The lake, situated between the residential areas Trollbäcken and Bollmora, empties into Lake Tyresö-Flaten through the brook Prästängsdiket.

[3] It is regarded as an important local recreational milieu.

[4] Barnsjön is a forest lake poor in nutrients.

With a low buffering capacity, the lake is sensitive to acid deposits and has an acidity at about pH 7.

It is therefore regarded as important to avoid contaminations within its catchment area.