R. Dijksma

Water flow and nitrate transport to a groundwater‐fed stream in the Belgian–Dutch chalk region

Human activities, such as high fertilizer applications, groundwater abstractions and land use changes, might have a negative impact on drinking water supply, environment and nature reserves of the chalk region in the Dutch–Belgian boundary area. In this area, the groundwater and surface water systems of the Noor experimental catchment have been monitored intensively and modelled to investigate groundwater flow and the transport of nitrate to groundwater-dominated streams. Groundwater flow towards the riparian area, the springs and Noor brook is strongly controlled by long-term variation of the groundwater recharge. Relatively small, but permanent changes in the recharge have a higher impact on spring flow and stream flow than current groundwater abstraction in this chalk catchment. Land use, groundwater flow patterns and the characteristics of the geological formations result in distinctly different groundwater types and associated nitrate concentrations. High concentrations were found under the plateau (median 55 mg · l−1) and extremely low in the riparian area (median 2 mg · l−1). The springs and thereby the Noor brook are mainly fed by groundwater directly coming from the plateau with a low probability of denitrification. This results in median NOconcentrations which are clearly above the drinking water standard of 50 mg · l−1 (i.e. 68 and 58 mg · l−1). Monitoring and modelling show that the north springs have higher NOconcentrations than the south springs due to different probabilities of denitrification. Nitrate in the groundwater-dominated Noor brook is hardly correlated with the discharge, but the major spring shows a clear upward trend in terms of nitrate concentrations, i.e. an increase by about 30 mg · l−1 from 1980 onwards. A policy of reduced nitrogen application will result in lower nitrate concentrations in the surface water system not earlier than in 10 or 20 years, because of the long travel times of water particles in the unsaturated and saturated zone. The north springs will react earlier than the south springs. Copyright

Hydrological classification, a practical tool for mangrove restoration

Mangrove restoration projects, aimed at restoring important values of mangrove forests after degradation, often fail because hydrological conditions are disregarded. We present a simple, but robust methodology to determine hydrological suitability for mangrove species, which can guide restoration practice. In 15 natural and 8 disturbed sites (i.e. disused shrimp ponds) in three case study regions in south-east Asia, water levels were measured and vegetation species composition was determined. Using an existing hydrological classification for mangroves, sites were classified into hydrological classes, based on duration of inundation, and vegetation classes, based on occurrence of mangrove species. For the natural sites hydrological and vegetation classes were similar, showing clear distribution of mangrove species from wet to dry sites. Application of the classification to disturbed sites showed that in some locations hydrological conditions had been restored enough for mangrove vegetation to establish, in some locations hydrological conditions were suitable for various mangrove species but vegetation had not established naturally, and in some locations hydrological conditions were too wet for any mangrove species (natural or planted) to grow. We quantified the effect that removal of obstructions such as dams would have on the hydrology and found that failure of planting at one site could have been prevented. The hydrological classification needs relatively little data, i.e. water levels for a period of only one lunar tidal cycle without additional measurements, and uncertainties in the measurements and analysis are relatively small. For the study locations, the application of the hydrological classification gave important information about how to restore the hydrology to suitable conditions to improve natural regeneration or to plant mangrove species, which could not have been obtained by estimating elevation only. Based on this research a number of recommendations are given to improve the effectiveness of mangrove restoration projects.