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The efficacy of water quality policies aiming to reduce or prevent nitrate contamination of waterbodies may be constrained by the inherent delay or 'time lag' of water and solute transport through unsaturated (soil) and saturated (groundwater) pathways. These delays must be quantified in order to determine realistic deadlines and thresholds, and to design effective best management practices. The objective of this review is to synthesise the current state of research on time lag, in both the European and North American/Canadian environmental and legislative contexts. The durations of time lags have been found to differ according to differing climatic, pedological, landscape and management scenarios, and elucidation of these driving factors on a watershed scale is therefore essential where water quality is impaired or at risk. Finally, acknowledgement and understanding of time lag is increasingly seen at policy level, and incorporated in the development of environmental legislation. However, it is not yet ubiquitously appreciated, and continued outreach and education in scientific, public and policy venues is still required.

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