Dynamics of nutrient mass transport : A river basin evaluation

• Human settlements and activities can dramatically change and accelerate the rate of eutrophication in a river basin. An undesirable increase in the input of nutrients, e.g. phosphorus and nitrogen, to a surface water causes adverse water quality conditions. These adverse conditions, in turn, can interfere significantly with human uses of the water resource.The river transport of nutrients at the outlet of a river basin is the final result of a number of mechanisms: diffuse discharges from catchment areas; point source emissions; and the interactions between the atmosphere, running waters and the sediments.The aim of the research presented in this thesis was to retrospectively analyse the relationship between changes in the transport of nutrients and different sources of phosphorus loadings on a river basin-scale (The Motala River Basin). The nutrient mass transports over a 20 year periodhave been calculated using methods that optimally combine frequently recorded water-flow data with less frequently recorded water-quality data.The results challenged prevailing opinions about large-scale transports:The transport of phosphorus and nitrogen along the river were much lower than could be expected when considering results from hectare-scale field experiments or estimations of point source discharges.The trend analysis of phosphorus concentrations showed a significant downward trend but no trends could be seen for nitrogen concentrations.There was also a surprising lack of correspondence between removal of point-source emissions and mass transports in the watercourse. In fact, the transport of phosphorus decreased markedly during the late 1960's, but this decrease coincided neither in space nor in time with the introduction of chemical precipitation of point source discharges. A striking imbalance between input and output of phosphorus was also found. Less than 5% of the net import to the study area could be recognized as any sort of loading to the surface water, and the mass transport via the outletof the main river was calculated to be less than 2% of the net import to the study area. The study indicated that prevailing agricultural practice caused a large-scale spatial redistribution and an accumulation of phosphorus in soil and sediments. The most important factors of large-scale land use changes coinciding with the transport drop in the late 1960's, seemed to be related to changes in transport mechanisms on a large scale rather than to large-scale pool changes.In general, the present study demonstrates the need for an extended space and time prespective in the evaluation of nutrient transports. This study also pin-points the need for objectives related to water quality standards rather than to emission standards.

Subject headings

SAMHÄLLSVETENSKAP  -- Annan samhällsvetenskap -- Tvärvetenskapliga studier inom samhällsvetenskap (hsv//swe)

SOCIAL SCIENCES  -- Other Social Sciences -- Social Sciences Interdisciplinary (hsv//eng)

Keyword

Motala ström

INTERDISCIPLINARY RESEARCH AREAS

TVÄRVETENSKAPLIGA FORSKNINGSOMRÅDEN

Publication and Content Type

vet (subject category)

dok (subject category)