River Paimiojoki area and Tammela tests with ferric sulphate

River Paimionjoki has a drainage basin of about 1000 km2 in SW Finland and it delivers a substantial nutrient load, 60 Mg yr-1 of total P and 530 Mg yr-1 of total N, to the Archipelago Sea. There are a number of animal farms in the area, and agriculture is estimated to contribute by 80% to the P load and by 68% to the N load to the Baltic Sea. The Association of River Paimionjoki has been working since 2010 in order to decrease the nutrient load to the river, and together with MTT and WWF conducted site survey for the selection test sites in this area.

An old sedimentation pond with lots of vegetation after which the Paimionjokipilot site P2 was built.

Aaro Närvänen, who has planned the ferix-doser, talking with the driver of the digger. Student Timo Alankomaa was helping.

Three sites in the community of Somero were found suitable for installation of ferric sulphate dosers (Table PAIM_1). Site P1 was installed below a small sedimentation pond, whilst the two other sites were a ditch draining from a field area that frequently receives pig slurry (P2) and a ditch that passes a piggery/cowshed yard. There was no opportunity to construct wetlands on any of these sites. The site P1 drained directly to the river Paimionjoki and the two other ditches drained to river Jaatilanjoki which is a sub-basin of the river Paimionjoki drainage area. Site characteristics of all selected ditches are shown in Table PAIM_1 and location of the sites in Fig. PAIM_1.

Table PAIM_1. Characteristic of the selected sites at the river Paimionjoki area and in Tammela.

Site Catchment area, ha Source of the loading Estimated chemical need/ recommended dosing period
P1 50 ha fields + 110 ha forest Fields and forest 1500kg/spring
P2 20 ha fields Piggery, fields receiving pig slurry 400 kg/spring
P3 15 ha fields + 5 ha forest Pasture, cowshed, piggery 800 kg/whole year
Tammela > 1 ha cowshed area, residential Manure storage-affected ditch, household wastewater 200 kg/whole year

There were two sizes of dosers built, depending on the expected water flow. The ditch draining from the sites P2 and P3 were equipped with 350 litre storage boxes whereas the ditch with the larger catchment (P1) was equipped with box having 600 litre storage capacity. All of these devices were equipped with one doser sock. Installation on the sites was done in September and October 2012, and tests were launched in October. The doser P3 was in use from October 8th and the two others (P1 and P2) from October 15th until November 5th 2012. Re-fill of the chemical storage and sampling of water was done by personnel from the Association of River Paimionjoki. Frost at the beginning of November ceased the tests so that the time for testing was only about a month. 

The bridge was constructed before the weir.

A large sand container (the Ferrix-3 doser)was put on the bridge.

The Tammela site, operated by MTT, was selected as a test site due to the very high (up to 19 mg l-1) concentrations of dissolved P in ditch water. The ditch, that was small in size and often dried during summers, obviously received liquors from a manure storage and a silo used as ensilage storage, plus wastewater from a house with insufficient and outdated sewage system. The Tammela site was equipped with a 150 litre doser unit.

For those sites (the Paimionjoki and Tammela, Nuutajärvi, and Ojainen sites) for which flow was only recorded during visits on the sites, the P fluxes to be presented are estimates based on an assumption that flow increases or decreases linearly between the recorded values. This, of course, is an incorrect assumption as peak flows (especially in small catchments) change rapidly, and these rapidly evolving peaks are not included in the flow and loading estimates, resulting in underestimation of actual water and nutrient flows. Hence, in the results sections dealing with the mentioned sites we focus on the concentration changes due to the chemical applications rather than the changes in loadings. However, estimates of P loading are also presented as P fluxes are needed for calculation of economy of the chemical usage. The changes in P fluxes obtained by chemicals were calculated using average percent change due to chemical treatment and the estimated total P flux through a given site. For the Ferix tests, dosing of the ferric sulphate was found to work better for the high-flow events than at low flow (as demonstrated at the Nautela site), and the estimated effects on P loading based on incomplete flow data are probably therefore rather underestimated than overestimated.



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