Suitability of Screened Monitoring Wells for Temperature Measurements Regarding Large-Scale Geothermal Collector Systems

Abstract

Groundwater temperature (GWT) is usually measured using screened monitoring wells (MWs). The aim of this study was to investigate whether MWs are suitable for monitoring the effects of large-scale geothermal collector systems (LSCs) on GWT, focusing on possible vertical flows within the MWs due to both natural and forced convection. Comparative temperature depth profiles were therefore recorded over a period of nine months in both shallow MWs and in small-diameter, non-screened temperature monitoring stations (TMSs), each of which was installed in a single borehole. Particularly high temperature deviations were measured in MWs in the upper part of the water column where the GWT reached up to 1.8 K warmer than in the surrounding subsurface. These deviations correlate unambiguously with the prevailing positive thermal gradients and are caused by thermal convection. Where forced convection occurred, the GWT was measured to be up to 0.8 K colder. Potential temperature deviations must be considered when monitoring very shallow GWT as thermal gradients can be particularly high in these zones. For monitoring concepts of LSCs, a combination of MW and TMS is proposed for GWT measurements decoupled by the effects of convection and in order to enable further investigations such as pumping tests.

Comparison of Measured and Derived Thermal Conductivities in the Unsaturated Soil Zone of a Large-Scale Geothermal Collector System (LSC)

“…These can be explained by the local decomposing to clayey 'Rotlehm', as in B4. In addition, the groundwater occurring in the partially decomposed and permeable basalt has high electrical conductivities, as was already measured in B3 [14] or is already known elsewhere in Bad Nauheim [67]. 2)-( 10) derived soil parameters of the individual ERT sections (Table 9) with the measured averaged and calculated values of ρ b , θ and λ listed in Tables 5 and 8.…”

“…Such considerations particularly concern the saturated zone, as thermal changes can result in hydrochemical and biological alterations of groundwater [10][11][12][13]. Therefore, a monitoring concept can be used to ensure the sustainable application of larger shallow geothermal systems [14,15]. Furthermore, the unsaturated zone and its thermal properties are essential, as the groundwater temperature down to a depth of 10 to 15 m is mainly influenced by vertical heat transport [16].…”

“…TP2 was conducted directly in the collector field and after backfilling with excavated material. Six 10.5 m deep boreholes for groundwater monitoring wells were installed around the LSC [14]. In addition, three more trial pits (TP4, TP5, TP6) were performed in the range of the low-temperature district heating network 170 to 300 m northwest of the LSC.…”

“…For the temperature correction, the average value of all temperature monitoring stations located at B1-B6 of the measurement on 22 April 2021 was formed, in this case T = 10.1 • C [14]. The electrical conductivity σ T is the reciprocal value of the measured electrical resistivity (ER).…”

Influence of Groundwater on the Very Shallow Geothermal Potential (vSGP) in the Area of a Large-Scale Geothermal Collector System (LSC)

“…Several infiltration tests with double rings [24] yielded higher values, but these are considered less reliable. The valley deposits overlay the 2.1 to 4.3 m thick bench gravel aquifer [25]. Due to the varying ground elevations, the distance between the groundwater and surface on site can range from 2.0 to 7.0 m. Since October 2020, groundwater levels have been measured with fluctuations of up to 1.9 m at six monitoring wells around the LSC (Figure 1a).…”