Groundwater bacterial content determination is a core component of water quality assessment, protecting public health by detecting indicator organisms such as total coliforms, Escherichia coli, thermotolerant coliforms, and heterotrophic bacteria. This guide explains internationally recognized testing standards—including GB/T 5750.12, GB/T 14848, ISO 9308, ISO 6222, and US EPA methods—along with sampling requirements, analytical procedures, acceptance limits, and typical applications for drinking water safety, environmental monitoring, and regulatory compliance.
Technical Overview
Bacteria are among the most common and most hazardous contaminants found in groundwater. Unlike chemical pollutants, microbial contamination is rarely uniform: bacteria occur in patches, multiply or die off rapidly under changing conditions, and can cause acute waterborne disease outbreaks even from a single exposure. Because pathogenic bacteria are difficult and expensive to enumerate directly, routine groundwater bacteriological analysis relies on indicator organisms—groups of bacteria whose presence signals possible fecal contamination and the potential presence of enteric pathogens such as Salmonella, Shigella, Campylobacter, and pathogenic strains of E. coli.
The principal indicators measured in groundwater bacterial content determination include the total plate count (total bacterial count), total coliforms, thermotolerant (fecal) coliforms, Escherichia coli, and fecal streptococci/enterococci. These parameters are regulated in China by GB 5749-2022 (Standards for Drinking Water Quality), GB/T 14848-2017 (Standard for Groundwater Quality), and tested according to GB/T 5750.12 (Standard Examination Methods for Drinking Water – Microbiological Parameters) and the HJ 347 series for surface and groundwater environmental monitoring. Equivalent international methods are published by ISO (ISO 9308 series, ISO 6222, ISO 8199, ISO 19458) and by US EPA (e.g., Method 1604).
All bacteriological examinations must be performed by accredited third-party laboratories operating under an ISO/IEC 17025 quality management system, using validated methods, calibrated incubators, sterile equipment, and certified reference strains for media performance verification. Correct sampling, preservation, and timely transport are equally critical, because bacteriological results reflect the condition of the sample at the moment of analysis—not necessarily the aquifer.
Testing Samples
Representative sampling is the single most important factor in obtaining meaningful bacteriological results. Samples are collected in pre-sterilized borosilicate glass or autoclaved plastic bottles (typically 100–500 mL), filled to approximately 80% of capacity to allow mixing, and opened only at the moment of collection. If the water has been disinfected, sodium thiosulfate must be added to the bottle beforehand to neutralize residual chlorine. Samples are held at 2–8 °C in the dark and should be analyzed within 6 hours whenever possible, and never later than 24 hours after collection, in accordance with ISO 19458 and HJ 493 preservation requirements.
Drinking Water and Domestic Supply Wells
Samples are taken from private wells, rural centralized supply wells, and municipal well fields, either directly from the wellhead tap or from a distribution point representing consumer exposure. Before sampling, the tap is disinfected (flamed or wiped with alcohol) and the water is flushed for 2–3 minutes to clear standing pipe water so that the sample reflects aquifer or source water quality. These samples are judged against the strict limits of GB 5749-2022.
Environmental Monitoring Wells
Dedicated monitoring wells at landfills, livestock operations, wastewater irrigation areas, and industrial sites are purged (typically 3–5 well volumes) until field parameters such as temperature, pH, and conductivity stabilize, after which a representative groundwater sample is collected using sterile bailers, bladder pumps, or low-flow sampling equipment. These samples support compliance monitoring under GB/T 14848-2017 and environmental impact assessments.
Agricultural, Livestock, and Irrigation Wells
Water used for crop irrigation, greenhouse operations, livestock watering, and aquaculture is tested for fecal indicator bacteria to prevent contamination of food products and animal health problems. Sampling frequency is typically aligned with the growing season and with the requirements of agricultural product certification schemes.
Background and Source-Tracing Samples
Up-gradient (background) wells and suspected contaminant source samples (septic leach fields, manure storage, leaking sewers) are collected in parallel to help identify contamination pathways and distinguish natural aquifer conditions from anthropogenic impacts during remediation or litigation support investigations.
Testing Items
The standard microbiological testing items for groundwater, together with their typical regulatory limits, are listed below:
- Total bacterial count (total plate count / colony count): a general hygiene indicator enumerating culturable heterotrophic bacteria; the drinking water limit in GB 5749-2022 is ≤ 100 CFU/mL.
- Total coliforms: the primary fecal contamination indicator; must be non-detectable in 100 mL for drinking water (GB 5749-2022); GB/T 14848-2017 requires non-detection for Class I groundwater and permits up to 3.0 MPN/100 mL for Class II and Class III waters.
- Thermotolerant (fecal) coliforms: coliforms that ferment lactose at 44.5 °C, indicating warm-blooded animal or human fecal origin; non-detectable in 100 mL for drinking water.
- Escherichia coli: the most specific fecal indicator, detected via beta-galactosidase and beta-glucuronidase activity or confirmatory biochemistry; non-detectable in 100 mL for drinking water.
- Fecal streptococci / Enterococci: supplementary indicators with extended environmental survival, useful for confirming historical or intermittent contamination and for fecal source discrimination (fecal coliform-to-fecal streptococci ratio).
- Heterotrophic plate count (HPC): used in ISO-based assessments (ISO 6222) to evaluate overall microbial load and the effectiveness of treatment and disinfection processes.
Testing Methods
Membrane Filtration Method
A measured volume of sample (typically 100 mL) is filtered through a sterile 0.45 µm membrane filter, which retains the bacteria. The membrane is then placed on a selective medium and incubated: m-Endo agar or chromogenic coliform agar (CCA) at 36 ± 1 °C for 18–24 hours for total coliforms, and mFC agar or EC medium at 44.5 ± 0.5 °C for fecal coliforms. Characteristic colonies are counted directly as CFU/100 mL, and presumptive E. coli colonies may be confirmed with oxidase, indole, or beta-glucuronidase tests. This method is specified in ISO 9308-1, US EPA Method 1604, GB/T 5750.12, and HJ 347.2, and is preferred for clean groundwater with low turbidity and low background flora.
Multiple-Tube Fermentation (Most Probable Number, MPN) Method
The sample is inoculated in a series of replicate tubes containing lactose lauryl sulfate tryptose broth and incubated at 36 ± 1 °C for 24–48 hours. Gas- and acid-positive tubes are subcultured into brilliant green bile broth and EC broth for presumptive and confirmed total coliforms and fecal coliforms, respectively. The combination of positive tubes is read against MPN statistical tables to yield an MPN/100 mL result. This classical method, described in ISO 9308-2, GB/T 5750.12, and HJ 347.2, tolerates turbidity better than membrane filtration and is widely used for routine surveillance.
Enzyme Substrate (Defined Substrate Technology) Method
The sample is mixed with a defined substrate medium containing ONPG (orthonitrophenyl-beta-D-galactopyranoside) and MUG (4-methylumbelliferyl-beta-D-glucuronide), then incubated at 36 ± 1 °C for 24 hours. Beta-galactosidase activity (yellow color) indicates total coliforms, while beta-glucuronidase activity (fluorescence under UV light at 365 nm) indicates E. coli. Using quantifying trays with 51 or 97 wells, results are reported as MPN/100 mL. The method is rapid, requires no confirmation steps, and is standardized in HJ 347.1 and ASTM-based enzyme substrate procedures; it is particularly suitable for high-throughput testing and for samples with high heterotrophic background.
Plate Count Methods (Pour Plate and Spread Plate)
For the total bacterial count, 1 mL of sample (or a serial dilution) is mixed with molten plate count agar at 44–46 °C (pour plate, ISO 6222 / GB/T 5750.12) or spread onto solidified agar and incubated at 36 ± 1 °C for 48 hours. All visible colonies are counted and the result is reported as CFU/mL. Dilutions are selected so that final plates contain between 30 and 300 colonies for statistical reliability.
Quality Assurance and Quality Control
- Field blanks and sterile-check controls are processed with every sampling campaign to verify the sterility of bottles and equipment.
- Positive and negative control strains (for example, E. coli as positive and Pseudomonas aeruginosa as negative for coliform media) are used to validate each batch of culture medium.
- Incubators are calibrated and monitored continuously, since fecal coliform results are highly sensitive to the 44.5 ± 0.5 °C incubation temperature.
- Duplicate samples and internal proficiency testing support the metrological traceability and ISO/IEC 17025 accreditation of the testing laboratory.
- Chain-of-custody documentation records sampling point, time, temperature, and analysis deadline to guarantee sample integrity.
Applications
Bacterial content determination of groundwater supports a wide range of regulatory, industrial, and public health objectives:
- Drinking water safety compliance: routine verification of well water and small centralized supplies against GB 5749-2022 and WHO guideline values before and after commissioning.
- National and regional groundwater monitoring: periodic assessment of aquifer quality classes under GB/T 14848-2017 for groundwater resource management and protection zoning.
- Contamination investigation and source tracing: identification of failing septic systems, manure leakage, sewer exfiltration, and landfill leachate impacts using multi-indicator comparison and background wells.
- Remediation and treatment validation: verification of disinfection (UV, chlorination, ozonation) performance and of wellhead treatment systems, including challenge testing and post-treatment confirmation sampling.
- Agriculture and food safety: certification of irrigation and livestock water for GAP (Good Agricultural Practices), organic production, and export-oriented food supply chains.
- Construction and engineering projects: water quality evaluation for dewatering discharge, geothermal systems, and environmental impact assessments required by environmental authorities.
FAQ
What is the acceptable bacterial level in groundwater used for drinking?
According to GB 5749-2022, total coliforms, thermotolerant coliforms, and E. coli must not be detected in any 100 mL sample, and the total plate count must not exceed 100 CFU/mL. Any detection of fecal indicator bacteria in drinking water requires immediate boil-water action, well disinfection, and follow-up testing.
How quickly must a groundwater sample be analyzed after collection?
Samples should be transported at 2–8 °C in the dark and analyzed within 6 hours of collection where feasible, with an absolute maximum hold time of 24 hours. Delays or warm transport allow bacteria to multiply or die, invalidating the result.
Why is sodium thiosulfate added to sampling bottles?
Sodium thiosulfate immediately neutralizes any residual chlorine or other oxidizing disinfectant in the water, preventing continued bacterial kill-off between sampling and analysis and ensuring that reported counts reflect the water at the moment of collection.
How often should a private well be tested for bacteria?
At minimum once per year, preferably after the rainy season, and additionally after flooding, well repair or pump replacement, any change in water taste, odor, or color, or any household illness consistent with waterborne infection.
What are the most common causes of bacterial contamination in wells?
Typical causes include inadequate wellhead sealing or damaged casing, proximity to septic tanks and leach fields, animal waste infiltration, surface runoff entering the well during heavy rain, and biofilm growth inside pipes and storage tanks.
Which method should be chosen: membrane filtration, MPN, or enzyme substrate?
Membrane filtration is the first choice for clear, low-turbidity groundwater and yields direct CFU counts; the MPN method is more robust for turbid samples with high background flora; the enzyme substrate method offers the fastest turnaround, simplest workflow, and simultaneous detection of total coliforms and E. coli, making it ideal for high-volume monitoring programs.