Who is drinking nitrate in their well water?

Introduction

Groundwater contamination with nitrate is a pervasive and serious problem. It is a health threat because of population expansion into rural areas, and an increasing population which relies heavily on private water systems. In Oregon, groundwater use is expected to expand in proportion to the rate of population growth (1). Little has been known about the quality of private well water supplies in Oregon because, until recently, these systems were not routinely monitored by their users or any governmental agency. Legislation in 1991 required testing of well water for nitrate and total coliform bacteria, but only when properties were sold or exchanged (2).

Approximately twenty years ago the World Health Organization recommended a maximum contaminant level (MCL) of 10 ppm nitrate nitrogen (nN) in drinking water (3). The United States Environmental Protection Agency (USEPA) and a majority of state agencies which oversee drinking water programs adopted this level as an indicator of potential health risk posed by drinking-water exposure to nitrate (4). The most common health concern is methemoglobinemia, or "blue-baby syndrome," a potentially fatal condition which inhibits the ability of red blood cells to bind and transport oxygen (5). Nitrate and nitrite have also been linked to such conditions as cancer, mutagenic and teratogenic effects (5,6), birth defects (6), behavioral and developmental abnormalities (7), and cardiovascular disease (8). A more detailed review of the health effects associated with nitrate exposure is found in a recent report published by the Oregon Water Resources Research Institute (9).

Groundwater Contamination in the LUB 

It has been known for several years that much of the groundwater in the Lower Umatilla Basin (LUB) of Umatilla and Morrow Counties of northeastern Oregon is contaminated with nitrate. A 1995 project report indicates that of 850 LUB groundwater samples for nitrate taken between 1990 and 1993, approximately 30% had nitrate levels over 10 ppm, 44% had nitrate levels between 2 ppm and 10 ppm, and the remaining 26% of the samples had nitrate levels under 2 ppm (10). This rural region also has a high concentration of private domestic water wells.

Land use is a major reason for current levels of nitrate contamination in the LUB. First, the area has a history of intensive farming practices using fertilizers and field irrigation methods. The nitrogen in nitrogen-containing fertilizers applied to crops, and not fully utilized by plants and soil organisms, may leach to groundwater. Second, a food processing industry operates within the same region resulting in an abundance of readily-available, nitrogen-rich plant residues and liquid wastes. If these wastes are land-applied to supplement chemical fertilizers, there is a tendency to over-fertilize and increase the potential for leaching of nitrate to groundwater. Third, septic systems are in common use in the LUB. Nitrate may leach to groundwater from leaking septic tanks, improperly constructed leach fields, failed systems, and even from properly operating septic systems.

Geology and precipitation also are important factors in nitrate contamination to groundwater. The LUB is an arid, 780-square-kilometer area, receiving an annual precipitation of less than 25 cm. The soils are predominantly sandy and the aquifers are shallow and unconfined. Irrigation water applied to crops under this set of conditions, if in surplus of plant requirements, can leach nitrate to groundwater. These relatively unprotected aquifers are also the most vulnerable to pollution and the buildup of contaminants over time.

Although high well water nitrate levels have been documented in some areas of the state, no studies have been conducted which evaluate the extent of population exposure to this nitrate source. The purpose of this study was, therefore, to document demographic characteristics of domestic well users in the LUB, and to evaluate whether or not this population is potentially at risk from drinking well water containing nitrate.

Methods

Three data sets were used in this study. Individuals living in rural residences in the LUB who used private wells were surveyed by telephone to gather information about their water consumption practices. This data comprises the first data set. The second data set, information on well water nitrate concentrations, was collected by the Oregon Department of Environmental Quality (DEQ) between July 1990 and October 1991. The third data set includes demographic information about Oregon residents taken from the 1990 U.S. census (11).

Surveys were administered to 83 residences in the LUB area. Although the Oregon DEQ had initially sampled 198 wells in this area, 80 wells, serving these 83 residences, were selected because they were the only private wells used for drinking water. Wells used exclusively for irrigation, stock watering, commercial or industrial supply purposes, or as public water supplies were not included, and people using these wells were not surveyed.

The telephone survey collected information regarding population demographics and socioeconomic data, dietary practices, experience of illness, the existence of alternative sources of water, water treatment devices in use, well siting practices, and other factors which could potentially affect well water nitrate exposure. Information was also gathered on potential local sources of nitrate contamination of wells. Data from these three sets were combined and described using median values, proportions, and frequency distributions. Because the data were not normally distributed, median rather than mean values were used as measures of central tendency.

Results

The 83 households contacted comprised a sample of 219 people. These residents represented approximately 0.8% of the total LUB population, and approximately 0.7% of the rural residents of the two counties. All residents of the households included in the survey, except for the residents of two households who drank bottled water or city water, used the water from their wells for drinking purposes. All residents prepared their food with well water.

The age distribution of the survey population is shown in Figure 1. The percentage of the population in each of the five age groups varied, with those over forty years in age comprising the largest group (62%). Twenty-eight percent of the over-forty group were sixty-five years in age or older. Only three infants, ages three months, eight months, and eleven months, were in the survey population. Approximately equal numbers of males and females were included in each age group. The median age for both genders was approximately 51 years.

The majority of respondents (63%) had lived elsewhere in the LUB prior to moving to their current location. Approximately 75% of the respondents had lived in their present location for more than ten years, and only seven percent reported that they had lived in their present location for fewer than two years. This indicates the potential for long-term exposure to nitrate from well water for these individuals.

Respondents were asked to identify local sources of potential well water nitrate contamination located within 30 m of their wells. Thirty-five (42%) of 83 households surveyed reported the presence of septic systems, agricultural crops, vegetable gardens, pastures, or barnyards within 30 m of their wells.

The respondents were also asked about the use of water treatment devices. Twenty-five households (30%) had installed such devices. Eight of the 25 households used reverse-osmosis systems, seven had installed charcoal-filter devices (which are not capable of removing nitrate), and the remaining ten households used alternative water treatment methods, such as sand filters or water softeners (which are also incapable of removing nitrate). In addition to the 25 households using water treatment devices, two households used bottled and/or city water because the Oregon Health Division (OHD) had previously alerted them that their well water nitrate levels exceeded the MCL. Because shallow wells are generally more at risk of nitrate contamination than deep wells, well depth and nitrate concentrations were compared for 78 of the wells in the LUB survey area (12,13). These results are shown in Figure 2. In general, shallow wells (less than 50 m in depth) are more likely to have nitrate levels of 10 ppm nN or more than are deep wells. This study, however, did not consider well construction and its possible effect on contamination at varying well depths.

Nitrate Exposure

Exposure was defined as any ingestion of well water containing nitrate, and no attempt was made to quantify the amount of nitrate ingested daily as a result of well water consumption. Given this definition of exposure, 57% (n=125) of the population was exposed to nitrate concentrations between [less than] 0.2 ppm and 4.9 ppm nN; 20% (n=44) were exposed to concentrations between 5.0 and 9.9 ppm nN; and 23% (n=50) were exposed to concentrations from 10.0 to 40.0 ppm nN [ILLUSTRATION FOR FIGURE 3 OMITTED!.

We were interested in the ages of people drinking water at these nitrate contaminated levels. Approximately 60% of the population exposed to 4.9 ppm nN or less in well water was over forty years old. Also among the group exposed to this same nitrate concentration were 28 persons nineteen to thirty-nine years old, nine youths ages six to eighteen, eight youngsters ages two to five, two infants six and eleven months old, and one three-month-old infant. Three women who were pregnant at the time of the survey were drinking well water with nitrate concentrations of 0.6 ppm, 4.5 ppm, and 4.9 ppm nN.

The three-month-old resided in a household where well water nitrate had been sampled at 4.5 ppm nN. At the time of the survey, the infant was being fed formula prepared with well water, supplemented daily with feedings of several ounces of plain well water or drinks mixed with well water. The two older infants, ages eight and eleven months, were exposed to 4.1 and 1.8 ppm nN respectively.

Approximately 55% of the people exposed to well water nitrate levels from 5.0 to 9.9 ppm were over forty years old [ILLUSTRATION FOR FIGURE 4 OMITTED!. Seventy-two percent of those exposed to 10 ppm nN or greater were over forty years old [ILLUSTRATION FOR FIGURE 5 OMITTED]. No children under two years old were exposed to nitrate concentrations above 4.5 ppm nN.

Discussion

Well Water Nitrate Levels

The median nitrate level for wells serving the surveyed LUB households (4.5 ppm nN) was nearly three times greater than national median levels of 1.6 ppm nN for community water systems and private wells combined (14). Nationally, 2.4% of all rural private wells exceed the 10 ppm nN MCL, whereas 25% of the LUB survey wells exceeded this level (14). This figure of 25% of wells above the MCL for nitrate is also considerably higher than that found in a study of private wells in agricultural areas of Pennsylvania, where 9.4% of private wells tested higher than 10 ppm nN (8).

The LUB survey findings were similar to those reported in an earlier study of well water nitrate in northern Malheur County, Oregon, where approximately 30% of the sites exceeded the 10 ppm nN MCL (1). The LUB and northern Malheur County areas were chosen by the Oregon DEQ for intensive study because of the known existence of relatively high groundwater nitrate levels. These focused efforts have led to a proposal to adopt a permanent maximum measurable level (MML) which would replace the interim MML for groundwater nitrate (15). Currently, the groundwater nitrate MML is used to trigger designation of groundwater management areas which occur when area-wide groundwater nitrate concentrations are detected in excess of 70% of the MML. Both the LUB and northern Malheur County have been declared groundwater management areas based on the interim MML of 10 ppm nN (15).

Thirty-three (41%) of the 80 wells surveyed were located within 30 m of potential sources of nitrate contamination, and 17 (52%) of these 33 wells had reported nitrate levels of 5 ppm nN or greater. These results suggest a possible relationship between local sources of nitrate contamination and nitrate concentrations in these wells. These findings do not appear unusual in light of previous studies which demonstrate relationships parallel to the findings of this study (12,16).

Data from this study also suggest that nitrate concentration may decrease with increasing well depth. If confirmed, this finding would be in agreement with prior studies with respect to the increased risk of nitrate contamination in shallow wells (although the definition of "shallow" is a relative term, depending on the range of well depths in each study area) (8,12,13).

Exposure and Risk

Although a high percentage (23%) of the residents in the survey area were exposed to nitrate at or above the 10 ppm MCL, 72% were over forty years old. Assuming these adults are healthy, they are unlikely to experience adverse health effects even if they consume water containing up to 20 ppm nN (15). Because no infants under six months old were exposed to well water nitrate above the MCL, the risk of infant methemoglobinemia was considered to be low among the population surveyed at the time of the study. Although the youngest infant in the survey population (age three months) was fed formula prepared with well water, and received supplemental feedings prepared with well water, she was exposed to waterborne nitrate which was well below the level considered to predispose infants to methemoglobinemia. Drinking water containing up to 10 ppm nitrate is considered by the majority of public health agencies in the United States to be safe for infants under six months old (15).

Of the eight women exposed to well water nitrate exceeding 10 ppm nN, only four were married and of childbearing age, and the remaining four were between the ages of eleven and eighteen years. Three women were pregnant at the time of the survey, but were exposed to well water nitrate below 10 ppm nN. Although methemoglobin levels rise during pregnancy, no cases of maternal methemoglobinemia during pregnancy have been documented in humans (4). However, pregnant women still appear to be at higher risk of methemoglobinemia than non-pregnant women, and they should be aware and cautious of the concentration of nitrate they are ingesting in their drinking water (15). The use of reverse-osmosis purifying devices and alternative water sources may have lessened exposures for some of the residents. Reverse-osmosis units were operating in eight households, five of which had reported nitrate levels over 10 ppm (19 to 40 ppm nN). The remaining three homes had nitrate levels of 1.8 ppm nN or less. Six of the eight reverse osmosis devices in use had been in service for three years or longer. Two other units had been in use less than one year. One respondent, who used one of the recently installed devices, stated that "poor taste" of the water, rather than high nitrate level (19 ppm nN), was the primary reason for installation of the filtering device.

One important factor may have been overlooked in the eight households in which reverse-osmosis devices had been installed. Residents of these households may have been unaware of the requirement for periodic maintenance of their units. Without proper maintenance, these units can lose their effectiveness.

Two households, with a combined total of five residents, used bottled water and/or city water to avoid excessive well water nitrate (31 ppm and 14 ppm nN). One three-year-old child resided in the home with the 14 ppm nN concentration. Both households had begun to use alternative water sources following notification by the OHD of well water nitrate which exceeded the standard.

It was found that 13 (72%) of 18 households drinking well water which exceeded the 10 ppm nN level did not use devices which effectively remove nitrate, nor were they using alternative water sources to avoid exposure. Again, most of the residents in these homes were over forty years in age. The adults in these households may have perceived themselves to be at low risk from exposure to well water nitrate because of their age. Also, because no infants under six months old resided in these homes, it may have been decided for these reasons to take no measures to reduce well water nitrate exposure. On the other hand, individuals whose water was treated by charcoal filters, paper filters, or water softeners may have erroneously believed that these devices would also remove nitrate.

Conclusions

This study investigated the demographics of well water users in the rural LUB of Oregon, and compared these data to identified well water nitrate levels to evaluate whether or not this population is potentially at risk from drinking water from their wells. Results of the study revealed that 25% of the domestic-use wells in the survey area had nitrate levels which were in excess of the USEPA 10 ppm MCL for drinking water. Twenty-three percent of the surveyed population itself was potentially exposed to nitrate concentrations in excess of the 10 ppm nN standard.

Although almost all the surveyed residents were using nitrate-contaminated wells for their sole source of drinking water, the risk of methemoglobinemia among this population remained low because few infants resided within the area. The degree of risk was probably not elevated for women who were pregnant at the time of the survey, nor would it have been a concern for their newborn infants, because these women were not living in homes with well water nitrate levels above the MCL. If demographic patterns remain the same, and there is no substantial increase in the number of young families moving into the area, the risk of methemoglobinemia should remain low. Therefore, no intervention measures appear to be warranted at this time in the LUB.

We agree with the OHD, which recommends that use of well water with nitrate levels in excess of the USEPA standard should be avoided (17). The best option, although not always a feasible solution, is for residents whose wells exceed the nitrate MCL to connect to an existing community water system. Bottled water may also be a viable alternative water source if the consumer can be assured that the bottled water meets water quality standards as determined by an appropriate agency, such as a state department of agriculture or health department. For those wishing to install treatment devices in their homes, ion exchange, distillation, or reverse osmosis will remove nitrate, but these devices require careful monitoring and maintenance. It is further recommended that if a home-treatment system is to be used, one certified by the National Sanitation Foundation International should be selected and properly maintained (17).

Additional monitoring of private wells by state and/or local agencies is recommended, especially in areas where domestic-use wells exceed 10 ppm nN. It is particularly important that test results be shared with local health agencies, extension offices, or public utilities, which may be able to provide direct follow-up services to affected residents. Educational programs for the local residents concerning their role in preventing groundwater contamination is urged. Education of health-care providers is also important, as the true incidence of methemoglobinemia may be underreported (18).

Acknowledgements

The authors gratefully acknowledge the assistance of Gerald Grondin of the Oregon DEQ for making available much of the data needed for this study, and Dennis Nelson of the Oregon Health Division for his helpful suggestions.

REFERENCES

1. Oregon Dept. of Environmental Quality (DEQ) (1990), 1990 Water Quality Status Assessment Report (305b Report), Portland, Or:DEQ, Water Quality Div.

2. Oregon Dept. of Environmental Quality (DEQ) (1995), Hydrogeology, Groundwater Chemistry and Land Uses in the Lower Umatilla Basin Groundwater Management Area Portland, Or:DEQ, Water Quality Div.

3. World Health Organization (WHO) (1978), Nitrates, Nitrites, and N-nitroso Compounds, Geneva, Switzerland:WHO.

4. U.S. Environmental Protection Agency (USEPA) (1987), Nitrate/Nitrite Health Advisory, Washington, D.C.:USEPA, Office of Drinking Water.

5. National Research Council (1981), The Health Effects of Nitrate, Nitrite, and N-nitroso Compounds, Washington, D.C.:National Academy Press.

6. Dorsch, M.M., P.A. Baghurst, K.F. Dyer, A.J. McMichael, and R.K. Scragg (1984), "Congenital Malformations and Maternal Drinking Water Supply in Rural South Australia: A Case-control Study," Am. J. Epidemiol., 119:473-486.

7. Gottlieb, M.S. (1988), An Epidemiologic Evaluation of the Role of Groundwater Nitrate on Development in Infancy: An Investigative Study in Portage County, Ph.D. diss., University of Wisconsin, Madison.

8. Swistock, B.R., R.D. Robillard, and W.E. Sharpe (1993), "A Survey of Lead, Nitrate and Radon Contamination of Private Individual Water Systems in Pennsylvania," J. Envtl. Health, 55(5):6-12.

9. Mitchell, T.J., and A.K. Harding (1996), An Evaluation of Well-water Nitrate Exposure and Related Health Risks in the Lower Umatilla Basin of Oregon, OWWR1 Special Report SR 96-1, Oregon State University, Corvallis, Or:Oregon Water Resources Research Institute.

10. Oregon Legislative Assembly, Legislative Counsel Committee (1991), 1991 Oregon Revised Statutes (Chapter 448, section 271), Salem, Or.

11. U.S. Bureau of the Census (1991), 1990 Census of Population - General Population Characteristics, Oregon, Washington, D.C.:Government Printing Office.

12. Hallberg, G.R. (1987), "Nitrates in Iowa Groundwater," Rural Groundwater Contamination, ed. F.M. D'Itri, and L.G. Lewis, Chelsea, Mi:Lewis Pub., pp.23-68.

13. Keeney, D. (1986), "Sources of Nitrate to Ground Water," Critical Reviews in Envtl. Control, 16:257-304.

14. U.S. Environmental Protection Agency (USEPA) (1990), National Pesticide Survey: Project Summary, NTIS No. PB91-125765, Washington, D.C.:Office of Water/Office of Pesticides and Toxic Substances.

15. Oregon Dept. of Environmental Quality (Oregon DEQ) (1995), Proposed Adoption of a Permanent Maximum Measurable Level for Groundwater Nitrate, public notice issued July 28, 1995 during public comment period, Portland, Or:Oregon DEQ, Water Quality Div.

16. Exner, M.E., and R.F. Spaulding (1985), "Ground-water Contamination and Well Construction in Southeast Nebraska," Ground Water, 23:26-33.

17. Oregon Health Div. (OHD) (June 1993), What is Nitrate? Fact sheet prepared by the Dept. of Human Resources, Portland, Or.

18. Johnson, C.J., and B.C. Kross (1990), "Continuing Importance of Nitrate Contamination of Groundwater and Wells in Rural Areas," Am. J. of Ind. Med. 18:449-456.

Corresponding Author: Anna K. Harding, Ph.D., R.S., Assistant Professor, Department of Public Health, 309 Waldo Hall, Oregon State University, Corvallis, OR 97331-6406.