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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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Landscapes typically deemed at risk from leached losses of nitrogen (N) and phosphorus (P) are those with short subsurface hydrologic time lags. Due to the short time it takes nutrients to move from a source to an area of concern, such sites are deemed perfect to test the efficacy of programmes of measures as management changes. However, a small subset of these sites can retain nutrients in soil/subsoil layers, which in turn are leached and can be either attenuated (e.g. nitrate converted to gaseous forms or immobilised in soil and P can be mineralised) or mobilised over time. This biogeochemical time lag can have long lasting effects on water quality. In an intensive agricultural karst oxidised aquifer setting, the aim of this study was to improve understanding of P and N inputs, retention, attenuation and subsurface pathway distribution and to inform how similar sites can be managed in the future. This was undertaken for the present site by integrating existing secondary and new primary datasets for both N and P. Results showed that in the years pre-2000 slurry from an on-site integrated pig production unit had been applied at rates of 33 t ha-1 annually, which supplied approximately 136 kg ha-1 total N and approximately 26 kg ha-1 total P annually. This practice contributed to large quantities of N (Total N and NH 4-N) and elevated soil test P (Morgan extractable P), present to a depth of 1 m. This store was augmented by recent surpluses of 263 kg N ha-1 , with leached N to groundwater of 82.5 kg N ha-1 with only 2.5 kg N ha-1 denitrified in the aquifer thereafter. High resolution spring data showed greatest percentage loss in terms of N load from small (54-88%) and medium fissure pathways (7-21%) with longer hydrologic time lags, with smallest loads from either large fissure (1-13%) or conduit (1-10%) pathways with short hydrologic time lags (reaction time at the spring from onset of a rainfall event is within hours). Although soils were saturated in P and in mobile forms to 0.5 m, dissolved reactive P concentrations in groundwater remained low due to Ca and Mg limestone chemistry. Depletion of the legacy store with no further inputs (taking 25% of available mass of soil organic N as available in 1 m of soil/subsoil to be 75 kg N ha-1) would take approximately 50 years, with NO 3-N concentrations in the source area dropping to levels that could sustain groundwater NO 3-N concentrations below admissible levels within 9 years. Biogeochemical time lags (decades) are longer than hydrologic time lags on this site (months to years). Future management should target farm surpluses that maintain a legacy store at or below a soil organic N mass of ~ 20 kg N ha-1. Incorporation of biogeochemical and hydrologic time lag principles into future water quality regulations will provide regulators with realistic expectations when implementing policies.

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This study investigated the extent of soil damage caused by field traffic associated with different levels of soil moisture deficit (SMD). The hybrid SMD model was used for computing temporal patterns of SMD which can be accurately predicted for a range of soil types in Ireland. The aim of this study was to determine SMD threshold limits to trafficability for incorporation into a decision support system for safe slurry spreading. A tractor and a fully loaded single-axle slurry tanker (total weight ca. 18 tonnes) were driven over well, moderate and poorly drained soils at SMD values of 0, 5, 10 and 20 mm during drying phases. The change in soil bulk density (SBD) was used as an indicator of soil compaction, and rut profile measurements were taken to determine soil deformation indicative of surface damage. The effect of traffic on the grass crop was determined by measuring dry matter yield at 30 and 60 days posttraffic in the wheel-rut and nontrafficked area. Results showed that the SMD at the time of traffic had a significant (P < 0.05) effect on the magnitude of the changes in SBD on soils of different drainage status, and on rut dimensions following traffic. DMY was significantly (P < 0.05) reduced on the wheeled compared with the nonwheeled soil. No differences in the magnitude of DMY loss were identified between the sites having different drainage status. An SMD value of 10 mm was suggested as an SMD threshold for trafficability for safe slurry spreading purposes.

High-resolution water quality monitoring indicates recurring elevation of stream phosphorus concentrations during low-flow periods. These increased concentrations may exceed Water Framework Directive (WFD) environmental quality standards during ecologically sensitive periods. The objective of this research was to identify source, mobilization, and pathway factors controlling in-stream total reactive phosphorus (TRP) concentrations during low-flow periods. Synoptic surveys were conducted in three agricultural catchments during spring, summer, and autumn. Up to 50 water samples were obtained across each watercourse per sampling round. Samples were analysed for TRP and total phosphorus (TP), along with supplementary parameters (temperature, conductivity, dissolved oxygen, and oxidation reduction potential). Bed sediment was analysed at a subset of locations for Mehlich P, Al, Ca, and Fe. The greatest percentages of water sampling points exceeding WFD threshold of 0.035 mg L −1 TRP occurred during summer (57%, 11%, and 71% for well-drained, well-drained arable, and poorly drained grassland catchments, respectively). These percentages declined during autumn but did not return to spring concentrations, as winter flushing had not yet occurred. Different controls were elucidated for each catchment: diffuse transport through groundwater and lack of dilution in the well-drained grassland, in-stream mobilization in the well-drained arable, and a combination of point sources and cumulative loading in the poorly drained grassland. Diversity in controlling factors necessitates investigative protocols beyond low-spatial and temporal resolution water sampling and must incorporate both repeated survey and complementary understanding of sediment chemistry and anthropogenic phosphorus sources. Despite similarities in elevation of P at low-flow, catchments will require custom solutions depending on their typology, and both legislative deadlines and target baselines standards must acknowledge these inherent differences.