Rising Demand, Sinking Land

Abstract

Groundwater extraction could be a crucial driver of subsidence; a phenomenon that has had, and still has, implications for the landscape, infrastructure, and environment. Vitens, a Dutch drinking water company, expects an increase of 10% in drinking water demand in 2040 for the Netherlands. This may intensify Subsidence due to Groundwater Extraction (SGE) in the future. The objective of this research is to quantify the historical and future effects of groundwater extraction on subsidence at Vitens extraction site ”Groenekan” in the Province of Utrecht. In this thesis, a methodology is developed to assess whether Vitens can responsibly expand Groenekan groundwater extraction from 5.0 Mm3/yr to 10.0 Mm3/yr. The groundwater extraction is situated in a rural polder landscape where two subsidence-sensitive soil layers are present: a 1 m thick Holocene peat/clay top layer and a 12 m thick aquitard (Waalre clay) at a depth of -48 m NAP. Under this aquitard, Vitens currently extracts ∼5.0 Mm3/yr of drinking water from the second aquifer. Next, a groundwater model is used to study the lowering of the groundwater level and hydraulic head due to the extraction. Subsequently, the effect of both lowerings has been quantified for four subsidence processes, 1) Biochemical Degradation of Organic Material (BDOM), 2) shrinkage, 3) consolidation, and 4) creep. Groundwater lowering affects all four processes, whereas head lowerings only affect consolidation and creep. Occurred subsidence from the start of the groundwater extraction and for eight future scenarios is calculated with the aid of the groundwater model output, analytical approaches and a 2D subsidence model. The cumulative subsidence of these four processes is labelled as SGE. From 1961 to 2023, the groundwater level at Groenekan is lowered with 33 and 60 cm for 5.0 and 10.0 Mm3/yr extraction discharges respectively. In the second aquifer, hydraulic head lowerings of 1.5 to 2.9 m with a symmetrical influence circle are observed at Groenekan. In a radius of 500 m around Groenekan, significant groundwater level lowerings are found, which enforces compaction processes in the form of BDOM and shrinkage. For the period 1961 to 2023, assuming a 1 m thick clean peat Holocene top layer reveals ∼16 cm of potential BDOM. However, potentially ∼11 or ∼18 cm of shrinkage would occur over the same period assuming a clean clay or organic clay Holocene soil type respectively. Compression of the Holocene and Waalre layers due to consolidation and creep has contributed ∼10 times less to SGE around Groenekan compared to compaction due to BDOM and shrinkage. Within a 500 m radius of Groenekan, SGE is noticeable; beyond that, SGE is negligible. The maximum total subsidence through compression is ∼1.0 cm over a period of 62 years (from the start in 1961 to 2023). Generally, consolidation contributes 75% to the total subsidence due to compression, whereas creep is responsible for 25% of the compression. Sensitivity analyses reveal that the Holocene soil type and the thickness of the soft soils are the most sensitive parameters in calculating the final subsidence. Clean peat soil gives a high potential subsidence rate, whereas sand soils hardly subside. Variations in groundwater level and head lowerings are found to be least sensitive. Within the Groenekan system, the 1 metre Holocene thickness is the limiting factor in the groundwater level lowering, since no additional subsidence effects are found if the groundwater level drops below the Holocene layer. The future scenario of 10.0 Mm3/yr + an extreme climate shows ∼1.2 cm of consolidation and creep in the Holocene and Waalre layers over the period 1961 to 2100 (139 years). The base scenario of 5.0 Mm3/yr gives of ∼0.9 cm of subsidence, which results in ∼0.3 cm of additional subsidence. This extra subsidence value is negligible over a period of 139 years compared to other locations in the Netherlands with dozens of centimeters of subsidence. Though, BDOM and shrinkage can potentially cause centimetres of subsidence in the Holocene on the long-term due to groundwater level lowering. In conclusion, expansion of Groenekan from 5.0 to 10.0 Mm3/yr is considered to be responsible from a subsidence perspective as long as BDOM and shrinkage are exercised with extreme caution. Future studies on the effects of extended droughts and the surface water system on the groundwater level are suggested to better distinguish the total subsidence from SGE. Lastly, it is recommended to Vitens to extract groundwater from deep, confined aquifers with a sandy top layer for maximal SGE mitigation.