Enhancing Freshwater Supply for Agriculture on Texel Island: Aquifer Storage and Recovery in a Saline Aquifer Using Horizontal Wells
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The Zoete Toekomst Texel (Fresh Future Texel) project aims to improve freshwater availability for farmers on Texel Island, the Netherlands, where agriculture relies entirely on rainwater due to the lack of alternative freshwater sources. Increasing drought frequency and intensity, driven by climate change, pose a growing challenge to sustainable farming. Aquifer Storage and Recovery (ASR) offers a potential solution by capturing excess tile drainage water in winter, storing it in the subsurface for use during the growing season. The project site’s specific hydrogeological conditions -a thin aquifer (8-10 m), low hydraulic conductivity (2-5 m/d), and high groundwater salinity (25-35 mS/cm)- favour horizontal wells over conventional vertical wells for higher recovery efficiency. After extensive hydrogeological assessments, including exploration drilling, cone penetration tests, geophysical surveys (DUALEM, ERT, borehole logging), pumping tests, and water quality analysis, the ASR system design was optimized and various operational scenarios tested with density-dependent groundwater flow modelling using MODFLOW 6 with the BUY-package. Between 2021 and 2024, four innovative horizontal wells (100-200 m screen length) were installed, each incorporating design improvements informed by operational experience and monitoring results, at 13-16 m below ground surface using horizontal directional drilling (HDD), for infiltration and extraction in an initially saline aquifer. However, two wells were abandoned due to construction failures. The remaining two connect to a slow sand filter with granular activated carbon for removal of pesticides and nutrients, which processes tile drainage water from a 30-ha catchment area. After 2.5 years of operation, the system has faced multiple challenges, including unintended seepage from hydraulic fracturing, strict waterboard regulations and high sand filter clogging rates limiting infiltration capacity. Despite these setbacks, the operational horizontal wells are capable of infiltrating ~20,000 m³/year, with 30-50% recovery within 1.6 mS/cm. A key innovation is the formation of a freshwater cooperation, where farmers jointly manage the system and share maintenance costs, improving cost-effectiveness. While challenges remain, this pilot provides valuable field-scale insights into horizontal ASR feasibility in saline environments, informing future applications.
