This master thesis was an integrated part of EUTROPIA. The specific aim of this present study is to enhance our understanding of the effects of reduction in acid rain on P-loss from agriculture soil due to reduced co-precipitation between P leached from agriculture soil and Al leaked from upstream acid forest soils. The study was carried out at two watersheds: Dalen, which is an aluminium rich, acidic forest stream water catchment upstream, and Støa1, which is a P-loaded agricultural water catchment in low lying areas. The Støa1 catchment is dominated by agriculture practices. Large P-loss to Western Vansjø (Vanemfjorden) has caused major eutrophication problems in the lake basin causing its water quality not to fulfill the requirements of good chemical and ecological status in the EU-Water Framework Directive(EU-WFD).Water samples were collected in the stream in the valley bottom in the Dalen forest and in stream water draining Støa1 catchment in low lying areas.Analysis of mixing ratios made of the two waters was performed to assess the Al-precipitation of P that occurs at the time of mixing between the two waters and the effect of Al-precipitation of P on the potential of agricultural water to carry P to Vanemfjorden.The results showed that aluminium precipitated P in the mixing ratios. The concentration of both inorganic(labile) aluminium(Ali) and phosphate(PO4) in the mixing ratios decreased with time. The more the Dalen-water there was in the mixing ratio, the bigger the amount of PO4 that was precipitated.A 1:1 mixture of Dalen and Støa1 water was analyzed for inorganic (labile) aluminium and PO4 at 4 ˚C. The results showed that Ali-precipitation of PO4 at 4 ˚C was lower compared to Ali-precipitation of PO4 at 25 ˚C.The situation with Ali- precipitation of PO4 in stream water as it was in the 1970s was stimulated in an experiment involving acidifying Dalen-water with H2SO4 prior to mix it with Støa1 water. It was found that during the seventies, Ali precipitated PO4 which stabilized around 0.25 μM. The result showed also that inorganic (labile) aluminium was the dominant species during the acid rain period contrary to the today´s situation where monomeric organic aluminium(Alo) is the predominant fraction.An experiment involving spiking of mixing ratios with 100 and 200 μg/L KAl(SO4)3.12H2O was run to test the effect of reduction in acid rain on the PO4-adsorption capacity of Al-oxy hydroxides at different pH-values. It was found that Al-oxy-hydroxides adsorbed PO4 at a maximum pH-value of 6.4. Over pH 6.5, the ability of Al-oxy hydroxides to precipitate PO4 decreased.The maximum PO4-adsorption capacity of Al- oxy-hydroxides was measured using fixed concentrations of Støa1-water(50 mL 353.4 μg TP/L) in 100 mL flasks added 5 mL 1.5, 10, 20, 40, 50, 70, 100, 150, 200, 250 and 300 μg/L KAl(SO4)2.12H2O solutions prior to dilution with Dalen- water. It was found that PO4- adsorption continued to a maximum concentration on Al-oxy- hydroxides.The chemistry of the two waters was analyzed because it is decisive for co-precipitation between aluminium and phosphate at the time Dalen and agriculture water mix. Conductivity and alkalinity in Støa1 water were measured at 391. 6μS/cm and 2.1 meq/L, respectively. Conductivity value measured in Dalen water was 43 μS/cm. A concentration of 1.10 SO42- mg/L was measured in Dalen- water. TOC values were found to be 30.95 mg/L and 7 mg/L for Dalen and Støa1, respectively. pH in Dalen and Støa1-water was measured at 4.5 and 6.9, respectively.