Coliform, E. coli, and Nitrates: Microbiological and Agricultural Contaminants

Total coliform: an indicator, not a pathogen

Total coliform bacteria are a broad group of gram-negative organisms that share certain biochemical characteristics. Most coliform species are not themselves pathogenic; they live harmlessly in soil, vegetation, and the digestive tracts of warm-blooded animals. Their relevance to drinking water testing is as an indicator: their presence in a water sample suggests that a pathway exists for contamination from the surface or shallow groundwater to enter the well.

For municipal water systems, EPA's Total Coliform Rule and the Revised Total Coliform Rule (RTCR, 2013) require routine sampling and corrective action when total coliform is detected at frequencies above defined thresholds. The MCL framework treats any total coliform detection as a notification trigger. For private wells, the CDC guidance is similar: any total coliform detection should prompt investigation of the well integrity, sanitary cap, casing, and surrounding area for contamination pathways.

E. coli: the priority indicator

Escherichia coli (E. coli) is a specific subset of total coliform that originates exclusively in the digestive tract of warm-blooded animals. E. coli detection in drinking water is a clear, unambiguous signal of fecal contamination. EPA classifies E. coli as a Tier 1 violation requiring same-day public notification by the utility.

For private well owners, E. coli detection on an annual test is a public health emergency. The well should be taken out of service for drinking and cooking until the contamination source is identified and the well is shock-chlorinated and re-tested with negative results. Many state DOH agencies provide free or subsidised testing in response to E. coli detection.

UV disinfection at 40 mJ/cm² is the standard residential treatment for E. coli and other coliform contamination. The EPA UV Disinfection Guidance Manual recognises UV-C at 254 nm and 40 mJ/cm² as effective against E. coli with a 4-log reduction (99.99 percent inactivation). UV does not address the underlying contamination pathway; remediation of the well or septic system is also required.

Nitrate: agricultural runoff and infant risk

Nitrate (NO3-) enters drinking water primarily from agricultural sources: nitrogen fertilisers, manure runoff, and septic system discharge. EPA's MCL for nitrate is 10 mg/L (as nitrate-nitrogen), reflecting protection against methemoglobinemia (blue baby syndrome) in formula-fed infants under six months of age. The MCLG is also 10 mg/L for nitrate.

Methemoglobinemia occurs when nitrate is converted to nitrite in the digestive tract and then to methemoglobin, which cannot transport oxygen. Infants under six months are particularly susceptible because their digestive systems convert nitrate to nitrite at higher rates than adults, and because formula prepared with high-nitrate water is the dominant exposure pathway. CDC and EPA both warn against using high-nitrate water for infant formula preparation.

Treatment options for nitrate are limited compared to other contaminants. Activated carbon does not remove nitrate. Conventional filtration does not remove nitrate. Three technologies are effective:

• Reverse osmosis: Achieves 60 to 90 percent nitrate rejection. POU RO at the kitchen sink is the most common residential nitrate-reduction approach.
• Anion exchange (specifically nitrate-selective resin): A dedicated treatment that removes nitrate via ion exchange, similar in mechanism to a softener but with a different resin chemistry.
• Distillation: Effective but energy-intensive; rarely used for whole-household nitrate removal.

The CDC annual well testing recommendation

CDC's Drinking Water - Guidelines for Testing Well Water page recommends annual testing of every private well for four parameters at minimum:

• Total coliform bacteria
• Nitrate (as nitrate-nitrogen)
• Total dissolved solids (TDS)
• pH

Additional testing is recommended when:

• Constructing a new well or rehabilitating an existing one
• Taste, odour, or colour changes appear
• Flooding or other contamination event in the area
• Family illness suspected to be water-related
• Pregnancy or infant in the household (test for nitrate, lead, and bacteria)
• Regional risk factors (arsenic, radon, pesticides, fluoride, uranium)

Testing must be done by a state-certified laboratory. The list of certified labs is available through your state Department of Health or through EPA's state lab certification program. Home test kits can flag obvious issues but are not suitable for compliance testing or treatment-system design.

Combined treatment for wells with co-occurring contamination

A private well with both bacterial and nitrate contamination needs a multi-stage train. UV disinfection alone does not remove nitrate; nitrate-removal technologies (RO, anion exchange) generally do not inactivate microbes. The standard configuration is:

• Spin-down sediment 50 micron (coarse pre-filter)
• Iron / manganese treatment if present
• Depth sediment 5 micron (UV pretreatment)
• UV disinfection at 40 mJ/cm² (whole-house pathogen inactivation)
• Point-of-use RO at the kitchen sink (drinking-water nitrate removal)

Whole-house anion exchange for nitrate is also possible but less common than POU RO; the resin requires periodic regeneration with sodium chloride brine, similar to a softener.

Related: UV disinfection at 40 mJ/cm², CDC well testing protocol, Well water guide.