Highway E18 Grimstad-Kristiansand; effects and quantification of acid runoff from deposits of sulphide-bearing rock

Abstract

The new highway E18 Grimstad-Kristiansand, constructed in the period 2006-2009, crosses through sulphide-bearing rock. Oxidation after blasting results in formation of sulphuric acid, which may enter the runoff. Blasted sulphide rock material was therefore deposited. Deposit M17 consisted of about 700.000 tonnes of rock material and covered 50.000 m2 of terrain. Although a total of 11.000 tonnes of shell sand and limestone gravel was used, runoff from deposit M17 became acid and aluminium-rich during the construction phase. pH at the outlet of the downstream situated Lake Lomtjenn was reduced to pH 4.0-4.6, and aluminium concentrations increased to very high levels (10-20 mg/l) in 2008/2009. Countermeasures in 2009 increased pH, and the Al-concentration decreased to below 5 mg/l. The water quality stayed toxic to fish, however, and the lake did not recover. It was estimated that the initial annual sulphur transport probably was in the order of 0.5 % of the total sulphur content in deposit M17. This weathering rate will result in ongoing consolidation of the rock pile. A plan for new measures at the deposit, launched by CJV in May 2011, is described, and a procedure for approaching good ecological status is proposed.