study in which the mother was given fairly high levels of perchlorate, also resulted in increased thyroid weight in the offspring and the mother (41). At this time, it is unclear whether lower doses of perchlorate would affect the thyroid of the developing fetus and young child and thus affect thyroid function at a time when normal thyroid hormone production is important to brain development.
There are animal studies underway which are exploring the toxicity of perchlorate, including effects on the immune system and developmental effects (see the Recommendations section at the end of the text for more information).
In 1992 and 1995, USEPA staff reviewed the perchlorate toxicology studies and derived a provisional reference dose (RfD) (12, 33). An RfD is a dose to which a person could be exposed over long-term period without having any appreciable risk of a noncancer health effect. The USEPA applied an uncertainty factor of 300 or 1000 to the No Observable Adverse Effect Level of 0.14 mg/kg/day (NOAEL) (33, 42) to derive an RfD of 1 to 5 x 10-4 mg/kg/day (12). (If one assumes that a person drinks 2 liters/day of water and weighs 70 kilograms, the reference dose range corresponds to an acceptable range of perchlorate in drinking water of 4 to 18 ppb).
The uncertainty factor of 300 or 1000 is derived from multiplying the following (12):
|*||An uncertainty factor of 10 to account for extrapolation from the acute exposure in the NOAEL study to chronic exposure of an RfD;
|*||An uncertainty factor for database deficiencies (3 or 10) to account for data limitations including limited data on subchronic and chronic exposure to low doses of perchlorate, limited data on other organ system effects, limited data on the effects on the hematopoietic system, and a lack of reproductive and multigenerational data;
|*||An uncertainty factor of 10 to protect sensitive subpopulations which would include groups such as hypothyroid patients and individuals with low iodine diets or with genetically impaired iodine accumulation.|
The only information about the possible carcinogenicity of perchlorate has to do with cancers of the follicular thyroid cells (12). Interference with the normal thyroid-pituitary feedback mechanism, such as that caused by perchlorate, can theoretically lead to thyroid follicular cell neoplasia. Several animal studies found that thyroid tumors were induced in both rats and mice by long-term administration of high doses of perchlorate. However, humans are not supposed to be as sensitive as the rat to thyroid cancer (43, 44). Since perchlorate's possible carcinogenic effects on the thyroid are based on the same mechanism (interfering with the thyroid-pituitary homeostasis) that determines its noncarcinogenic effects, it may be appropriate to consider the RfD as a dose which does not pose a significant risk of thyroid cancer (33).
It is even harder to determine whether or not perchlorate exposure can cause any other type of cancer. If a link is discovered, it will probably be based on perchlorate acting not as a mutagen (causing genetic changes) but rather as a growth promoter, an effect associated with a threshold. In other words, below a certain threshold, perchlorate would not have cancer-causing effects. More toxicological information is needed to ascertain whether perchlorate can cause cancer and if it can, at what dose this effect may start occurring.
Using USEPA's provisional reference dose (0.0001 to 0.0005 mg/kg/day) based on perchlorate's effect on the thyroid (12), CDHS evaluated the noncancer (thyroid) health impact of the exposure to perchlorate-contaminated water from Mather's Main Base water system that occurred prior to the discovery and discontinued use of the perchlorate-contaminated wells (Table 2). We evaluated this completed exposure pathway for four different receptor populations: worker in the Main Base Area, frequent adult customer/visitor to a business served by the Main Base well system, adult using the transitional housing, and adult patient at the McClellan Hospital in the Main Base Area (Table 2).
Since the water that services the Main Base comes a storage tank where water from the wells may be mixed and well usage was rotated among the four wells, it is hard to estimate what concentration of perchlorate was delivered to the user. It may be possible to recreate past exposures through a time intensive analysis of the historical documentation of the Main Base well logs and other water system documentation. However, for this health consultation, we will instead evaluate three well contribution scenarios: Main Base well #1 was delivering 100% of the water, Main Base well #2 was delivering 100% of the water, and all wells were equally contributing to the water being delivered to the user (so the concentration of perchlorate being delivered to the user in the third scenario is the average of the four well levels). By evaluating these three scenarios, we will be considering the worst case scenario, when well #2 was the lead well; the second worst situation, when well #1 was the lead well; and a rough approximation of the automated, rotational use of the wells with the storage tank being the place where the blending of the water occurs.
CDHS will use the concentrations of perchlorate measured in the wells when DDW sampled in March 1997 (Table 1), since these are the measurements that correspond most closely in time with when these wells were taken off-line (Main Base wells #1 and 2) or put in the backup mode (Main Base well #3). Thus we will be evaluating exposure based on recent perchlorate concentrations. It does seem that the perchlorate levels in Mather wells (Table 1) and in other water purveyor wells (see other health consultations) have fluctuated a bit over the past several months, but on the whole seem to relatively constant. This would mean that the dose estimates that we calculate may reflect exposures that have occurred in the past. However, because it is expected that the concentrations do change over a period of several years, and there is no good perchlorate concentration information prior to 1997, it is not possible to evaluate exposures that occurred as long ago as 1988-1994
Though it is possible to estimate a dose for a child drinking the Mather Main Base water, CDHS did not calculate this dose because we are not confident about how to interpret the dose estimate. To compare the estimate of a child's dose with toxicological information based on adult exposure ignores the fact that a child is not a small adult, especially when it comes to the importance of the thyroid in normal brain development (see above). Thus, until there is more information about perchlorate's effect on children, CDHS is not able to evaluate past and current exposures to a young child drinking the Mather Main Base water whether visiting the base or staying at the transitional housing.
Worker exposure at a Main Base Area business: CDHS estimated the exposure for a worker who worked eight hours a day, five days a week, for 50 weeks of the year (assumes a two week vacation) at a business that is served by the Main Base water system (Table 3 is a list of the exposure parameters used in the toxicological evaluation). CDHS assumed that the worker is involved in manual labor and thus drank a relatively large quantity of water (3.7 liters/day, the equivalent of 15.6 cups/day) (45). CDHS estimated the dose if the worker was exposed to water as described in the three water contribution scenarios described above.
The estimated dose for worker exposure to water from the Main Base water system in each of the three well contribution scenarios (0.00081, 0.0015, and 0.00062 mg/kg/day, respectively) exceeds the provisional reference dose range (0.0001 to 0.0005 mg/kg/day) which means that noncancer (thyroid depression) health effects may have occurred when workers in the Mather Base Area were exposed to water from these wells. However, because there is a very large uncertainty factor associated with the provisional reference dose and the estimated doses do not approach the NOAEL (0.14 mg/kg/day), it is unlikely that these exposures did cause any noncancer health effects.
Frequent Adult customer or visitor exposure at Main Base businesses: CDHS estimated the exposure for a adult visitor or adult customer who went once a day, five days a week, for 50 weeks of the year (assumes a two week vacation) to a business in the Main Base Area (Table 3 is a list of the exposure parameters used in the toxicological evaluation). CDHS will assume that the adult visitor/customer drank a cup of water (0.24 liters) per trip to the business. CDHS estimated the dose if the frequent Adult customer/visitor was exposed to water as described in the three water contribution scenarios described above.
The estimated dose for frequent Adult customer/visitor exposure to water from the Main Base water system in each of the three well contribution scenarios (0.00016, 0.00028, and 0.00012 mg/kg/day, respectively) does not exceed the provisional reference dose range (0.0001 to 0.0005 mg/kg/day). This means that noncancer (thyroid depression) health effects would not have occurred to the frequent Adult customer/visitor drinking or washing with water from the Main Base water system.
Temporary Adult resident at the transitional housing in Main Base Area: CDHS estimated the exposure for a person who used the transitional housing, for the maximum allowable time, 2 years. We assumed that the temporary Adult resident may spend as much as 24 hours per day during their stay at the transitional housing (Table 3 is a list of the exposure parameters used in the toxicological evaluation). CDHS estimated the dose if the temporary Adult resident is exposed to water as described in the three water contribution scenarios described above.
The estimated dose for temporary Adult resident exposure to water from the Main Base water system in each of the three well contribution scenarios (0.0012, 0.0021, and 0.00091 mg/kg/day, respectively) exceeds the provisional reference dose range (0.0001 to 0.0005 mg/kg/day) which means that noncancer (thyroid depression) health effects may have occurred when temporary Adult residents of the transitional housing were exposed to water from the Main Base wells. However, because there is a very large uncertainty factor associated with the provisional reference dose and the estimated doses do not approach the NOAEL (0.14 mg/kg/day), it is unlikely that these exposures did cause any noncancer health effects.
Patient at the McClellan Hospital in the Main Base Area: CDHS estimated the exposure for a person who stayed at the hospital for a two-weeks (Table 3 is a list of the exposure parameters used in the toxicological evaluation). CDHS estimated the dose if the adult patient is exposed to to water as described in the three water contribution scenarios described above.
The estimated dose for the adult patient's exposure to water from the Main Base water system in each of the three well contribution scenarios (0.0019, 0.0034, and 0.0015 mg/kg/day, respectively) exceeds the provisional reference dose range (0.0001 to 0.0005 mg/kg/day) which means that noncancer (thyroid depression) health effects may have occurred when adult patient at the McClellan Hospital was exposed to water from the Main Base wells. Since a patient's health is probably already compromised, the large uncertainty factor built into the provisional reference dose may not necessarily ensure that a health impact did not occur.