Axial-radial influence of porous cup soil solution samplers in a sandy soil

Porous cup soil solution samplers (PCS) have been extensively used to assess movement of solutes through the vadose zone. The ease of installation, simplicity of design, and low cost make PCS attractive and perhaps useful tools. Quality of PCS data is often suspect, and it is difficult to determine solute flux because the volume of soil sampled is unknown. We compared the performance of PCS constructed of ceramic and stainless steel porous cups in Sparta sand (mesic, uncoated Typic Quartzipsamment) to assess their axial-radial influence under transient water flow conditions. Bromide tracer was used to mimic water transport. Replicated (two) samplers of each porous material were plated at 25-, 60-, and 140-cm depths. A time domain reflectometry system was utilized to measure water content adjacent to the samplers at 15-min intervals. Axial-radial PCS influence estimates were obtained by three independent methods: (i) an analytical approach based on a soil hydraulic conductivity function and sample volume; (ii) a mass balance of estimated water flux and volume collected by each sampler; and (iii) a mass balance of collected vs. leached Br-. Calculated axial-radial spheres were lognormally distributed. Significant differences (P < 0.05; type 1 error) were observed for mean axial-radius estimates obtained by the three analytical methods. The mean axial radius for the ceramic and stainless steel porous cup samplers within each depth of sampling was also significantly different (P < 0.05; type 1 error). Combining data from both ceramic and stainless steel samplers across all three methods of analysis and depth of sampling resulted in a mean axial radius of approximate to 24 mm.