Effects of forest fire and fire-fighting operations on water chemistry in Tyresta National Park, Stockholm, Sweden

Abstract
After a large forest fire in Tyresta National Park, Sweden, monitoring programs were started to study the consequences on the biology and chemistry in the burned area. To study the effects on the water chemistry, four brooks in the same water system were chosen for further studies. Samples were also taken in a limed lake downstream the brooks. The sites were sampled 10 times each year, starting the month after the fire, and a range of chemical analyses where performed. Pre-fire data exists for 2 of the brooks and the lake due to former studies in the area. In the brooks drastic changes of the water chemistry were seen immediately after the fire. pH in the brooks deereased with up to 2 pH-units from over 6 to about 4,5 and labile inorganic aluminium increased to over 400  g/l in the most affected brooks. Other determined metals and nutrients in the brooks also increased. In the lake no changes in pH or aluminium were measured, but significant (p <0,01) increases of chloride, sulphate, magnesium, potassium, sodium, manganese, zinc and cadmium where observed.

The water chemical effects may be explained by release of e.g. hydrogen- and aluminium ions through burning of organic matter in the acidified soil, but also by the large amounts of brackish water used during the fire-fighting operations. The extent of the effects from the fire are yet to be seen, as the concentrations of many water chemical variables still are increased three years after the fire.