Dounreay Site Restoration Ltd

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Water

The Burn of Isauld flows into Sandside Bay about 4km to the west of Dounreay and the River Forss flows into Crosskirk Bay 4km to the east.

Neither watercourse catchment overlaps the site.

The Dounreay Burn, also known as the Mill Lade, flows through the site and is classified as being of good quality status in accordance with the Water Framework Directive.

The burn been entirely modified by artificial channelling and local culverting around field boundaries and along the edges of the local roads. The banks have been reinforced by block walling of the local Caithness flags.

Age, vegetation succession and the many gaps between the blocks of stone have much ameliorated the built structures and the stream as a whole provides a good quality aquatic habitat with stocks of brown trout and stickleback.

The occasional salmon and sea trout have been recorded in the lower reach through the Dounreay site.

Within the site, the wetted surface of the channel varies between 0.5-1.5 metres in width with average water depths of around 25-30 cms. There are, however, deeper pools with slow-flow conditions and water depths of up to 75 cm. The velocity of the current in the riffle-pool sequences varies from 0.1 to 0.4 metres/second. Locally emergent vegetation, particularly within the Dounreay site, reinforces and accelerates the local flow rate to form faster flowing riffles between the vegetation stands where current velocities may reach 0.55 m/s.

The substrate is dominated by gravel formed from the Caithness flags but the waters appear rich in organic particles and the still or slow-flowing pools have a substrate of fine silts and muds. Algal mats on the larger stones and occasional beds of water crowfoot and willow moss provide additional microhabitats for aquatic invertebrates.

Caddis flies are particularly well represented in the invertebrate fauna and include filter feeders, omnivores, detritivores and predatory species. Good numbers of other typical clean-water fauna such as stoneflies and mayflies are present.

The range of flow conditions, pools and riffles provide for a range of invertebrates with species of stronger currents present together with those more typical of slow waters such as the diving beetles and water mites. The abundance of certain molluscs is suggestive of neutral or calcareous waters.

The coastal waters around Dounreay are considered to be of high quality status according to SEPA in relation to the Water Framework Directive.

Authorised liquid discharges from the Dounreay site are all to sea.

At depth within the basement rocks, the groundwater is saline. All waters are of meteoric origin with minimal seawater addition.

Flows in the Devonian rocks tend to be associated with lithological features, whereas flows in the basement rocks are related to structural features, particularly around fault zones.

Interpretation of the hydrogeological tests has indicated that nearly all the flow within the Devonian and basement rocks is through fractures, with the flowing fractures in the Devonian rocks being well connected and those in the basement rocks having limited connectivity

On the gently sloping ground along the coast, a number of springlines arise draining northwards towards the sea, particularly to the north of Dounreay. These ground conditions have no doubt hindered land-claim here for arable farming and the vegetation here retains the characteristics of wet heath and mire that may be lightly grazed at certain times of year. Some of the grasslands adjacent to the wet heath, both on the cliff edges and on the inland edge may be species-rich where agricultural improvements have been of low intensity.