Reply to Louloudis et al. Comments on “Tzampoglou, P.; Loupasakis, C. Hydrogeological Hazards in Open Pit Coal Mines–Investigating Triggering Mechanisms by Validating the European Ground Motion Service Product with Ground Truth Data. Water 2023, 15, 1474”

The comments of Louloudis, Roumpos, Mertiri, and Kostaridis [1] refer to data related to previous studies and not the one presented in the commented paper [2]. These specific studies have been published and evaluated in numerous articles and international conferences, certifying their validity. Also, a multitude of researchers and organizations align with our viewpoint that overexploitation of the aquifer leads to the activation of land subsidence phenomena [3,4,5,6,7]. The scope of this study is to combine the already verified and published ground truth datasets acquired for the study of mining geohazards with the European Ground Motion Service (EGMS) InSAR dataset, aiming to cross-verify them.

Before responding, we wish to point that the authors of the comments have disclosed a clear conflict of interest, as they were employed by the Public Power Corporation of Greece and were involved as hydrogeological, mining, or geological experts in the engineering study of the Amyntaion mine.

Focusing on the current study, its scope is to combine the ground truth datasets acquired for the study of mining geohazards, namely land subsidence and landslide, with the European Ground Motion Service (EGMS) InSAR dataset, aiming to cross-verify their outcomes. The main objectives of the study are to (a) estimate the spatial distribution of the vertical deformation due to land subsidence and landslide by evaluating the EGMS InSAR datasets; (b) correlate the above data with the data obtained through the geotechnical research highlighting opposite driving failures mechanisms and main causal factors and (c) estimate the interaction between these two catastrophic events.

Regarding the accuracy of the ground water level measurements, the extent of the depression cone, and the equal-drawdown contour maps, we would like to emphasize that the wider mining area has been thoroughly studied for the past 20 years by the authors, and several field work campaigns were conducted in which over 40 boreholes were measured at the wider perimeter of the mine. The equal-drawdown contour lines maps were designed by subtracting the piezometric curves before the start of the mining activities, as reported in the PhD thesis [8] of, at that time, employee of the hydrogeological department of PPC, with the piezometric surfaces that resulted from our campaigns. These maps, along with the methodology used to create them, have been presented in five peer-reviewed papers [9,10,11,12,13] and six international conferences [14,15,16,17,18,19].

Regarding the irrigation drills, a drills census was conducted during our field work campaigns. The census proved that almost all the drills at the perimeter of the mine, and at more than 2 km, were inactive due to the intensive ground water drawdown. Aiming to cover the irrigation water needs, the farmers at the perimeter of the mine began pumping water from the draining canal network using electricity from the old power supply network, installed initially for the operation of the deep wells’ pumps. So, the electricity consumption, used by the authors of the comments [1] as proof of pumping activity, provides false information. This fact has been repeatedly reported to the PPC’s scientific personnel and it was also introduced during common on-site field visits.

As far as the extent of the depression cone caused by the mining activities, besides our above reported accurately designed piezometric contour maps, it is clearly presented in maps and cross-sections included in a recent publication [20] cowritten by the, at that time, director of the hydrogeological department of PPC, claiming that it was caused by the practically inactive irrigation drills. So, we both agree that the depression cone exists, although we disagree on its cause.

Please note that it is not just the authors’ opinion that the water table drawdown extends to a distance of 2 km, affecting the settlements located at the perimeter of the mine. It is commonly accepted and published in several papers and studies [3,4,5,6,7] that the open pit coal mine hydraulically resembles a huge well with a diameter of 3 to 4 km and appears to be the main cause of the groundwater table level drop, not only in its immediate vicinity but also in a wider area [6], triggering land subsidence phenomena.

Regarding the landslide event, in our publication [2], we identify several preparatory and triggering factors contributing to the triggering of the failure. Although each factor was individually studied, the synergistic interaction among them was not adequately assessed, ultimately leading to the failure’s manifestation. As far as the geometry of the slopes, it is provided by simple calculations. Knowing, from high-resolution satellite images, the horizontal distance between the base of the slope and the crown of the benches, as well as the height of the benches and the total depth of the mine, the steepness was calculated. A clear description of the applied procedure can be found in a previous publication [13] and is not included in this manuscript as this is not in the scope of this paper.

The statement that “the groundwater pore pressure at this depth (at the depth of 200 m) is not influenced by the mine’s pumping activities in the upper aquifer. Consequently, the 2017 landslide is not related to the pumping operations at the perimeter of the mine.” proves that, as abovementioned, the multiple causal and triggering factors were not properly evaluated. According to their statement, it is clear that they do not consider the interaction of the extensive faults intersecting the site, the high piezometric loads underneath the floor of the open pit, originating from a deeper karstic aquifer, or the occurrence of the pre-existing sheared zones within the Neogene formations that all together interact and co-act with the pore pressure of the shallow semi-confined aquifer. Also, if the stability of the slopes, as stated by the comments [1], “is not related to the pumping operations at the perimeter of the mine”, they should not have applied all the draining and pumping measures from the beginning of its operation, avoiding at the same time the occurrence of the land subsidence.

Our findings—derived through a rigorous series of processes—are thoroughly corroborated by EGMS satellite data records, confirming their validity. This particular aspect, which also constitutes the central aim of the original article under discussion, was completely overlooked by the authors of the comment report.

Finaly, we underscore that the EGMS datasets constitute a significant and reliable resource for disentangling the various causal mechanisms of ground deformations that have impacted the study area. Their provision by an independent and internationally recognized authority ensures that the conclusions drawn are scientifically robust and impartial.