Rainfall simulator - grid lysimeter system for preferential solute transport studies using, large, intact soil blocks

Abstract

A grid lysimeter system, sample collection, containment and storage techniques were developed for detailed laboratory studies of water and solute movement through large, intact soil blocks. Procedures are given for field-isolation and containment of 46-cm intact soil cubes within a polyurethane foam shell. The polyurethane formed a stable, intimate bond with the soil, was impermeable to water and was strong enough to support a large soil block, but sufficiently elastic to accommodate shrink-swell of the soil with changing water content without rupturing. In the laboratory, the soil blocks were instrumented with solution delivery, collection and monitoring systems. A dripper-based rainfall simulator was used to deliver steady rainfall rates ranging from 4.8 to 30 mm hr-1 with a uniformity coefficient > 95%. The solution collection system consisted of a 10 x 10 grid of 3.8 x 3.8 x 1.3-cm deep cells in an aluminum block which individually drained into collection tubes housed within a vacuum chamber. The collection grid permitted characterization of spatial and temporal movement of water and solute through the soil block. The solution monitoring system consisted of sideby-side pairs of tensiometers and TDR probes inserted horizontally through the foam shell at four depths in the soil block. To test the operation of the lysimeter system, a bromide tracer breakthough curve was generated using the intact soil block, at a constant simulated rainfall rate of 19.2 mm hr-1. The flow data indicated that 85% of the water in the block was "bypassed" by the bromide, and that over 99% of the water flow passed through only 26% of the basal area of the soil block. The pattern of water flow in the solution collector was random with no evidence of preferential flow along the interface between the soil and the outer polyurethane shell. These results indicate that the lysimeter system was operating effectively and as required.