This thesis work was conducted in Vansjø-Hobøl catchment (Morsa), which is one of the most studied watercourse, situated south east Norway, for its poor water quality status. The poor water quality status is due to the high diffuse phosphorus (P) loss from the agricultural watershed; and this causes major eutrophication problems in surface waters. The thesis work was part of a big EUTROPIA project funded by Research Council of Norway (RCN) (190028/S30). The main aim of the project was broadly to better understand the hydro-biogeo-chemical processes governing mobility of P into lake Vansjø. The specific objective of this thesis is to better understand the effect of land use on the transport and mobility of nutrients P in particular.
As part of the Morsa monitoring programme, water samples from selected inlet streams draining into western Vansjø were collected over an extended period. The samples were collected from three streams: Dalen, which is a generic of an aluminium (Al) rich first order stream draining forested sub-catchment with shallow acid sensitive soil; Støabbakken1, which represents P rich and neutral agricultural stream; and Huggenes, draining mixed land use. In addition to these stream water, soil water samples were also collected at one of the sampling streams (Dalen) to capture spatial difference of nutrients which contribute to the stream water chemistry. The collected stream water samples were analyzed for pH, total suspended solids (TSS), dissolved organic carbon (DOC), conductivity, major cations and anions and for tot-P along with the operationally defined P fractions. The operationally defined P fractions are particulate P (PP), organically bound P (DOM-P) and orthophosphate (PO4-P). Statistical analysis was performed on measured parameters to show correlation among explanatory variables. Temporal variations in the data have been related to hydrological conditions and spatial variation to differences in land use in the catchments.
The water chemistry for the three streams is presented on median values. The pH for Støabbakken1, Huggenes and Dalen was 7.3, 7.1 and 4.6; conductivity (µS/cm) 397, 259 and 46.6, respectively. Sum of cations (Ca2+, Mg2+, Na+ and K+) were 2650, 1950 and 242µeq/l and sum of anions (SO42-, NO3- and Cl-) were 2560, 1970 and 182µeq/l, respectively. Alkalinity values for Støabbakken1and Huggenes were 1597 and 687µeq/l, respectively. The results of tot-P content and contribution of P fractions were found to be greatly influenced with land-use distribution. It was found that the stream draining agricultural has elevated levels of tot-P (median 113µg/l) of which PP is the dominant fraction (55%) and significant amounts of PO4-P (37%), whereas the lowest tot-P (median 8.3µg/l) content was found in the stream draining forest with 60% DOM-P. Tot-P content in the stream draining mixed land-use was 45.3µg/l with 63% in PP fraction. The stream draining forest showed considerable amounts of DOC (32mg/l) and DOC concentrations of Støabbakken1and Huggenes were 10.7mg/l and 15mg/l respectively. Moreover, the results of P contents from lysimeter soil water in Dalen showed DOM-P (40 to 90%) as a major fraction of tot-P in all soil plots of the soil horizons.
In the stream draining mixed land use (Huggenes), the results of tot-P content showed intermediate in rank between the two streams. Moreover, when runoff water from Dalen with low pH, aluminium rich and low ionic strength were conservatively mixed with runoff water from Støabbakken1 having high P content, in a ratio similar to agriculture to forest land-use in Huggenes, it was observed that measured tot-P to be 45% lower than what was calculated as a conservative mixture. Calculated concentrations of P fractions in the conservative mixture was higher than the measured concentration of P fractions at Huggenes and that confirms increased precipitation of P with aluminium. Calculated DOC concentration of the mixed water was observed to increase by 13% from the measured DOC content at Huggenes which implies increased leaching of DNOM which is due to the effect of reduction in acid rain.
The results also showed that tot-P content is positively correlated (r=0.881 at Støa1 and r=0.783 at Hug) with runoff rate in the streams draining agriculture indicating the P transport is more favoured with hydrology. Tot-P transport by month showed clear seasonal variation with generally large portion transported during periods of high flow events-spring and autumn (78 to 82% for Støabbakken1and Huggenes), suggesting the impact of runoff in the transport of P. TSS was best correlated to tot-P in the streams draining agriculture which indicates the case that sediment derived P transport is an important factor. It was further observed that the ratio of tot-P/TSS for Støabekken1 (12‰), Huggenes (6.74‰) and Dalen (1.6‰) on average which shows for a mass soil particle loss an amount of P is lost.