Rainfall Simulator-Grid Lysimeter System for Solute Transport Studies Using Large, Intact Soil Blocks

A grid lysimeter system and sample collection, containment, and storage techniques were developed for detailed laboratory studies of water and solute movement through intact soil blocks. This was done because existing designs and techniques had important deficiencies and were limited in their range of capabilities. Intact 46-cm soil cubes were isolated, then contained within a polyurethane foam shell, which formed a stable, intimate soil bond, was impermeable to water and strong enough to support a large soil block, while sufficiently elastic to accommodate soil shrink-swell with changing water content without rupturing. The soil blocks were instrumented with solution delivery, collection, and monitoring systems. A dripper-based simulator delivered steady rainfall ranging from 4.8 to 30.0 mm h⁻¹. The solution collection system was a 10 by 10 grid of cells (3.8 by 3.8 by 1.3 cm deep) milled into an aluminum block, which individually drained into collection tubes housed within a vacuum chamber. The collection grid permitted characterization of spatial and temporal water and solute movement through the block. The solution monitoring system consisted of side-by-side tensiometer pairs and time domain reflectometry (TDR) probes inserted horizontally through the foam shell at four depths in the block. As a partial test of the system, a bromide (Br⁻) tracer breakthough curve (saturated flow) was generated at a simulated rainfall rate of 19.2 mm h⁻¹. Flow data indicated that 85% of the water in the block was bypassed by the Br⁻, and that >99% of the water flow passed through only 26% of the basal area of the block. The water flow pattern in the solution collector exhibited no evidence of preferential flow along the interface between the soil and the outer polyurethane shell. It was concluded that the rainfall simulator-grid lysimeter system was operating effectively.