The importance of exfiltrating nutrient-rich groundwater to the ammonium load of surface water in the Flevoland Polders, the Netherlands

In many regions of the Netherlands, ammonium concentrations in surface water exceed the EU Water Framework Directive standard of 0.3 mg NH4-N/L, which has an ecological origin. In many Dutch polders, there is significant seepage of groundwater that may contain naturally high concentrations of ammonium. This combination leads to a background load that is high compared to anthropogenic loads by agriculture, waste water discharge and other sources. The Flevoland Polders that were established in the 1960s, are known to be such an example.

This study investigated the water quality and surface water load with respect to ammonium for the Flevoland Polders for which large datasets on groundwater quality and surface water quality are available. We aim to characterise : 1. the variation of ammonium concentration in surface water, seasonally and between dry and wet years, 2. the spatial variation of groundwater ammonium concentration for different time intervals, and 3. the contribution of ammonium from groundwater to the surface water system compared to other sources such as waste water treatment plants, drain water from agricultural land, and leaching from nature areas.

The typical concentration ranges for ammonium are as follows: 1. about 0 – 8 mg NH₄/L for ditches and canals, 2. from 0 to 60 mg/L or even higher for groundwater, and 3. approximately 0 – 8 mg/L for drain water in agricultural areas. This illustrates the major role of groundwater exfiltration in the ammonium load of surface water taking into account groundwater exfiltration rates compared to recharge by net precipitation. More detailed data analysis indicates 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. 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. Further, the water analyses show that groundwater ammonium concentrations were relatively stable over time, except for data prior to 1980. Mass balance calculations indicate that groundwater seepage is the major source of ammonium  followed by drain water from agricultural land. Waste water 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. It also shows that background load should be taken into account when establishing water quality standards for surface water.