Filter press tests are widely used for field quality control of bentonite slurry in slurry trench excavations and often are incorporated in technical specifications for assessing filtrate properties of initial (freshly mixed) slurry and occasionally for in-trench slurry. In some cases, the results of filter press tests conducted on in-trench slurry are directly or indirectly used to assess the hydraulic performance of the cutoff wall. However, specifications for maximum filtrate loss from filter press testing of in-trench slurry can result in unnecessary construction delays and costs. The authors examine the reasons for specifying filtrate parameters for slurry and present the results of filter press tests conducted on lab and field samples to evaluate the utility of filtrate loss as an appropriate quality control measure for soil-bentonite cutoff walls. These results show no evidence to support that excessive filtrate loss of in-trench slurry (i.e., above the maximum value employed in typical specifications) compromises trench stability or contributes to an overall higher permeability of the completed vertical barrier. Higher in-trench filtrate loss is associated with higher sand content, which results in higher slurry density and improved trench stability. Furthermore, the permeability of all filter cake specimens created in this study from in-trench slurries with sand contents as high as 35 percent were less than 10-7 cm/s, which is the lowest target permeability typically specified for soil-bentonite cutoff wall backfill in hydraulic or geoenvironmental containment applications. The authors do not recommend using filtrate loss of in-trench slurry as a quality control parameter for assessing the effectiveness of slurry trench cutoff walls.