Nano scale zero valent iron (NZVI) is a promising material for DNAPL (Dense Non-Aqueous Phase Liquid) source zone treatment with high reactivity for reducing chlorinated organic compounds to less toxic dechlorinated products. Source zone treatment requires that NZVI be mobile in groundwater, and this requires nanoparticle surface modification by polymers, mainly anionic polyelectrolytes, that minimize NZVI aggregation and deposition to sand grains. Chemical and physical heterogeneity in subsurface sites such as groundwater pH and the presence of fines and clays could be factors that reduce colloidal mobility of anionic polyelectrolyte coated NZVIs. A fundamental understanding of how these factors affect NZVI mobility will aid in planning an injection or placement scheme for a desired NZVI distribution and treatment effectiveness. The effect of pH (5 to 8) in the absence of fines and clays, and the pH-dependent effect of silica sand fines (2 wt%) and clay fines (kaolin) (2 wt%) on the mobility of polyelectrolyte modified nanoiron were evaluated in water-saturated sand columns to test the hypothesis that positively charged edge sites on clay particles, the magnitude of which depends on pH, may limit the mobility of polyelectrolyte modified NZVI. Modifiers included a high molecular weight (125 kg/mol) poly(methacrylic acid)-b-(methy methacrylate)-b-(styrenesulfonate) triblock co-polymer and low molecular weight polyaspartate. Kaolin-NZVI heteroaggregation was studied by monitoring sedimentation of particles over the pH range 5 to 8 to confirm the attachment of modified NZVI to kaolinite at low pH. pH had a significant effect on the transport especially below pH 7. This was mainly caused by NZVI aggregation. Transport of polyaspartate-modified NZVI was significantly more pH-sensitive than the triblock copolymer-modified NZVI due to the strong polyelectrolyte character of the latter. While 2 wt% fines did not show a significant effect, 2 wt% kaolin decreased mobility of NZVI substantially, with the copolymer-modified NZVI showing less sensitivity to kaolin than the polyaspartate-modified NZVI. These results indicate that differences in subsurface hydrogeochemical conditions can limit the transport of NZVI and must be considered carefully when designing site-specific remediation strategies.