E. coli in Iowa lakes and streams, 2020-2023
Dan Haug, Prairie Rivers of Iowa
This is a prototype for an interactive map that would inform recreational users about the water quality at both swimming beaches and rivers in Iowa. Like the state’s 2024 Integrated Report, it uses data from the 2020-2022 recreational seasons to evaluate which lakes and rivers meet recreational standards. However, the purpose is to educate the public rather than making regulatory determinations. Toward that end, it offers the following improvements.
- The map shows sites in DNR’s AQuIA database that were monitored for E. coli bacteria in 2020-2022. This makes missing data more obvious. A map of impaired waters does not distinguish between waters that are not impaired because they met water quality standards, and waters that are not impaired because they were not assessed.
- The map is color coded to show which sites meet primary and secondary recreation standards. Which standard is relevant to you depends on water levels, whether you have small children, and whether you plan on getting submerged in the water. For waters that are not a downstream of a regulated facility, the designated uses may not have been assessed.
- The most recent season (2023) is also included.
Standards
E. coli bacteria criteria for recreational waters apply from March 15-November 15, when the water is warm enough that recreational use is possible. A minimum of seven samples are needed in a season to apply the standards. The standard has two parts:
1) A geometric mean for the season should not exceed 126 colonies/100mL (primary contact recreation) or 630 colonies/100 mL (secondary contact recreation).
2) No single sample should exceed 235 colonies/100 mL (primary contact) or 2,880 colonies/100mL (secondary contact). If more than 9 samples are collected, no more than 10% of samples should exceed this threshold.
The color coding on the maps is based on the geometric mean. Use the radio buttons to view different seasons. Click on a point to get additional information, including whether the single-sample maximum was violated.
Green = Meets primary contact recreation standard (<126 colonies/100mL)
Yellow = Exceeds primary contact standard but meets secondary contact recreation standard (126-630)
Red = Exceeds secondary contact recreation standard (> 630)
Lakes
Streams