What’s the problem with nitrate/nitrite in drinking water and what is the best way to manage it?
– New York
Nitrates and nitrites have received much attention as drinking water and dietary contaminants with concerns about potential health risks. Nitrate, and sometimes nitrite, are frequently present in drinking water sources and they are common problems particularly in smaller public water supplies and probably more so in home well waters. The Environmental Protection Agency (EPA) has reported that eight states have more than 10 percent of their areas with groundwater nitrate concentrations that exceed 5 mg/L as N (half of the drinking water standard) (www2.epa.gov/nutrient-policy-data/estimated-nitrate-concentrations-groundwater-used-drinking). The principle concern is for people who produce drinking water from their own wells because that water is seldom treated — except perhaps softening — and almost never disinfected for nitrate removal. Nitrate and nitrite can occur in surface waters and groundwaters. Surface waters can contain them from runoff of agricultural fertilizers, which could have periodic peaks associated with planting season or rainfall, or from upstream sewage discharges. Groundwaters can be generally contaminated from surface fertilizer uses, as well as locally from residential septic tank outflows, which would also contribute microbial contamination.
In some water supplies, particularly where chloramine residuals are used, there can be production of small amounts of nitrite at the extremes of the system with long residence times. Generally, the principle source of human exposure is dietary from consumption of vegetables and cured meats; however, water can become an important source for bottle fed infants less than three to six months old. The risk is significantly increased when the child has a gastrointestinal infection, which can increase the conversion of nitrate to nitrite in the stomach. Those infections can be induced by consumption of microbially contaminated drinking water that may concurrently contain nitrate/nitrite.
Drinking water standards
The drinking water standards in the U.S. are: Nitrate, 10 mg/L as N, ( mg/L as NO3–); nitrite, 1 mg/L as N, (~ 3 mg/L as NO2–). The standard for the combined total is 10 mg/L as N. The basis for the drinking water standards and the traditional concern about nitrate and nitrite in drinking water is the risk of infant methemoglobinemia (aka, blue baby). Nitrate is converted to nitrite under reducing conditions — it can oxidize a portion of the ferrous iron in blood hemoglobin to ferric iron. That reduces its ability to release oxygen to tissues and a sufficient percentage level of methemoglobin results in oxygen starvation and possibly death.
Typical background levels of blood methemoglobin are less than 2 percent; levels approaching 10 percent are a significant concern, especially for pregnant women. Fortunately, methemoglobinemia is treatable when recognized and serious illness and deaths are rare in the U.S., but more frequent in some other parts of the world. When it is water-related it is due to groundwater contamination, often from residential septic tanks.
Nitrate and nitrite are difficult to remove from water. However, disinfection alone will oxidize nitrite to nitrate, which is the less hazardous form. Disinfection is especially important because it also reduces the risk of gastrointestinal infections.
Conventional coagulation/sedimentation/filtration water treatment technologies are not effective because nitrate salts are very soluble in water. Municipal plants have used anion exchange (IX), reverse osmosis (RO), electrodialysis (EDR) or biological denitrification. None of these are traditional simple technologies or low cost. EPA has published guidance for water treatment technologies (Point-of-use or Point-of-entry Treatment Options for Small Drinking Water Systems, EPA 815-R-06-010).
POU and POE technologies are particularly appropriate for home well contamination situations, but they may also be cost effective in small community environments. A detailed study of a community-wide POU implementation of decentralized treatment for arsenic removal has demonstrated the methodology and success of the concept (Feasibility of an Economically Sustainable Point-of-Use/Point-of-Entry Decentralized Public Water System. EPA Grant X82952301).
Anion exchange, RO and distillation are available for POU and/or POE applications. Since the health risk for water from drinking and cooking is the concern, POU will be most cost effective because only a few liters of water will be needed each day for those purposes. Several RO and IX units have been certified to ANSI/NSF 58 or 53 by CSA, IAPMO, NSF, UL or WQA. Although anion exchange activated alumina or iron-based media are quite effective technologies for removal of anions like fluoride and arsenic, nitrate is less efficiently bound to the media. The New Hampshire Department of Environmental Services (2006) has concluded that RO is the most cost effective of the three options for a small required water volume considering all factors, although RO has lower water recovery and higher wastage. Disinfection might also be necessary in many cases, although the RO might also remove most microorganisms.
Water quality and testing
It is important to understand the nitrate and composition microbial status of the water to be treated to determine the challenge as well as to periodically test treated water to be sure that the water does not exceed a drinking water standard. There are simple to use and inexpensive colormetric test kits available for quick measurements of nitrate and nitrite. For example, some companies sell suitable test strips and color comparison tests for drinking water that cost about $1 per test. Carefully follow the directions and be sure whether the reading is as nitrate, NO3–; nitrite, NO2–; or as N. If a more accurate test result is required, water samples can be sent to a certified laboratory for analysis, but those costs can be about $15 per sample. There are also simple tests for coliform bacteria that can be conducted at commercial or local public health laboratories, or even on-site using a test kit.
Information is available from community drinking water supplies on the composition of their drinking waters and compliance with the drinking water regulations. A Consumer Confidence Report (CCR) is usually required annually that includes their status and water quality and violations. Most of them can be found online by searching Consumer Confidence Report with the name of the community.
Drinking water health issues and the press
There are occasional reports in the press of studies associating nitrate/nitrite in drinking water and various health outcomes, including cancer and birth defects. These can create anxieties among the public because they are frequently not balanced by contrary information. The weight of evidence does not seem to support the occasionally reported associations with nitrate. Here are several quotes from the World Health Organization Guidelines for Drinking Water Quality 4th edition, 2011, at pp.398-403:
- “The weight of evidence indicates that there is unlikely to be a causal association between gastric cancer and nitrate in drinking water.”
- “There have been suggestions that nitrate in drinking water could be associated with congenital malformations, but the overall weight of evidence does not support this.”
- “There have been suggestions of an association between nitrate in drinking water and the incidence of childhood diabetes mellitus. However, subsequent studies have not found a significant relationship…”
In a 2006 review by U.S. Centers for Disease Control and Prevention epidemiologists, they concluded, “The current literature does not provide sufficient evidence of a causal relationship between exposure to nitrates in drinking water and adverse reproductive effects.” (Env. Health Perspectives 114(3): 320-327)
In the last few months there was a published report that was widely disseminated in the national press that there was a higher incidence of certain birth defects, including spina bifida, limb deficiencies, cleft palate and cleft lip, associated with offspring from mothers who consumed more than 5 to 5.42 mg of nitrate per day from drinking water in the first trimester compared to controls (<1 mg/day). This surprising “association” was reported in the press as confirming that there were birth defect risks associated with nitrate in drinking water even at very low levels. The above reported breakpoint exposure was about 6 percent of the nitrate intake for someone drinking two liters per day of water at the current drinking water standard. The lead author stated that there were insufficient numbers of cases to determine a dose response. The authors also stated that the reported association with nitrate could have been caused by some other component in the water.
However, in a study from most of the same authors reported a few months prior, they concluded: “Overall, odds of neural tube defects, oral clefts or limb deficiencies did not appear to be significantly associated with estimated dietary intake of nitrite, nitrite and nitrosamines.”
Nitrate from either food or water would behave similarly (identically?) in the GI tract after consumption. Thus, this earlier negative finding for total daily nitrate intake from dietary food and water, a larger amount than from water alone, is inconsistent with the later positive water association report and, therefore, renders it questionable, at best. So, it is appropriate to be cautious when interpreting unbalanced media reports and to carefully read published literature reports of adverse effects of this sort from drinking water.