Leaching of the geochemical buffer capacity in the subsurface

The geochemical buffer capacity of the subsurface assists in breaking down pollutants before they reach drinking water abstraction zones. But this involves the generation of reaction products that sometimes pose a bigger problem for drinking water treatment than the original pollutant – for example, by increasing the groundwater’s hardness. A literature study was carried out in this project to get an overview of the current state of knowledge about geochemical leaching.

Leaching threatens the subsurface buffer capacity

Thanks to a geochemical buffer capacity, the subsurface in many parts of the Netherlands and Flanders has the property of containing or breaking down in a natural manner certain agricultural and industrial pollutants, such as sulphuric acid, nitrate, chlorinated hydrocarbons and pesticides. At the same time, these buffer reactions can also create problems, such as a deteriorating water quality, the necessary implementation of more advanced instead of simple treatment techniques, or an increase in the seriousness and incidence of well clogging. Because the soil leaching that occurs during the geochemical processes is for the most part irreversible, the soil’s buffer capacity can diminish, which has implications for the drinking water utilities. Given that the drinking water sector, through activities such as groundwater abstraction, artificial recharge, river-bank filtration, subsurface iron removal and ASR, is (partly) responsible for the leaching of the buffer capacity, it is important that a better understanding of the question be achieved. This research contributed to this by surveying the current knowledge about the leaching of the subsurface’s  geochemical buffer capacity and its impact on water quality.

Reactive components protect the soil

The inventory showed that the subsurface in the Netherlands generally contains sufficient reactive components to protect wells for tens to hundreds of years against the breakthrough of acidifying and oxidising substances or organic/inorganic micropollutants. This applies both to the regional groundwater systems and to artificial recharge systems, which involve much higher flow rates. Reaction products of geochemical buffer reactions (Ca, Fe, SO4, trace elements such as arsenic) can create problems for groundwater quality that are greater than those caused by the diminution of the buffer itself. The reaction products can have undesirable effects, such as a more intense treatment effort and in some cases well clogging. The risks and associated extra costs have hitherto often been neglected.

Reliable field measurements desired

The results can for instance give cause, in specific situations, to increase the geochemical buffer capacity of the subsurface, for instance, in the subsurface purification of groundwater and in the management of sludge layers in recharge basins.

In order to predict the macro-chemical groundwater composition and the behaviour of trace elements and organic micropollutants in groundwater, drinking water utilities need data about the geochemical composition and reactivity of the subsurface. This refers in particular to the concentrations and reactivity of SOM, pyrite, Fe hydroxides, siderite, lime and clay minerals. These parameters are hardly ever measured. It is therefore desirable that supplementary field measurements be taken. This can moreover be effectively combined with the replacement or the construction of new extraction and monitoring wells.

Illustration of geochemical reactions in subsurface water storage.

Illustration of geochemical reactions in subsurface water storage