Safety_Series_015_1965 - gnssn - International Atomic Energy ...

This publication is not longer validPlease see http://www-ns.iaea.org/standards/1 m below the su rfa ce and the w aste was continuously w et. In thehigher areas the position was im proved with the water table at a depthof 3-5 m. Owing to the low perm eability of the shale and the slopingground p rofile, any relea se of activity has been caused m ore by su r­face water than by su b-su rface flow paths into neighbouring stream s.R ainfall has p erco la te d the p orou s tr e n ch -fillin g and accu m u lateduntil it overflow ed at the low er end, and only where the stream hasbeen v e ry c lo s e to a tren ch (3 m ) has th ere been any undergroundseepage.In spite of the apparent esca p e of ra d ioa ctivity fro m the soliddisposal area, it is significant that the contribution of contaminationto the main drainage creek is seldom detectable above the backgroundactivity previously introduced upstream from a water-treatm entplant.R elatively high concentrations of Na+, NOg and S O ^ions found in contaminatedground water sam ples indicate that ch em ical leaching hasaccom panied the relea se of radion u clides.An im proved type of disposal trench was introduced roughly 3 mw id eX 4 . 5 m deep (F ig. 23a and b). H ere the depth o f the tren ch ,though variable, was determ ined solely by the depth to ground waterand not by the maximum height to which the sid e-w alls rem ainedfree-standing. The base was covered with 15 cm of gravel and slopedat one end to an asphalt-lined sump in which a perforated casing wasinstalled for water sampling or rem oval. After the waste was dumpedvoid s w ere fille d with shale excavated fro m a neigh bou rin g tren chand the top la y er was com pacted to rece iv e the capping, com p osedo f 2. 5 cm o f asphalt sprayed on to a la y er of gravel.At Savannah R iver both liquid and solid disposal have been p ra c­tised for many years at a site where the soil is rather less perm eablethan at the other establishm ents cited owing to a higher clay contentin the predom inantly granular soil. The w ater table lie s gen erallyat a depth o f 13 m in deep sed im en tary m a teria ls o v e rly in g m eta -m orphic basem ent rock at a depth o f 300 m. The annual rain fall isroughly 125 cm /y e a r. Solid waste has accounted fo r the m ajor p o r­tion of the radioactive m aterials disposed into the ground since 1953.The burial ground covers roughly 0. 5 km2, is bare of vegetationand has a low r e lie f (m axim um c r o s s fa ll 7 m ). Surface ru n -o ff iscollected in three drainage ditches and because of the puddling actiono f heavy equipm ent the sandy clay adm ixture has form ed a su rfacelayer of low p erm eability that has in creased ru n -off and reduced infiltration[99].83