Macro chemistry, nutrients, and heavy metals in River Bø-elva upstream and downstream Bø sewage treatment plant - 2023

Abstract

During the period 26.04.2023 – 06.11.2023, 20 water samples have been taken weekly in River Bø-elva , both upstream and downstream Bø sewage plant (BSP), located in Midt-Telemark municipality, Norway. The upstream site was located about 100 m upstream the discharge point from BSP, the downstream site about 300m downstream. According to the results, the concentrations were significantly higher downstream BSP for the following compounds, ranked according to concentrations: Cl- > Na+ > Ca2+ > NO3--N > K+ > Mg2+ > NH4+-N = Al > Mn.

Except for Mn and Fe, the concentrations of heavy metals in River Bø-elva were never recorded above detection limits for the analysis method, neither upstream nor downstream. Also, the concentration of Tot-P was almost identical at both sampling sites, i.e. 5,7 ± 1,1 mg P L-1, primarily because of the effective phosphorous precipitation by use of poly-aluminium chloride (PAX-18) at BSP. Tot-N the concentrations varied a lot at the two sites during the investigated period, i.e. from 674 - 2040 mg N L-1 upstream BSP and from 598 - 4340 mg N L-1. Because of these large variations, primarily in the concentration of organic N, it was not possible to reveal significant differences in Tot-N between the two sampling sites.

According to the Norwegian classification guidelines, the overall chemical pollution status of River Bø-elva, according to Tot-N was "Bad" at both locations. Regarding Tot-P, acidification and heavy metal, the river was classified as "Very Good".