Strategic framework document for liming of acidified

waters in Stockholm County

This report assesses trends in acidification and the status of acidified lakes

and rivers in Stockholm County (SE Sweden). It also presents a framework

for liming of lakes, which includes a liming strategy, an action plan, an

environmental quality monitoring plan and a policy for government

subsidies for liming. The action plan follows the liming strategy presented

in the Swedish national liming guidelines (Swedish Environmental

Protection Agency, Handbook 2002:1).

In order to combat acidification, approximately 30 lakes in Stockholm

County have been treated with lime since the late 1970s. Conditions in the

lakes have clearly improved, and acidic lakes are rare in Stockholm County

today. In eight of the nine reference lakes used for baseline monitoring, pH

values had increased since 1990. On average, pH values in the reference

lakes have increased by 0.013 units per year.

Treatment with lime has been so successful that Stockholm County has now

achieved one of the national interim targets for the Swedish environmental

quality objective “

Natural acidification only”. According to the interim

target, “By 2010 not more than 5 % of all lakes and 15 % of the total length

of running water in the country will be affected by anthropogenic

acidification”.

The lakes of Stockholm County are less affected by acidification than the

average Swedish lake, and are more resistant to deposition of sulphur and

nitrogen. In a study of 164 lakes, acid deposition exceeded the critical load

(the load below which inputs of acidic substances can be neutralized by soilweathering

processes) in eight lakes only. We estimate that at current levels

of acidic deposition, only 8 % of the lakes (66 of all 790 lakes in the county)

risk becoming acidic. This is equivalent to 66 km

2 of lake area, or 2 % of

the total lake area in the county (excluding Lake Mälaren).