Effects of precipiation and spatial factors on tile-drainage and surface water salinity in a coastal deltaic island (Goeree-Overflakkee, The Netherlands).

Abstract

Due to its low lying properties, surface water in Goeree-Overflakkee (GO) is under the influence of saline seepage. Seepage enters the surface water directly and indirectly via tile drainage systems. Drain tiles salinities were sampled in four sites during spring in order to find spatial features which can be used to predict effluent salinity in response to precipitation events. From literature, important identified factors were seepage intensity, drainage depth, drainage area per tile and precipitation-evaporation surplus. A shortage in precipitation caused insignificant results in temporal variability in the measurements, making it impossible to quantify changes in salinity. However, drain tiles and measurement sites were significantly different suggesting an effect of spatial features to drainage salinity. Seepage intensity was identified as the only significant factor for causing differences between sites (large scale variability). A validation attempt to test the model to surface water concentrations in GO did not result in a good prediction, leaving room for future research. For variation between drain tiles, small scale variability, two mechanisms can be identified 1) boils are suspected to cause small scale spatial differences in seepage intensities, 2) cracks in the soils are thought to cause preferential flow in the rain water lens, resulting in drain tile specific mixture ratios of seepage, RW-lens water and precipitation. These ratios can cause drain tiles to respond differently to incoming precipitation and contribute to drain tile specific salinities. A spatial analysis into the salinities of surface water monitoring points did not result in identifying important factors. Future research should focus in area specific monitoring of the salinity response per area. This can give detailed information about the specific precipitation responses and can help with managing water quality according the WFD. For future research, a longer period is necessary to investigate precipitation responses in detail.