Weathering and riverine fluxes in pristine and controlled river catchments in Iceland

Anthropogenic water management has extensively altered the worlds’ river systems through impoundments and channel diversions to meet mankind’s increasing demand for water, energy, and transportation. One of these altered river systems is the, now dammed, glacial river Jökulsá á Dal in Eastern Iceland. Construction of the dam (Kárahnjúkar) in the highlands East of Vatnajökull ice cap was from 2004 to 2007. An initial baseline study of Jökulsá á Dal, prior to dam construction, was conducted from 1998 to 2003 in order to constrain the natural discharge regime and fluxes of suspended- and dissolved material. This river monitoring resumed in 2007 in order to assess the impact of damming Jökulsá á Dal and the construction of Hálslón reservoir and Kárahnjúkar power plant on these riverine fluxes. Monitoring lasted until 2013. The dataset collected before dam construction was used to demonstrate natural changes within the river catchments and to assess the effect of climate on chemical weathering rates within the catchment. There is a positive correlation between riverine discharge and suspended load but the correlation is negative between discharge and the concentrations of most dissolved elements (e.g. SiO2, Na, Ca, Mg, DIC, SO4, Cl, F, Sr, Mo). However, some elements are controlled by other factors, such as sunlight (e.g. NO3) and the redox potential of the water (e.g. Fe and Mn). The results showed a positive correlation between the climate parameters (water temperature and river discharge) and riverine fluxes of suspended- and dissolved material, and thus on the flux of many essential nutrients to coastal waters. This potentially adds to the negative feedback between chemical weathering of silicates and atmospheric CO2 through photosynthesis in the oceans. Before damming, the glacial river Jökulsá á Dal carried a large suspended sediment load (~5.8 Mt/yr), the majority of which is now deposited in Hálslón reservoir (~5 Mt/yr). However, the finest grains (<60 µm) remain suspended and are transported with the diverted water from Hálslón reservoir, through headrace tunnels to the power plant, and into the Lagarfljót river. The result of these anthropogenic interventions is increased discharge and concentration of suspended matter in Lagarfljót. The concentrations of the riverine dissolved elements have changed less than the particle concentration. However, since damming, there has been an overall increase in the flux of dissolved constituents due to i) changed saturation state of secondary minerals, ii) decreased nutrient uptake in Lagarfljót owing to less transparency in the lagoon, iii) increased chemical dissolution of suspended material in Hálslón and Lagarfljót, and iv) less metal adsorption on suspended material in Jökulsá á Dal. The dam construction has also changed the timing of riverine fluxes, but not the timing of the spring floods, which are an important flux of nutrients to the diatom spring bloom in the coastal waters, because the spring melt occurs at relatively low elevations, below the dam. Other anthropogenic alterations can have an impact on river regimes and the environment in more general terms. Precipitation in South Iceland has been monitored for decades, providing the opportunity to study long term changes. This precipitation dataset has enabled the direct measurement of acid rain, widespread in the 1970’s. This acid rain caused environmental damage on the continents but, after new regulations limiting industrial emissions of anthropogenic sulphur from Europe and North America, pH in the precipitation from South Iceland increased (from 5.0 to 5.7) between 1980 and 1998. However, since 1998 the pH has once again started to decrease (from 5.7 to 5.4). The timing of this acidification of precipitation is concurrent with increased riverine sulphur fluxes in the region. Proposed causes of these changes are: the development of the Nesjavellir geothermal power plant in the vicinity, which started producing electricity in 1998; and the construction and operation of the Hellisheidi geothermal power plant since 2006.

Author dc:creator

• Eydís Salome Eiríksdóttir 1972-

Contributors dc:contributor

• Háskóli Íslands

• Doktorsritgerðir

Language dc:language.iso

en

Handle dc:identifier.uri

http://hdl.handle.net/1946/23831

OAI identifier oai:identifier

oai:skemman.is:1946/23831