Review of Iowa Lakes Using Proposed Water Quality Standards

The Iowa Department of Natural Resources (IDNR) has proposed a revision to the water quality standards used to assess Iowa lakes. [Read more about the proposed standards] In conjunction with this rulemaking process, IDNR has published a review of existing data sets on Iowa lakes to evaluate how well lakes in the State currently comply with the current and the proposed standards.

The report, “How Many Lakes Will Meet the New Criteria”, compares sampling data from 132 lakes with both the proposed criteria and the criteria currently used to decide whether a lake fully supports swimming. The Trophic State Index (TSI) is used to quantify whether a lake meets the narrative water quality criteria contained in Iowa’s water quality standards such as a lake must be free of nuisance aquatic life. The TSI measures water quality parameters such as nutrient concentrations, transparency and chlorophyll-a concentrations. algae blooms. These narrative criteria apply to all surface waters.

In summary, of the 132 lakes reviewed, 64% do not meet the TSI criteria. A total of 88% of these lakes do not meet the proposed criteria. Most of the lakes that do not meet the proposed criteria would not meet either the transparency or the chlorophyll-a criteria.

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