EPA National Lake Assessment Survey Lower 48 States Lake Chemistry

EPA and its state and tribal partners conducted a survey of the nation's lakes, ponds and
reservoirs (>10 acres, using NHD) in 2007. This National Lakes Assessment was designed to
provide statistically valid regional and national estimates of the condition of lakes. It used a
probability-based sampling design (5 size classes of lakes, 9 agglomerated Omernik ecoregions,
and spatially distributed across the lower 48 US states) to represent the condition of all lakes
in similar regions sharing similar ecological characteristics. Consistent sampling and
analytical procedures ensured that the results could be compared across the country. The NLA
helped build state and tribal capacity for monitoring and assessment and promoted collaboration
across jurisdictional boundaries in the assessment of water quality. 1000 total lakes were
included in the NLA (909 lakes visited once and 91 that were revisited), however, the dbase also
includes additional lakes for any states that performed an ‘intensification’ in
order to do a state-wide lake assessment. Finally, there are lakes that are designated as
‘reference’ lakes that were not chosen in the same probabilistic fashion, but were
chosen state-by-state using a variety of methods. For more info on the lakes and survey design
see:
http://water.epa.gov/type/lakes/upload/2006_05_05_lakes_lakessurvey_site_selection_factsheet.pdf