The background contribution of exfiltrating, ammonium-rich groundwater to the exceedance of the national ammonium standard for surface water bodies in Flevoland Polder (the Netherlands)

Abstract

In many regions of the Netherlands, ammonium concentrations in surface water exceed the standard of 0.3 mg NH4-N/L. In some polders, there is significant groundwater seepage that may contain relatively high natural concentrations of ammonium. This combination can result in substantial background nutrient loads that may or may not exceed the anthropogenic loads in surface water systems. Therefore, the Flevoland polder, where there is both ammonium exceedance and groundwater exfiltration, was chosen as the study area to address three primary questions: 1) the variation of ammonium concentration in surface water, seasonally and between dry and wet years 2) the variation of groundwater ammonium concentration over different time periods and spatially, and 3) the contribution of ammonium from groundwater to surface water system compared to other sources such as wastewater treatment plants, drain water from agriculture area, and nature land. In Flevoland polder, the typical concentration ranges for ammonium are as follows: for surface water, it is about 0 – 8 mg/L; for groundwater, it ranges from 0 to 60 mg/L or even higher; for drain water in agricultural areas, it is approximately 0 – 8 mg/L. To address the first question, surface water data from Dutch Water Information House were applied to make interpolation maps by ArcGIS Pro, and time series analyses were conducted using R. Meteorological data from KNMI was used to make annual precipitation bar chart by R language. Results indicated seasonal variations in surface water ammonium concentrations, with higher levels in winter, likely due to reduced microbial activity, and lower levels in summer, although occasional summer peaks were observed, potentially due to higher evapotranspiration and weaker dilution. No significant patterns were found between wet and dry years, possibly due to the lack of extreme wet or dry conditions during the study period. The second question was explored by creating interpolation maps in ArcGIS, using Dino and Bro datasets. The results reveal that groundwater ammonium concentrations were relatively stable over time, except for data prior to 1980. The analysis also showed that the data density for shallow groundwater (0 – 8 m-NAP) was insufficient for reliable interpolation. The distribution of ammonium concentrations in the 0 – 30 m-NAP interval was primarily influenced by deeper groundwater (8 – 30 m-NAP). To answer the last question, a mass balance approach was made to calculate the contributions of different sources to the ammonium load in surface water system. In addition to the aforementioned data sources, data representing leaching from agricultural fields provided by the National Minerals Policy Network (LMM) and data from the Lizard data warehouse & analytics platform were also used. The results indicated that groundwater seepage was the dominant source of ammonium, followed by drain water from agricultural. Wastewater treatment plants and nature areas contributed the least. Overall, this study highlights the significant role of groundwater in contributing to ammonium loads in surface water and underscores the importance of considering both temporal and spatial variations in managing water quality.