Most Common Lake Impairment Causes

Research question

Which impairment causes appear most often in state lake assessments?

Methodology

The ranking reads the national cause rollup: for every impairment cause states have listed on a lake-type assessment unit it carries the number of waterbodies carrying that cause, the number of states that list it, and how many of those listings have a management action attached. The causes are ordered by waterbody count and the ten most-listed are shown. The rollup is rebuilt from the assessment records each time the dataset is refreshed.

A cause is a label a state chose, not a measurement this site made. ATTAINS records the pollutant or condition the state named when it decided a waterbody does not support a designated use, and PlainLakes carries that label across unchanged apart from de-shouting the all-capitals source text into readable case. Where a scientific name has a fixed spelling, that spelling is preserved: pH is not "Ph", PCBs is not "Pcbs", and Escherichia coli keeps its lower-case species epithet.

States do not share one vocabulary, and the top of this ranking shows it. Mercury reaches the list under several distinct labels, including a fish-tissue reading and a fish-consumption advisory, because different states file the same underlying problem under different names. Read the ranking as the most frequently FILED labels rather than a settled national tally of causes: a reader who wants mercury's true footprint has to add the mercury-family rows together, and this page does not silently do that merge for them.

The states column is the more informative half of each row. A cause listed on thousands of waterbodies by six states describes those states' listing conventions; a cause listed by thirty-nine states describes a condition that assessors across the country encounter. The two shapes look identical in a single-column ranking, which is why the state count sits beside the waterbody count on every row.

A waterbody is counted once per cause, not once per assessment. Where a state has listed the same cause on the same assessment unit across several reporting cycles or several designated uses, the rollup counts the unit a single time, so the number reads as "how many lakes carry this cause" rather than "how many times it was written down".

The action column counts listings with a management action attached, of which 8,801 of 9,219 are a TMDL, the pollution budget written for an impaired water, usually by the reporting agency and on 531 of them by EPA itself, which the Clean Water Act empowers to establish a load where a state does not submit one or where EPA disapproves the one it submits. The rest are other Clean Water Act instruments EPA types separately, such as a category 4B restoration approach, which exists precisely where a TMDL is not required. It records that a plan exists in the record, not that the water has improved, and the share of listings carrying one varies widely between causes.

Refresh cadence follows the source. States report assessment decisions to EPA on their own cycles, typically every two years, and some file late, so a cause listed on a lake can date from an earlier period. The database behind this page spans reporting cycles 1987 through 2026, and the data-vintage date shown on the page is the vintage PlainLakes has loaded, not a promise about how quickly a new EPA release appears here.

Absence is not evidence of cleanliness. A cause that ranks low may be genuinely rare, or may be one that few states monitor for; a lake with no listed cause may be unimpaired or simply unassessed, and most lake-type units in the file carry no assessment at all. This ranking measures what states have looked for and recorded, and nothing about what they have not.

Every figure here is traceable. Each cause row aggregates records that remain on the individual lake pages, the citation block names the dataset and its vintage, and EPA publishes the same assessment records through its own ATTAINS interface, so any total on this page can be checked against the source without going through PlainLakes.

PlainLakes is published by an organization, not by a newsroom of named reporters, and no individual byline or human review is claimed for these pages. The ordering is produced by the documented query above rather than by editorial selection. Corrections are welcome through the address in the footer; a substantive correction is reconciled against the upstream record and carried at the next refresh.

See the methodology page for the complete data-update process, source vintage, and field definitions.