Chromium concentrations of up to 91 mg l⁻¹ were found by ICP-OES for ground water from nine boreholes at four landfill sites in an area of S.E. Glasgow/S. Lanarkshire where high-lime chromite ore processing residue (COPR) from a local chemical works had been deposited from 1830 to 1968. Surface water concentrations of up to 6.7 mg l⁻¹ in a local tributary stream fell to 0.11 mg l⁻¹ in the River Clyde. Two independent techniques of complexation/colorimetry and speciated isotope dilution mass spectrometry (SIDMS) showed that Cr was predominantly (>90%) in hexavalent form (Cr^VI) as CrO₄²⁻, as anticipated at the high pH (7.5–12.5) of the sites. Some differences between the implied and directly determined concentrations of dissolved Cr^III, however, appeared related to the total organic carbon (TOC) content. This was most significant for the ground water from one borehole that had the highest TOC concentration of 300 mg l⁻¹ and at which <3% of Cr was in the form of Cr^VI. Subsequent ultrafiltration produced significant decreases in Cr concentration with decreasing size fractions, e.g. <0.45 µm, <100 kDa, <30 kDa and <1 kDa by the tangential-flow method. As this appeared related more to concentrations of humic substances than of TOC per se, horizontal bed gel electrophoresis of freeze-dried ultrafilter retentates was carried out to further characterise the Cr^III-organic complex. This showed for the main Cr-containing fraction, 100 kDa–0.45 µm, that the Cr was associated with a dark brown band characteristic of organic (humic) matter. Comparison of gel electrophoresis and FTIR results for ultrafilter retentates of ground water from this borehole with those for a borehole at another site where Cr^VI predominated suggested the influence of carboxylate groups, both in reducing Cr^VI and in forming soluble Cr^III-humic complexes. The implications of this for remediation strategies (especially those based on the addition of organic matter) designed to reduce highly mobile and carcinogenic Cr^VIO₄²⁻ to the much less harmful Cr^III as insoluble Cr(OH)₃ are discussed.