Evaluating appropriate maximum holding times for private well water samples

Privately maintained groundwater wells are often located at a great distance from laboratories, creating a barrier to bacteriological testing (a necessity for determining drinking water potability). Extending the acceptable holding time between testing and collection could potentially diminish this barrier. Using seven Escherichia coli strains isolated from private well waters, the acceptability of Ontario's current allowable sample maximum holding time (48 h) was compared with time of collection. Additionally, the acceptability of extending the holding time from 48 h to 72 h was investigated. All analyses were performed using noninferiority statistical approaches to determine if later holding times had no meaningful difference in bacterial growth (determined by colony forming unit enumeration). All strains did not statistically decrease below the acceptable 10% difference during the two time periods. However, variations in the survival rates of isolates were observed, suggesting that a risk management approach should be employed when determining maximum holding times.