Sorption of 10(-18)-10(-5) M neptunium (Np) to goethite was examined using liquid scintillation counting and gamma spectroscopy. A combination approach using Np-239 and long lived Np-237 was employed to span this wide concentration range. Np-239 detection limits were determined to be 2 x 10(-18) M and 3 x 10(-17) M for liquid scintillation counting and gamma spectroscopy, respectively. Sorption was found to be linear below 10(-11) M, in contrast to the non-linear behavior observed at higher concentrations both here and in the literature. 2-site and 3-site Langmuir models were used to simulate sorption behavior over the entire 10(-18)-10(-5) M range. The 3-site model fit yielded Type I and II site densities of 3.56 sites/nm(2) (99.6%) and 0.014 +/- 0.007 sites/nm(2) (0.4 +/- 0.1%), consistent with typical "high affinity" and "low affinity" sites reported in the literature [21]. Modeling results for both models suggest that sorption below similar to 10(-11) M is controlled by a third (Type III) site with a density on the order of similar to 7 x 10(-5) sites/nm(2) (similar to 0.002%). While the nature of this "site" cannot be determined from isotherm data alone, the sorption data at ultra-low Np concentrations indicate that Np(V) sorption to goethite at environmentally relevant concentrations will be (1) linear and (2) higher than previous (high concentration) laboratory experiments suggest. (C) 2012 Elsevier Inc. All rights reserved.