Unexpected impact of agricultural land-use practices on the concentration and trend in hardness of groundwater abstracted for drinking water supply

Diffuse pollution of shallow groundwater as a result of leaching of substances from agricultural soils has a negative impact on groundwater quality. In the drinking water sector, the focus has traditionally been on nitrogen (nitrate) and crop protection products, which are subject to strict standards. Geochemical buffering processes in the subsurface convert a large part of the nitrate load that leaches to groundwater from agricultural soils. However, these processes can often lead to an increase in the hardness (sum of the calcium and magnesium concentrations) of groundwater, which is undesirable for drinking water and domestic use.

Recent research shows that 70% of phreatic groundwater extraction locations in the Netherlands show a significant increasing trend in hardness. However, quantitative insight into the relationship between spatial characteristics (land use, soil type and geochemical composition of the subsoil) and hardness has so far been lacking. In this research we analyzed a long time series of data (since 1900) from shallow (< 25 m below ground surface) phreatic and semi-confined groundwater extraction locations in the Netherlands. The trends in hardness and partial pressure of CO₂ (P_CO2) and relationship with spatial characteristics of the extractions is presented.

A clear influence of agriculture activities in groundwater protection areas was observed; the hardness in agricultural-dominated extraction sites was 2.5 times higher compared to nature-dominated extraction sites, while the trend in the increase in hardness was almost 3 times higher. The trend is observed in both calcium carbonate-rich and calcium carbonate-poor soils. In carbonate-rich areas, the hardness of groundwater is determined by the addition of acid, from atmospheric deposition, agricultural activities such as fertilization and crop harvesting and by weak acid (CO₂) contributions from root respiration and mineralization of organic matter. In carbonate-poor soils hardness sources are the use of calcium and magnesium salts and the application of manure on agricultural land. 

In carbonate-rich systems, slightly less than half of the groundwater hardness was found to be due to limescale weathering due to strong acid input, while slightly more than half of the hardness was due to weathering from CO₂. Higher P_CO2 levels and trends in agricultural-dominated extraction sites comparted to nature-dominated sites reveals an impact of intensive agricultural production on the CO₂ production is soils, and thereby on groundwater quality, that have not been considered so far.

To minimize hardness as a result of soil acidification it is recommended to reduce nitrogen deposition, limiting nitrate leaching and limit the use of fertilizers that acidify the soil. To minimize hardness as a result of weak acid (CO₂) weathering, more extensive agricultural practices to reduce root respiration should be adopted, and the degradation of soil organic matter can be limited by preventing (short-term) lowering of groundwater levels in organic rich soils. The results indicate that land use has a significant effect on the hardness and P_CO2 in groundwater. Mitigating measures should consider an area-based approach, taking into account the land-use, soil type and geochemical characteristics of the subsurface to limit the impacts of increased hardness and P_CO2 as a result of agricultural activities.