This invention relates to the treatment of contaminated materials including soils and solids to reduce its permeability, and in particular to chemical treatment of waste or solids containing high levels of heavy metals such as lead, by treating the waste or solid with a single additive or combination of additives including calcium carbonate; reactive magnesium carbonate; reactive calcium magnesium carbonate; magnesium oxide; magnesium hydroxide; ammonium phosphate; diammonium phosphate; phosphoric acid; boric acid; metallic iron; polyphosphoric acid; sodium carbonate; sodium bicarbonate; calcium oxide; calcium hydroxide; sodium hydrogen phosphate; triple super phosphate (TSP); superphosphate; polyphosphoric acid; phosphoric acids; salts of phosphoric acids; P.sub.2 O.sub.5 ; phosphate rock; monoammonium phosphate; disodium hydrogen phosphate; trisodium phosphate; water soluble phosphate; Fe(II); Fe(III); Al(III); chloride; Fe(NO.sub.3).sub.3 ; FeSO.sub.4 ; Fe.sub.2 (SO.sub.4).sub.3 ; FeCl.sub.3 ; Al.sub.2 (SO.sub.4) .sub.3 ; Al.sub.2 O.sub.3 ; aluminosilicate; calcium sulfate; gypsum; sulfuric acid; alum; trisodium phosphate; potassium phosphate; monobasic, dibasic and tribasic calcium phosphates; lime; halites; sulphates of iron; sulphates of aluminum; pyrophosphates and potassium phosphates.