The movement and accumulation of sediment suspended in water used for recharge were determined by radiotracer techniques and by examining thin sections of recharge basin material with a petrographic microscope. The purpose of the study was to determine suspended sediment accumulation and movement, loci of accumulation, and its effect on the recharge basin life.

The radiotracer study showed that 50% of the sediment suspended in the recharge water moved deeper than 33 cm when naturally occurring large pores were allowed to remain at the basin surface, but only 10% moved deeper than 2.5 cm when these pores were destroyed.

Horizontal and vertical thin sections were made from cores taken from the upper 30 cm of a recharge basin in which the large pores at the surface were destroyed, and that had recharged over 91 m of turbid water in 6 cycles. The nature of the sediment accumulations allowed identification in thin sections of the types, loci, and amounts. Three types of accumulations were observed: (1) flakelike structures on the upper 2.5 cm; (2) two horizontal sheetlike structures, 0.1 mm thick, between the depths of 5 to 8 cm and 8 to 12 cm, respectively; and (3) fillings in voids, mostly between 2.5 and 23 cm. The average pore volume lost between a depth of 2.5 and 5 cm was 1.5%, and below, 5 cm, less than 1%. The volume of accumulated sediment was 23% in the upper 2.5 cm, 5.5% between the depths of .5 and 5 cm, and less than 0.5% below 5 cm. These data agree with the data obtained from the radiotracer study.

Even though the material above 2.5 cm accumulated a large amount of sediment, porosity was maintained by freezing and thawing, and wetting and drying. Infiltration rates of the basin have not been noticeably reduced. The success of excavated basins for recharge of

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turbid water should not be limited by the movement and accumulation of suspended sediment in the basin material.

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