DRINKING WATER SUPPLY Geophysical Investigation of Subsidence around Wells in the Ridderkerk Well Field The Bohrlochmessung-Storkow GmbH investigated six selected groundwater wells from Oasen drinking water company in Ridderkerk, The Netherlands (Figure 1). The reason for the geophysical logging was to investigate the condition of several wells, where subsidence of annular space material occured at the surface after regenerations. n 2012, subsidence of annular space a result of the discussion it was planned to The objectives of the measurement Imaterial at the surface was discovered at organize a logging campaign in Ridderkerk. campaign were: a number of wells in the Ridderkerk well For the investigation of the annular 1. to determine the condition of the well field. It was assumed that the subsidence spaces of the wells the use of active radio- casings was related to sand production during metric measurements was necessary 2. to determine the condition of the annular regular pumping and well regenerations. (gamma-gamma density and neutron- space (find and test the hydraulic sealings, As a result, Oasen was looking for methods neutron). Thus, the Bohrlochmessung- find filling gaps, find clogged parts) to further investigate the settling features Storkow GmbH applied for a permission 3. to check for short-circuit flow (inside in the annular space. One of the main re- to handle sealed radioactive sources in the casing, and in the annular space) search questions was whether or not the Netherlands. The permit from the Dutch well construction is still hydrologically reli- Radiation Protection Authority was ob- Groundwater extraction site able after subsidence has occurred. tained in June 2013. Subsequently a mee- Ridderkerk First contact between Oasen and Bohr- ting was arranged at Oasen in Gouda, The Ridderkerk well field is located 10 km lochmessung-Storkow GmbH emerged on where the study objectives were clearly south-east of Rotterdam (Figure 1). The the Berlin Brandenburger Brunnentage in defined and a customized study program well field consists of 30 wells that mainly 2012. Here, the companies discussed suit- was created. In April 2014 six wells were extract riverbank filtrate, which infiltrates able geophysical logging methods to in- geophysically investigated during the log- from the Nieuwe Maas and Noord rivers. vestigate the subsidence phenomenon. As ging campaign. The Nieuwe Maas and Noord rivers are branches of the Rhine, which splits into numerous forearms in the coastal delta area. This deltaic depositional environ- ment in combination with transgressive and regressive phases of the Northern Sea level is responsible for the geological structure of the subsurface in the Rid- derkerk water catchment. The schematic structure of the geological subsurface in the Ridderkerk groundwater extraction field is shown in Figure 3. Three aquifers are present up to a depth of 100 m. The confining layer is formed by clay deposits and peat. The underlying first aquifer consists of medium to coarse grained sand and gravel, has a high content of organic matter, and locally contains silt or clay lenses. Below the first aquifer follows the about 20 m thick second aquiclude, which forms a continuous hydrologic bar- rier with the second aquifer. Beneath this aquiclude, the lithology is very heterogene- ous. The second and third aquifers mainly Figure 1: The Ridderkerk water catchment is located about 10 km south-east of the city center consist of fine sands, alternated with silt and of Rotterdam. The groundwater production well gallery is marked with the red ellipse clay layers. The aquiclude between the sec- ond and third aquifer is not continuously 48 gwf-Wasser | Abwasser International Issue 1/2015 DRINKING WATER SUPPLY present, and only locally forms an effective hydrological barrier. The presence of fine particles and or- ganic matter in the aquifers causes severe mechanical clogging of the wells. As a result a typical Ridderkerk well has to be regenerated once in three years. Well design and geophysical investigation program Two of the six geophysically investigated wells have their filter sections installed in aquifer one (shallow). The filter sections of the remaining four wells are located in the aquifers two or three (deep). A schematic overview of a general deep well design is shown in Figure 4. Figure 2: Blowout of surface annular space sealing in well RK-P34 after chemical regeneration The investigated wells have different comp letion characteristics which are sum- marized in Table 1. The logging program had to be adapted to these variable char- acteristics, whereas specific measurement methods could not be performed in all wells due to technical limitations (Packer- Test, Gas-Dynamic-Test – not possible in wells with casing diameters > 450 mm). In Table 2, the performed measurement methods and their investigation objectives are listed. As already noted, subsidence may be related to sand production. Interesting in that respect is to note that after regenerations in 2011 and 2012 sand was found in wells P09 (amount unknown), P15 (4 m) and P20 (3 m). At P21 and P34 the use of hypochlorite and Figure 3: Schematic overview of the geological setting and groundwater flow directions in peroxide during regeneration caused strong the Ridderkerk well field. Filter sections of the groundwater production wells are marked by reactions due to the presence of organic rectangular boxes with dashed blue lines matter and fine particles. At P21 this resulted Table 1: Design of investigated wells in water catchment Ridderkerk Well RK-P09 RK-P34 RK-P15 RK-P20 RK-P21 RORK-P38 construction date [year] 2001 1999 2003 1970 1970 2012 drilling diameter [mm] 700 / 572 1000 600 600 600 600 steel/ steel/ casing material stainless steel PVC PVC PVC stainless steel* stainless steel* casing diameter [mm] 508 475 230 254 / 155 254 / 155 315 filter depth [m] 13–19 15–20 45–57 49–59** 41–102 73–84 gravel pack grain size 1.0–1.6 1.0–1.6 1.0–2.0 0.8–1.25 0.8–1.25 1.0–1.6 [mm] * = extension pipe material is steel with bitumen coating, filter material is stainless steel ** = well is filled with cement beneath 59 m gwf-Wasser | Abwasser International Issue 1/2015 49 DRINKING WATER SUPPLY in the blowout of water. At P34 this resulted in a complete blowout of the surface sealing (Figure 2, 2005). Logging results The well casings of the oldest wells P20 and P21 are in a bad overall condition. The bitu- men coatings of these wells show serious damages, whereas significant signs of cor- rosion are visible at the exposed parts of the steel casing material. Several leaking casing joints between 13–45 m were detected along the riser pipes of these wells. The ca- sings of the other wells are in good overall condition; no leakages were detected. In the deeper wells (P15, P20, P21) subsi- dence in the annular space is proven by depths shifts of the clay sealings compared to with their projected intervals according to the construction plan. Also changes in Figure 4: Schematic overview of a “deep” groundwater production well design in the the respective clay sealing thicknesses as Ridderkerk water catchment well as intercalated sand at the top and base of the clay sealings are evidences for settling (P15, Figure 5). Furthermore, in well P20 the clay sealing between the first and second aquifer is ineffective due to subsidence, resulting in short circuit flow between the two upper aquifers. In wells P20 and P21 the aquifers two and three are already hydrau- lically connected, since no clay sealing was installed during construction of the well. A short circuit flow of about 1 m³/h through the blind filter parts was measured in well P21 under no-production conditions. In shallow wells P09 and P34, clay sea- lings were also found at different depths than specified in the construction plan. For P34 this was expected, since the clay sealing was repaired after the blowout in 2005. The sea- ling of P34 is considered hydraulically func- tional based on the geophysical measure- ments. At well P09 a clear indication of subsidence was found. The base of the sea- ling clay was detected 1.5 m lower than expected, and the clay was mixed with sand. In 2012 the upper part of the clay sealing of P09 was repaired, since the sealing was hy- draulically permeable. The newly installed Figure 5: Comparison between planned well design and actual construction state according clay sealing contains very high amounts of to the geophysical logging results. Settlement in the annular space is proven by the depth water, and is therefore rated suspension-like. displacement and the intercalated sand at the top and the bottom of the clay sealing. Further, The sealing of P09 is probably ineffective, the Gas-Dynamic-Test (injection of nitrogen into the annular space via the upper filter slots) despite of the repair works. shows that the clay sealing is hydraulically ineffective at least up to a depth of 33 m (accu- mulation in the annular space marks ascension path of injected nitrogen, orange shading.) The overall condition of the new well P38 is very good, as expected. No subsi- 50 gwf-Wasser | Abwasser International Issue 1/2015 DRINKING WATER SUPPLY dence or short circuit flow was detected Table 2: Overview of conducted borehole geophysical measurements and investigation purposes at this well. Oasen used magnetic clay sea- Logging Method Investigation Purpose lings at P38, which were clearly visible in casing diameter, pipe connections, screen posi- Caliper-Log (CAL) the MAL-, SGL- and NN-Logs. Since no sub- tion, deposits/corrosion, damages, obstacles in PVC-casings determination between sidence had occurred, the boundaries Focused-Electrical-Log (FEL) – PVC casing and filter (FEL), inspection of leaka- between the clay sealings and the annular Packer-Test (PA-T) – Steel ges at pipe connections gravel fillings were sharp.