Undersøgelse af renseeffekten af Stokbæk okkersø

Studenteropgave: Professionsbachelor

  • Emil Eg Møller
  • Rikke Nørregaard Hansen
7. semester, Kemi og Bioteknologi, Diplomingeniør (Diplomuddannelse)
This project treats the problem regarding an in increased amount of ferrous iron in aquatic environments in
western Denmark. The problem is mainly due to oxidation of pyrite caused by farmers lowering the water
table, when draining arable land for cultivation as well as straightening of creeks and rivers.
The study focuses specifically on establishing a way of measuring the actual residence time of water, in lakes
made for ocher cleaning, as well as describing the status of Stokbæk okkersø, regarding the water quality.
This is done in order to find out, if there is a potential for establishing living conditions for aquatic life, such
as trout. As the biggest threat is the ferro iron, the cleaning effect is examined, by comparing water from the
inlet to that of the outlet.
An examination of Stokbæk creek, was performed downstream from the lake in order to find out, if the
location of the lake secures, that all point sources of ferrous iron is lead through the lake.
At last, testing of the lake sediments is carried out, in order to find out, whether the sediment can be used
as fertilizer on fields, or whether the amount of heavy metals excess the amounts, which can be used for that
purpose. At the same time any potential patterns in sedimentation is sought.
The main findings of the study have been obscured, by the fact, that the autumn of 2019 has brought the
largest amount of precipitation on record, so that the flow through the lake, exceeds its capacity. It is
however concluded, that under the given conditions, the cleaning effect of the lake on ferrous iron is at 18%,
which is among the lowest observed values since the establishment of the lake.
By applying small amounts of sodium chloride to the inlet, and measuring the differences of conductivity, an
approximate residence time, was found to be approximately 5 hours. This low value corresponds well to the
low cleaning effect and is low compared to the theoretically calculated residence time of 7 hours 40 minutes.
When analyzing the iron content in in- and outlet by ICP, a negative cleaning percentage was measured. This
observation was made on water samples collected shortly after an intense downpour. There were indications
of a possible inlet of water from a drainpipe, with a high iron content, that may have been the cause of the
negative cleaning. No direct measurements were performed on the inlet of drain water, as the inlet was
located under the water table.
The quality of the data obtained from sediment samples was not good enough to find any significant
differences between the sedimentation contents in different parts of the lake. The chosen significance level
was 0.05. There were, however some indications, that there is a difference between the amount of
precipitated metals between the northern and southern part of the lake.
While investigating possible point sources of iron downstream from the lake, 3 sources was located. This
indicates the need of either blocking off the sources or making an additional cleaning facility further
Udgivelsesdato10 jan. 2020
Antal sider83
Ekstern samarbejdspartnerVarde Kommune
Vandløbsmedarbejder Flemming Sørensen flso@varde.dk
ID: 319228088