Effect of successive small hydropower plants on water quality

Abstract

The aim of this work was to evaluate the influence of two successive small hydropower plants (SHPs) on the water quality of the Vokė river (Lithuania). Two SHPs (‘Vokė’ and ‘Grigiškės’) is situated in the 7 km long section of the river, close to Vilnius, the capital and the biggest Lithuanian city. Water samples were taken in the dams above SHPs, in the rapids immediately below SHPs and 2 km below each SHP. The concentrations of nitrates (NO3--N), nitrites (NO2--N), ammonium (NH4+-N) and phosphates (PO43+-P), and chemical oxygen demand (COD) were investigated. An increase in N and P compounds was detected in the dams above both SHPs. The highest increases were characteristic for phosphates (~40%, p <0.05) and nitrates (34% and 41%, p <0.05, in the ‘Vokė’ and ‘Grigiškės’ dams, respectively). Strong and statistical significant increase in the level of ammonium was observed only in the ‘Grigiškės’ dam (31%). Contrary to NO3--N, NH4+-N and PO43+-P, concentration of NO2--N was lower in the dams of both SHPs. Strong and significant increase in COD (33%, p <0.05) was detected only in the dam of ‘Vokė’ SHP. Considering the river sections 2 km below the SHPs, water quality changed negligibly. The levels of biogenic N and P compounds tended to decrease; however, sharp increase in PO43+-P concentration was detected below ‘Vokė’ SHP, indicating an impact of subsequent ‘Grigiškės’ SHP. The levels of nitrates and phosphates were higher in the downstream ‘Grigiškės’ SHP, as compared to ‘Vokė’ SHP, both in the dams and in the subsequent rapids. The results of this study have shown that the levels of biogenic and/or organic compounds tend to increase in the dams above SHP. The concentrations of phosphates and nitrates further increase downstream due to the successive SHPs.