Nanoscale iron particles (NIP) have been found effective for remediation of contaminated groundwater, but their transport in subsurface after injection is limited due to aggregation and sedimentation. Lactate-modified NIP (LMNIP) are found to be effective for remediation as well as possess adequate transport characteristics in subsurface after injection. Advection-dispersion transport with adsorption and decay equation is used to simulate NIP and LMNIP transport in saturated sand (groundwater). However, significant uncertainty exists in selecting values of important input parameters such as dispersion coefficient, retardation coefficient, and first-order decay constant. The objective of this study is to assess the variability of these transport parameters and perform a reliable assessment of NIP and LMNIP transport behavior through groundwater. Probabilistic modeling is performed using reliability-based analysis with first-order reliability method (FORM) to account for the uncertainty in the parameters for NIP and LMNIP transport. The mean and standard deviation values of transport parameters were chosen based on the statistical analysis of the results of previous column experiments. Results indicated that the effect of dependency among the random transport parameters on reliability index for LMNIP and NIP transport should be given due consideration when designing contaminant remediation strategies. The reliability analysis also indicated that the coefficient of variation (COV) associated with the dispersion coefficient is relatively the most influential parameter compared with retardation coefficient and decay constant. The breakthrough time curves presented in the form of design charts are useful to determine extent and time required for NIP/LMNIP transport through soils. © 2018 American Society of Civil Engineers.