THE EFFECT OF MICROORGANISMS, SALINITY AND TURBIDITY ON HYDRAULIC CONDUCTIVITY OF IRRIGATION CHANNEL SOIL

The introduction of polysaccharide producing benthic algae and bacteri a could provide a low cost technique for seepage control in irrigation channels. The ability of algae and bacteria to produce polysaccharide s proved to be successful in reducing the hydraulic conductivity of ir rigation channel soil. Hydraulic conductivity was reduced to less than 22% of its original value within a month of inoculating soil columns with algae. Chlorophyll and polysaccharide concentrations in irrigatio n channel soil were measured in order to assess the growth of algae an d extent of polysaccharide production, and their correlation with hydr aulic conductivity of channel soil. Increases in polysaccharide occurr ed in the top layer (0-5 mm) of the soil column. The reduction of hydr aulic conductivity was highly correlated with the amount of polysaccha rides produced (r2 = 0.92). Hydraulic conductivity decreased with incr easing algal and bacterial numbers. The first few millimetres of the s oil core where microbial activity was concentrated, seemed effective i n controlling seepage. Incorporation of extra nitrate and phosphate in to algal medium did not increase the production of polysaccharides by algae in channel soil. The effect of salinity and turbidity of irrigat ion channel water on channel seepage was studied by measuring the effe cts on hydraulic conductivity of channel soils. When the electrical co nductivity (EC) of the water increased above a threshold value, the hy draulic conductivity increased because of the flocculating effects on clay particles in channel soils. A relationship between sodium adsorpt ion ratio (SAR) and EC of the channel water was established which indi cated 15% increase in channel seepage due to increases in salinity. In creasing the turbidity of irrigation water (by increasing the concentr ation of dispersed clay) resulted in lowering the hydraulic conductivi ty of the channel soil due to the sealing of soil pores by dispersed c lay particles. When the turbidity of the water was 10 g clay 1(-1), th e hydraulic conductivity was reduced by 100%. An increase in clay conc entration above 1 g 1(-1) resulted in significant reduction in hydraul ic conductivity. Soil bowl experiments indicated that clay sealing wit h a coating of hydrophobic polymer on the surface could also effective ly prevent seepage of saline water.