THE HUMIC LAKE ACIDIFICATION EXPERIMENT (HUMEX) - MAIN PHYSICOCHEMICAL RESULTS AFTER 5 YEARS OF ARTIFICIAL ACIDIFICATION

The HUMEX-project is a whole catchment manipulation experiment where t he effects of the addition of H2SO4 and NH4NO3 to a humic-rich lake, L ake Skjervatjern, and its catchment were studied. The lake was physica lly divided into an experimental lake (Basin A) and a control lake (Ba sin B). Two years after the division, Basin A and its catchment were a rtificially acidified. Hydrological data, meteorology, precipitation, and runoff chemistry collected during a 2-y pre-acidification period a nd during 5 y of acidification were evaluated. Randomized intervention analysis (RIA) was used to evaluate statistical significant differenc es between runoff chemistry from the two basins before and after the a cidification. RIA showed significantly higher concentrations of SO42- H+, NH4+, NO3-, Al-n+, Ca2+, Mg2+, total reactive Al (RAL), and labile Al (LAL) in Basin A after treatment compared with the control basin. After the treatment, the acid neutralizing capacity (ANC) in Basin A w as significantly lower than in Basin B. However, the average ANC is su bstantially higher in the control basin after acidification compared w ith the two years before acidification, while unchanged in the manipul ated catchment. The main reason for this is the long lasting effect of Na leakage after seasalt-episodes. No significant changes were observ ed regarding the amount of total organic carbon (TOC), water color, or UV-absorbency after the treatment, but the anion deficiency (A(-)) wa s significantly lower in the treated basin. This indicates that the or ganic acids are more protonated in the treated basin compared with the control basin. After a cold winter in 1993/94, an extreme NH4+ increa se was observed in runoff water from Basin A. This increase was accomp anied by increases in water color and UV-absorbency, but without any i ncrease in TOC.